High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian hydroelectric reservoir

Quadra, Gabrielle R.; Sobek, Sebastian; Paranaíba, José R.; Isidorova, Anastasija; Roland, Fábio; do Vale, Roseilson; Mendonça, Raquel

doi:https://doi.org/10.5194/bg-17-1495-2020

Articles | Volume 17, issue 6

https://doi.org/10.5194/bg-17-1495-2020

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

https://doi.org/10.5194/bg-17-1495-2020

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

Articles | Volume 17, issue 6

Research article

|

25 Mar 2020

Research article |

| 25 Mar 2020

High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian hydroelectric reservoir

Gabrielle R. Quadra, Sebastian Sobek, José R. Paranaíba, Anastasija Isidorova, Fábio Roland, Roseilson do Vale, and Raquel Mendonça

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Final revised paper (published on 25 Mar 2020)
Supplement to the final revised paper
Preprint (discussion started on 05 Jul 2019)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review of: High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian reservoir by Gabrielle R. Quadra et al.', Maciej Bartosiewicz, 12 Aug 2019
- AC1: 'RESPONSE TO REVIEW', Gabrielle Quadra, 06 Sep 2019
RC2: 'Review of “High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian reservoir”', Anonymous Referee #2, 17 Aug 2019
- AC2: 'RESPONSE TO REVIEW', Gabrielle Quadra, 07 Sep 2019

Peer-review completion

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

ED: Reconsider after major revisions (17 Sep 2019) by Clare Woulds

AR by Gabrielle Quadra on behalf of the Authors (23 Oct 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Nov 2019) by Clare Woulds

RR by Anonymous Referee #3 (19 Dec 2019)

Suggestions for revision or reasons for rejection

This is a revised version of a manuscript focusing on sediment C accumulation and sediment CH4 concentration in a tropical reservoir. The study is an important addition to the literature about the net C and greenhouse gas emissions caused by hydropower reservoirs and dams. Valuable work has been done to estimate the sediment C accumulation rates and spatial patterns in this reservoir. The current work is not capable of closing the full C or GHG budget, but perhaps, that is the ultimate goal of the researchers. The work is topical as it seems that hydropower business is increasing, particularly in the tropics. I agree with the reviewers 1 and 2 that the study deserves to be published. I also agree with the criticisms raised by those reviewers.

It looks that the authors have improved the manuscript quite well according to the comments on the earlier version by the reviewers 1 and 2. I have, however, still some reservations. I think that the presentation needs some work to be publishable and I suggest a rewrite. I suggest introducing the big questions behind and how sediment C accumulation and potential for CH4 emissions relate to that. Is the question about the GHG emissions caused by hydropower reservoirs and dams and/or how sediment C accumulation affect catchment C and GHG budgets, or something else? In addition, I found problematic how the CH4 data were dealt. Authors considered the CH4 concentration data as an indicator of potential for CH4 ebullition if concentration high enough for bubble formation. This is a bit problematic, because CH4 concentration, or CH4 saturation if high enough, may not translate to actual CH4 emission. The ‘potential’ is not considered in quantitative manner in this study and the approach needs a motivation. I suggest that authors could elaborate the analysis or the text to the direction of quantitative analysis how the sediment and C accumulation and qualitative sediment properties determine the sediment CH4 concentration/CH4 saturation. I am confident that the authors can improve the presentation. I have listed some detailed comments and suggestions below.

The title is awkward, because the emissions vs accumulation aspect was not really studied in the current study. How about: High sediment carbon accumulation and high sediment methane concentrations in an Amazonian hydroelectric reservoir. Other thing, I am more used to the term “carbon accumulation” rather than “C burial”. I.e. sediment accumulation rate (SAR), carbon ccumulation rate (CAR), but up to you.

Abstract 31-34. Difficult to read. Maybe make two sentences.
36. CUN, explain
46-47. It is not only transport from land to the Ocean, but also net sequestration by autotrophs at places.
49. But see also Kortelainen et al. 2004, GCB
61. You just gave an estimate so rewrite to express the uncertainty
75- Overall C accumulation is one component in the C and GHG budgets and you shoul acknowledge also the other parts. Examining the the sediment C accumulation is important, though.
83. There are also older references
87-88. Awkward, suggest editing. To capture the total emissions both high emission and not so high emissions sites count.
94. Note that CH4 emission may not affect the C pool much even though it has significange as a greenhouse gas. How about CO2 and CH4 emissions.
96- Shoudn’t you formulate research questions or hypotheses?
122-124. here and elsewhere, unnecessary use of parentheses.
Fig S1. Why not in the main document.
140. Delete (see…)
153-159. Specify that sampling capaigns were targeted to rising and falling water periods?
158. Quite large GPS error!
184. Unnecessary parentheses
185. Explain, how saturation concentration defined
192. Be specific, how many cores were measured for [CH4]
199. Injected unnecessary > …and analyzed for CH4 concentration within the same day using…
208. was lost
218. do you mean that ‘therefore, the mean accumulation rate cannot reveal short-term …’
226. The average C accumulation rate (xx) was calculated dividing the total C mass (g C m-2) by the accumulation time
231. We used empirical relationship between SAR and CAR (y=..) to estimate the CAR for the reamining…
235. 237. Suggestion, ..’to produce maps of SAR and CAR’.
245. was > were?
260. Sediment and C accumulation rates
262. Why supplement if the main result? To my opinnion belongs to the main document
276. The same here
278-279. to the site description
286. Isn’t this unnecessary reference to the supplement, because you have fig. 3?
289. Is the test mentioned in the M&M?
292. ‘this indicates…’ belongs to the discussion
294. S5 unnecessary?
295. So the same areas have also high CAR. Maybe emphasize.
317. What you mean with ‘margins’?
323. What the ‘muddy lake area’ actually means?
328. I think that the ref to S6 is unnecessary
352. If CUN has high CAR (& other rates), why it is important? How these findings will improve our understanding on the reservoir systems?
386. Can you refer to the Fig 1 instead of the supplement?
388- I found this section unclear
398. Can you include S6 in the main document
401- Write open the idea behind
417. Mention that the C:N in the sediment is a mixture
444. How about diffusive transport? Totally neglected
Fig5. A scatterplot instead?
448-457. I found this only marginally relevant
458. What the earlier CUN study says about the CH4 ebullition? Do you have own measurements? If [CH] stays the same, what it means? A stable reservoir with no release but no production either / continuous production and emission at same rates/
474. But there are earlier CH4 flux measurements.
475- Net effect on what? Reservoir radiative forcing, catchment C balance? What are the other components in the budgets?

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RR by Anonymous Referee #4 (20 Dec 2019)

Suggestions for revision or reasons for rejection

This paper estimates the storage of sediment OC in an Amazonian reservoir. It also shows the CH4 ebullition potential (high). The data set is sound and the conclusions more or less appropriate. This paper was reviewed before in the open review process at BG, and this version is solid and addressed the reviewer concerns. I still had a lot of comments, but these are less about the scientific merit of the work, but rather ways to improve the presentation of the work. I have no major concerns, but plenty of minor ones.

69. The “However…” does not really follow from the first sentence. as the thread goes from reservoir to temperature and runoff

78. delete it has been shown that

96 Because both OC…

116. SD on the right of the plus/minus sign?

215. It is best to use variables in equations and not text. But his eqution is so simple, it might be easiest to simply leave the text as is in lines 212 and 213.

230. Use variables, the numerator in this equation looks like it reads “organic carbon mass in the post minus flooding sediment”, which of course is meaningless.. Use multiplication sign, not *.

231 Saying correlation and using regression is incorrect. If correlation analysis simply state r and maybe a p value. 5 significant digits is way too high for high uncertainty parameters such as these.

243 Delete in order

254. See 116

285 Isn’t M supposed to be written as mol L-1?

289. This statistical method needs to be described in the methods. Simply saying “We did a t-test for …” is fine. But with the SD higher than the mean, and no way for negative numbers, the data are assuredly no normally distributed thus preventing the application of a t test. log transform might do the trick.

262. Is it possible to estimate a confidence interval for this flux? It might be more difficult than plus/minus 2*SD/sqrt(n) due to skewed distribution of rates and spatial autocorrelation.

265 ditto

291. What is saturation conc? Never mind, I see it on the figure.

293 In most of the

Fig 3. This figure could use some work to improve readability. Make font nearly the same size for axis labels and titles ( 9-11 pt). No need for bold font, it distracts readers to the titles and labels. And because all the axes are the same, they do not all need to be titled. One y and x axis title is enough. The numbers following the F and R don’t mean much to readers (37? 21?); they seems like some sort of internal labeling code. Rename them.

311. A leading paragraph in the discussion outlining the main findings would be useful. The first sentence, that water slows entering a reservoir, is not really a finding from the study.

362. What are units of mineralization?

363. What is the x here? A multiplication sign? Confusing way to write math.

364. Use the prediction uncertainty from the regression to derive uncertainty on the 325. I note that Cardosa et al regressed log mineralization with temp, and they have prediction error from 100-1100 at 29degC (wow what a nice analysis in that paper). That makes burial efficiency range from 8 to 48%, a huge range. I do think the authors can safely say that the range is not the 67 or 87% found in other places, but by scaling the uncertainty makes them more certain in that conclusion!

380. This conclusion seems dicey. I would want rather see carbon isotope evidence for this point.

412 delete “is known to”

421. I was hoping there were such data. A figure with a simple carbon budget for this reservoir would be interesting, I realize that seston input might be unknown, but with sedimentation rate, mineralization and CO2 flux would make a nice picture.

Fig 5. This figure is tough to interpret. The binning seem arbitrary and can skew the picture depending on how binned. Why not simply plot the proportion of samples > saturation vs burial rate? Or the actual % saturation vs burial rate. That way there is a continuous relationship. It is likely not going to be a clean line, but it might be a triangle, where high CH4 relates with low to high burial rate, but low CH4 is always low burial rate. Just a guess, but I think there are much better ways to show this plot.

469-471. This sentence seems to say “we have no idea of the C source” since the first part says the forest input is high and the second part says aquatic is high. I am totally fine with this interpretation; these data do not really allow assessing the source (see 412). Are there primary production data from this reservoir?

476. Yes and also to scale how the building of reservoirs will alter regional and global carbon budgets. Based on the work here the ocean is missing 0.3 Tg of its riverine carbon input.

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ED: Publish subject to minor revisions (review by editor) (10 Jan 2020) by Clare Woulds

AR by Gabrielle Quadra on behalf of the Authors (24 Jan 2020) Author's response Manuscript

ED: Publish subject to technical corrections (17 Feb 2020) by Clare Woulds

AR by Gabrielle Quadra on behalf of the Authors (20 Feb 2020) Author's response Manuscript

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Short summary

Hydropower is expanding in the Amazon Basin, but the potential effects of river damming on carbon fluxes cannot be gauged due to a lack of studies. We quantified, for the first time in an Amazonian reservoir, both organic carbon burial and the concentrations of methane in the sediments. We found that the dual role of sediments as both a carbon sink and methane source may be particularly pronounced in this Amazonian reservoir.