Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates

Clayer, François; Gélinas, Yves; Tessier, André; Gobeil, Charles

doi:https://doi.org/10.5194/bg-17-4571-2020

Articles | Volume 17, issue 18

https://doi.org/10.5194/bg-17-4571-2020

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

https://doi.org/10.5194/bg-17-4571-2020

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

Articles | Volume 17, issue 18

Research article

|

18 Sep 2020

Research article |

| 18 Sep 2020

Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates

François Clayer, Yves Gélinas, André Tessier, and Charles Gobeil

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Final revised paper (published on 18 Sep 2020)
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Preprint (discussion started on 31 Jan 2020)
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Interactive discussion

Status: closed

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

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RC1: 'Review Clayer et al', Anonymous Referee #1, 23 Mar 2020
- AC1: 'Response to comments - Reviewer 1', Francois Clayer, 13 May 2020
RC2: 'Superb dataset requires careful interpretations', Anonymous Referee #2, 15 Apr 2020
- AC2: 'Response to comments - Reviewer 2', Francois Clayer, 13 May 2020
RC3: 'Review of Mineralization of organic matter in boreal lake sediments', Anonymous Referee #3, 22 Apr 2020
- AC3: 'Response to comments - Reviewer 3', Francois Clayer, 13 May 2020

Peer-review completion

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

ED: Reconsider after major revisions (26 May 2020) by Perran Cook

AR by Francois Clayer on behalf of the Authors (25 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Jun 2020) by Perran Cook

RR by Anonymous Referee #2 (09 Jul 2020)

Suggestions for revision or reasons for rejection

The revised manuscript is improved in many respects. While it still reads somewhat heavy, it is now easier to follow in the aspects that relate to its central theme, methanogenesis. I still have a problem with the sulfur part of the story, however.

Regarding the dissolution of iron sulfides, the authors refer to Couture et al. 2016. That study shows that FeS dissolves in Basic B of Lake Tantare, while in Basin A the dissolution rate is significantly smaller (their Fig. 7) According to Couture et al. 2016, while FeS (as an unstable AVS fraction) dissolves, FeS2 is formed instead. (Curiously, neither of their modeled sulfate profiles actually reproduced the increasing sulfate trend. The dominant pool of sulfur in these sediments was inferred to be in organic form, which is not even considered here but can be potentially significant and can even generate sulfate through mineralization; e.g. Fakhraee et al. 2017)
This study, in contrast, claims not only that the FeS dissolution is highly active in Basin A, but also that the precipitation of stable sulfides can be ignored. This is problematic even for the internal logic of the paper: If pyrite precipitation is ignored because iron sulfides are dissolving in these sediments, what is the basis for using the reaction r8 with an end product that cannot form?

And yet, this still does not explain the accumulation of sulfate, as the dissolution of sulfides generates only the reduced form, sulfide. Line 238 states: sulfide oxidation by iron oxides is a source of sulfate (r8). This reaction is considered to generate pyrite and sulfate. This is not an actual reaction, however. Oxidation of sulfide produces elemental sulfur. A quarter of it may be converted to sulfate via disproportionation (with the other three quarters regenerating sulfide). But sulfate could hardly be expected to accumulate in the anoxic sediment, as sulfate reduction is a much more efficient sink for sulfate than disproportionation is a source.

The sulfate profiles are claimed to be successfully reproduced with the model that is based on Clayer et al. 2016. The description of the model in Clayer et al. 2016, however, is incomplete and does not specify the reactions in the sulfur cycle. So while I tried to understand the processes that could lead to sulfate accumulation, I could not identify the specific reactions on which the simulation was based. The only reaction listed in Clayer et al. 2016 that produces sulfate is the aerobic oxidation of sulfide, which obviously cannot operate below the depth of oxygen penetration.

Thus the puzzle of the sulfur cycle remains. It seems to me, however, that this cycle is invoked in the paper mainly to justify the source of H2 (line 610). But there are other ways to generate hydrogen, as already acknowledged in the paper. Perhaps the authors might consider making their argument without an explicit consideration of the cryptic sulfur cycling, or otherwise they need to describe it in more realistic detail.

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ED: Reconsider after major revisions (15 Jul 2020) by Perran Cook

AR by Francois Clayer on behalf of the Authors (15 Jul 2020) Author's response Manuscript

ED: Publish as is (06 Aug 2020) by Perran Cook

AR by Francois Clayer on behalf of the Authors (14 Aug 2020)

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Short summary

Here, we quantified the sediment production of methane and carbon dioxide in lake sediments to better characterize the nature of the organic matter at the origin of these two greenhouse gases. We demonstrate that the production of these gases is not adequately represented in models for deep lake sediments. We thus propose to improve the representation of organic matter degradation reactions in current models for improving predictions of greenhouse gas cycling in aquatic sediments.