the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Feb 2022
Recent significant decline of strong carbon peat accumulation rates in tropical Andes related to climate change and glacier retreat
Abstract. Climate change has altered precipitation and temperature patterns in the tropical Andes. As a result, tropical glaciers have retreated significantly over the past 50 years and have even disappeared in some areas. Andean peatlands, one of the most important Andean carbon reservoirs, also seem to be affected by these climate changes, since glaciers have been recognized as one of their vital water sources. Here, we point out the important role of Andean peatlands on carbon accumulation rates (CAR), one of the highest in the world, and the impact of climate on carbon storage over the last 65 years, using four peat cores. The peat cores were radiocarbon-dated and ages were post-bomb calibrated and chronological models indicated basal ages (30 cm depth) ranging from 1957 to 1972 CE, where accumulation rates reached up 1.7 cm yr−1. For both peatlands, carbon accumulation rates are high (mean of 470 and 220 g C m−2 yr−1 at APA 1 and APA 2 sites, respectively) and can reach up to 1010 g C m−2 yr−1. Distichia muscoides is the dominant species in the Peruvian Central Andes peatlands and the high CAR, among other factors, is a characteristic of this species. Our results point out that a marked decrease of CAR after the early 1980s at both peatlands is likely related to an increase in annual temperature, which is responsible for the retreat of glaciers. We use a new high-resolution proxy (Skrzypek et al., 2011) based on the δ13C of Distichia along the cores to evaluate the temperature variability at the site. We observed a general trend of increase in the reconstructed temperature from both studied peatlands from 1.9 to 2 ºC for the period 1970–2015 CE. Comparison with air temperature data from the NCEP-NCAR reanalysis for the higher resolution cores shows a good relationship and an increase of 2.15 °C for the same period. Temperature increase may directly affect CAR by an increase in organic matter degradation rates. The decrease in CAR during the period of study may also be due to a decrease in melt water inflow generated by the retreat of glaciers that have almost disappeared today in the catchments as a consequence of regional warming. Our findings emphasize that marked changes in carbon accumulation rates demonstrate the high ecological sensitivity of tropical high-Andean peatlands, endangering their outstanding role in the regional (and even global) C cycle as large C sinks that contribute to the mitigation of global climate change.
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RC1: 'Comment on bg-2022-47', Anonymous Referee #1, 10 Mar 2022
General comments
Llanos et al. present a record from a sedimentary core from the Apacheta region in the central Peruvian Andes. Four peat cores from high-Andean Distichia cushion-plant peatlands were radiocarbon-dated and C accumulation rates, TOC and C stable isotope composition are presented for the four 29-35 cm long peat cores. Based on the presumption by Skrzypek et al. (2011), who interpreted growing season temperature as a determining factor for δ13C in a high-Andean Distichia peatland in Peru, the authors reconstruct temperature from both studied peatlands for the period 1970-2015 CE.
The presented research would potentially represent an important contribution of paleodata in a region, where paleoenvironmental data is still very scarce. However, the presentation and interpretation of the data need significant improvement, and currently, the presented research does not represent a sound and elaborate work. Overall presentation, methodological concept, and data interpretation are not ready yet for publication. As the presented research work needs significant improvement on several topics, I do not recommend it for publication in Biogeosciences.
Specific comments
1. The exact coordinates of the investigated sites are missing. However, the sites can somewhat be located with help of Figure 1. By checking the "sub-catchments", I absolutely do not agree with the presented "sub-catchment" area of 130 km² for APA 1, which is situated in a kar valley of Nevado Portuguesa (aka Chicllarazo or Apacheta) and has no connection to the yellow-shaded area. However, without exact coordinates, this remains unclear.
2. The "study area" chapter lacks important information. Distichia muscoides is the only plant species mentioned. In the central Peruvian Andes, cushion-plant peatlands are often dominated by Distichia, but accompanied by other species, which - depending on site factors - might dominate specific areas of the peatlands (other cushion-formers like Plantago rigida, Zameioscirpus muticus, Phylloscirpus deserticola or reed grasses like Deyeuxia/Calamagrostis) or grow into the Distichia cushions. Further, these peatlands are usually characterized by shallow pools, which form between the cushions (Coronel et al. 2004). No information is given on that, nor on the topography of the peatland, nor on the influence of grazing or other impact by the local population. Further, no information is provided on the possible influence of geothermal springs, which might contribute to the springwater. The presented study did not conduct analysis of the peatlands` spring and surface water (at least pH and conductivity), which is a prerequisite for any peatland study. Noble & McKee (1982) mention geothermal springs for the Nevado Portuguesa area. Can the influence of geothermal water be excluded?
3. The authors point out the relation of carbon peat accumulation rates and glacier retreat, since glaciers have been recognized as "the main water source" for high-Andean peatlands. Line 266 says: "The subsequent reduction in peat growth rates could have been due in part to the decrease in the rate of water inflow from nearby glaciers to peatlands after their complete disappearance." In point of fact, I cannot detect glaciers within the upper catchments of both investigated peatland sites. Many peatlands in the tropical Andes are fed by glacial meltwater. However, the majority of high-Andean peatlands is fed by permafrost (Ruthsatz et al. 2020), and water originates from high-elevation cryogenic soils and glaciolithic deposits (Trombotto 2000). This is the case for the two investigated peatlands (as far as I presume from Figure 1 and Google Earth). Therefore, the whole climate change-related argumentation should not solely focus on glaciers, but also on the very important role of permafrost.
4. For radiocarbon dates, the authors use the SH calibration dataset. Due to a significant influx of Northern Hemisphere air masses and moisture over a substantial part of the continent, especially the tropical central Andes, during the South American Summer Monsoon (SASM), Marsh et al. (2018) recommend using a mixed calibration curve. During the austral spring and summer seasons, the south shift in the ITCZ brings atmospheric CO2 from the Northern Hemisphere to the Andes, which is taken up by the vegetation during the growing season (Schittek et al. 2016). How do the authors explain the use of the SH calibration set?
5. The authors do not pay attention to the effect that bulk peat stable carbon isotopes may reflect the dilution of atmospheric δ13CO2 and the effects of early stage kinetic fractionation during diagenesis (Esmeijer-Liu et al. 2012) or other factors like dust influx or vegetational changes. For a scientifically sound reconstruction of paleotemperatures, this has to be taken into account.
6. A scientifically sound "high-resolution" record concerning the past 50 years would require age control by applying the Pb/Cs dating method rather than applying CaliBomb upon radiocarbon dating for only 3 samples per core.
7. The stable isotope measurement method is described in only one sentence. How about the use of calibrated laboratory standards and what is the analytical uncertainty?
line 43: change "High-altitude" into "High-elevation"
line 46: "Their most important ecological role..." This sentence should be reworked. First, it should not be evaluated what is the most important ecological role of peatlands. Second, tropical peatlands do not control decomposition processes in the soil!!!
lines 52-75: The focus should be rather on permafrost than on glaciers as the investigated peatlands seemingly are not fed by glacial meltwater.
line 81: Vegetation is not dominated by Distichia muscoides! (What is meant by "vegetation"??? Peatland? Steppe?). This is only the case for peatland areas with permanent saturation. The cited literature in this paragraph has no relation to the Apacheta region.
line 90: "The climate of the Apacheta peatlands..." I do not agree with this statement. First, it should be "Apacheta region", second, there definitely is a rainy and a dry season, as this area is affected by the South American summer monsoon.
line 199: Chimner
line 200: Oxychloe
line 200: Azorella is typical for high-Andean steppe vegetation and never grows inside a peatland.
line 250: "...which are typically associated with glacial dynamics..." I do not agree. Distichia muscoides is associated to permanent saturation above 4000 m asl. Its distribution is not restricted to the presence of glaciers.
line 292: "good relationship" What does that mean? Did you conduct any correlation analysis?
Figure 8: The reconstructed air temperatures of the two presented cores, in some parts, differ significantly, although the two coring sites are very close to each other. How do the authors explain this? How about the other two retrieved cores? Is there any results for them?
The following publications are mentioned in the manuscript, but not listed in the references:
Salvador et al. 2014, Huaman et al. 2020, Thompson et al. 2006, Kalnay et al. 1996, Hribljan et al. 2015, Hribljan et al. 2016, Drexler et al. 2015, Cooper et al. 2010, Lourencato et al. 2017, Roa-Garcia et al. 2016, Lähteenoja et al. 2013, Hapsari et al. 2017, Craft & Richardson 1993, Tolonen & Turunen 1996, Turunen et al. 2001, Chimner & Cooper 2003, Turunen et al. 2004, Beilman et al. 2009, Van Bellen et al. 2011, Nakatsubo et al. 2014, Chimner et al. 2016, Bao et al. 2010, Mitsch & Gosselink 2007References:
Coronel J.S., Declerck S., Maldonado M., Ollevier, F. & Brendonck L. (2004): Temporary shallow poolsin high-Andes bofedal peatlands: a limnological characterization at different spatial scales. Archives des Sciences 57: 85-96.Noble D.C. & McKee E.H. (1982): Nevado Portugueza volcanic center, central Peru; a Pliocene central volcano-collapse caldera complex with associated silver mineralization. Economic Geology 77(8): 1893-1900.
Ruthsatz B., Schittek K. & Backes B. (2020): The vegetation of cushion peatlands in the Argentine Andes and changes in their floristic composition across a latitudinal gradient from 39°S to 22°S. Phytocoenologia 50(3): 249-278.
Trombotto, D. (2000): Survey of cryogenic processes, periglacial forms and permafrost conditions in South America. Revista do Instituto Geológico 21: 33–55.
Marsh E.J., Bruno M.C., Fritz S.C., Baker P, Capriles J.M. & Hastorf C.A. (2018): IntCal, SHCal, or a Mixed Curve? Choosing a 14C Calibration Curve for Archaeological and Paleoenvironmental Records from Tropical South America. Radiocarbon 60(3): 925-940.
Schittek, K., Kock, S.T., Lücke, A., Hense, J., Ohlendorf, C., Kulemeyer, J.J., Lupo, L.C. & Schäbitz, F. 2016. A high-altitude peatland record of environmental changes in the NW Argentine Andes (24°S) over the last 2100 years. Climate of the Past 12: 1165–1180.
Esmeijer-Liu A.J., Kürschner W.M., Lotter A.F., Verhoeven J.T.A. & Goslar T. (2012): Stable carbon and nitrogen isotopes in a peat profile are influenced by early stage diagenesis and changes in atmospheric CO2 and N deposition. Water Air Soil Pollut 223: 2007-2022.
Citation: https://doi.org/10.5194/bg-2022-47-RC1 -
AC1: 'Reply on RC1', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the reviewer's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
GENERAL COMMENTS
Llanos et al. present a record from a sedimentary core from the Apacheta region in the central Peruvian Andes. Four peat cores from high-Andean Distichia cushion-plant peatlands were radiocarbon-dated and C accumulation rates, TOC and C stable isotope composition are presented for the four 29-35 cm long peat cores. Based on the presumption by Skrzypek et al. (2011), who interpreted growing season temperature as a determining factor for δ13C in a high-Andean Distichia peatland in Peru, the authors reconstruct temperature from both studied peatlands for the period 1970-2015 CE.
The presented research would potentially represent an important contribution of paleodata in a region, where paleoenvironmental data is still very scarce. However, the presentation and interpretation of the data need significant improvement, and currently, the presented research does not represent a sound and elaborate work. Overall presentation, methodological concept, and data interpretation are not ready yet for publication. As the presented research work needs significant improvement on several topics, I do not recommend it for publication in Biogeosciences.
We thank the referee for their positive comments, their detailed review and for the constructive recommendations. We respond thereafter to each of their comments.
SPECIFIC COMMENTS
1. The exact coordinates of the investigated sites are missing. However, the sites can somewhat be located with help of Figure 1. By checking the "sub-catchments", I absolutely do not agree with the presented "sub-catchment" area of 130 km² for APA 1, which is situated in a kar valley of Nevado Portuguesa (aka Chicllarazo or Apacheta) and has no connection to the yellow-shaded area. However, without exact coordinates, this remains unclear.
We thank the referee for this comment and in the new version we have included the location in the manuscript.
The exact coordinates of the investigated sites are now specified and we detected a mistake in APA1 position on the map figure 1. APA 1 Cores: Lat. -13,35128; Long. -74,65882; APA 2 Cores: Lat. -13,34324; Long. -74,66140.
Now the sub-catchment areas have been recalculated after a supplementary field work (March-April 2022) and indeed APA1 is much smaller since it is not drained by Apacheta River which is located several meters below the peatland. The sub-catchment area of APA1 is only 3.3 Km2 while for APA2, the sub-catchment area, inserted in APA2 one, is 2.14 km2 (please see Figures RC1.1: The sub-catchment area of APA1, and Figure RC1.2: The sub-catchment area of APA2, in SUPPLEMENT).
None of the peatlands studied is located in the valley of Nevado Portuguesa, the location of the APA1 point on the map (figure 1B) was wrong, but it has already been rectified and the coordinates have also been specified in the paper.
2. The "study area" chapter lacks important information. Distichia muscoides is the only plant species mentioned. In the central Peruvian Andes, cushion-plant peatlands are often dominated by Distichia, but accompanied by other species, which - depending on site factors - might dominate specific areas of the peatlands (other cushion-formers like Plantago rigida, Zameioscirpus muticus, Phylloscirpus deserticola or reed grasses like Deyeuxia/Calamagrostis) or grow into the Distichia Further, these peatlands are usually characterized by shallow pools, which form between the cushions (Coronel et al. 2004). No information is given on that, nor on the topography of the peatland, nor on the influence of grazing or other impact by the local population. Further, no information is provided on the possible influence of geothermal springs, which might contribute to the springwater. The presented study did not conduct analysis of the peatlands` spring and surface water (at least pH and conductivity), which is a prerequisite for any peatland study. Noble & McKee (1982) mention geothermal springs for the Nevado Portuguesa area. Can the influence of geothermal water be excluded?
We thank you for these recommendations in order to provide the reader with more information about our study area.
Accompanying species found in the study area will be added.
“In this region, Distichia muscoides Nees & Meyen (Juncaceae) is the predominant cushion peatland species, and it is present on most high-elevation peatlands in the central Peruvian Andes (Schittek et al., 2015), however other plant species are also found, such as Plantago tubulosa, Aciachne pulvinata, Scirpus rigidus, Calamagrostis rigescens, Calamagrostis spp., Hypochaeris sessiliflora, Hypsela reniformis. Distichia cushions are surrounded by little shallow pools (around 50 cm-large).”
We have also included information about the topography and soils of the study area:
“The Apacheta region is characterized by being a mountainous area, with peatlands located in the valleys and sections with gentle slope, at altitudes above 4100 m asl. Edaphologically, the study area is mainly composed of relatively medium texture deep soils developed upon volcanic rocks (porphyritic andesite) from Apacheta formation (Nm-ap_s) (INGEMMET, 2002). In this area, the main economic activities of the local population are agriculture and livestock. Agriculture takes place at lower altitudes than peatlands and grazing of livestock occurs in the peatland zone, because peatlands provide year-round forage production for grazing native domestic camelids (llama and alpaca) and for livestock species (particularly sheep). Evidence of grazing activity has been observed in the study area although with little visible impact on peatlands.”
About the influence of geothermal water on the study area:
We think that the study area is not influenced by geothermal activity. But as mentioned by Noble & McKee (1982), there are thermal springs in the surrounding region. Also, according to the database of INGEMET (National Geological, Mining and Metallurgical Institute) of Peru there are 2 thermal springs:
- Niñobamba (-13,334°; -74,581°; 3670 m asl) which is located more than 7 km downstream from the Apacheta River and 300 m of altitude difference, so it would not have influence in the study zone.
- Licapa (-13,361°, -74,871°, 4100 m asl) located approximately 23 km west of the study area and belongs to another hydrological basin, so the influence of a geothermal spring is ruled out.
About the analysis of the peatlands` spring and surface water, sorry for our negligence. We have measured pH and conductivity and will add this information in the text:
“Between the cushions of APA 1 an APA 2 peatlands we found small and shallow pools of water that are characteristic of this type of ecosystem. The mean pH and conductivity, measured in these pools during the campaign, were 5.93 and 45.4 µS cm-1 for APA 1 and 6.01 and 39.2 µS cm-1for APA2, respectively.”
2. The authors point out the relation of carbon peat accumulation rates and glacier retreat, since glaciers have been recognized as "the main water source" for high-Andean peatlands. Line 266 says: "The subsequent reduction in peat growth rates could have been due in part to the decrease in the rate of water inflow from nearby glaciers to peatlands after their complete disappearance." In point of fact, I cannot detect glaciers within the upper catchments of both investigated peatland sites. Many peatlands in the tropical Andes are fed by glacial meltwater. However, the majority of high-Andean peatlands is fed by permafrost (Ruthsatz et al. 2020), and water originates from high-elevation cryogenic soils and glaciolithic deposits (Trombotto 2000). This is the case for the two investigated peatlands (as far as I presume from Figure 1 and Google Earth). Therefore, the whole climate change-related argumentation should not solely focus on glaciers, but also on the very important role of permafrost.
Indeed nowadays there is no glacier in the area and, as the temperature is always positive (see MAAT curves) very probably no permafrost exist today. According to Chadbrun et al. (2017), permafrost requires an average annual temperature of less than -2°C. It is not the case in our study area as we can see with the annual temperature data shown in Figure RC1.3 (Mean annual temperature (°C) over the period 1958-2018 of the four pixels of TerraClimate datasets covering the two sub-catchments, in SUPPLEMENT), so the probability of having permafrost at present is low.
The TerraClimate dataset comprises a global dataset based on reanalysis data since 1958, with a 4 km grid size at a monthly time scale. This dataset was validated with the Global Historical Climatology Network using 3,230 stations for temperature (r =0.95; mean absolute error 0.32°C) and 6,102 stations for precipitation (r =0.90; mean absolute error 9.1%) (Abatzoglou et al., 2018).
The development of permafrost is badly known in the Andes (Trombotto, 2000) and cryogenic soils are formed above 4600m in Peruvian and Bolivian Cordilleras (Trombotto, 2000). So we cannot exclude that permafrost has played some role in the past in supplying groundwaters to the peatlands during period of warming climate. The manuscript have been modified in this sense.
4. For radiocarbon dates, the authors use the SH calibration dataset. Due to a significant influx of Northern Hemisphere air masses and moisture over a substantial part of the continent, especially the tropical central Andes, during the South American Summer Monsoon (SASM), Marsh et al. (2018) recommend using a mixed calibration curve. During the austral spring and summer seasons, the south shift in the ITCZ brings atmospheric CO2 from the Northern Hemisphere to the Andes, which is taken up by the vegetation during the growing season (Schittek et al. 2016). How do the authors explain the use of the SH calibration set?
We thank the referee for this observation and, indeed, as the region rainfall is associated to the South American Monsoon a mixed atmospheric post-bomb 14C curve between Northern and Southern Hemisphere must be used. We recalibrated the age with the most recent curve published by Hua et al. (2021) using the mixed curve recommended for South American Monsoon region (Bomb21SH3). All other data have been recalculated in agreement of this.
The new age models are shown in Figure RC1.4 (in SUPPLEMENT) and are very similar to the old ones.
5. The authors do not pay attention to the effect that bulk peat stable carbon isotopes may reflect the dilution of atmospheric δ13CO2 and the effects of early stage kinetic fractionation during diagenesis (Esmeijer-Liu et al. 2012) or other factors like dust influx or vegetational changes. For a scientifically sound reconstruction of paleotemperatures, this has to be taken into account.
Esmeijer-Liu et al.(2012) have studied a peat core from Northern Finland and observed an increase of ∂13C toward the top. The linear trend of ∂13C increase is 0.0072 ‰ yr-1, to be compare to the 0.047‰ yr-1 (APA1- C5) and 0.044‰ yr-1 (APA2- C4) we observed in our cores. The most recent atmospheric ∂13C - CO2 data (Graven et al., 2017) indicate a trend of 0,0078‰ yr-1 for the same period, very close to Esmeijer-Liu et al (2012). So, the observed variation in Finland core can be explained by the atmospheric trend alone. However, the authors consider that there is also an effect of ∂13C decrease during the diagenesis of organic matter. Considering the trends value, this effect must be very low compare with the changes we measured. However, the referee is right in remembering this « Suess » effect on atmospheric ∂13C - CO2 and we will integrate it in our calculations.
Distichia remains are observable in great quantity from the top to the base of our cores and there is no evidence of vegetation change during the study period. We did not find any data about recent dust deposition in the region but it will be surprising that dust deposition has decrease in the las decades.
6. A scientifically sound "high-resolution" record concerning the past 50 years would require age control by applying the Pb/Cs dating method rather than applying CaliBomb upon radiocarbon dating for only 3 samples per core.
Dating of peat using 210Pb or 137Cs is a tricky problem, especially in highly porous peatlands like those dominated by Distichia where lead is likely to move. The roots of Distichia are likely to transport lead to deeper areas (Benavides et al., 2015). Most importantly, however, dating from Distichia peatlands in Colombia (Benavides et al., 2015) has shown very low unsupported lead values, close to the background of supported lead, when the sedimentation rate is very high. Since the sedimentation rates we observe are even stronger we believe that this method could not have been used.
7. The stable isotope measurement method is described in only one sentence. How about the use of calibrated laboratory standards and what is the analytical uncertainty?
The analytical standard used was “High Organic Sediment Standard – OAS” that consists of batch of high organic content sediments standard traceable to IAEA-CH-6, prepared and certified by Elemental Microanalysis Ltd, Okehampton, Devon. The analytical uncertainty is 0.2‰.
line 43: change "High-altitude" into "High-elevation"
We agree.
line 46: "Their most important ecological role..." This sentence should be reworked. First, it should not be evaluated what is the most important ecological role of peatlands. Second, tropical peatlands do not control decomposition processes in the soil!!!
Our sentence is not clear, of course we cannot evaluate the most important ecological role of peatlands. They just have a great potential to accumulate organic carbon due to the slow decomposition rate of the plants that give origin to the peat. We have changed the text.
lines 52-75: The focus should be rather on permafrost than on glaciers as the investigated peatlands seemingly are not fed by glacial meltwater.
See point 3.
line 81: Vegetation is not dominated by Distichia muscoides! (What is meant by "vegetation"??? Peatland? Steppe?). This is only the case for peatland areas with permanent saturation. The cited literature in this paragraph has no relation to the Apacheta region.
Distichia is the dominant plant species in cushion peatlands in this region and, according to Schitteck et al 2015, in the Central Andes as a whole.
line 90: "The climate of the Apacheta peatlands..." I do not agree with this statement. First, it should be "Apacheta region", second, there definitely is a rainy and a dry season, as this area is affected by the South American summer monsoon.
We fully agree, this is what we meant when we wrote about “large seasonal precipitation variability” with a total rainfall of 830 mm with 82% of precipitation during the rainy season from october to march.
line 199: Chimner
We agree.
line 200: Oxychloe
We agree.
line 200: Azorella is typical for high-Andean steppe vegetation and never grows inside a peatland.
We agree.
line 250: "...which are typically associated with glacial dynamics..." I do not agree. Distichia muscoides is associated to permanent saturation above 4000 m asl. Its distribution is not restricted to the presence of glaciers.
You are right. Tropical glaciers are an important source of water only for peatlands that are close to them, within their watershed. We have changed the text.
line 292: "good relationship" What does that mean? Did you conduct any correlation analysis?
The comparison is only visual, correlation calculation for this kind of data with different dates in each curve is not very possible. We compared the trends for the NCEP data and reconstructed temperature.
Figure 8: The reconstructed air temperatures of the two presented cores, in some parts, differ significantly, although the two coring sites are very close to each other. How do the authors explain this? How about the other two retrieved cores? Is there any results for them?
As we explained, one of the problems is that the ∂13C and thus temperature values are obtained for different dates on the different cores. Another point is that the growth rates are different in the two cores, meaning that each sample does not correspond to the same interval of time. However, it is certain that this cannot explain all the differences between the curves and those other parameters than temperature also influence the ∂13C. This is part of the uncertainty on the temperature reconstruction that we consider nevertheless to be the main factor influencing the ∂13C.
For the two other cores the sampling interval is too large (~3 cm) for a good temporal resolution.
The following publications are mentioned in the manuscript, but not listed in the references: Salvador et al. 2014, Huaman et al. 2020, Thompson et al. 2006, Kalnay et al. 1996, Hribljan et al. 2015, Hribljan et al. 2016, Drexler et al. 2015, Cooper et al. 2010, Lourencato et al. 2017, Roa-Garcia et al. 2016, Lähteenoja et al. 2013, Hapsari et al. 2017, Craft & Richardson 1993, Tolonen & Turunen 1996, Turunen et al. 2001, Chimner & Cooper 2003, Turunen et al. 2004, Beilman et al. 2009, Van Bellen et al. 2011, Nakatsubo et al. 2014, Chimner et al. 2016, Bao et al. 2010, Mitsch & Gosselink 2007
We have corrected that, thank you.
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AC1: 'Reply on RC1', Romina Llanos, 01 Jun 2022
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RC2: 'Comment on bg-2022-47', Anonymous Referee #2, 14 Mar 2022
Dear Editor,
Now I can inform you about the paper titled “Recent significant decline of strong carbon peat accumulation rates in the tropical Andes related to climate change and glacier retreat” by Romina Llanos et al.
In this work, four-peat cores from high-Andean Distichia cushion-plant peatlands close to tropical glacial were radiocarbon-dated to estimate the C accumulation rates. The paper would potentially contribute to paleoenvironmental data since they are scarce. However, the data interpretation is highly speculative. For this reason and those explained below, I suggest rejecting the manuscript.
This work does not present hypotheses: The authors state that the retreat of the glaciers could have affected the rate of C accumulation due to temperature change from the 1970s, but this effect could have impacted both sites where the carbon accumulate were lower in the southern sites than the northern ones (only 6 km away and similar elevation, nothing is said about how far or glacial description). As the authors state, the increase in temperature could have impacted the primary production rates and decomposition rates. However, neither of these were measured; therefore, it is difficult to sustain that the temperature change was the primary driver because both sites received a similar impact (Figure 8 shows a similar average), and other factors such as topography and drainage conditions, other potential factors mentioned were not measured either or described properly. In general, this paper is highly speculative, and it lacks rugosity with many imprecise sentences and often confusing ones (see below).
Other important and minor details
Abstract
L.15-17 “…Here, we point out the important role of Andean peatlands on carbon accumulation rates (CAR), one of the highest in the world, and the impact of climate on carbon storage over the last 65 years, using four peat cores”. From the sentence above it is not clear what is the highest in the world, the Andean peatlands in general, or your study using four-peat cores?
- 19 “For both peatlands”: Never mention before the two peatlands sites.
- 25 Where did depth accumulation rates reach up to? What is CE?
- L.20 Annual mean temperature cannot be responsible; only humans are responsible for something.
- L.25 The authors indicate a decrease in CAR during the study period may be due to a decrease in meltwater by the retreat of the glaciers and the increase in temperature (the last tested); however, an increase in temperature is not the only factor even when you do not mention if there was a type of control to confirm your findings. For comparison you have to be sure that the primary productivity was similar 50-60 years ago.
Introduction
- 38 say: …researches, …must say: researches, however,..
- L.76-103 move this section to M&M. The authors need to clearly describe the differences between APA-1 and APA-2 in the results section, as the calibrated age from APA-1 and APA-2 are compared.
M&M
I generally miss the statistical analysis for setting the differences of CAR and depths.
- L.105 says: between 29 and 35 cm-long, it must say: intervals layers between 2 and 31 cm depth.
- L.105-107 The authors need to clarify how they named the samples in Table 1. In M&M, there is no clear description.
- L. 114 says: accelerator mass spectroscopy, it must say: accelerator mass spectrometry. This mistake comes from another article, Xing et al. (2015) that used the same terminology.
- L. 127 says C stable isotope. It must say. The natural abundance of stable isotope…
- L. 131-132 Even though you are citing a source, please give the equation and units of each variable. How were C accumulation rates calculated? It is not straightforward and familiar for all readers. By the way, Lähteenoja et al., 2009 and Cooper et al., 2015 are not listed in the reference.
- L.133 says: strong. It must say: significant and positive (or negative) …
- L.136-137 says: …can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat, as they mentioned. It must say: can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat during the growth season (See Skrzypek et al. 2011).
- L. 138-139 Please expand the explanation about the resolution used because I understand that NCP-NCAR uses 5ºxº5 pixel. I know you cite Kalnay et al., 1996; however, the last reference is not in the list of references.
Results
- 146-147 The authors say “…an abrupt change occurred at the end of the 1970s when the rates visibly decreased”… Compared with? APA-2 ? I see such abrupt change from Fig 2 if I only compare APA-2 with APA-1.
- L161. “Mean TOC content…” Figure 3: Neither the text tells us if these results average the three depths or only the upper part? The authors refer to supplementary information to prompt the reader to seek information, but this must be carried over to the main text.
- L.174 The authors say, “…CAR varied depending on age and elevation” however, the elevation of these sites is similar (see sites description).
- L. 184. It is hard to see differences without statistical analysis. The variability is so high.
- L193. I do not see the difference for APA-2, even when it was the site that present lower CAR.
Discussion
- L.197 The authors introduce Fig 6 for tropical versus boreal and temperate climate; however tropical high latitude presents an enormous error bar, invalidating the comparison. Please remove this Figure from the Discussion.
- L. 234-235 “…The author says: ...differences found in CAR (Fig. 4) ...were related to the different drainage area surfaces, much more prominent for APA-1 than for APA-2. These differences must be described in the site descriptions first and later discussed.
- L 237. Again other differences that were not described “…specific topographic factors,...”
- L.239-240 “…Although there is a similar downward trend in the CAR at both sites after the early 1980s,..” I do not see the difference in APA-1 in Fig. 2.
- L. 256. Move Fig. 7 to the results section.
- L.255-260 What about photosynthesis. The increase in CO2 must have a consequence?
- L.280-285. Ok, here photosynthesis is discussed.
- L.283 “…The strong gradients in δ13C…” Insist I do not see this gradient in APA-2 having a similar temperature.
- 286 Figure 8 should be the first figure that the authors must show in the result section.
- L. 292 Say: “showed a good relationship especially in trends”. It must say: showed a good relationship”
- L. 291-294 “…this comparison is difficult because the NCEP data … because we do not know precisely what time period each peat sample corresponds to”, this sentence is not clear.
- L. 296 “between 1.9 and 2ºC” is different than “from 1.9 and 2ºC” what do you mean?
Conclusions
- L 307-311 Sentences are more summary than conclusions.
- L. 314-316. “…This decline in C accumulation was mainly related to the temperature rise which increases the organic matter degradation rate…”
The lower CAR probably comes from a lower primary biomass production in APA-2, which was not measured neither discussed. This may have shed light on the input, prevented speculation such as high decomposition rate, and reduced water supply from glacier retreats. The hypothesis that the temperature causes the differences been CARs in my view has not been demonstrated.
Citation: https://doi.org/10.5194/bg-2022-47-RC2 -
AC2: 'Reply on RC2', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the referee's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
Dear Editor,
Now I can inform you about the paper titled “Recent significant decline of strong carbon peat accumulation rates in the tropical Andes related to climate change and glacier retreat” by Romina Llanos et al.
In this work, four-peat cores from high-Andean Distichia cushion-plant peatlands close to tropical glacial were radiocarbon-dated to estimate the C accumulation rates. The paper would potentially contribute to paleoenvironmental data since they are scarce. However, the data interpretation is highly speculative. For this reason and those explained below, I suggest rejecting the manuscript.
This work does not present hypotheses: The authors state that the retreat of the glaciers could have affected the rate of C accumulation due to temperature change from the 1970s, but this effect could have impacted both sites where the carbon accumulate were lower in the southern sites than the northern ones (only 6 km away and similar elevation, nothing is said about how far or glacial description). As the authors state, the increase in temperature could have impacted the primary production rates and decomposition rates. However, neither of these were measured; therefore, it is difficult to sustain that the temperature change was the primary driver because both sites received a similar impact (Figure 8 shows a similar average), and other factors such as topography and drainage conditions, other potential factors mentioned were not measured either or described properly. In general, this paper is highly speculative, and it lacks rugosity with many imprecise sentences and often confusing ones (see below).
We thank the referee for their positive comments and suggestions.
There are two aspects to the referee's remarks: one is how to explain the differences between the two sites and the other is why there were changes over time.
As the other referees pointed out, the detailed description of the sites is insufficient to understand their differences. We returned to the field to a better understanding of the relationship between the Apacheta River and our peatlands. The two peatlands have similar vegetation. APA2 is located at 4420m on a gentle slope of the valley, and APA1 is at 4200m and is located on a glacial terrace now incised by the Rio Apacheta which is a few meters lower. So, the river does not supply water to APA2 and it turns out that the drainage areas of the two peatlands are not as different as we thought.
The ratio of drainage area to peatland area explains well the difference in accumulation rate between the peatlands and in net primary production that has been estimated by MODIS as you suggested (please see Table RC2.1: Data comparison between both peatlands (APA1 and APA2), in SUPPLEMENT).
With respect to changes in accumulation over time, the observed trend is the same in all 4 cores with a reduction in peat growth rates and carbon accumulation starting in the 1980s (please see Figure RC2.1: Carbon Accumulation Rates for the 4 cores, in SUPPLEMENT). For such short time scales, it is not differences in topography or drainage area that may have influenced the observed changes and no anthropogenic action on the drainage network was observed. The most likely hypothesis is that climate change has caused this reduction. It may have intervened directly, through changes in temperature or precipitation, or indirectly through reductions in snow, glaciers and permafrost. It is these hypotheses that we test here using the available data.
OTHER IMPORTANT AND MINOR DETAILS
ABSTRACT
L.15-17 “…Here, we point out the important role of Andean peatlands on carbon accumulation rates (CAR), one of the highest in the world, and the impact of climate on carbon storage over the last 65 years, using four peat cores”. From the sentence above it is not clear what is the highest in the world, the Andean peatlands in general, or your study using four-peat cores?
Both are very high. CAR for Andean peatlands in general are high (Benavides et al., 2013; Benavides, 2014; Cooper et al., 2015) and in our study CAR values are even higher.
1. L 19 “For both peatlands”: Never mention before the two peatlands sites.
Thank you, we have took this into account to improve the manuscript.
2. L 25 Where did depth accumulation rates reach up to? What is CE?
Highest CARs are found at the base of the cores.
CE: Common Era.
Copernicus English Standard: “CE (common era) and BCE (before the common era) should be used instead of AD and BC since CE and BCE are more appropriate in interfaith dialogue and science”.
3. L20 Annual mean temperature cannot be responsible; only humans are responsible for something.
We agree.
4. L25 The authors indicate a decrease in CAR during the study period may be due to a decrease in meltwater by the retreat of the glaciers and the increase in temperature (the last tested); however, an increase in temperature is not the only factor even when you do not mention if there was a type of control to confirm your findings. For comparison you have to be sure that the primary productivity was similar 50-60 years ago.
This is the characteristic of all paleo-environmental studies: we cannot be sure whether primary production has changed or not in the past, but if it has changed it is probably due to climate change. The MODIS satellite productivity data (figure below), while clearly showing the difference between the two peatlands, only shows a slight upward trend over the past 20 years. For precipitation no clear tendency appears, only a very recent increasing trend (Fig. 7). For this reason, we believe that temperature is the main driver of the observed changes, either directly or indirectly.
INTRODUCTION
L38 say: …researches, …must say: researches, however,..
We agree.
L.76-103 move this section to M&M.
OK.
The authors need to clearly describe the differences between APA-1 and APA-2 in the results section, as the calibrated age from APA-1 and APA-2 are compared.
According to the suggestions made by Referee 1, we recalibrated the age with the most recent curve published by Hua et al. (2021) using the mixed curve recommended for South American Monsoon region (Bomb21SH3). All other data as been recalculated in agreement.
The new age models are shown in Figure RC2.2 (in SUPPLEMENT) and are very similar to the old ones.
M&M
I generally miss the statistical analysis for setting the differences of CAR and depths.
Please see Figure RC2.3 (Statistics for the two periods, before and after the transition for the 4 cores, in SUPPLEMENT).
L.105 says: between 29 and 35 cm-long, it must say: intervals layers between 2 and 31 cm depth.
To clarify this point, we have changed the sentence to: “For this study, four peat cores were collected: APA1-C1(34 cm) and APA1-C5 (29 cm) from the site APA1 located at 4200 m, and APA2-C3 (35 cm) and APA2-C4 (34 cm) from the APA2 site at 4420 m.”
L.105-107 The authors need to clarify how they named the samples in Table 1. In M&M, there is no clear description.
We have added more information about the two sites in M&M. And we have added an extra column in Table 1 to identify the differences between the two sites (APA1 and APA2). The description of the two sites is now more detailed.
L.114 says: accelerator mass spectroscopy, it must say: accelerator mass spectrometry. This mistake comes from another article, Xing et al. (2015) that used the same terminology.
We agree with the referee. This was a mistake. It is now corrected.
L.127 says C stable isotope. It must say. The natural abundance of stable isotope…
We agree. It is now corrected: “The natural abundance of C stable isotope was determined using an isotope mass spectrometer … “.
L.131-132 Even though you are citing a source, please give the equation and units of each variable. How were C accumulation rates calculated? It is not straightforward and familiar for all readers. By the way, Lähteenoja et al., 2009 and Cooper et al., 2015 are not listed in the reference.
Sorry, we have added the two references in the list.
“Carbon accumulation rates (CAR in g C m-2 yr-1) were determined using the mathematic equation (Eq. 2) (Lähteenoja et al., 2009; Cooper et al., 2015; Xing et al., 2015):
CAR = BD * GT * TOC (Eq. 2)
Where: CAR is the carbon accumulation rate (gC m−2 yr−1); BD is the bulk density of the bulk peat samples (g cm−3); GT is the growth rate (cm yr-1); TOC is the total organic carbon content (%).”
We have changed the term accumulation rate to growth rate, which is more appropriate for peatlands.
L.133 says: strong. It must say: significant and positive (or negative)…
We agree with the referee. The term "strong" is too much. “We used new stable isotope paleoclimate proxy (δ13C) based on a positive significant relationship found between the C stable isotope composition of Distichia and air temperature (Skrzypek et al. 2011).”
L.136-137 says: …can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat, as they mentioned. It must say: can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat during the growth season (See Skrzypek et al. 2011).
We agree.
L.138-139 Please expand the explanation about the resolution used because I understand that NCP-NCAR uses 5ºxº5 pixel. I know you cite Kalnay et al., 1996; however, the last reference is not in the list of references.
We are sorry for this oversight. The NCEP/NCAR reanalyses data have a latitude-longitude 2.5° grid spatial resolution (Kalnay et al., 1996).
The reference is below.
Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R., and Joseph, D.: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–472, doi:10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2, 1996.
RESULTS
L 146-147 The authors say “…an abrupt change occurred at the end of the 1970s when the rates visibly decreased”… Compared with? APA-2 ? I see such abrupt change from Fig 2 if I only compare APA-2 with APA-1.
The four cores show a marked decrease in carbon accumulation rates from the early 1980s. We interpret this decrease as being the consequence of a decrease in nutrient input from the melting and retreat of the glaciers over tiem which has caused a decrease in productivity. And this is originally related to a rise in temperature in the Andes.
L161. “Mean TOC content…” Figure 3: Neither the text tells us if these results average the three depths or only the upper part? The authors refer to supplementary information to prompt the reader to seek information, but this must be carried over to the main text.
We will transfer the Figure 1 from the Supplementary Material to the main text.
L.174 The authors say, “…CAR varied depending on age and elevation” however, the elevation of these sites is similar (see sites description).
Although the difference between the altitudes of APA1 and APA2 are only 220 m, for the Andes this difference is important for temperature and productivity.
L.184. It is hard to see differences without statistical analysis. The variability is so high.
We thought that the variability between the two sites was visible from the graphs, but at the request of the referee we present the table below with the mean and standard deviation of the CARs before and after the transition year. We hope this data is enough to present this transition (please see Table RC2.2: Statistics for the two periods, before and after the transition for the 4 cores). We think that the data in the table highlights this transition.
L193. I do not see the difference for APA-2, even when it was the site that present lower CAR.
We believe that the table mentioned above (Table RC2.2: Statistics for the two periods, before and after the transition for the 4 cores) highlights these differences.
DISCUSSION
L.197 The authors introduce Fig 6 for tropical versus boreal and temperate climate; however tropical high latitude presents an enormous error bar, invalidating the comparison. Please remove this Figure from the Discussion.
These are not error bars, but extreme values. We will remove and modify this figure.
L.234-235 “…The author says: ...differences found in CAR (Fig. 4) ...were related to the different drainage area surfaces, much more prominent for APA-1 than for APA-2. These differences must be described in the site descriptions first and later discussed.
We thank the referee for this comment and in the new version we have included these data in the text. Now the sub-catchment areas have been recalculated after a supplementary field work (March and April 2022), indeed APA1 is much smaller since it is not drained by Apacheta River which is located several meters below the peatland. The sub-catchment area of APA1 is only 3.3 Km2 while for APA2, the sub-catchment area, inserted in APA2 one, is 2.14 km2. Plear see Figure RC2.3: The sub-catchment area of APA1, and Figure RC2.4: The sub-catchment area of APA2, in SUPPLEMENT.
L 237. Again other differences that were not described “…specific topographic factors,...”
We have included information about the topography and soils of the study area:
“The Apacheta region is characterized by being a mountainous area, with peatlands located in the valleys and sections with gentle slope, at altitudes above 4100 m asl. Edaphologically, the study areas are mainly composed of relatively medium texture deep soils developed upon volcanic rocks (porphyritic andesite) from Apacheta formation (Nm-ap_s) (INGEMMET, 2002). In this area, the main economic activities of the local population are agriculture and livestock. Agriculture takes place at lower altitudes than peatlands and grazing of livestock occurs in the peatland zone, because peatlands provide year-round forage production for grazing native domestic camelids (llama and alpaca) and for livestock species (particularly sheep). Evidence of grazing activity has been observed in the study area although with little visible impact on peatlands."
L.239-240 “…Although there is a similar downward trend in the CAR at both sites after the early 1980s,..” I do not see the difference in APA-1 in Fig. 2.
Statistics are in the table RC2.2 (Statistics for the two periods, before and after the transition for the 4 cores, in SUPPLEMENT).
L.256. Move Fig. 7 to the results section.
We agree.
L.255-260 What about photosynthesis. The increase in CO2 must have a consequence?
L.280-285. Ok, here photosynthesis is discussed.
Ok.
L.283 “…The strong gradients in δ13C…” Insist I do not see this gradient in APA-2 having a similar temperature.
The gradient we are talking about is the one established by Skrzypek et al. (2011).
L286 Figure 8 should be the first figure that the authors must show in the result section.
The main objective of our study is to estimate the rates of recent carbon accumulation in these peatlands. It is only in a second step that we formulate hypotheses as to the cause of the decrease of these rates after 1980.
L.292 Say: “showed a good relationship especially in trends”. It must say: showed a good relationship”
We agree.
L.291-294 “…this comparison is difficult because the NCEP data … because we do not know precisely what time period each peat sample corresponds to”, this sentence is not clear.
The problem is that the ∂13C and thus temperature values are obtained for different dates on the different cores. Another point is that the growth rates are different in the two cores, meaning that each sample do not correspond to the same interval of time.
L.296 “between 1.9 and 2ºC” is different than “from 1.9 and 2ºC” what do you mean?
Sorry for the English redaction. We observed an increase of 1.9 °C for a core and 2°C for the other.
CONCLUSIONS
L 307-311 Sentences are more summary than conclusions.
We will reformulate those sentences.
L.314-316. “…This decline in C accumulation was mainly related to the temperature rise which increases the organic matter degradation rate…” The lower CAR probably comes from a lower primary biomass production in APA-2, which was not measured neither discussed. This may have shed light on the input, prevented speculation such as high decomposition rate, and reduced water supply from glacier retreats. The hypothesis that the temp We have estimated the primary production from MODIS satellite data (2000-2021) and can observe the variations between the two sites (see results in graph below) erature causes the differences been CARs in my view has not been demonstrated.
We appreciate the suggestion.
We have estimated the primary production from MODIS satellite data (2000-2021) and can observe the variations between the two sites (please see Figure RC2.5: Primary production from MODIS satellite data (2000-2021) for the study area, in SUPPLEMENT). APA1 has an average net primary production (NPP) for the period of 0.37 kg C m-2 and APA2 of 0.27 kg C m-2.
The two peatlands have similar vegetation, APA2 is located at 4420m on a gentle slope of the valley, and APA1 is at 4200m and is located on a glacial terrace now incised by the Rio Apacheta which is a few meters lower. So the river does not supply water to APA2 and it turns out that the drainage areas of the two peatlands are not as different as we thought and that the ratio of drainage area to peatland area explains well the difference in accumulation rate between the peatlands.
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RC3: 'Comment on bg-2022-47', Anonymous Referee #3, 14 Mar 2022
The authors present an interesting study on the impact of climate change on Andean peatlands carbon storage. Given the importance of peatlands for the global carbon cycle and climate, I believe this paper is of interest to the readership of this journal. Although the methodology is not particularly novel, the Andean peatlands appear poorly studied (after a quick search on web of science) and any good evidence of climate change impact on this ecosystem woudl be worth being published.
I have some major comments on the methodology and interpretation of the data that led to certain conclusions and some minor comments to help improve the manuscript overall.
- I have some concerns about the derivation of accumulation rates and carbon accumulation rates and the logic behind these estimations. First, how is the accumulation rate obtained? This is not presented. Second, why is the CAR computed directly from the accumulation rate? Is the assumption behind this step that carbon moves only top to bottom? I am not able to determine because this methodology is only briefly mentioned here. But if this is the case, what about carbon released from the roots? Could not plant release carbon directly at depth as root exudates? We just need more details and discussion of the assumptions to better evaluate conclusions originating from this approach.
- The authors somewhat try to infer the evolution of soil carbon over time as a result of the balance between inputs (from plants) and outputs (decomposition). Is there any estimate of how plant productivity changed over time? Although only from year 2000, MODIS from NASA could help.
- Most importantly, I believe the authors should considerably improve their discussion of the methodology, give more context about these peatlands, and elaborate more on their research question.
Minor comments:
- There are some issues with the abstract. First, in line 16 the authors introduce carbon accumulation rates (CAR), but then the following line they quantify accumulation rates. From reading the rest of the paper these two quantities are different and have different units. From the abstract it seems they are the same quantity, and it is measured in cm per year. Second, there is no reason to mention APA1 and APA2 here, because a reader does not know what they are at this point. Third, the sentence in lines 20-23 on Distichia muscoides does not seem to fit here. This seems a preliminary information that could be mentioned earlier, if necessary. In summary, I would simplify the abstract and keep only information and conclusions that are needed to invite a reader to look at the entire paper.
- Give at least a brief description of the age-depth model.
- Rather than just a simple map, Figure 1 could be used to introduce also trends in temperature and other relevant preliminary information about the sites (e.g., what is presented in fig. 7).
- Line 174. replace “that” with “than”.
- Line 242. Maybe topographic “location” is better than “conditions”.
- Lines 276-285. Could you rephrase this whole paragraph?
Citation: https://doi.org/10.5194/bg-2022-47-RC3 -
AC3: 'Reply on RC3', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the referee's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
The authors present an interesting study on the impact of climate change on Andean peatlands carbon storage. Given the importance of peatlands for the global carbon cycle and climate, I believe this paper is of interest to the readership of this journal. Although the methodology is not particularly novel, the Andean peatlands appear poorly studied (after a quick search on web of science) and any good evidence of climate change impact on this ecosystem woudl be worth being published.
We really appreciate your comments and positive suggestions that will improve our manuscript.
I have some major comments on the methodology and interpretation of the data that led to certain conclusions and some minor comments to help improve the manuscript overall.
1. I have some concerns about the derivation of accumulation rates and carbon accumulation rates and the logic behind these estimations. First, how is the accumulation rate obtained? This is not presented. Second, why is the CAR computed directly from the accumulation rate? Is the assumption behind this step that carbon moves only top to bottom? I am not able to determine because this methodology is only briefly mentioned here. But if this is the case, what about carbon released from the roots? Could not plant release carbon directly at depth as root exudates? We just need more details and discussion of the assumptions to better evaluate conclusions originating from this approach.
We have changed the term accumulation rate to growth rate, which is more appropriate since all of the material is peat.
Thank you for your observation, there is a little mistake in the formula: CAR was obtained multiplying the growth rate (cm yr-1) by the bulk density (g cm-3 yr-1) and the TOC (%).
The carbon is not supplied from top to bottom. The peat is a mixed system where the plants of the peatlands grow vertically leaving behind the dead and decomposed organic matter. To this system is added a sedimentary system, not very important quantitatively in our case, whose sediments will be deposited on the surface of the peat. This type of system produces extremely coherent 14C ages over periods of time ranging from ten to thousands of years, showing the progressive net accumulation of organic matter by these systems.
2. The authors somewhat try to infer the evolution of soil carbon over time as a result of the balance between inputs (from plants) and outputs (decomposition). Is there any estimate of how plant productivity changed over time? Although only from year 2000, MODIS from NASA could help.
We thank you for this suggestion.
We have estimated the primary production from MODIS satellite data (2000-2021) and can observe the variations between the two sites (please see Figure RC3.1: Primary production from MODIS satellite data (2000-2021) for the study area, in SUPPLEMENT). APA1 has an average net primary production (NPP) for the period of 0.37 kg C m-2 and APA2 of 0.27 kg C m-2.
The two peatlands have similar vegetation, APA2 is located at 4420m on a gentle slope of the valley, and APA1 is at 4200m and is located on a glacial terrace now incised by the Rio Apacheta which is a few meters lower. So the river does not supply water to APA2 and it turns out that the drainage areas of the two peatlands are not as different as we thought and that the ratio of drainage area to peatland area explains well the difference in accumulation rate between the peatlands. Please see Table RC3.1: Data comparison between both peatlands (APA1 and APA2) in SUPPLEMENT.
3. Most importantly, I believe the authors should considerably improve their discussion of the methodology, give more context about these peatlands, and elaborate more on their research question.
We agree with the referee and will change the text according to this remark.
MINOR COMMENTS
1. There are some issues with the abstract. First, in line 16 the authors introduce carbon accumulation rates (CAR), but then the following line they quantify accumulation rates. From reading the rest of the paper these two quantities are different and have different units. From the abstract it seems they are the same quantity, and it is measured in cm per year. Second, there is no reason to mention APA1 and APA2 here, because a reader does not know what they are at this point. Third, the sentence in lines 20-23 on Distichia muscoides does not seem to fit here. This seems a preliminary information that could be mentioned earlier, if necessary. In summary, I would simplify the abstract and keep only information and conclusions that are needed to invite a reader to look at the entire paper.
We took this into account to improve the manuscript.
2. Give at least a brief description of the age-depth model.
We will add this information.
3. Rather than just a simple map, Figure 1 could be used to introduce also trends in temperature and other relevant preliminary information about the sites (e.g., what is presented in fig. 7).
We agree with the suggestion. We will make a new figure 1.
4. Line 174. replace “that” with “than”.
We agree.
5. Line 242. Maybe topographic “location” is better than “conditions”.
We agree.
6. Lines 276-285. Could you rephrase this whole paragraph?
We will change the text according to this remark and those from other referees.
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RC4: 'Comment on bg-2022-47', Anonymous Referee #4, 01 Apr 2022
The authors use 4 short peat cores from 2 high-elevation Peruvian peatlands to discuss recent changes in carbon accumulation rates (CARs). They found very high apparent rates of carbon accumulation at these sites. They document a decrease in CAR after 1980 that could be caused by an increase in annual temperature; they corroborate this hypothesis with d13C values from the peat cores. The authors assume that d13C values are proxies for temperature (this assumption comes from a different article). Overall, this is an interesting study that could be appropriate for CofP readership, but I am afraid it is not ready for publication. I have numerous methodological issues (some may be flaws), which are described below. In particular, the CAR calculations may not represent true recent changes in carbon dynamics. Likewise, I have serious doubt about the application of the temperature transfer function to the d13C of bulk (?) peat. I hope the comments below are of use to the authors, who should definitely review their study and submit at a later time.
Idea: It would be interesting to read about why you think these young peat deposits started developing less than 100 years ago -- could make for a good discussion of the article.
GENERAL COMMENTS:
Study area: I would have liked to read more about the hydrology of the area, its vegetation, whether it is pristine or impacted by local communities (and animals), etc. Why did you choose those 2 sites should also be covered, as well as brief descriptions of those 2 sites (including the coring sites themselves). As of now, this section lacks important information.
Methods: lots of important information is missing (see the list below).
Results: (1) the trends in CAR that "slow down" in the early 1980s might be due to an autogenic process: the young peat has not decomposed and compacted yet, making TOC values smaller than the older peat. This would potentially yield lower CARs... (2) the changes in d13C are not considering the Suess effect. Other factors impacting d13C should also be tested/discussed, including hydrological changes.
SPECIFIC COMMENTS:
Abstract:
lines 14-15: "...since glaciers have been recognized as one of their vital water sources" -- this is true for some Andean peatlands, but certainly not for MANY of them. This statement is therefore too general and misleading.
line 23: a "new" proxy... the reference you are using is over 10 years old! not so "new" (and they were not the first ones to use it either...)
Introduction:
line 35: This statement about "all carbon in the atmosphere" is incorrect!! "The amount of C stored in peatlands is similar to the total C stocks in all living biomass or in the atmosphere"
line 68: what are you referring to here? "between 500 and 700m in length"
line 72: this is not a "new" method!!
Study Area
line 77 / Figure 1: I'm a bit surprised by your delienation of the watersheds; I'm not familiar with this region, but why is watershed 1 so large and watershed 2 so small? Is watershed 2 in fact part of watershed 1? I don't understand why you are reporting the size of the watersheds...
Methods
line 105: why are the cores shorter than the PVC tubes? Are the peat deposits only 29 to 35 deep? If so, you need to mention this important "detail".
line 110-113: please add references to the methods you describe.
line 120: why not use a Bayesian approach? It seems like the standard in paleoecological studies these days. Bacon can accomodate for your postbomb dates.
line 129: by convention, you must report against which international standard your d13C values were calibrated against! (VPDB?). Also, and perhaps more importantly, how did you sample and prepare for d13C measurements? Did you measure the bulk peat, the Distichia leaves, or something else? Did you extract the cellulose or not? What weight did you use in the lab? A lot of information missing here... that would hinder replication of your study.
line 131: you should use organic matter density (rather than dry bulk density) to truly estimate CAR... You can do it since you have OM% from the LOI measurements...
lines 134-135: you need to explain the mechanism that links d13C with temperature... If I remember the Skrzypek study, they used an elevational gradient to build their relationship, which means that temperature may NOT be the main factor, but rather changes in pressure...
lines 135-136: this sentence does not make sense to me; what do you mean? "This value was similar to the previously reported range for other species (included Sphagnum peat: -0.5 to -0.6‰/°C)"
line 139: why did you use 600 mb in this case? Are there known limitations/issues with using the NCEP/NCAR reanalysis in the high Andes that should be documented?
Results:
Table 1: you say that you report "2 sigmas", but clearly you do not. Instead, you only report the calibrated age - it's unclear if this is the mean, median, or most probable age provided by Clam. Since those are post-bomb dates, it would be useful to know the most plausible age ranges (on either sides of the postbomb calibration curve).
Figure 2: I cannot tell which dates (and error bars) belong to which cores! would it be possible to have 4 panels (one for each core)? It could go in the supplementary file...
lines 163-165: you say that "there was a general upward trend in TOC content from the peat basal depth of the cores from both studied peatlands to approximately 13 cm (the early 1980s) and then the TOC values decreased to the top of the cores (2015 CE)". This is likely because the uppermost samples are "fresher", being that they have not undergone decomposition and compaction. This is likely why your recent CARs are lower than your older CARS... In other words, this could all be an autogenic signal that has nothing to do with a temperature change.
line 174: mean CAR were higher at APA1 than at APA2 - probably because APA1 has high bulk density?! It would be worth to calculate organic matter density for a fairer comparison of those sites.
line 189: did you consider the Suess effect at your sites? It is expected that d13C become more negative over time because of fossil fuels mixing in the global atmosphere... "At both peatlands, there was a general trend to more negative δ13C values from the basal depth to the top of the cores". Getting rid of the Suess effect would be very useful. Then, I see that your 2 cores tell different stories: one of them (the red line on Fig 5) would show increasing d13C values vs. the blue curve would show a decreasing d13C trend. As mentioned in my intro, I am not convinced that these are temperature records. These could relate to hydrological changes: could it be that one site is becoming wetter )blue line) vs the other one is becoming drier (red line)? Please look into the literature that discusses stomatal closure.
Discussion:
lines ~ 200: you should read the paper by Benfield and Yu, Distichia deposits from Columbia were analyzed... You'll see that they also document very high recent CARs.
lines 195-205: you cannot compare your core tops with Holocene-aged cores and say that your cores have greater CARs! This is obvious: short peat hasn't decomposed much, especially compared to old sites... Figure 6 is a misrepresentation and flawed way to compare these data. For a fairer discussion, only look at recent CARs from around the world... There are plenty of data to play with!
I did not comment on the rest of the discussion, as I question the validity of the results.
Citation: https://doi.org/10.5194/bg-2022-47-RC4 -
AC4: 'Reply on RC4', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the referee's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
The authors use 4 short peat cores from 2 high-elevation Peruvian peatlands to discuss recent changes in carbon accumulation rates (CARs). They found very high apparent rates of carbon accumulation at these sites. They document a decrease in CAR after 1980 that could be caused by an increase in annual temperature; they corroborate this hypothesis with d13C values from the peat cores. The authors assume that d13C values are proxies for temperature (this assumption comes from a different article). Overall, this is an interesting study that could be appropriate for CofP readership, but I am afraid it is not ready for publication. I have numerous methodological issues (some may be flaws), which are described below. In particular, the CAR calculations may not represent true recent changes in carbon dynamics. Likewise, I have serious doubt about the application of the temperature transfer function to the d13C of bulk (?) peat. I hope the comments below are of use to the authors, who should definitely review their study and submit at a later time.
Idea: It would be interesting to read about why you think these young peat deposits started developing less than 100 years ago -- could make for a good discussion of the article.
We thank the referee for their comments to improve the manuscript.
We also apologize for the fact that we did not make it clear, that our Andean peatlands did not appear only 100 years ago. In fact, several papers (Engel et al., 2014; Schitteck et al., 2016) based on long core studies in the Andes show that peatlands existed throughout the Holocene. In APA1 peatland, we sampled, for example, core peat that are 2500 cal years BP (Huaman et al., 2020).
However, we aimed to study a much more recent period, so we chose to make short cores based on other studies (p. e., Benavides et al., 2013; Benavides, 2014) which showed that cores of about 30 cm covered the last ~100 years.
GENERAL COMMENTS:
STUDY AREA: I would have liked to read more about the hydrology of the area, its vegetation, whether it is pristine or impacted by local communities (and animals), etc. Why did you choose those 2 sites should also be covered, as well as brief descriptions of those 2 sites (including the coring sites themselves). As of now, this section lacks important information.
We chose to work at these sites due to the dominance of Distichia in both peatlands, the difference in altitude and because this species is well preserved in this Andean environments.
We will add more information about the study area (see below).
“The Apacheta region is characterized by being a mountainous area, with peatlands located in the valleys and sections with gentle slope, at altitudes since 4100 m asl. In this area, the main economic activities of the local population are agriculture and livestock. Agriculture takes place at low altitudes and grazing of livestock in the peatland zone, due peatlands provide year-round forage production for grazing native domestic camelids (llama and alpaca) and for livestock species (particularly sheep). Evidence of grazing activity has been observed in the study area although with little visible impact on peatlands.
Two tropical high-elevation peatlands (APA 1: 13º 21' 4.61'' S, 74º 39' 31.75'' W, 4200 m asl; APA 2: 13º 20' 35.66'' S, 74º 39' 41.04'' W, 4420 m asl) were studied in Apacheta region in the central Andes of Peru (Fig. 1). APA 1 is located in a sub-catchment with an area of 3.3 km2, with an elevation ranging from 4200 to 5000 m asl, while APA 2 sub-catchment had an area of only 2.14 km2, with an elevation ranging from 4350 to 4850 m asl. The topographic relief is variable, ranging from soft to slightly undulating and overlain with slopes of moderate to strong decline. Edaphologically, the study sites are mainly composed of relatively medium texture deep soils developed upon volcanic rocks (porphyritic andesite) from Apacheta formation (Nm-ap_s) (INGEMMET, 2002).
In this region, Distichia muscoides Nees & Meyen (Juncaceae) is the predominant cushion peatland species, and it is present on most high-elevation peatlands in the central Peruvian Andes (Schittek et al., 2015), however other plant species are also found, such as Plantago tubulosa, Aciachne pulvinata, Scirpus rigidus, Calamagrostis rigescens, Calamagrostis spp., Hypochaeris sessiliflora, Hypsela reniformis. D. muscoides is a dioecious semiaquatic plant that grows in dense cushions (Buffen et al., 2009; Skrzypek et al., 2011; Schittek et al., 2018). Distichia leaves are 3 to 7 mm-long, are inserted densely along the stem and form hard monticules (diameter: ~2 m), which are compact mats surrounded by flooded hollows that are permanently bare of vegetation (Balslev, 1996). This species is well adapted to the high-elevation Andean climate and is able to survive diurnal cycles of freezing and thawing (Buffen et al., 2009). The cushion-forming species Distichia muscoides Nees & Meyen dominates many high elevation bofedales in Chile (Squeo et al., 2006), Peru (Skrzypek et al., 2011; Salvador et al., 2014), Ecuador (Chimner and Karberg, 2008), and Colombia (Benavides et al., 2013). Distichia cushions may have started to form as a single individual, producing a large number of shoots and rhizomes, which later transformed into smaller groups as the underground parts of the plant died off (Schittek et al., 2018). The cushion-forming species Distichia muscoides Nees & Meyen dominates many high elevation bofedales in Chile (Squeo et al., 2006), Peru (Skrzypek et al., 2011; Salvador et al., 2014), Ecuador (Chimner and Karberg, 2008), and Colombia (Benavides et al., 2013).
Between the cushions of APA 1 an APA 2 peatlands we found small and shallow pools of water, that are characteristic of this type of ecosystem. The mean pH and conductivity, measured in these pools during the fieldwork, were 5.93 and 45.4 µS cm-1 for APA 1 and 6.01 and 39.2 µS cm-1for APA2, respectively.
The climate of the Apacheta region is typical of tropical high mountains with little seasonal variations in temperature and large seasonal precipitation variability with rainy and dry seasons. Based on data from the Apacheta station located near the two Andean peatlands of this study, about 1.6 km, at 13° 20' 51" S, 74° 38' 44" W and 4150 m asl, the study area presents an average annual precipitation of 830 mm for the period 1991–2012 CE and is marked by seasonal precipitation, with the wettest months being from October to March (monthly average: 114 mm). This seasonal pattern of rainfall reflects the occurrence of South American Monsoon during South Hemisphere summer. The mean annual temperature of the upper part of the basin is 6.4 ºC for the period 2000–2014 CE, with monthly averages ranging from 4.8 to 7.6 ºC; and the annual average relative humidity is 70.3% (for the period 2009–2013 CE) (GORE Ayacucho, 2015).”
METHODS: Lots of important information is missing (see the list below).
RESULTS:
(1) the trends in CAR that "slow down" in the early 1980s might be due to an autogenic process: the young peat has not decomposed and compacted yet, making TOC values smaller than the older peat. This would potentially yield lower CARs...
We found the opposite. Constant high TOC values over the two APA1 cores, and high values with a rising trend after the 1980's for the two APA2 cores (Figure 1 in SM).
(2) the changes in d13C are not considering the Suess effect. Other factors impacting d13C should also be tested/discussed, including hydrological changes.
The problem is: there is no precipitation changes registered in the climate data, so it is difficult to use this argument for ∂13C changes.
The most recent atmospheric ∂13C-CO2 data for the period 1850-2015 (Graven et al., 2017) indicate a trend of -0,01 ‰ yr-1 to be compare to the -0.047‰ yr-1 (APA1-C5) and -0.044 ‰ yr-1 (APA2-C4) we observed in our cores. However the referee is right in remembering the Suess effect on atmospheric ∂13C-CO2 and we will integrate it in our calculations. This will not change the temperature values much because they are not based on absolute ∂13C values but on the differences between ∂13C values from one sample to another.
SPECIFIC COMMENTS
ABSTRACT:
lines 14-15: "...since glaciers have been recognized as one of their vital water sources" -- this is true for some Andean peatlands, but certainly not for MANY of them. This statement is therefore too general and misleading.
We agree.
line 23: a "new" proxy... the reference you are using is over 10 years old! not so "new" (and they were not the first ones to use it either...).
For us, 11 years is not particularly old, but we have no problem removing the term "new".
INTRODUCTION:
line 35: This statement about "all carbon in the atmosphere" is incorrect!! "The amount of C stored in peatlands is similar to the total C stocks in all living biomass or in the atmosphere"
According to Yu et al., (2016) the information we put in the manuscript is valid, it refers to the quantity of carbon, not CO2.
line 68: what are you referring to here? "between 500 and 700m in length"
This is a citation from Rabatel et al. (2013) who measured a retreat of the front of the glacier of 500 to 700m. This means that the length of the glaciers has decreased by 500 to 700m.
line 72: this is not a "new" method!!
OK.
STUDY AREA
line 77 / Figure 1: I'm a bit surprised by your delienation of the watersheds; I'm not familiar with this region, but why is watershed 1 so large and watershed 2 so small? Is watershed 2 in fact part of watershed 1? I don't understand why you are reporting the size of the watersheds...
We thank the referee for this comment and in the new version we have included these data in the text.
The exact coordinates of the investigated sites are now specified and we detected a mistake in APA1 position on the map figure 1. APA 1 Cores: Lat. -13,35128; Long. -74,65882; APA 2 Cores: Lat. -13,34324; Long. -74,66140.
Now the sub-catchment areas have been recalculated after a supplementary field work (March-April 2022) and indeed APA1 is much smaller since it is not drained by Apacheta River which is located several meters below the peatland. The sub-catchment area of APA1 is only 3.3 Km2 while for APA2, the sub-catchment area, inserted in APA2 one, is 2.14 km2 (please see Figure RC4.1: The sub-catchment area of APA1, and Figure RC4.2: The sub-catchment area of APA2, in SUPPLEMENT).
The purpose of delimiting the two basins is to be able to compare peatland areas and carbon accumulation rates (please see Table RC4.1: Data comparison between both peatlands (APA1 and APA2)).
METHODS:
line 105: why are the cores shorter than the PVC tubes?
The size of the tubes (50 cm) is larger than the cores because it is necessary to leave a part of the tubes outside the peat in order to remove them manually.
We chose to work in a more current period to emphasize this period, based on the Huaman et al 2020 core that worked in the same place with a longer core (2500 years BCE) and that highlighted the big differences between the last 60 years and the rest of the core. To clarify this point, we will change the sentence to: “For this study, four peat cores were collected: APA1-C1(34 cm) and APA1-C5 (29 cm) from the site APA1 located at 4200 m, and APA2-C3 (35 cm) and APA2-C4 (34 cm) from the APA2 site at 4420 m.”
Are the peat deposits only 29 to 35 deep? If so, you need to mention this important "detail".
No, the peat deposits are longer at APA1 site (see Huaman et al 2020) and APA2 (no published data), but our aim objective was to work the most recent periods (last 60 years) so we only made core samples of 30 cm.
line 110-113: please add references to the methods you describe.
We agree.
line 120: why not use a Bayesian approach? It seems like the standard in paleoecological studies these days. Bacon can accomodate for your postbomb dates.
The first assumption of Bayesian method for age models is that “the chronology can be broken down into events” (Bronk Ramsey, 2019, Radiocarbon 51:337-360). It means that the sedimentation is formed by a succession of discrete sedimentary events, i.e. sediment layers that constitute the sedimentary sequence. This is generally true for sedimentary deposits but does not apply to peat growth. Mainly in tropical peat and in Distichia peat in particular, which is growing all along the year. For that reason, the Bayesian method cannot be applied in these peats and the best way to build the age model is by spline cubic interpolation.
line 129: by convention, you must report against which international standard your d13C values were calibrated against! (VPDB?).
Yes, δ13C values were expressed relative to international standards VPDB.
Also, and perhaps more importantly, how did you sample and prepare for d13C measurements? Did you measure the bulk peat, the Distichia leaves, or something else?
We measured the δ13C in the bulk peat.
Did you extract the cellulose or not?
No, we do not extract the cellulose.
What weight did you use in the lab?
For the δ13C analysis we use about 2 - 3.5 mg of material. All analyses were performed in duplicates.
A lot of information missing here... that would hinder replication of your study.
We apologize, but in the new version of the manuscript the answers or precisions to all of these comments will be included.
line 131: you should use organic matter density (rather than dry bulk density) to truly estimate CAR... You can do it since you have OM% from the LOI measurements...
To calculate the CAR based on growth rate (cm yr-1) we have multiplicated it by the bulk density to obtain the total accumulation rate (g cm-2 yr -1) and then by OM percentage multiplicating by 100-LOI%. Bulk density by OM% is the OM bulk density, so, in fact our calculation of the CAR is based on the organic matter density.
lines 134-135: you need to explain the mechanism that links d13C with temperature... If I remember the Skrzypek study, they used an elevational gradient to build their relationship, which means that temperature may NOT be the main factor, but rather changes in pressure...
According to Skrzypek et al. (2011) and Engel et al. (2014) temperature is the main factor responsible for ∂13C variations (see below):
“Several authors have investigated the meaning of the ∂13C value in peat and peat-forming plants, especially in the Sphagnum genera (including Menot-Combesetal., 2004; Loader et al., 2007; Moschen et al., 2009; Brader et al., 2010; Tillman et al., 2010; Skrzypek et al., 2013) and have considered environmental factors such as air temperature, humidity, precipitation, vapour pressure deficit and atmospheric CO2 concentration. Despite the possibility of a combined influence of a few factors, the air temperature of the growth season seems to be the major factor governing ∂13C of Distichia macrofossils well pre- served in peat sediments (Skrzypek et al., 2011). Our initial calibration, based on an altitudinal transect that intersects the core collection site, indicated that the observed decrease of ~0.97 ± 0.23‰ in the stable carbon isotope composition of Distichia peat reflects a 1°C increase in the mean air temperature of the growing seasons at the ground level. The temperature at the ground level at high altitudes largely reflects insolation. In contrast, no obvious relationship was observed between precipitation and the stable carbon isotope composition of Distichia peat (Skrzypek et al., 2011).” Engel et al., 2014.
lines 135-136: this sentence does not make sense to me; what do you mean? "This value was similar to the previously reported range for other species (included Sphagnum peat: -0.5 to -0.6‰/°C)"
Other peatland species also exhibit a negative ∂13C gradient as a function of temperature, e.g. -0.5 to -0.6‰/°C for Sphagnum peat (Skrzypek et al., 2010).
line 139: why did you use 600 mb in this case?
Due to the altitude of our study area. It is located at 4000 - 4200 m asl, and this is related to a barometric pressure of approximately 600 mb (West, 1996; Paul & Ferl, 2005). Salzmann et al. (2013) in a study on glacier changes and climate trends in southern Peruvian Andes, also used NCEP/NCAR data for their analysis.
Are there known limitations/issues with using the NCEP/NCAR reanalysis in the high Andes that should be documented?
Of course there are many limitations in NCEP reanalysis, principally in a mountainous area, because these reanalysis data have a poor special resolution and then the topography is not well represented. This product is unable to take into account local process, like mountain breeze. In fact, our objective was to compare our data with regional climatological data to understand the relationship between regional climate and the peat functioning. The hypothesis is that recent climate changes have influence the peat development. Higher resolution data which, in reality, are produced by a downscaling with the same initial number of observation data, do not seems better in this case.
RESULTS
Table 1: you say that you report "2 sigmas", but clearly you do not. Instead, you only report the calibrated age - it's unclear if this is the mean, median, or most probable age provided by Clam. Since those are post-bomb dates, it would be useful to know the most plausible age ranges (on either sides of the postbomb calibration curve).
We are sorry, we forgot to give the two sigmas values, this will be corrected. The calibration has been done again using the most recent pos-bomb curve (Hua et al., 2021). We used Oxcal to calibrate the 14C data. Here are the new age models.
Figure 2: I cannot tell which dates (and error bars) belong to which cores! would it be possible to have 4 panels (one for each core)? It could go in the supplementary file...
According to the suggestions made by Referee 1, we recalibrated the age with the most recent curve published by Hua et al. (2021) using the mixed curve recommended for South American Monsoon region (Bomb21SH3). All other data has been recalculated in agreement.
New age models (new Figure 2) are shown in Figure RC4.3 (in SUPPLEMENT). No significant changes were found.
lines 163-165: you say that "there was a general upward trend in TOC content from the peat basal depth of the cores from both studied peatlands to approximately 13 cm (the early 1980s) and then the TOC values decreased to the top of the cores (2015 CE)". This is likely because the uppermost samples are "fresher", being that they have not undergone decomposition and compaction. This is likely why your recent CARs are lower than your older CARS... In other words, this could all be an autogenic signal that has nothing to do with a temperature change.
The content of organic matter and thus carbon increases towards the top of the APA2 cores and is relatively constant for APA1 cores. The peat decomposition during time would lower the CAR accumulation downcore (please see Figure RC4.4: Organic matter content (%) for the 4 cores, in SUPPLEMENT).
line 174: mean CAR were higher at APA1 than at APA2 - probably because APA1 has high bulk density?! It would be worth to calculate organic matter density for a fairer comparison of those sites.
Yes, the mean bulk density is 44% higher at APA1 (0.11 g cm-3) than APA 2 (0.076 g cm-3), but this is not the only reason why the CAR is higher, the mean growth rate is 54% higher at APA1 (0.87 cm yr-1) than APA2 (0.57 cm yr-1). APA1 is also richer in organic matter (96.2%) than APA2 (91.7%). This leads to a difference in MO densities (0.10 gMO cm-3 for APA1 and 0.07 gMO cm-3 for APA2) that has been taken into account in the calculations as explained above.
line 189: did you consider the Suess effect at your sites? It is expected that d13C become more negative over time because of fossil fuels mixing in the global atmosphere... "At both peatlands, there was a general trend to more negative δ13C values from the basal depth to the top of the cores". Getting rid of the Suess effect would be very useful. Then, I see that your 2 cores tell different stories: one of them (the red line on Fig 5) would show increasing d13C values vs. the blue curve would show a decreasing d13C trend. As mentioned in my intro, I am not convinced that these are temperature records. These could relate to hydrological changes: could it be that one site is becoming wetter )blue line) vs the other one is becoming drier (red line)? Please look into the literature that discusses stomatal closure.
The most recent atmospheric ∂13C CO2 data for the period 1850-2015 (Graven et al., 2017) indicate a trend of -0,01 ‰ yr-1 to be compare to the -0.047‰ yr-1 (C5) and -0.044 ‰ yr-1 (C4) we observed in our cores. However, the referee is right in remembering the Suess effect on atmospheric CO2 ∂13C and we will integrate it in our calculations. This will not change the temperature values much because they are not based on absolute ∂13C values but on the differences between ∂13C values from one sample to another.
As explained above there is no change in precipitation in the reanalyses and we do not see how to explain that APA1 would become wetter while APA2 would become drier. For such short time scales, it is not differences in topography or drainage area that may have influenced the observed changes and no anthropogenic action on the drainage network was observed. Skrzypek et al. (2011) has shown that temperature influences ∂13C of this kind of peatlands and others, and this explains well the variations observed in our cores.
DISCUSSION
lines ~ 200: you should read the paper by Benfield and Yu, Distichia deposits from Columbia were analyzed... You'll see that they also document very high recent CARs.
Yes, we know this paper. Those authors found high accumulation rates for the Colombian site that confirms our findings. This reference will be included in the discussion.
lines 195-205: you cannot compare your core tops with Holocene-aged cores and say that your cores have greater CARs! This is obvious: short peat hasn't decomposed much, especially compared to old sites... Figure 6 is a misrepresentation and flawed way to compare these data. For a fairer discussion, only look at recent CARs from around the world... There are plenty of data to play with!
Ok, we will redraw this figure with recent CAR data only.
I did not comment on the rest of the discussion, as I question the validity of the results.
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AC4: 'Reply on RC4', Romina Llanos, 01 Jun 2022
Status: closed
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RC1: 'Comment on bg-2022-47', Anonymous Referee #1, 10 Mar 2022
General comments
Llanos et al. present a record from a sedimentary core from the Apacheta region in the central Peruvian Andes. Four peat cores from high-Andean Distichia cushion-plant peatlands were radiocarbon-dated and C accumulation rates, TOC and C stable isotope composition are presented for the four 29-35 cm long peat cores. Based on the presumption by Skrzypek et al. (2011), who interpreted growing season temperature as a determining factor for δ13C in a high-Andean Distichia peatland in Peru, the authors reconstruct temperature from both studied peatlands for the period 1970-2015 CE.
The presented research would potentially represent an important contribution of paleodata in a region, where paleoenvironmental data is still very scarce. However, the presentation and interpretation of the data need significant improvement, and currently, the presented research does not represent a sound and elaborate work. Overall presentation, methodological concept, and data interpretation are not ready yet for publication. As the presented research work needs significant improvement on several topics, I do not recommend it for publication in Biogeosciences.
Specific comments
1. The exact coordinates of the investigated sites are missing. However, the sites can somewhat be located with help of Figure 1. By checking the "sub-catchments", I absolutely do not agree with the presented "sub-catchment" area of 130 km² for APA 1, which is situated in a kar valley of Nevado Portuguesa (aka Chicllarazo or Apacheta) and has no connection to the yellow-shaded area. However, without exact coordinates, this remains unclear.
2. The "study area" chapter lacks important information. Distichia muscoides is the only plant species mentioned. In the central Peruvian Andes, cushion-plant peatlands are often dominated by Distichia, but accompanied by other species, which - depending on site factors - might dominate specific areas of the peatlands (other cushion-formers like Plantago rigida, Zameioscirpus muticus, Phylloscirpus deserticola or reed grasses like Deyeuxia/Calamagrostis) or grow into the Distichia cushions. Further, these peatlands are usually characterized by shallow pools, which form between the cushions (Coronel et al. 2004). No information is given on that, nor on the topography of the peatland, nor on the influence of grazing or other impact by the local population. Further, no information is provided on the possible influence of geothermal springs, which might contribute to the springwater. The presented study did not conduct analysis of the peatlands` spring and surface water (at least pH and conductivity), which is a prerequisite for any peatland study. Noble & McKee (1982) mention geothermal springs for the Nevado Portuguesa area. Can the influence of geothermal water be excluded?
3. The authors point out the relation of carbon peat accumulation rates and glacier retreat, since glaciers have been recognized as "the main water source" for high-Andean peatlands. Line 266 says: "The subsequent reduction in peat growth rates could have been due in part to the decrease in the rate of water inflow from nearby glaciers to peatlands after their complete disappearance." In point of fact, I cannot detect glaciers within the upper catchments of both investigated peatland sites. Many peatlands in the tropical Andes are fed by glacial meltwater. However, the majority of high-Andean peatlands is fed by permafrost (Ruthsatz et al. 2020), and water originates from high-elevation cryogenic soils and glaciolithic deposits (Trombotto 2000). This is the case for the two investigated peatlands (as far as I presume from Figure 1 and Google Earth). Therefore, the whole climate change-related argumentation should not solely focus on glaciers, but also on the very important role of permafrost.
4. For radiocarbon dates, the authors use the SH calibration dataset. Due to a significant influx of Northern Hemisphere air masses and moisture over a substantial part of the continent, especially the tropical central Andes, during the South American Summer Monsoon (SASM), Marsh et al. (2018) recommend using a mixed calibration curve. During the austral spring and summer seasons, the south shift in the ITCZ brings atmospheric CO2 from the Northern Hemisphere to the Andes, which is taken up by the vegetation during the growing season (Schittek et al. 2016). How do the authors explain the use of the SH calibration set?
5. The authors do not pay attention to the effect that bulk peat stable carbon isotopes may reflect the dilution of atmospheric δ13CO2 and the effects of early stage kinetic fractionation during diagenesis (Esmeijer-Liu et al. 2012) or other factors like dust influx or vegetational changes. For a scientifically sound reconstruction of paleotemperatures, this has to be taken into account.
6. A scientifically sound "high-resolution" record concerning the past 50 years would require age control by applying the Pb/Cs dating method rather than applying CaliBomb upon radiocarbon dating for only 3 samples per core.
7. The stable isotope measurement method is described in only one sentence. How about the use of calibrated laboratory standards and what is the analytical uncertainty?
line 43: change "High-altitude" into "High-elevation"
line 46: "Their most important ecological role..." This sentence should be reworked. First, it should not be evaluated what is the most important ecological role of peatlands. Second, tropical peatlands do not control decomposition processes in the soil!!!
lines 52-75: The focus should be rather on permafrost than on glaciers as the investigated peatlands seemingly are not fed by glacial meltwater.
line 81: Vegetation is not dominated by Distichia muscoides! (What is meant by "vegetation"??? Peatland? Steppe?). This is only the case for peatland areas with permanent saturation. The cited literature in this paragraph has no relation to the Apacheta region.
line 90: "The climate of the Apacheta peatlands..." I do not agree with this statement. First, it should be "Apacheta region", second, there definitely is a rainy and a dry season, as this area is affected by the South American summer monsoon.
line 199: Chimner
line 200: Oxychloe
line 200: Azorella is typical for high-Andean steppe vegetation and never grows inside a peatland.
line 250: "...which are typically associated with glacial dynamics..." I do not agree. Distichia muscoides is associated to permanent saturation above 4000 m asl. Its distribution is not restricted to the presence of glaciers.
line 292: "good relationship" What does that mean? Did you conduct any correlation analysis?
Figure 8: The reconstructed air temperatures of the two presented cores, in some parts, differ significantly, although the two coring sites are very close to each other. How do the authors explain this? How about the other two retrieved cores? Is there any results for them?
The following publications are mentioned in the manuscript, but not listed in the references:
Salvador et al. 2014, Huaman et al. 2020, Thompson et al. 2006, Kalnay et al. 1996, Hribljan et al. 2015, Hribljan et al. 2016, Drexler et al. 2015, Cooper et al. 2010, Lourencato et al. 2017, Roa-Garcia et al. 2016, Lähteenoja et al. 2013, Hapsari et al. 2017, Craft & Richardson 1993, Tolonen & Turunen 1996, Turunen et al. 2001, Chimner & Cooper 2003, Turunen et al. 2004, Beilman et al. 2009, Van Bellen et al. 2011, Nakatsubo et al. 2014, Chimner et al. 2016, Bao et al. 2010, Mitsch & Gosselink 2007References:
Coronel J.S., Declerck S., Maldonado M., Ollevier, F. & Brendonck L. (2004): Temporary shallow poolsin high-Andes bofedal peatlands: a limnological characterization at different spatial scales. Archives des Sciences 57: 85-96.Noble D.C. & McKee E.H. (1982): Nevado Portugueza volcanic center, central Peru; a Pliocene central volcano-collapse caldera complex with associated silver mineralization. Economic Geology 77(8): 1893-1900.
Ruthsatz B., Schittek K. & Backes B. (2020): The vegetation of cushion peatlands in the Argentine Andes and changes in their floristic composition across a latitudinal gradient from 39°S to 22°S. Phytocoenologia 50(3): 249-278.
Trombotto, D. (2000): Survey of cryogenic processes, periglacial forms and permafrost conditions in South America. Revista do Instituto Geológico 21: 33–55.
Marsh E.J., Bruno M.C., Fritz S.C., Baker P, Capriles J.M. & Hastorf C.A. (2018): IntCal, SHCal, or a Mixed Curve? Choosing a 14C Calibration Curve for Archaeological and Paleoenvironmental Records from Tropical South America. Radiocarbon 60(3): 925-940.
Schittek, K., Kock, S.T., Lücke, A., Hense, J., Ohlendorf, C., Kulemeyer, J.J., Lupo, L.C. & Schäbitz, F. 2016. A high-altitude peatland record of environmental changes in the NW Argentine Andes (24°S) over the last 2100 years. Climate of the Past 12: 1165–1180.
Esmeijer-Liu A.J., Kürschner W.M., Lotter A.F., Verhoeven J.T.A. & Goslar T. (2012): Stable carbon and nitrogen isotopes in a peat profile are influenced by early stage diagenesis and changes in atmospheric CO2 and N deposition. Water Air Soil Pollut 223: 2007-2022.
Citation: https://doi.org/10.5194/bg-2022-47-RC1 -
AC1: 'Reply on RC1', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the reviewer's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
GENERAL COMMENTS
Llanos et al. present a record from a sedimentary core from the Apacheta region in the central Peruvian Andes. Four peat cores from high-Andean Distichia cushion-plant peatlands were radiocarbon-dated and C accumulation rates, TOC and C stable isotope composition are presented for the four 29-35 cm long peat cores. Based on the presumption by Skrzypek et al. (2011), who interpreted growing season temperature as a determining factor for δ13C in a high-Andean Distichia peatland in Peru, the authors reconstruct temperature from both studied peatlands for the period 1970-2015 CE.
The presented research would potentially represent an important contribution of paleodata in a region, where paleoenvironmental data is still very scarce. However, the presentation and interpretation of the data need significant improvement, and currently, the presented research does not represent a sound and elaborate work. Overall presentation, methodological concept, and data interpretation are not ready yet for publication. As the presented research work needs significant improvement on several topics, I do not recommend it for publication in Biogeosciences.
We thank the referee for their positive comments, their detailed review and for the constructive recommendations. We respond thereafter to each of their comments.
SPECIFIC COMMENTS
1. The exact coordinates of the investigated sites are missing. However, the sites can somewhat be located with help of Figure 1. By checking the "sub-catchments", I absolutely do not agree with the presented "sub-catchment" area of 130 km² for APA 1, which is situated in a kar valley of Nevado Portuguesa (aka Chicllarazo or Apacheta) and has no connection to the yellow-shaded area. However, without exact coordinates, this remains unclear.
We thank the referee for this comment and in the new version we have included the location in the manuscript.
The exact coordinates of the investigated sites are now specified and we detected a mistake in APA1 position on the map figure 1. APA 1 Cores: Lat. -13,35128; Long. -74,65882; APA 2 Cores: Lat. -13,34324; Long. -74,66140.
Now the sub-catchment areas have been recalculated after a supplementary field work (March-April 2022) and indeed APA1 is much smaller since it is not drained by Apacheta River which is located several meters below the peatland. The sub-catchment area of APA1 is only 3.3 Km2 while for APA2, the sub-catchment area, inserted in APA2 one, is 2.14 km2 (please see Figures RC1.1: The sub-catchment area of APA1, and Figure RC1.2: The sub-catchment area of APA2, in SUPPLEMENT).
None of the peatlands studied is located in the valley of Nevado Portuguesa, the location of the APA1 point on the map (figure 1B) was wrong, but it has already been rectified and the coordinates have also been specified in the paper.
2. The "study area" chapter lacks important information. Distichia muscoides is the only plant species mentioned. In the central Peruvian Andes, cushion-plant peatlands are often dominated by Distichia, but accompanied by other species, which - depending on site factors - might dominate specific areas of the peatlands (other cushion-formers like Plantago rigida, Zameioscirpus muticus, Phylloscirpus deserticola or reed grasses like Deyeuxia/Calamagrostis) or grow into the Distichia Further, these peatlands are usually characterized by shallow pools, which form between the cushions (Coronel et al. 2004). No information is given on that, nor on the topography of the peatland, nor on the influence of grazing or other impact by the local population. Further, no information is provided on the possible influence of geothermal springs, which might contribute to the springwater. The presented study did not conduct analysis of the peatlands` spring and surface water (at least pH and conductivity), which is a prerequisite for any peatland study. Noble & McKee (1982) mention geothermal springs for the Nevado Portuguesa area. Can the influence of geothermal water be excluded?
We thank you for these recommendations in order to provide the reader with more information about our study area.
Accompanying species found in the study area will be added.
“In this region, Distichia muscoides Nees & Meyen (Juncaceae) is the predominant cushion peatland species, and it is present on most high-elevation peatlands in the central Peruvian Andes (Schittek et al., 2015), however other plant species are also found, such as Plantago tubulosa, Aciachne pulvinata, Scirpus rigidus, Calamagrostis rigescens, Calamagrostis spp., Hypochaeris sessiliflora, Hypsela reniformis. Distichia cushions are surrounded by little shallow pools (around 50 cm-large).”
We have also included information about the topography and soils of the study area:
“The Apacheta region is characterized by being a mountainous area, with peatlands located in the valleys and sections with gentle slope, at altitudes above 4100 m asl. Edaphologically, the study area is mainly composed of relatively medium texture deep soils developed upon volcanic rocks (porphyritic andesite) from Apacheta formation (Nm-ap_s) (INGEMMET, 2002). In this area, the main economic activities of the local population are agriculture and livestock. Agriculture takes place at lower altitudes than peatlands and grazing of livestock occurs in the peatland zone, because peatlands provide year-round forage production for grazing native domestic camelids (llama and alpaca) and for livestock species (particularly sheep). Evidence of grazing activity has been observed in the study area although with little visible impact on peatlands.”
About the influence of geothermal water on the study area:
We think that the study area is not influenced by geothermal activity. But as mentioned by Noble & McKee (1982), there are thermal springs in the surrounding region. Also, according to the database of INGEMET (National Geological, Mining and Metallurgical Institute) of Peru there are 2 thermal springs:
- Niñobamba (-13,334°; -74,581°; 3670 m asl) which is located more than 7 km downstream from the Apacheta River and 300 m of altitude difference, so it would not have influence in the study zone.
- Licapa (-13,361°, -74,871°, 4100 m asl) located approximately 23 km west of the study area and belongs to another hydrological basin, so the influence of a geothermal spring is ruled out.
About the analysis of the peatlands` spring and surface water, sorry for our negligence. We have measured pH and conductivity and will add this information in the text:
“Between the cushions of APA 1 an APA 2 peatlands we found small and shallow pools of water that are characteristic of this type of ecosystem. The mean pH and conductivity, measured in these pools during the campaign, were 5.93 and 45.4 µS cm-1 for APA 1 and 6.01 and 39.2 µS cm-1for APA2, respectively.”
2. The authors point out the relation of carbon peat accumulation rates and glacier retreat, since glaciers have been recognized as "the main water source" for high-Andean peatlands. Line 266 says: "The subsequent reduction in peat growth rates could have been due in part to the decrease in the rate of water inflow from nearby glaciers to peatlands after their complete disappearance." In point of fact, I cannot detect glaciers within the upper catchments of both investigated peatland sites. Many peatlands in the tropical Andes are fed by glacial meltwater. However, the majority of high-Andean peatlands is fed by permafrost (Ruthsatz et al. 2020), and water originates from high-elevation cryogenic soils and glaciolithic deposits (Trombotto 2000). This is the case for the two investigated peatlands (as far as I presume from Figure 1 and Google Earth). Therefore, the whole climate change-related argumentation should not solely focus on glaciers, but also on the very important role of permafrost.
Indeed nowadays there is no glacier in the area and, as the temperature is always positive (see MAAT curves) very probably no permafrost exist today. According to Chadbrun et al. (2017), permafrost requires an average annual temperature of less than -2°C. It is not the case in our study area as we can see with the annual temperature data shown in Figure RC1.3 (Mean annual temperature (°C) over the period 1958-2018 of the four pixels of TerraClimate datasets covering the two sub-catchments, in SUPPLEMENT), so the probability of having permafrost at present is low.
The TerraClimate dataset comprises a global dataset based on reanalysis data since 1958, with a 4 km grid size at a monthly time scale. This dataset was validated with the Global Historical Climatology Network using 3,230 stations for temperature (r =0.95; mean absolute error 0.32°C) and 6,102 stations for precipitation (r =0.90; mean absolute error 9.1%) (Abatzoglou et al., 2018).
The development of permafrost is badly known in the Andes (Trombotto, 2000) and cryogenic soils are formed above 4600m in Peruvian and Bolivian Cordilleras (Trombotto, 2000). So we cannot exclude that permafrost has played some role in the past in supplying groundwaters to the peatlands during period of warming climate. The manuscript have been modified in this sense.
4. For radiocarbon dates, the authors use the SH calibration dataset. Due to a significant influx of Northern Hemisphere air masses and moisture over a substantial part of the continent, especially the tropical central Andes, during the South American Summer Monsoon (SASM), Marsh et al. (2018) recommend using a mixed calibration curve. During the austral spring and summer seasons, the south shift in the ITCZ brings atmospheric CO2 from the Northern Hemisphere to the Andes, which is taken up by the vegetation during the growing season (Schittek et al. 2016). How do the authors explain the use of the SH calibration set?
We thank the referee for this observation and, indeed, as the region rainfall is associated to the South American Monsoon a mixed atmospheric post-bomb 14C curve between Northern and Southern Hemisphere must be used. We recalibrated the age with the most recent curve published by Hua et al. (2021) using the mixed curve recommended for South American Monsoon region (Bomb21SH3). All other data have been recalculated in agreement of this.
The new age models are shown in Figure RC1.4 (in SUPPLEMENT) and are very similar to the old ones.
5. The authors do not pay attention to the effect that bulk peat stable carbon isotopes may reflect the dilution of atmospheric δ13CO2 and the effects of early stage kinetic fractionation during diagenesis (Esmeijer-Liu et al. 2012) or other factors like dust influx or vegetational changes. For a scientifically sound reconstruction of paleotemperatures, this has to be taken into account.
Esmeijer-Liu et al.(2012) have studied a peat core from Northern Finland and observed an increase of ∂13C toward the top. The linear trend of ∂13C increase is 0.0072 ‰ yr-1, to be compare to the 0.047‰ yr-1 (APA1- C5) and 0.044‰ yr-1 (APA2- C4) we observed in our cores. The most recent atmospheric ∂13C - CO2 data (Graven et al., 2017) indicate a trend of 0,0078‰ yr-1 for the same period, very close to Esmeijer-Liu et al (2012). So, the observed variation in Finland core can be explained by the atmospheric trend alone. However, the authors consider that there is also an effect of ∂13C decrease during the diagenesis of organic matter. Considering the trends value, this effect must be very low compare with the changes we measured. However, the referee is right in remembering this « Suess » effect on atmospheric ∂13C - CO2 and we will integrate it in our calculations.
Distichia remains are observable in great quantity from the top to the base of our cores and there is no evidence of vegetation change during the study period. We did not find any data about recent dust deposition in the region but it will be surprising that dust deposition has decrease in the las decades.
6. A scientifically sound "high-resolution" record concerning the past 50 years would require age control by applying the Pb/Cs dating method rather than applying CaliBomb upon radiocarbon dating for only 3 samples per core.
Dating of peat using 210Pb or 137Cs is a tricky problem, especially in highly porous peatlands like those dominated by Distichia where lead is likely to move. The roots of Distichia are likely to transport lead to deeper areas (Benavides et al., 2015). Most importantly, however, dating from Distichia peatlands in Colombia (Benavides et al., 2015) has shown very low unsupported lead values, close to the background of supported lead, when the sedimentation rate is very high. Since the sedimentation rates we observe are even stronger we believe that this method could not have been used.
7. The stable isotope measurement method is described in only one sentence. How about the use of calibrated laboratory standards and what is the analytical uncertainty?
The analytical standard used was “High Organic Sediment Standard – OAS” that consists of batch of high organic content sediments standard traceable to IAEA-CH-6, prepared and certified by Elemental Microanalysis Ltd, Okehampton, Devon. The analytical uncertainty is 0.2‰.
line 43: change "High-altitude" into "High-elevation"
We agree.
line 46: "Their most important ecological role..." This sentence should be reworked. First, it should not be evaluated what is the most important ecological role of peatlands. Second, tropical peatlands do not control decomposition processes in the soil!!!
Our sentence is not clear, of course we cannot evaluate the most important ecological role of peatlands. They just have a great potential to accumulate organic carbon due to the slow decomposition rate of the plants that give origin to the peat. We have changed the text.
lines 52-75: The focus should be rather on permafrost than on glaciers as the investigated peatlands seemingly are not fed by glacial meltwater.
See point 3.
line 81: Vegetation is not dominated by Distichia muscoides! (What is meant by "vegetation"??? Peatland? Steppe?). This is only the case for peatland areas with permanent saturation. The cited literature in this paragraph has no relation to the Apacheta region.
Distichia is the dominant plant species in cushion peatlands in this region and, according to Schitteck et al 2015, in the Central Andes as a whole.
line 90: "The climate of the Apacheta peatlands..." I do not agree with this statement. First, it should be "Apacheta region", second, there definitely is a rainy and a dry season, as this area is affected by the South American summer monsoon.
We fully agree, this is what we meant when we wrote about “large seasonal precipitation variability” with a total rainfall of 830 mm with 82% of precipitation during the rainy season from october to march.
line 199: Chimner
We agree.
line 200: Oxychloe
We agree.
line 200: Azorella is typical for high-Andean steppe vegetation and never grows inside a peatland.
We agree.
line 250: "...which are typically associated with glacial dynamics..." I do not agree. Distichia muscoides is associated to permanent saturation above 4000 m asl. Its distribution is not restricted to the presence of glaciers.
You are right. Tropical glaciers are an important source of water only for peatlands that are close to them, within their watershed. We have changed the text.
line 292: "good relationship" What does that mean? Did you conduct any correlation analysis?
The comparison is only visual, correlation calculation for this kind of data with different dates in each curve is not very possible. We compared the trends for the NCEP data and reconstructed temperature.
Figure 8: The reconstructed air temperatures of the two presented cores, in some parts, differ significantly, although the two coring sites are very close to each other. How do the authors explain this? How about the other two retrieved cores? Is there any results for them?
As we explained, one of the problems is that the ∂13C and thus temperature values are obtained for different dates on the different cores. Another point is that the growth rates are different in the two cores, meaning that each sample does not correspond to the same interval of time. However, it is certain that this cannot explain all the differences between the curves and those other parameters than temperature also influence the ∂13C. This is part of the uncertainty on the temperature reconstruction that we consider nevertheless to be the main factor influencing the ∂13C.
For the two other cores the sampling interval is too large (~3 cm) for a good temporal resolution.
The following publications are mentioned in the manuscript, but not listed in the references: Salvador et al. 2014, Huaman et al. 2020, Thompson et al. 2006, Kalnay et al. 1996, Hribljan et al. 2015, Hribljan et al. 2016, Drexler et al. 2015, Cooper et al. 2010, Lourencato et al. 2017, Roa-Garcia et al. 2016, Lähteenoja et al. 2013, Hapsari et al. 2017, Craft & Richardson 1993, Tolonen & Turunen 1996, Turunen et al. 2001, Chimner & Cooper 2003, Turunen et al. 2004, Beilman et al. 2009, Van Bellen et al. 2011, Nakatsubo et al. 2014, Chimner et al. 2016, Bao et al. 2010, Mitsch & Gosselink 2007
We have corrected that, thank you.
-
AC1: 'Reply on RC1', Romina Llanos, 01 Jun 2022
-
RC2: 'Comment on bg-2022-47', Anonymous Referee #2, 14 Mar 2022
Dear Editor,
Now I can inform you about the paper titled “Recent significant decline of strong carbon peat accumulation rates in the tropical Andes related to climate change and glacier retreat” by Romina Llanos et al.
In this work, four-peat cores from high-Andean Distichia cushion-plant peatlands close to tropical glacial were radiocarbon-dated to estimate the C accumulation rates. The paper would potentially contribute to paleoenvironmental data since they are scarce. However, the data interpretation is highly speculative. For this reason and those explained below, I suggest rejecting the manuscript.
This work does not present hypotheses: The authors state that the retreat of the glaciers could have affected the rate of C accumulation due to temperature change from the 1970s, but this effect could have impacted both sites where the carbon accumulate were lower in the southern sites than the northern ones (only 6 km away and similar elevation, nothing is said about how far or glacial description). As the authors state, the increase in temperature could have impacted the primary production rates and decomposition rates. However, neither of these were measured; therefore, it is difficult to sustain that the temperature change was the primary driver because both sites received a similar impact (Figure 8 shows a similar average), and other factors such as topography and drainage conditions, other potential factors mentioned were not measured either or described properly. In general, this paper is highly speculative, and it lacks rugosity with many imprecise sentences and often confusing ones (see below).
Other important and minor details
Abstract
L.15-17 “…Here, we point out the important role of Andean peatlands on carbon accumulation rates (CAR), one of the highest in the world, and the impact of climate on carbon storage over the last 65 years, using four peat cores”. From the sentence above it is not clear what is the highest in the world, the Andean peatlands in general, or your study using four-peat cores?
- 19 “For both peatlands”: Never mention before the two peatlands sites.
- 25 Where did depth accumulation rates reach up to? What is CE?
- L.20 Annual mean temperature cannot be responsible; only humans are responsible for something.
- L.25 The authors indicate a decrease in CAR during the study period may be due to a decrease in meltwater by the retreat of the glaciers and the increase in temperature (the last tested); however, an increase in temperature is not the only factor even when you do not mention if there was a type of control to confirm your findings. For comparison you have to be sure that the primary productivity was similar 50-60 years ago.
Introduction
- 38 say: …researches, …must say: researches, however,..
- L.76-103 move this section to M&M. The authors need to clearly describe the differences between APA-1 and APA-2 in the results section, as the calibrated age from APA-1 and APA-2 are compared.
M&M
I generally miss the statistical analysis for setting the differences of CAR and depths.
- L.105 says: between 29 and 35 cm-long, it must say: intervals layers between 2 and 31 cm depth.
- L.105-107 The authors need to clarify how they named the samples in Table 1. In M&M, there is no clear description.
- L. 114 says: accelerator mass spectroscopy, it must say: accelerator mass spectrometry. This mistake comes from another article, Xing et al. (2015) that used the same terminology.
- L. 127 says C stable isotope. It must say. The natural abundance of stable isotope…
- L. 131-132 Even though you are citing a source, please give the equation and units of each variable. How were C accumulation rates calculated? It is not straightforward and familiar for all readers. By the way, Lähteenoja et al., 2009 and Cooper et al., 2015 are not listed in the reference.
- L.133 says: strong. It must say: significant and positive (or negative) …
- L.136-137 says: …can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat, as they mentioned. It must say: can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat during the growth season (See Skrzypek et al. 2011).
- L. 138-139 Please expand the explanation about the resolution used because I understand that NCP-NCAR uses 5ºxº5 pixel. I know you cite Kalnay et al., 1996; however, the last reference is not in the list of references.
Results
- 146-147 The authors say “…an abrupt change occurred at the end of the 1970s when the rates visibly decreased”… Compared with? APA-2 ? I see such abrupt change from Fig 2 if I only compare APA-2 with APA-1.
- L161. “Mean TOC content…” Figure 3: Neither the text tells us if these results average the three depths or only the upper part? The authors refer to supplementary information to prompt the reader to seek information, but this must be carried over to the main text.
- L.174 The authors say, “…CAR varied depending on age and elevation” however, the elevation of these sites is similar (see sites description).
- L. 184. It is hard to see differences without statistical analysis. The variability is so high.
- L193. I do not see the difference for APA-2, even when it was the site that present lower CAR.
Discussion
- L.197 The authors introduce Fig 6 for tropical versus boreal and temperate climate; however tropical high latitude presents an enormous error bar, invalidating the comparison. Please remove this Figure from the Discussion.
- L. 234-235 “…The author says: ...differences found in CAR (Fig. 4) ...were related to the different drainage area surfaces, much more prominent for APA-1 than for APA-2. These differences must be described in the site descriptions first and later discussed.
- L 237. Again other differences that were not described “…specific topographic factors,...”
- L.239-240 “…Although there is a similar downward trend in the CAR at both sites after the early 1980s,..” I do not see the difference in APA-1 in Fig. 2.
- L. 256. Move Fig. 7 to the results section.
- L.255-260 What about photosynthesis. The increase in CO2 must have a consequence?
- L.280-285. Ok, here photosynthesis is discussed.
- L.283 “…The strong gradients in δ13C…” Insist I do not see this gradient in APA-2 having a similar temperature.
- 286 Figure 8 should be the first figure that the authors must show in the result section.
- L. 292 Say: “showed a good relationship especially in trends”. It must say: showed a good relationship”
- L. 291-294 “…this comparison is difficult because the NCEP data … because we do not know precisely what time period each peat sample corresponds to”, this sentence is not clear.
- L. 296 “between 1.9 and 2ºC” is different than “from 1.9 and 2ºC” what do you mean?
Conclusions
- L 307-311 Sentences are more summary than conclusions.
- L. 314-316. “…This decline in C accumulation was mainly related to the temperature rise which increases the organic matter degradation rate…”
The lower CAR probably comes from a lower primary biomass production in APA-2, which was not measured neither discussed. This may have shed light on the input, prevented speculation such as high decomposition rate, and reduced water supply from glacier retreats. The hypothesis that the temperature causes the differences been CARs in my view has not been demonstrated.
Citation: https://doi.org/10.5194/bg-2022-47-RC2 -
AC2: 'Reply on RC2', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the referee's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
Dear Editor,
Now I can inform you about the paper titled “Recent significant decline of strong carbon peat accumulation rates in the tropical Andes related to climate change and glacier retreat” by Romina Llanos et al.
In this work, four-peat cores from high-Andean Distichia cushion-plant peatlands close to tropical glacial were radiocarbon-dated to estimate the C accumulation rates. The paper would potentially contribute to paleoenvironmental data since they are scarce. However, the data interpretation is highly speculative. For this reason and those explained below, I suggest rejecting the manuscript.
This work does not present hypotheses: The authors state that the retreat of the glaciers could have affected the rate of C accumulation due to temperature change from the 1970s, but this effect could have impacted both sites where the carbon accumulate were lower in the southern sites than the northern ones (only 6 km away and similar elevation, nothing is said about how far or glacial description). As the authors state, the increase in temperature could have impacted the primary production rates and decomposition rates. However, neither of these were measured; therefore, it is difficult to sustain that the temperature change was the primary driver because both sites received a similar impact (Figure 8 shows a similar average), and other factors such as topography and drainage conditions, other potential factors mentioned were not measured either or described properly. In general, this paper is highly speculative, and it lacks rugosity with many imprecise sentences and often confusing ones (see below).
We thank the referee for their positive comments and suggestions.
There are two aspects to the referee's remarks: one is how to explain the differences between the two sites and the other is why there were changes over time.
As the other referees pointed out, the detailed description of the sites is insufficient to understand their differences. We returned to the field to a better understanding of the relationship between the Apacheta River and our peatlands. The two peatlands have similar vegetation. APA2 is located at 4420m on a gentle slope of the valley, and APA1 is at 4200m and is located on a glacial terrace now incised by the Rio Apacheta which is a few meters lower. So, the river does not supply water to APA2 and it turns out that the drainage areas of the two peatlands are not as different as we thought.
The ratio of drainage area to peatland area explains well the difference in accumulation rate between the peatlands and in net primary production that has been estimated by MODIS as you suggested (please see Table RC2.1: Data comparison between both peatlands (APA1 and APA2), in SUPPLEMENT).
With respect to changes in accumulation over time, the observed trend is the same in all 4 cores with a reduction in peat growth rates and carbon accumulation starting in the 1980s (please see Figure RC2.1: Carbon Accumulation Rates for the 4 cores, in SUPPLEMENT). For such short time scales, it is not differences in topography or drainage area that may have influenced the observed changes and no anthropogenic action on the drainage network was observed. The most likely hypothesis is that climate change has caused this reduction. It may have intervened directly, through changes in temperature or precipitation, or indirectly through reductions in snow, glaciers and permafrost. It is these hypotheses that we test here using the available data.
OTHER IMPORTANT AND MINOR DETAILS
ABSTRACT
L.15-17 “…Here, we point out the important role of Andean peatlands on carbon accumulation rates (CAR), one of the highest in the world, and the impact of climate on carbon storage over the last 65 years, using four peat cores”. From the sentence above it is not clear what is the highest in the world, the Andean peatlands in general, or your study using four-peat cores?
Both are very high. CAR for Andean peatlands in general are high (Benavides et al., 2013; Benavides, 2014; Cooper et al., 2015) and in our study CAR values are even higher.
1. L 19 “For both peatlands”: Never mention before the two peatlands sites.
Thank you, we have took this into account to improve the manuscript.
2. L 25 Where did depth accumulation rates reach up to? What is CE?
Highest CARs are found at the base of the cores.
CE: Common Era.
Copernicus English Standard: “CE (common era) and BCE (before the common era) should be used instead of AD and BC since CE and BCE are more appropriate in interfaith dialogue and science”.
3. L20 Annual mean temperature cannot be responsible; only humans are responsible for something.
We agree.
4. L25 The authors indicate a decrease in CAR during the study period may be due to a decrease in meltwater by the retreat of the glaciers and the increase in temperature (the last tested); however, an increase in temperature is not the only factor even when you do not mention if there was a type of control to confirm your findings. For comparison you have to be sure that the primary productivity was similar 50-60 years ago.
This is the characteristic of all paleo-environmental studies: we cannot be sure whether primary production has changed or not in the past, but if it has changed it is probably due to climate change. The MODIS satellite productivity data (figure below), while clearly showing the difference between the two peatlands, only shows a slight upward trend over the past 20 years. For precipitation no clear tendency appears, only a very recent increasing trend (Fig. 7). For this reason, we believe that temperature is the main driver of the observed changes, either directly or indirectly.
INTRODUCTION
L38 say: …researches, …must say: researches, however,..
We agree.
L.76-103 move this section to M&M.
OK.
The authors need to clearly describe the differences between APA-1 and APA-2 in the results section, as the calibrated age from APA-1 and APA-2 are compared.
According to the suggestions made by Referee 1, we recalibrated the age with the most recent curve published by Hua et al. (2021) using the mixed curve recommended for South American Monsoon region (Bomb21SH3). All other data as been recalculated in agreement.
The new age models are shown in Figure RC2.2 (in SUPPLEMENT) and are very similar to the old ones.
M&M
I generally miss the statistical analysis for setting the differences of CAR and depths.
Please see Figure RC2.3 (Statistics for the two periods, before and after the transition for the 4 cores, in SUPPLEMENT).
L.105 says: between 29 and 35 cm-long, it must say: intervals layers between 2 and 31 cm depth.
To clarify this point, we have changed the sentence to: “For this study, four peat cores were collected: APA1-C1(34 cm) and APA1-C5 (29 cm) from the site APA1 located at 4200 m, and APA2-C3 (35 cm) and APA2-C4 (34 cm) from the APA2 site at 4420 m.”
L.105-107 The authors need to clarify how they named the samples in Table 1. In M&M, there is no clear description.
We have added more information about the two sites in M&M. And we have added an extra column in Table 1 to identify the differences between the two sites (APA1 and APA2). The description of the two sites is now more detailed.
L.114 says: accelerator mass spectroscopy, it must say: accelerator mass spectrometry. This mistake comes from another article, Xing et al. (2015) that used the same terminology.
We agree with the referee. This was a mistake. It is now corrected.
L.127 says C stable isotope. It must say. The natural abundance of stable isotope…
We agree. It is now corrected: “The natural abundance of C stable isotope was determined using an isotope mass spectrometer … “.
L.131-132 Even though you are citing a source, please give the equation and units of each variable. How were C accumulation rates calculated? It is not straightforward and familiar for all readers. By the way, Lähteenoja et al., 2009 and Cooper et al., 2015 are not listed in the reference.
Sorry, we have added the two references in the list.
“Carbon accumulation rates (CAR in g C m-2 yr-1) were determined using the mathematic equation (Eq. 2) (Lähteenoja et al., 2009; Cooper et al., 2015; Xing et al., 2015):
CAR = BD * GT * TOC (Eq. 2)
Where: CAR is the carbon accumulation rate (gC m−2 yr−1); BD is the bulk density of the bulk peat samples (g cm−3); GT is the growth rate (cm yr-1); TOC is the total organic carbon content (%).”
We have changed the term accumulation rate to growth rate, which is more appropriate for peatlands.
L.133 says: strong. It must say: significant and positive (or negative)…
We agree with the referee. The term "strong" is too much. “We used new stable isotope paleoclimate proxy (δ13C) based on a positive significant relationship found between the C stable isotope composition of Distichia and air temperature (Skrzypek et al. 2011).”
L.136-137 says: …can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat, as they mentioned. It must say: can be used to estimate relative paleotemperature changes recorded in Andean Distichia peat during the growth season (See Skrzypek et al. 2011).
We agree.
L.138-139 Please expand the explanation about the resolution used because I understand that NCP-NCAR uses 5ºxº5 pixel. I know you cite Kalnay et al., 1996; however, the last reference is not in the list of references.
We are sorry for this oversight. The NCEP/NCAR reanalyses data have a latitude-longitude 2.5° grid spatial resolution (Kalnay et al., 1996).
The reference is below.
Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R., and Joseph, D.: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–472, doi:10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2, 1996.
RESULTS
L 146-147 The authors say “…an abrupt change occurred at the end of the 1970s when the rates visibly decreased”… Compared with? APA-2 ? I see such abrupt change from Fig 2 if I only compare APA-2 with APA-1.
The four cores show a marked decrease in carbon accumulation rates from the early 1980s. We interpret this decrease as being the consequence of a decrease in nutrient input from the melting and retreat of the glaciers over tiem which has caused a decrease in productivity. And this is originally related to a rise in temperature in the Andes.
L161. “Mean TOC content…” Figure 3: Neither the text tells us if these results average the three depths or only the upper part? The authors refer to supplementary information to prompt the reader to seek information, but this must be carried over to the main text.
We will transfer the Figure 1 from the Supplementary Material to the main text.
L.174 The authors say, “…CAR varied depending on age and elevation” however, the elevation of these sites is similar (see sites description).
Although the difference between the altitudes of APA1 and APA2 are only 220 m, for the Andes this difference is important for temperature and productivity.
L.184. It is hard to see differences without statistical analysis. The variability is so high.
We thought that the variability between the two sites was visible from the graphs, but at the request of the referee we present the table below with the mean and standard deviation of the CARs before and after the transition year. We hope this data is enough to present this transition (please see Table RC2.2: Statistics for the two periods, before and after the transition for the 4 cores). We think that the data in the table highlights this transition.
L193. I do not see the difference for APA-2, even when it was the site that present lower CAR.
We believe that the table mentioned above (Table RC2.2: Statistics for the two periods, before and after the transition for the 4 cores) highlights these differences.
DISCUSSION
L.197 The authors introduce Fig 6 for tropical versus boreal and temperate climate; however tropical high latitude presents an enormous error bar, invalidating the comparison. Please remove this Figure from the Discussion.
These are not error bars, but extreme values. We will remove and modify this figure.
L.234-235 “…The author says: ...differences found in CAR (Fig. 4) ...were related to the different drainage area surfaces, much more prominent for APA-1 than for APA-2. These differences must be described in the site descriptions first and later discussed.
We thank the referee for this comment and in the new version we have included these data in the text. Now the sub-catchment areas have been recalculated after a supplementary field work (March and April 2022), indeed APA1 is much smaller since it is not drained by Apacheta River which is located several meters below the peatland. The sub-catchment area of APA1 is only 3.3 Km2 while for APA2, the sub-catchment area, inserted in APA2 one, is 2.14 km2. Plear see Figure RC2.3: The sub-catchment area of APA1, and Figure RC2.4: The sub-catchment area of APA2, in SUPPLEMENT.
L 237. Again other differences that were not described “…specific topographic factors,...”
We have included information about the topography and soils of the study area:
“The Apacheta region is characterized by being a mountainous area, with peatlands located in the valleys and sections with gentle slope, at altitudes above 4100 m asl. Edaphologically, the study areas are mainly composed of relatively medium texture deep soils developed upon volcanic rocks (porphyritic andesite) from Apacheta formation (Nm-ap_s) (INGEMMET, 2002). In this area, the main economic activities of the local population are agriculture and livestock. Agriculture takes place at lower altitudes than peatlands and grazing of livestock occurs in the peatland zone, because peatlands provide year-round forage production for grazing native domestic camelids (llama and alpaca) and for livestock species (particularly sheep). Evidence of grazing activity has been observed in the study area although with little visible impact on peatlands."
L.239-240 “…Although there is a similar downward trend in the CAR at both sites after the early 1980s,..” I do not see the difference in APA-1 in Fig. 2.
Statistics are in the table RC2.2 (Statistics for the two periods, before and after the transition for the 4 cores, in SUPPLEMENT).
L.256. Move Fig. 7 to the results section.
We agree.
L.255-260 What about photosynthesis. The increase in CO2 must have a consequence?
L.280-285. Ok, here photosynthesis is discussed.
Ok.
L.283 “…The strong gradients in δ13C…” Insist I do not see this gradient in APA-2 having a similar temperature.
The gradient we are talking about is the one established by Skrzypek et al. (2011).
L286 Figure 8 should be the first figure that the authors must show in the result section.
The main objective of our study is to estimate the rates of recent carbon accumulation in these peatlands. It is only in a second step that we formulate hypotheses as to the cause of the decrease of these rates after 1980.
L.292 Say: “showed a good relationship especially in trends”. It must say: showed a good relationship”
We agree.
L.291-294 “…this comparison is difficult because the NCEP data … because we do not know precisely what time period each peat sample corresponds to”, this sentence is not clear.
The problem is that the ∂13C and thus temperature values are obtained for different dates on the different cores. Another point is that the growth rates are different in the two cores, meaning that each sample do not correspond to the same interval of time.
L.296 “between 1.9 and 2ºC” is different than “from 1.9 and 2ºC” what do you mean?
Sorry for the English redaction. We observed an increase of 1.9 °C for a core and 2°C for the other.
CONCLUSIONS
L 307-311 Sentences are more summary than conclusions.
We will reformulate those sentences.
L.314-316. “…This decline in C accumulation was mainly related to the temperature rise which increases the organic matter degradation rate…” The lower CAR probably comes from a lower primary biomass production in APA-2, which was not measured neither discussed. This may have shed light on the input, prevented speculation such as high decomposition rate, and reduced water supply from glacier retreats. The hypothesis that the temp We have estimated the primary production from MODIS satellite data (2000-2021) and can observe the variations between the two sites (see results in graph below) erature causes the differences been CARs in my view has not been demonstrated.
We appreciate the suggestion.
We have estimated the primary production from MODIS satellite data (2000-2021) and can observe the variations between the two sites (please see Figure RC2.5: Primary production from MODIS satellite data (2000-2021) for the study area, in SUPPLEMENT). APA1 has an average net primary production (NPP) for the period of 0.37 kg C m-2 and APA2 of 0.27 kg C m-2.
The two peatlands have similar vegetation, APA2 is located at 4420m on a gentle slope of the valley, and APA1 is at 4200m and is located on a glacial terrace now incised by the Rio Apacheta which is a few meters lower. So the river does not supply water to APA2 and it turns out that the drainage areas of the two peatlands are not as different as we thought and that the ratio of drainage area to peatland area explains well the difference in accumulation rate between the peatlands.
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RC3: 'Comment on bg-2022-47', Anonymous Referee #3, 14 Mar 2022
The authors present an interesting study on the impact of climate change on Andean peatlands carbon storage. Given the importance of peatlands for the global carbon cycle and climate, I believe this paper is of interest to the readership of this journal. Although the methodology is not particularly novel, the Andean peatlands appear poorly studied (after a quick search on web of science) and any good evidence of climate change impact on this ecosystem woudl be worth being published.
I have some major comments on the methodology and interpretation of the data that led to certain conclusions and some minor comments to help improve the manuscript overall.
- I have some concerns about the derivation of accumulation rates and carbon accumulation rates and the logic behind these estimations. First, how is the accumulation rate obtained? This is not presented. Second, why is the CAR computed directly from the accumulation rate? Is the assumption behind this step that carbon moves only top to bottom? I am not able to determine because this methodology is only briefly mentioned here. But if this is the case, what about carbon released from the roots? Could not plant release carbon directly at depth as root exudates? We just need more details and discussion of the assumptions to better evaluate conclusions originating from this approach.
- The authors somewhat try to infer the evolution of soil carbon over time as a result of the balance between inputs (from plants) and outputs (decomposition). Is there any estimate of how plant productivity changed over time? Although only from year 2000, MODIS from NASA could help.
- Most importantly, I believe the authors should considerably improve their discussion of the methodology, give more context about these peatlands, and elaborate more on their research question.
Minor comments:
- There are some issues with the abstract. First, in line 16 the authors introduce carbon accumulation rates (CAR), but then the following line they quantify accumulation rates. From reading the rest of the paper these two quantities are different and have different units. From the abstract it seems they are the same quantity, and it is measured in cm per year. Second, there is no reason to mention APA1 and APA2 here, because a reader does not know what they are at this point. Third, the sentence in lines 20-23 on Distichia muscoides does not seem to fit here. This seems a preliminary information that could be mentioned earlier, if necessary. In summary, I would simplify the abstract and keep only information and conclusions that are needed to invite a reader to look at the entire paper.
- Give at least a brief description of the age-depth model.
- Rather than just a simple map, Figure 1 could be used to introduce also trends in temperature and other relevant preliminary information about the sites (e.g., what is presented in fig. 7).
- Line 174. replace “that” with “than”.
- Line 242. Maybe topographic “location” is better than “conditions”.
- Lines 276-285. Could you rephrase this whole paragraph?
Citation: https://doi.org/10.5194/bg-2022-47-RC3 -
AC3: 'Reply on RC3', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the referee's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
The authors present an interesting study on the impact of climate change on Andean peatlands carbon storage. Given the importance of peatlands for the global carbon cycle and climate, I believe this paper is of interest to the readership of this journal. Although the methodology is not particularly novel, the Andean peatlands appear poorly studied (after a quick search on web of science) and any good evidence of climate change impact on this ecosystem woudl be worth being published.
We really appreciate your comments and positive suggestions that will improve our manuscript.
I have some major comments on the methodology and interpretation of the data that led to certain conclusions and some minor comments to help improve the manuscript overall.
1. I have some concerns about the derivation of accumulation rates and carbon accumulation rates and the logic behind these estimations. First, how is the accumulation rate obtained? This is not presented. Second, why is the CAR computed directly from the accumulation rate? Is the assumption behind this step that carbon moves only top to bottom? I am not able to determine because this methodology is only briefly mentioned here. But if this is the case, what about carbon released from the roots? Could not plant release carbon directly at depth as root exudates? We just need more details and discussion of the assumptions to better evaluate conclusions originating from this approach.
We have changed the term accumulation rate to growth rate, which is more appropriate since all of the material is peat.
Thank you for your observation, there is a little mistake in the formula: CAR was obtained multiplying the growth rate (cm yr-1) by the bulk density (g cm-3 yr-1) and the TOC (%).
The carbon is not supplied from top to bottom. The peat is a mixed system where the plants of the peatlands grow vertically leaving behind the dead and decomposed organic matter. To this system is added a sedimentary system, not very important quantitatively in our case, whose sediments will be deposited on the surface of the peat. This type of system produces extremely coherent 14C ages over periods of time ranging from ten to thousands of years, showing the progressive net accumulation of organic matter by these systems.
2. The authors somewhat try to infer the evolution of soil carbon over time as a result of the balance between inputs (from plants) and outputs (decomposition). Is there any estimate of how plant productivity changed over time? Although only from year 2000, MODIS from NASA could help.
We thank you for this suggestion.
We have estimated the primary production from MODIS satellite data (2000-2021) and can observe the variations between the two sites (please see Figure RC3.1: Primary production from MODIS satellite data (2000-2021) for the study area, in SUPPLEMENT). APA1 has an average net primary production (NPP) for the period of 0.37 kg C m-2 and APA2 of 0.27 kg C m-2.
The two peatlands have similar vegetation, APA2 is located at 4420m on a gentle slope of the valley, and APA1 is at 4200m and is located on a glacial terrace now incised by the Rio Apacheta which is a few meters lower. So the river does not supply water to APA2 and it turns out that the drainage areas of the two peatlands are not as different as we thought and that the ratio of drainage area to peatland area explains well the difference in accumulation rate between the peatlands. Please see Table RC3.1: Data comparison between both peatlands (APA1 and APA2) in SUPPLEMENT.
3. Most importantly, I believe the authors should considerably improve their discussion of the methodology, give more context about these peatlands, and elaborate more on their research question.
We agree with the referee and will change the text according to this remark.
MINOR COMMENTS
1. There are some issues with the abstract. First, in line 16 the authors introduce carbon accumulation rates (CAR), but then the following line they quantify accumulation rates. From reading the rest of the paper these two quantities are different and have different units. From the abstract it seems they are the same quantity, and it is measured in cm per year. Second, there is no reason to mention APA1 and APA2 here, because a reader does not know what they are at this point. Third, the sentence in lines 20-23 on Distichia muscoides does not seem to fit here. This seems a preliminary information that could be mentioned earlier, if necessary. In summary, I would simplify the abstract and keep only information and conclusions that are needed to invite a reader to look at the entire paper.
We took this into account to improve the manuscript.
2. Give at least a brief description of the age-depth model.
We will add this information.
3. Rather than just a simple map, Figure 1 could be used to introduce also trends in temperature and other relevant preliminary information about the sites (e.g., what is presented in fig. 7).
We agree with the suggestion. We will make a new figure 1.
4. Line 174. replace “that” with “than”.
We agree.
5. Line 242. Maybe topographic “location” is better than “conditions”.
We agree.
6. Lines 276-285. Could you rephrase this whole paragraph?
We will change the text according to this remark and those from other referees.
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RC4: 'Comment on bg-2022-47', Anonymous Referee #4, 01 Apr 2022
The authors use 4 short peat cores from 2 high-elevation Peruvian peatlands to discuss recent changes in carbon accumulation rates (CARs). They found very high apparent rates of carbon accumulation at these sites. They document a decrease in CAR after 1980 that could be caused by an increase in annual temperature; they corroborate this hypothesis with d13C values from the peat cores. The authors assume that d13C values are proxies for temperature (this assumption comes from a different article). Overall, this is an interesting study that could be appropriate for CofP readership, but I am afraid it is not ready for publication. I have numerous methodological issues (some may be flaws), which are described below. In particular, the CAR calculations may not represent true recent changes in carbon dynamics. Likewise, I have serious doubt about the application of the temperature transfer function to the d13C of bulk (?) peat. I hope the comments below are of use to the authors, who should definitely review their study and submit at a later time.
Idea: It would be interesting to read about why you think these young peat deposits started developing less than 100 years ago -- could make for a good discussion of the article.
GENERAL COMMENTS:
Study area: I would have liked to read more about the hydrology of the area, its vegetation, whether it is pristine or impacted by local communities (and animals), etc. Why did you choose those 2 sites should also be covered, as well as brief descriptions of those 2 sites (including the coring sites themselves). As of now, this section lacks important information.
Methods: lots of important information is missing (see the list below).
Results: (1) the trends in CAR that "slow down" in the early 1980s might be due to an autogenic process: the young peat has not decomposed and compacted yet, making TOC values smaller than the older peat. This would potentially yield lower CARs... (2) the changes in d13C are not considering the Suess effect. Other factors impacting d13C should also be tested/discussed, including hydrological changes.
SPECIFIC COMMENTS:
Abstract:
lines 14-15: "...since glaciers have been recognized as one of their vital water sources" -- this is true for some Andean peatlands, but certainly not for MANY of them. This statement is therefore too general and misleading.
line 23: a "new" proxy... the reference you are using is over 10 years old! not so "new" (and they were not the first ones to use it either...)
Introduction:
line 35: This statement about "all carbon in the atmosphere" is incorrect!! "The amount of C stored in peatlands is similar to the total C stocks in all living biomass or in the atmosphere"
line 68: what are you referring to here? "between 500 and 700m in length"
line 72: this is not a "new" method!!
Study Area
line 77 / Figure 1: I'm a bit surprised by your delienation of the watersheds; I'm not familiar with this region, but why is watershed 1 so large and watershed 2 so small? Is watershed 2 in fact part of watershed 1? I don't understand why you are reporting the size of the watersheds...
Methods
line 105: why are the cores shorter than the PVC tubes? Are the peat deposits only 29 to 35 deep? If so, you need to mention this important "detail".
line 110-113: please add references to the methods you describe.
line 120: why not use a Bayesian approach? It seems like the standard in paleoecological studies these days. Bacon can accomodate for your postbomb dates.
line 129: by convention, you must report against which international standard your d13C values were calibrated against! (VPDB?). Also, and perhaps more importantly, how did you sample and prepare for d13C measurements? Did you measure the bulk peat, the Distichia leaves, or something else? Did you extract the cellulose or not? What weight did you use in the lab? A lot of information missing here... that would hinder replication of your study.
line 131: you should use organic matter density (rather than dry bulk density) to truly estimate CAR... You can do it since you have OM% from the LOI measurements...
lines 134-135: you need to explain the mechanism that links d13C with temperature... If I remember the Skrzypek study, they used an elevational gradient to build their relationship, which means that temperature may NOT be the main factor, but rather changes in pressure...
lines 135-136: this sentence does not make sense to me; what do you mean? "This value was similar to the previously reported range for other species (included Sphagnum peat: -0.5 to -0.6‰/°C)"
line 139: why did you use 600 mb in this case? Are there known limitations/issues with using the NCEP/NCAR reanalysis in the high Andes that should be documented?
Results:
Table 1: you say that you report "2 sigmas", but clearly you do not. Instead, you only report the calibrated age - it's unclear if this is the mean, median, or most probable age provided by Clam. Since those are post-bomb dates, it would be useful to know the most plausible age ranges (on either sides of the postbomb calibration curve).
Figure 2: I cannot tell which dates (and error bars) belong to which cores! would it be possible to have 4 panels (one for each core)? It could go in the supplementary file...
lines 163-165: you say that "there was a general upward trend in TOC content from the peat basal depth of the cores from both studied peatlands to approximately 13 cm (the early 1980s) and then the TOC values decreased to the top of the cores (2015 CE)". This is likely because the uppermost samples are "fresher", being that they have not undergone decomposition and compaction. This is likely why your recent CARs are lower than your older CARS... In other words, this could all be an autogenic signal that has nothing to do with a temperature change.
line 174: mean CAR were higher at APA1 than at APA2 - probably because APA1 has high bulk density?! It would be worth to calculate organic matter density for a fairer comparison of those sites.
line 189: did you consider the Suess effect at your sites? It is expected that d13C become more negative over time because of fossil fuels mixing in the global atmosphere... "At both peatlands, there was a general trend to more negative δ13C values from the basal depth to the top of the cores". Getting rid of the Suess effect would be very useful. Then, I see that your 2 cores tell different stories: one of them (the red line on Fig 5) would show increasing d13C values vs. the blue curve would show a decreasing d13C trend. As mentioned in my intro, I am not convinced that these are temperature records. These could relate to hydrological changes: could it be that one site is becoming wetter )blue line) vs the other one is becoming drier (red line)? Please look into the literature that discusses stomatal closure.
Discussion:
lines ~ 200: you should read the paper by Benfield and Yu, Distichia deposits from Columbia were analyzed... You'll see that they also document very high recent CARs.
lines 195-205: you cannot compare your core tops with Holocene-aged cores and say that your cores have greater CARs! This is obvious: short peat hasn't decomposed much, especially compared to old sites... Figure 6 is a misrepresentation and flawed way to compare these data. For a fairer discussion, only look at recent CARs from around the world... There are plenty of data to play with!
I did not comment on the rest of the discussion, as I question the validity of the results.
Citation: https://doi.org/10.5194/bg-2022-47-RC4 -
AC4: 'Reply on RC4', Romina Llanos, 01 Jun 2022
In order to be able to respond to each of the referee's observations and comments, I will put all of them in "normal" font, and our responses to them in bold italics, to make sure we respond to everything.
The authors use 4 short peat cores from 2 high-elevation Peruvian peatlands to discuss recent changes in carbon accumulation rates (CARs). They found very high apparent rates of carbon accumulation at these sites. They document a decrease in CAR after 1980 that could be caused by an increase in annual temperature; they corroborate this hypothesis with d13C values from the peat cores. The authors assume that d13C values are proxies for temperature (this assumption comes from a different article). Overall, this is an interesting study that could be appropriate for CofP readership, but I am afraid it is not ready for publication. I have numerous methodological issues (some may be flaws), which are described below. In particular, the CAR calculations may not represent true recent changes in carbon dynamics. Likewise, I have serious doubt about the application of the temperature transfer function to the d13C of bulk (?) peat. I hope the comments below are of use to the authors, who should definitely review their study and submit at a later time.
Idea: It would be interesting to read about why you think these young peat deposits started developing less than 100 years ago -- could make for a good discussion of the article.
We thank the referee for their comments to improve the manuscript.
We also apologize for the fact that we did not make it clear, that our Andean peatlands did not appear only 100 years ago. In fact, several papers (Engel et al., 2014; Schitteck et al., 2016) based on long core studies in the Andes show that peatlands existed throughout the Holocene. In APA1 peatland, we sampled, for example, core peat that are 2500 cal years BP (Huaman et al., 2020).
However, we aimed to study a much more recent period, so we chose to make short cores based on other studies (p. e., Benavides et al., 2013; Benavides, 2014) which showed that cores of about 30 cm covered the last ~100 years.
GENERAL COMMENTS:
STUDY AREA: I would have liked to read more about the hydrology of the area, its vegetation, whether it is pristine or impacted by local communities (and animals), etc. Why did you choose those 2 sites should also be covered, as well as brief descriptions of those 2 sites (including the coring sites themselves). As of now, this section lacks important information.
We chose to work at these sites due to the dominance of Distichia in both peatlands, the difference in altitude and because this species is well preserved in this Andean environments.
We will add more information about the study area (see below).
“The Apacheta region is characterized by being a mountainous area, with peatlands located in the valleys and sections with gentle slope, at altitudes since 4100 m asl. In this area, the main economic activities of the local population are agriculture and livestock. Agriculture takes place at low altitudes and grazing of livestock in the peatland zone, due peatlands provide year-round forage production for grazing native domestic camelids (llama and alpaca) and for livestock species (particularly sheep). Evidence of grazing activity has been observed in the study area although with little visible impact on peatlands.
Two tropical high-elevation peatlands (APA 1: 13º 21' 4.61'' S, 74º 39' 31.75'' W, 4200 m asl; APA 2: 13º 20' 35.66'' S, 74º 39' 41.04'' W, 4420 m asl) were studied in Apacheta region in the central Andes of Peru (Fig. 1). APA 1 is located in a sub-catchment with an area of 3.3 km2, with an elevation ranging from 4200 to 5000 m asl, while APA 2 sub-catchment had an area of only 2.14 km2, with an elevation ranging from 4350 to 4850 m asl. The topographic relief is variable, ranging from soft to slightly undulating and overlain with slopes of moderate to strong decline. Edaphologically, the study sites are mainly composed of relatively medium texture deep soils developed upon volcanic rocks (porphyritic andesite) from Apacheta formation (Nm-ap_s) (INGEMMET, 2002).
In this region, Distichia muscoides Nees & Meyen (Juncaceae) is the predominant cushion peatland species, and it is present on most high-elevation peatlands in the central Peruvian Andes (Schittek et al., 2015), however other plant species are also found, such as Plantago tubulosa, Aciachne pulvinata, Scirpus rigidus, Calamagrostis rigescens, Calamagrostis spp., Hypochaeris sessiliflora, Hypsela reniformis. D. muscoides is a dioecious semiaquatic plant that grows in dense cushions (Buffen et al., 2009; Skrzypek et al., 2011; Schittek et al., 2018). Distichia leaves are 3 to 7 mm-long, are inserted densely along the stem and form hard monticules (diameter: ~2 m), which are compact mats surrounded by flooded hollows that are permanently bare of vegetation (Balslev, 1996). This species is well adapted to the high-elevation Andean climate and is able to survive diurnal cycles of freezing and thawing (Buffen et al., 2009). The cushion-forming species Distichia muscoides Nees & Meyen dominates many high elevation bofedales in Chile (Squeo et al., 2006), Peru (Skrzypek et al., 2011; Salvador et al., 2014), Ecuador (Chimner and Karberg, 2008), and Colombia (Benavides et al., 2013). Distichia cushions may have started to form as a single individual, producing a large number of shoots and rhizomes, which later transformed into smaller groups as the underground parts of the plant died off (Schittek et al., 2018). The cushion-forming species Distichia muscoides Nees & Meyen dominates many high elevation bofedales in Chile (Squeo et al., 2006), Peru (Skrzypek et al., 2011; Salvador et al., 2014), Ecuador (Chimner and Karberg, 2008), and Colombia (Benavides et al., 2013).
Between the cushions of APA 1 an APA 2 peatlands we found small and shallow pools of water, that are characteristic of this type of ecosystem. The mean pH and conductivity, measured in these pools during the fieldwork, were 5.93 and 45.4 µS cm-1 for APA 1 and 6.01 and 39.2 µS cm-1for APA2, respectively.
The climate of the Apacheta region is typical of tropical high mountains with little seasonal variations in temperature and large seasonal precipitation variability with rainy and dry seasons. Based on data from the Apacheta station located near the two Andean peatlands of this study, about 1.6 km, at 13° 20' 51" S, 74° 38' 44" W and 4150 m asl, the study area presents an average annual precipitation of 830 mm for the period 1991–2012 CE and is marked by seasonal precipitation, with the wettest months being from October to March (monthly average: 114 mm). This seasonal pattern of rainfall reflects the occurrence of South American Monsoon during South Hemisphere summer. The mean annual temperature of the upper part of the basin is 6.4 ºC for the period 2000–2014 CE, with monthly averages ranging from 4.8 to 7.6 ºC; and the annual average relative humidity is 70.3% (for the period 2009–2013 CE) (GORE Ayacucho, 2015).”
METHODS: Lots of important information is missing (see the list below).
RESULTS:
(1) the trends in CAR that "slow down" in the early 1980s might be due to an autogenic process: the young peat has not decomposed and compacted yet, making TOC values smaller than the older peat. This would potentially yield lower CARs...
We found the opposite. Constant high TOC values over the two APA1 cores, and high values with a rising trend after the 1980's for the two APA2 cores (Figure 1 in SM).
(2) the changes in d13C are not considering the Suess effect. Other factors impacting d13C should also be tested/discussed, including hydrological changes.
The problem is: there is no precipitation changes registered in the climate data, so it is difficult to use this argument for ∂13C changes.
The most recent atmospheric ∂13C-CO2 data for the period 1850-2015 (Graven et al., 2017) indicate a trend of -0,01 ‰ yr-1 to be compare to the -0.047‰ yr-1 (APA1-C5) and -0.044 ‰ yr-1 (APA2-C4) we observed in our cores. However the referee is right in remembering the Suess effect on atmospheric ∂13C-CO2 and we will integrate it in our calculations. This will not change the temperature values much because they are not based on absolute ∂13C values but on the differences between ∂13C values from one sample to another.
SPECIFIC COMMENTS
ABSTRACT:
lines 14-15: "...since glaciers have been recognized as one of their vital water sources" -- this is true for some Andean peatlands, but certainly not for MANY of them. This statement is therefore too general and misleading.
We agree.
line 23: a "new" proxy... the reference you are using is over 10 years old! not so "new" (and they were not the first ones to use it either...).
For us, 11 years is not particularly old, but we have no problem removing the term "new".
INTRODUCTION:
line 35: This statement about "all carbon in the atmosphere" is incorrect!! "The amount of C stored in peatlands is similar to the total C stocks in all living biomass or in the atmosphere"
According to Yu et al., (2016) the information we put in the manuscript is valid, it refers to the quantity of carbon, not CO2.
line 68: what are you referring to here? "between 500 and 700m in length"
This is a citation from Rabatel et al. (2013) who measured a retreat of the front of the glacier of 500 to 700m. This means that the length of the glaciers has decreased by 500 to 700m.
line 72: this is not a "new" method!!
OK.
STUDY AREA
line 77 / Figure 1: I'm a bit surprised by your delienation of the watersheds; I'm not familiar with this region, but why is watershed 1 so large and watershed 2 so small? Is watershed 2 in fact part of watershed 1? I don't understand why you are reporting the size of the watersheds...
We thank the referee for this comment and in the new version we have included these data in the text.
The exact coordinates of the investigated sites are now specified and we detected a mistake in APA1 position on the map figure 1. APA 1 Cores: Lat. -13,35128; Long. -74,65882; APA 2 Cores: Lat. -13,34324; Long. -74,66140.
Now the sub-catchment areas have been recalculated after a supplementary field work (March-April 2022) and indeed APA1 is much smaller since it is not drained by Apacheta River which is located several meters below the peatland. The sub-catchment area of APA1 is only 3.3 Km2 while for APA2, the sub-catchment area, inserted in APA2 one, is 2.14 km2 (please see Figure RC4.1: The sub-catchment area of APA1, and Figure RC4.2: The sub-catchment area of APA2, in SUPPLEMENT).
The purpose of delimiting the two basins is to be able to compare peatland areas and carbon accumulation rates (please see Table RC4.1: Data comparison between both peatlands (APA1 and APA2)).
METHODS:
line 105: why are the cores shorter than the PVC tubes?
The size of the tubes (50 cm) is larger than the cores because it is necessary to leave a part of the tubes outside the peat in order to remove them manually.
We chose to work in a more current period to emphasize this period, based on the Huaman et al 2020 core that worked in the same place with a longer core (2500 years BCE) and that highlighted the big differences between the last 60 years and the rest of the core. To clarify this point, we will change the sentence to: “For this study, four peat cores were collected: APA1-C1(34 cm) and APA1-C5 (29 cm) from the site APA1 located at 4200 m, and APA2-C3 (35 cm) and APA2-C4 (34 cm) from the APA2 site at 4420 m.”
Are the peat deposits only 29 to 35 deep? If so, you need to mention this important "detail".
No, the peat deposits are longer at APA1 site (see Huaman et al 2020) and APA2 (no published data), but our aim objective was to work the most recent periods (last 60 years) so we only made core samples of 30 cm.
line 110-113: please add references to the methods you describe.
We agree.
line 120: why not use a Bayesian approach? It seems like the standard in paleoecological studies these days. Bacon can accomodate for your postbomb dates.
The first assumption of Bayesian method for age models is that “the chronology can be broken down into events” (Bronk Ramsey, 2019, Radiocarbon 51:337-360). It means that the sedimentation is formed by a succession of discrete sedimentary events, i.e. sediment layers that constitute the sedimentary sequence. This is generally true for sedimentary deposits but does not apply to peat growth. Mainly in tropical peat and in Distichia peat in particular, which is growing all along the year. For that reason, the Bayesian method cannot be applied in these peats and the best way to build the age model is by spline cubic interpolation.
line 129: by convention, you must report against which international standard your d13C values were calibrated against! (VPDB?).
Yes, δ13C values were expressed relative to international standards VPDB.
Also, and perhaps more importantly, how did you sample and prepare for d13C measurements? Did you measure the bulk peat, the Distichia leaves, or something else?
We measured the δ13C in the bulk peat.
Did you extract the cellulose or not?
No, we do not extract the cellulose.
What weight did you use in the lab?
For the δ13C analysis we use about 2 - 3.5 mg of material. All analyses were performed in duplicates.
A lot of information missing here... that would hinder replication of your study.
We apologize, but in the new version of the manuscript the answers or precisions to all of these comments will be included.
line 131: you should use organic matter density (rather than dry bulk density) to truly estimate CAR... You can do it since you have OM% from the LOI measurements...
To calculate the CAR based on growth rate (cm yr-1) we have multiplicated it by the bulk density to obtain the total accumulation rate (g cm-2 yr -1) and then by OM percentage multiplicating by 100-LOI%. Bulk density by OM% is the OM bulk density, so, in fact our calculation of the CAR is based on the organic matter density.
lines 134-135: you need to explain the mechanism that links d13C with temperature... If I remember the Skrzypek study, they used an elevational gradient to build their relationship, which means that temperature may NOT be the main factor, but rather changes in pressure...
According to Skrzypek et al. (2011) and Engel et al. (2014) temperature is the main factor responsible for ∂13C variations (see below):
“Several authors have investigated the meaning of the ∂13C value in peat and peat-forming plants, especially in the Sphagnum genera (including Menot-Combesetal., 2004; Loader et al., 2007; Moschen et al., 2009; Brader et al., 2010; Tillman et al., 2010; Skrzypek et al., 2013) and have considered environmental factors such as air temperature, humidity, precipitation, vapour pressure deficit and atmospheric CO2 concentration. Despite the possibility of a combined influence of a few factors, the air temperature of the growth season seems to be the major factor governing ∂13C of Distichia macrofossils well pre- served in peat sediments (Skrzypek et al., 2011). Our initial calibration, based on an altitudinal transect that intersects the core collection site, indicated that the observed decrease of ~0.97 ± 0.23‰ in the stable carbon isotope composition of Distichia peat reflects a 1°C increase in the mean air temperature of the growing seasons at the ground level. The temperature at the ground level at high altitudes largely reflects insolation. In contrast, no obvious relationship was observed between precipitation and the stable carbon isotope composition of Distichia peat (Skrzypek et al., 2011).” Engel et al., 2014.
lines 135-136: this sentence does not make sense to me; what do you mean? "This value was similar to the previously reported range for other species (included Sphagnum peat: -0.5 to -0.6‰/°C)"
Other peatland species also exhibit a negative ∂13C gradient as a function of temperature, e.g. -0.5 to -0.6‰/°C for Sphagnum peat (Skrzypek et al., 2010).
line 139: why did you use 600 mb in this case?
Due to the altitude of our study area. It is located at 4000 - 4200 m asl, and this is related to a barometric pressure of approximately 600 mb (West, 1996; Paul & Ferl, 2005). Salzmann et al. (2013) in a study on glacier changes and climate trends in southern Peruvian Andes, also used NCEP/NCAR data for their analysis.
Are there known limitations/issues with using the NCEP/NCAR reanalysis in the high Andes that should be documented?
Of course there are many limitations in NCEP reanalysis, principally in a mountainous area, because these reanalysis data have a poor special resolution and then the topography is not well represented. This product is unable to take into account local process, like mountain breeze. In fact, our objective was to compare our data with regional climatological data to understand the relationship between regional climate and the peat functioning. The hypothesis is that recent climate changes have influence the peat development. Higher resolution data which, in reality, are produced by a downscaling with the same initial number of observation data, do not seems better in this case.
RESULTS
Table 1: you say that you report "2 sigmas", but clearly you do not. Instead, you only report the calibrated age - it's unclear if this is the mean, median, or most probable age provided by Clam. Since those are post-bomb dates, it would be useful to know the most plausible age ranges (on either sides of the postbomb calibration curve).
We are sorry, we forgot to give the two sigmas values, this will be corrected. The calibration has been done again using the most recent pos-bomb curve (Hua et al., 2021). We used Oxcal to calibrate the 14C data. Here are the new age models.
Figure 2: I cannot tell which dates (and error bars) belong to which cores! would it be possible to have 4 panels (one for each core)? It could go in the supplementary file...
According to the suggestions made by Referee 1, we recalibrated the age with the most recent curve published by Hua et al. (2021) using the mixed curve recommended for South American Monsoon region (Bomb21SH3). All other data has been recalculated in agreement.
New age models (new Figure 2) are shown in Figure RC4.3 (in SUPPLEMENT). No significant changes were found.
lines 163-165: you say that "there was a general upward trend in TOC content from the peat basal depth of the cores from both studied peatlands to approximately 13 cm (the early 1980s) and then the TOC values decreased to the top of the cores (2015 CE)". This is likely because the uppermost samples are "fresher", being that they have not undergone decomposition and compaction. This is likely why your recent CARs are lower than your older CARS... In other words, this could all be an autogenic signal that has nothing to do with a temperature change.
The content of organic matter and thus carbon increases towards the top of the APA2 cores and is relatively constant for APA1 cores. The peat decomposition during time would lower the CAR accumulation downcore (please see Figure RC4.4: Organic matter content (%) for the 4 cores, in SUPPLEMENT).
line 174: mean CAR were higher at APA1 than at APA2 - probably because APA1 has high bulk density?! It would be worth to calculate organic matter density for a fairer comparison of those sites.
Yes, the mean bulk density is 44% higher at APA1 (0.11 g cm-3) than APA 2 (0.076 g cm-3), but this is not the only reason why the CAR is higher, the mean growth rate is 54% higher at APA1 (0.87 cm yr-1) than APA2 (0.57 cm yr-1). APA1 is also richer in organic matter (96.2%) than APA2 (91.7%). This leads to a difference in MO densities (0.10 gMO cm-3 for APA1 and 0.07 gMO cm-3 for APA2) that has been taken into account in the calculations as explained above.
line 189: did you consider the Suess effect at your sites? It is expected that d13C become more negative over time because of fossil fuels mixing in the global atmosphere... "At both peatlands, there was a general trend to more negative δ13C values from the basal depth to the top of the cores". Getting rid of the Suess effect would be very useful. Then, I see that your 2 cores tell different stories: one of them (the red line on Fig 5) would show increasing d13C values vs. the blue curve would show a decreasing d13C trend. As mentioned in my intro, I am not convinced that these are temperature records. These could relate to hydrological changes: could it be that one site is becoming wetter )blue line) vs the other one is becoming drier (red line)? Please look into the literature that discusses stomatal closure.
The most recent atmospheric ∂13C CO2 data for the period 1850-2015 (Graven et al., 2017) indicate a trend of -0,01 ‰ yr-1 to be compare to the -0.047‰ yr-1 (C5) and -0.044 ‰ yr-1 (C4) we observed in our cores. However, the referee is right in remembering the Suess effect on atmospheric CO2 ∂13C and we will integrate it in our calculations. This will not change the temperature values much because they are not based on absolute ∂13C values but on the differences between ∂13C values from one sample to another.
As explained above there is no change in precipitation in the reanalyses and we do not see how to explain that APA1 would become wetter while APA2 would become drier. For such short time scales, it is not differences in topography or drainage area that may have influenced the observed changes and no anthropogenic action on the drainage network was observed. Skrzypek et al. (2011) has shown that temperature influences ∂13C of this kind of peatlands and others, and this explains well the variations observed in our cores.
DISCUSSION
lines ~ 200: you should read the paper by Benfield and Yu, Distichia deposits from Columbia were analyzed... You'll see that they also document very high recent CARs.
Yes, we know this paper. Those authors found high accumulation rates for the Colombian site that confirms our findings. This reference will be included in the discussion.
lines 195-205: you cannot compare your core tops with Holocene-aged cores and say that your cores have greater CARs! This is obvious: short peat hasn't decomposed much, especially compared to old sites... Figure 6 is a misrepresentation and flawed way to compare these data. For a fairer discussion, only look at recent CARs from around the world... There are plenty of data to play with!
Ok, we will redraw this figure with recent CAR data only.
I did not comment on the rest of the discussion, as I question the validity of the results.
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AC4: 'Reply on RC4', Romina Llanos, 01 Jun 2022
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