Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarém, Brazil

Sanders, Luciana M.; Taffs, Kathryn; Stokes, Debra; Sanders, Christian J.; Enrich-Prast, Alex; Amora-Nogueira, Leonardo; Marotta, Humberto

doi:https://doi.org/10.5194/bg-15-447-2018

Articles | Volume 15, issue 2

https://doi.org/10.5194/bg-15-447-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-15-447-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 2

Research article

|

22 Jan 2018

Research article |

| 22 Jan 2018

Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarém, Brazil

Luciana M. Sanders, Kathryn Taffs, Debra Stokes, Christian J. Sanders, Alex Enrich-Prast, Leonardo Amora-Nogueira, and Humberto Marotta

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Final revised paper (published on 22 Jan 2018)
Preprint (discussion started on 17 May 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review', Jack Hutchings, 21 Jul 2017
- AC1: 'Responce to Reviewer', Luciana Sanders, 04 Aug 2017
RC2: 'Review of Sanders et al, bg-2017-151', Anonymous Referee #2, 08 Aug 2017

Peer-review completion

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

ED: Reconsider after major revisions (17 Aug 2017) by Steven Bouillon

AR by Luciana Sanders on behalf of the Authors (19 Sep 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (03 Oct 2017) by Steven Bouillon

RR by Anonymous Referee #2 (16 Oct 2017)

Suggestions for revision or reasons for rejection

I have reviewed the first version of this paper, and in my first review, I had concerns about the dating methods. Unfortunately, I do not find that the authors have addressed my concerns appropriately in their revision.

The problem is the following: two different dating methods are used, the Pu method and the Pb method.

The Pu method gives information about the average sediment accumulation rate since ~1950, but does not allow for finer temporal resolution. The authors use it in that sense, and that’s fine. AD 1950 was located at ~20 cm depth.

The Pb method can resolve temporal changes in sediment accumulation rate at the scale of years, but in the study lake, the authors state that the Pb profile was disturbed in the upper 20 cm, but declining linearly in the 20-60 layer layer. Therefore, they did not calculate sediment accumulation from the Pb data in upper 20 cm of sediment, and they present an average sediment accumulation rate of 4 mm/yr in the 20-60 cm layer.

The chronology given in this paper is therefore (a) an average sediment accumulation rate since 1950 (0-20 cm sediment depth) from the Pu dating, and (b) an average sediment accumulation rate prior to 1950 (20-60 cm sediment depth) from the Pb dating. The presented data does not give information about how sediment accumulation might have varied during the 1950-present period, or during the ~1850-1950 period. Contrary to what the authors claim (L225-227), the fact that the average sediment accumulation rates for these two periods are similar does not indicate that there was little change of sediment accumulation over time; within each of the time periods, sediment accumulation rates might varied at the scale of years. In fact, when looking at the 210Pb(ex) profile (Fig 4b), there is an intriguing increase in unsupported Pb over the 3-20 cm horizon, possibly indicating non-constant sediment accumulation. Similarly, 210Pb(ex) varies between layers in the 20-60 cm horizon, indicating quite variable sediment accumulation rate, and not a constant sediment accumulation rate, as the authors claim (L212-215).

The lack of time-resolved chronologies is serious when the authors plot chemical data over age (Fig. 7), since there is no information about the exact age each respective layer of sediment. In Figure 7, the authors should instead plot chemical data against sediment depth, and indicate the AD 1950 layer.

The same problem appears when changes in OC burial are attributed to changes in land use (Fig.8B). Over the period where land use change was most intense, e.g. the past 50 years, the authors have only one average value of sediment accumulation rate to rely on, and the values of OC burial cannot be confidently attributed to a certain year. In fact, the calculations of OC burial use one average sediment accumulation rate, and using such averaged OC burial rates to illustrate temporal changes in burial seems unwarranted. Analysis of temporal patterns can only be done with temporally resolved data.

In my opinion, the authors can go two ways: either present the Pb chronologies (i.e. plot depth against age, based on both CIC and CRS models) in order to present evidence for the attribution of individual sediment layers to certain years. I asked for the Pb chronologies in my first review, but the authors have not responded to that. The other potential way to deal with this issue would be to stay away from giving distinct ages to distinct sediment layers, and analyze the differences between pre-1950 and post-1950 layers (for which there is good data from the Pu dating). The average sediment accumulation rates could still be used to speak about indicative ages, i.e. since the long-term average sediment accumulation was ~4 mm/yr, the 0-4 cm layer may be regarded to represent approximately the past 10 yrs.

Whichever way the authors choose, the ms needs to be revised accordingly prior to publication.

Another comment: I found the hypothesis a bit vaguely formulated, “related to the carbon burial capacity” leaves room for interpretation. The authors could be more definitive in their prediction: “historically documented increases in deforestation have increased OC burial rates in the studied floodplain lake” or something similar.

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RR by Jack Hutchings (03 Nov 2017)

Suggestions for revision or reasons for rejection

General Comments:

Sanders et al. have revised their manuscript on the organic matter accumulation rates within a floodplain lake in the Amazon. This revised manuscript presents an improved analysis of the data and presentation of methods. In addition to a few small comments below, I have one major question: does any of this impact how these systems should be managed? Currently, neither the abstract nor the conclusion tackle this question. It may not be completely answerable within the scope of this study, but I would welcome an attempt at question of land use management. I believe this would improve the quality of the manuscript, especially as there is significant text spent analyzing the land use change component of this study system.

Specific Comments:

Lines 115-120: I did not catch this in my initial review, but the d15N results should be interpreted with caution based on this pre-treatment method. The relative differences between samples is possibly retained, but the absolute value of d15N following acid pre-treatment is suspect. In addition, C:N can also be affected as acidification and should be interpreted with caution. I do not believe this invalidates the findings here, however these caveats should be considered when interpreting the data.
Here are two recent references that document these issues:

Brodie, C.R., Casford, J.S.L., Lloyd, J.M., Leng, M.J., Heaton, T.H.E., Kendrick, C.P., Yongqiang, Z., 2011. Evidence for bias in C/N, δ13C and δ15N values of bulk organic matter, and on environmental interpretation, from a lake sedimentary sequence by pre-analysis acid treatment methods. Quaternary Science Reviews 30, 3076–3087.

Kim, M.S., Lee, W.S., Suresh Kumar, K., Shin, K.H., Robarge, W., Kim, M., Lee, S.R., 2016. Effects of HCl pretreatment, drying, and storage on the stable isotope ratios of soil and sediment samples. Rapid Communications in Mass Spectrometry 1567–1575.

Lines 270-283: This fleshed out handling of the statistical methods if welcome. However, I wonder if some of this detail is not better fit for the methods and/or results sections so that the discussion can focus on the implications of the findings.

Figure 7: Please consider adding a secondary Y-axis indicating the depth associated with the year.

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ED: Reconsider after major revisions (09 Nov 2017) by Steven Bouillon

AR by Luciana Sanders on behalf of the Authors (21 Nov 2017)

ED: Publish subject to technical corrections (28 Nov 2017) by Steven Bouillon

AR by Luciana Sanders on behalf of the Authors (02 Dec 2017) Author's response Manuscript

Short summary

The Amazon rainforest produce large quantities of carbon, a portion of which is deposited in floodplain lakes. This research shows a potentially important spatial dependence of the carbon deposition in the Amazon lacustrine sediments in relation to deforestation rates in the catchment. The findings presented here highlight the effects of abrupt and temporary events in which some of the biomass released by the deforestation reach the depositional environments in the Amazon floodplains.