Reconstructing past variations in environmental conditions and paleoproductivity over the last  ∼ 8000 years off  north-central Chile (30°&thinsp;S)

Muñoz, Práxedes; Rebolledo, Lorena; Dezileau, Laurent; Maldonado, Antonio; Mayr, Christoph; Cárdenas, Paola; Lange, Carina B.; Lalangui, Katherine; Sanchez, Gloria; Salamanca, Marco; Araya, Karen; Jara, Ignacio; Easton, Gabriel; Ramos, Marcel

doi:https://doi.org/10.5194/bg-17-5763-2020

Articles | Volume 17, issue 22

https://doi.org/10.5194/bg-17-5763-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-17-5763-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 22

Research article

|

24 Nov 2020

Research article |

| 24 Nov 2020

Reconstructing past variations in environmental conditions and paleoproductivity over the last ∼ 8000 years off north-central Chile (30° S)

Práxedes Muñoz, Lorena Rebolledo, Laurent Dezileau, Antonio Maldonado, Christoph Mayr, Paola Cárdenas, Carina B. Lange, Katherine Lalangui, Gloria Sanchez, Marco Salamanca, Karen Araya, Ignacio Jara, Gabriel Easton, and Marcel Ramos

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Final revised paper (published on 24 Nov 2020)
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Interactive discussion

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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Interesting data but a proper paleoclimate discussion is missing', Anonymous Referee #1, 12 Oct 2018
- AC1: 'General comments', Praxedes Muñoz, 12 Dec 2018
- AC3: 'corrected version', Praxedes Muñoz, 12 Dec 2018
RC2: 'Review of - Reconstructing past variations in environmental conditions and paleoproductivity over the last ~8000 years off Central Chile (30° S)', Anonymous Referee #2, 22 Oct 2018
- AC2: 'general coments rev#2', Praxedes Muñoz, 12 Dec 2018
- AC4: 'corrected version', Praxedes Muñoz, 12 Dec 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (16 Dec 2018) by Markus Kienast

AR by Praxedes Muñoz on behalf of the Authors (08 Feb 2019) Author's response

ED: Referee Nomination & Report Request started (20 Feb 2019) by Markus Kienast

RR by Anonymous Referee #1 (06 Mar 2019)

Suggestions for revision or reasons for rejection

The manuscript has been greatly improved. All the methodological concerns that I had have been clarified. Some work is still needed, however, before the manuscript can be published.
Some general comments:
- The text is too long and should be reduced by 20-30%. The results and the discussion in particular should be cleaned of all the textbook comments and context generalities and be focused on the data.
The data interpretation is not satisfying. It is generally very confusing and some important issues are not discussed.
- The climate interpretation takes shortcuts more based on widely repeated scenarios than their data.
- The differences between the two cores are described. However, the authors do not acknowledge or dicuss the fact that these differences, which underline the importance of local processes, greatly limit the geographical extent of the interpretations that can be made from those records. The authors draw large conclusions from the opal and diatom records in one core, which are precisely the proxies without equivalent in the other core.
- The records of productivity and oxygenation are expected be related as it is observed in sediment cores from Peru (Gutierrez et al., 2009; Salvatecci et al., 2014). However, the opal flux curve shows a sharp decrease at 6500 BP that is not reproduced in U, Re or Mo records. In the recent period (last ~200years), oxygenation seems to increase as well as productivity, which seems contradictory. The authors should discuss the relationship between oxygen and productivity proxies in their dataset and discuss why productivity and oxygenation which are both related to the upwelling intensity show here different behaviors.
- The discussion about the upwelling intensity is only based on the productivity proxy while oxygenation proxies are left aside.
- The logic of the discussion does not respect the time line of events in the literature nor in their own records. A supposed increase in ENSO is mentioned to explain both the drop in productivity at 6500BP and the increased oxygenation at 2000 BP. And the increasing humidity through the Holocene. Finally, ENSO is invoked to explain everything without citing the articles which properly reconstruct ENSO using monthly resolved marine records.

Detail comments:
Abstract:
L37: ENSO corresponds to inter-annual variability. Interdecadal variability would be related to the PDO.
L49: a “period” does not have a maximum. I think I understand what the authors mean but it needs to be rephrased.
L51: I suppose “moisture levels” refers to the climate on land, but it should be clarified.
L52: what is being stronger? Please clarify
L53: “associated with greater El Niño frequencies”. See comments about the discussion.
What about the 2 other high productivity periods?

Introduction:
Half of the introduction is dedicated to ENSO but the bibliography about paleo-ENSO is incomplete and does not reflect correctly the current knowledge.
L99 – L108: “Paleo-ENSO reconstructions indicate attenuated ENSO events 99 before the mid-Holocene (last 5000 years) and increasing from this period towards the 100 present (Marchant et al., 1999; Koutavas et al., 2006; Vargas et al., 2006), consistent 101 with paleoceanographic and paleoclimate interpretations (Rodbell et al., 1999; Rein et 102 al., 2005).”
This part incorrectly reflects the literature. The onset of ENSO 5000 years ago is an old hypothesis that had support from low resolution continental rainfall-derived indicators, but that has been contradicted by high resolution marine proxies adapted to ENSO reconstruction. Marchant et al., 1999, show an increased variability at 3ka, not 5ka. Koutavas et al., 2012 showed a minimum in ENSO activity from 6-3 ka. This was confirmed by mollusk shells (Carré et al., 2014), corals across the Pacific (Emile-Geay et al., 2016) and sediments off Peru (Rein, 2007).
The Lake Pallcacocha record (Rodbell et al., 1999; Moy et al., 2002) has been shown to reflect glacier activity related to tempêrature and the south American monsoon (hence the precession signal) instead of ENSO (Rodbell et al., 2008).
So, if a date for modern ENSO activity is to be proposed, that would be 3000 BP.
L124-126: please mention the time period recorded by the sediment cores.
L245: Add “ 14C” before “ age value”.

5.3. Anoxia is mentioned when metal concentrations show more reducing conditions. Anoxia corresponds to the complete absence of oxygen. Since the authors cannot reconstruct oxygen levels quantitatively, they should not use the term “anoxia” and describe changes in terms of more or less oxygenation.

5.4. In the first version, the pollen record was barely mentioned. In the revised version, it represents the largest part of the climate discussion.

L824: How an increase in ENSO would explain lower productivity without producing higher oxygenation? In addition, ENSO has been shown to be decreasing at 6700 BP, to enter its minimum activity between 6000 and 3000 BP (Koutavas et al., 2012; Carré et al., 2014).

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RR by Anonymous Referee #2 (13 Mar 2019)

ED: Reconsider after major revisions (18 Mar 2019) by Markus Kienast

AR by Praxedes Muñoz on behalf of the Authors (13 Jul 2019) Manuscript

ED: Referee Nomination & Report Request started (25 Jul 2019) by Markus Kienast

RR by Anonymous Referee #2 (24 Aug 2019)

ED: Reconsider after major revisions (01 Sep 2019) by Markus Kienast

AR by Praxedes Muñoz on behalf of the Authors (22 Nov 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Dec 2019) by Markus Kienast

RR by Anonymous Referee #3 (11 Dec 2019)

Suggestions for revision or reasons for rejection

Reconstructing past variations in environmental conditions and paleoproductivity over the last ~8000 years off Central Chile (30° S)

The manuscript by Muñoz et al. presents a multiproxy approach analysis of two short sediment cores retrieved off central Chile to reconstruct paleoceanographic and paleoclimatic variability during the last 8000 years. While several paleoceanographic studies have been conducted in southern Chile and off Peru, paleoceanographic records from central Chile covering the Holocene are extremely scarce. Thus, this study presents valuable data to understand millennial-scale changes in the Eastern South Pacific that can potentially attract the paleoceanographic community. I agree with most of the comments and observations made by the two anonymous reviewers on the first manuscript. After reading the revised version of the manuscript I think that the manuscript still needs substantial work before it is considered for publication in Biogeosciences.

My main criticism, also mentioned by both anonymous reviewers, is that the manuscript lacks a proper paleoceanographic interpretation combining relevant paleorecords from the Peruvian Upwelling system and from southern Chile. This could help to disentangle high latitude versus tropical forcing driving the observed changes in the data. During the last decade multiple studies focused on the Peruvian upwelling system were published that could help to gain insights on the mechanisms driving millennial-scale changes off Central Chile. Surprisingly these studies were not included in the revised version of the manuscript. The second main problem that I see with the manuscript is that the Discussion lacks focus, probably because of the lack of a clear scientific question and the multiple types of proxies presented by Muñoz et al. I suggest including a clear scientific question in the Introduction to better guide the reader throughout the manuscript.

Some other issues that need to be addresses are:

1) I am completely surprised that coastal sediment cores of just ~110 cm long can contain records of ~8000 years, especially in areas associated with high marine productivity. After examining the age models and Fig. 5, it is evident that the sediment cores present large discontinuities or sections with extremely low sedimentation rates like during the mid-Holocene in core BGGC5. This observation merits an in-depth discussion in the manuscript given that Holocene records from the Peruvian Upwelling systems are well known for discontinuities and erosion events (e.g. Erdem et al. 2016). Specifically, what is the role of winnowing on the proxies presented in this manuscript?

2) The authors show records of Mo, U and Re for both cores to infer changes in paleo-oxygenation. The use of Mo/U systematics is a powerful tool to reconstruct sub-oxic and anoxic conditions (Algeo and Tribovillard, 2006; Scholz et al., 2011; Salvatteci et al. 2016;). I recommend the authors to use this strategy to infer paleo-oxygenation (Section 5.3).

3) The results of some paleoceanographic studies need to be compared and contrasted with the results presented by Muñoz et al. to gain a mechanistic understanding of the processes driving the temporal trends in the data. For example, there are very few records of Mo, U and Re in the Eastern South Pacific (e.g. Muratli et al., 2010; Scholz et al., 2014; Salvatteci et al., 2016). A comparison and discussion with these records seems mandatory to better understand the mechanisms driving oxygen changes off central Chile. Other relevant studies to be consulted are: Doering et al. (2016), Mollier-Vogel et al. (2018), Salvatteci et al. (2016); Salvatteci et al. (2019).

4) The authors should carefully revise the manuscript and the references for mistakes. There are several typos in the text (e.g. the use of BC and BP; misspelling names in the references, etc).

References

Algeo, T.J., Lyons, T.W., 2006. Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions. Paleoceanography, 21, PA1016.
Doering, K., Erdem, Z., Ehlert, C., Fleury, S., Frank, M., Schneider, R., 2016. Changes in diatom productivity and upwelling intensity off Peru since the Last Glacial Maximum: Response to basin-scale atmospheric and oceanic forcing. Paleoceanography, 31.
Erdem, Z., Schönfeld, J., Glock, N., Dengler, M., Mosch, T., Sommer, S., Elger, J., Eisenhauer, A., 2016. Peruvian sediments as recorders of an evolving hiatus for the last 22 thousand years. Quaternary Science Reviews, 137, 1-14.
Muratli, J., Chase, Z., Mix, A.C., McManus, J., 2010. Increased glacial-age ventilation of the Chilean margin by Antarctic Intermediate Water Nature geoscience, 3, 23-26.
Scholz, F., Hensen, C., Noffke, A., Rohde, A., Liebetrau, V., Wallmann, K., 2011. Early diagenesis of redox-sensitive trace metals in the Peru upwelling area – response to ENSO-related oxygen fluctuations in the water column. Geochimica et Cosmochimica Acta, 75, 7257-7276.
Scholz, F., McManus, J., Mix, A., Hensen, C., Schneider, R., 2014. The impact of ocean deoxygenation on the ocean's iron supply. Nature geoscience, 7, 433-437.
Salvatteci, R., Gutierrez, D., Sifeddine, A., Ortlieb, L., Druffel, E., Boussafir, M., Schneider, R., 2016. Centennial to millennial-scale changes in oxygenation and productivity in the Eastern Tropical South Pacific during the last 25 000 years. Quaternary Science Reviews, 131, 102-117.
Salvatteci, R., Schneider, R.R., Blanz, T., Mollier-Vogel, E., 2019. Deglacial to Holocene Ocean Temperatures in the Humboldt Current System as Indicated by Alkenone Paleothermometry. Geophysical Research Letters, 46.

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RR by Anonymous Referee #1 (17 Dec 2019)

Suggestions for revision or reasons for rejection

The paper has been greatly improved. I recommend to publish it with the following minor revisions:
- The abstract should be corrected for english. Several inacurate choices of words.
- the introduction is still too long. I suggest deleting all the unfocused generalities from line 73 to line 111.
- the description of the study area is also long and includes uncecessary generalities. Delete from line 182 to line 190, at least.
- There is a confusion with the calculated reservoir age: it is presented as a local deviation (DR) in the method (line 241), and as a reservoir age (R) in the results (line 318-322). The values in Carré et al., 2016 and Merino-Campos et al., 2018 are DR values. Check what the age of 411 years really corresponds to (DR or R) and correct the text accordingly (and the age model if necessary)
- line 585-586: I am surprised that the author do not relate the large Cu increase (and to a lesser extent Fe and Mn) in the last century to mining activities. It is said that it indicates an increase of productivity but this is not consistent with other productivity indicators, nor, as they mention, with the increase in oxygenation.
- the discussion about climatic implications (section 5.3.) is mostly focused on ENSO, which is somewhat speculative since the record does not have an interannual resolution, while in the other hand changes in the mean state, which is indeed what sediments most clearly record on the first order, is not sufficiently discussed. The most robust result here is the long-term trend from dry, disoxigenated, productive conditions in the early Holocene, towards more humid, more oxigenated and less productive conditions at present. This is a strong support for the scenario proposing that early-midHolocene was characterizaed by a La Nina like mean conditions in response to insolation.
The La Niña-like conditions is mentionned in the discussion, but strangely, the authors cite De Pol-Holz et al., 2006 and Kaiser et al., 2008, who found warmer and more oxigenated conditions between 10 and 5ka further offshore in that area, in contradiction with the results presented here. The authors should discuss this discrepancy and present their results in the context of the early to mid Holocene La Niña-like hypothesis which was shown by Koutavas et al., Science, 2002, and by Carré et al., Quat. Int., 2012. This latter paper already discussed some coastal vs offshore discrepancies observed in Chile in this period.
- Line 616-617: the La Niña-like conditions may be associated to reduced ENSO and a shifted ITCZ, but we cannot say it is caused by them.

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ED: Reconsider after major revisions (17 Dec 2019) by Markus Kienast

AR by Praxedes Muñoz on behalf of the Authors (16 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Jun 2020) by Markus Kienast

RR by Anonymous Referee #3 (07 Jul 2020)

Suggestions for revision or reasons for rejection

The authors did a good job addressing most of the comments raised by me and the other reviewers. Specifically, I greatly appreciate the more cautionary approach taken by the authors to construct the age models. Before the paper can be accepted the following observations need to be addressed:

- The paragraphs in the Discussion section are extremely long, I suggest dividing the long paragraphs into two or three to facilitate the readability of the manuscript.
- Lines 410-412: Move “We use Al as a normalizing parameter for the enrichment/depletion of elements due to its conservative behavior. The elements are presented as metal/Al ratios” the Methods section
- Line 438: add “in core BTGC8” after “were lower”
- Lines 440-446: Move the first 3 sentences to the Methods section.
- Lines 652: Yes, it seems to be contradictory, but a similar trend (trend towards higher productivity associated with decreasing reducing conditions) was observed off Peru (Salvatteci et al., 2014; Cardich et al., 2019). These authors based their observations on d15N, redox sensitive metals and benthic foraminifera assemblages. I suggest citing these papers and discuss whether the mechanisms proposed by these authors can explain or not the observed pattern off Guanaqueros Bay (BGGC5) and Tongoy Bay (BTGC8).
- Lines 693-696. The westerlies were located in a more poleward position during the mid-Holocene (Lamy et al. 2001). Mollier-Vogel et al. (2019) erroneously indicate, citing Lamy et al. (2001), that the during the mid-Holocene the Westerlies were situated in a more northward position (see Salvatteci et al., 2019, for further discussion on this topic). This needs to be corrected in the manuscript and the original citation must be also included (Lamy et al., 2011).
- Lines 725-730. The authors need to briefly compare and contrast their results with the reconstructed shifts of the storm tracks of the westerlies (Lamy et al., 2001).
- Lines 763-767. Mollier et al. (2019) show evidence for decreased denitrification during the Mid-Holocene based on multiple d15N records along the coast. They do not show evidence for changes in sediment redox conditions. Rephrase this sentence or provide a better reference.

References
Cardich, J., Sifeddine, A., Salvatteci, R., Romero, D., Briceño-Zuluaga, F., Graco, M., Anculle, T., Almeida, C., Gutiérrez, D., 2019. Multidecadal changes in marine subsurface oxygenation off central Peru during the last ca. 170 years. Frontiers in Marine Science 6.
Lamy, F., Hebbeln, D., Röhl, U., Wefer, G., 2001. Holocene rainfall variability in southern Chile: a marine record of latitudinal shifts of the Southern Westerlies. Earth and Planetary Science Letters 185, 369–382.
Mollier-Vogel, E., Martinez, P., Blanz, T., Robinson, R., Desprat, S., Etourneau, J., Charlier, K., Schneider, R., 2019. Mid-Holocene deepening of the Southeast Pacific oxycline. Global and Planetary Change 172, 365–373.
Salvatteci, R., Gutierrez, D., Field, D., Sifeddine, A., Ortlieb, L., Bouloubassi, I., Boussafir, M., Boucher, H., Cetin, F., 2014. The response of the Peruvian Upwelling Ecosystem to centennial-scale global change during the last two millennia. Climate of the Past 10, 715–731. https://doi.org/10.5194/cp-10-715-2014
Salvatteci, R., Schneider, R.R., Blanz, T., Mollier-Vogel, E., 2019. Deglacial to Holocene Ocean Temperatures in the Humboldt Current System as Indicated by Alkenone Paleothermometry. Geophysical Research Letters 46. https://doi.org/10.1029/2018gl080634

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ED: Publish subject to minor revisions (review by editor) (13 Jul 2020) by Markus Kienast

AR by Praxedes Muñoz on behalf of the Authors (04 Aug 2020)

ED: Publish as is (19 Aug 2020) by Markus Kienast

AR by Praxedes Muñoz on behalf of the Authors (27 Aug 2020) Author's response Manuscript

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Praxedes Muñoz on behalf of the Authors (11 Nov 2020) Author's adjustment Manuscript

EA: Adjustments approved (12 Nov 2020) by Markus Kienast

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Short summary

We analyze marine sedimentary records to study temporal changes in oxygen and productivity in marine waters of central Chile. We observed increasing oxygenation and decreasing productivity from 6000 kyr ago to the modern era that seem to respond to El Niño–Southern Oscillation activity. In the past centuries, deoxygenation and higher productivity are re-established, mainly in the northern zones of Chile and Peru. Meanwhile, in north-central Chile the deoxygenation trend is maintained.