Carbon mineralization in Laptev and East Siberian sea shelf and slope sediment

Brüchert, Volker; Bröder, Lisa; Sawicka, Joanna E.; Tesi, Tommaso; Joye, Samantha P.; Sun, Xiaole; Semiletov, Igor P.; Samarkin, Vladimir A.

doi:https://doi.org/10.5194/bg-15-471-2018

Articles | Volume 15, issue 2

https://doi.org/10.5194/bg-15-471-2018

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

Special issue:

Climate–carbon–cryosphere interactions in the...

https://doi.org/10.5194/bg-15-471-2018

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

Articles | Volume 15, issue 2

Research article

|

25 Jan 2018

Research article |

| 25 Jan 2018

Carbon mineralization in Laptev and East Siberian sea shelf and slope sediment

Volker Brüchert, Lisa Bröder, Joanna E. Sawicka, Tommaso Tesi, Samantha P. Joye, Xiaole Sun, Igor P. Semiletov, and Vladimir A. Samarkin

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Final revised paper (published on 25 Jan 2018)
Supplement to the final revised paper
Preprint (discussion started on 13 Apr 2017)

Interactive discussion

Status: closed

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

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RC1: 'Carbon mineralization in Laptev and East Siberian Sea shelf and slope sediment', Anonymous Referee #1, 07 May 2017
- AC1: 'Responses to referees 1 and 2', Volker Brüchert, 02 Jul 2017
RC2: 'Reviewer comment on "Carbon mineralization in Laptev and East Siberian Sea shelf and slope sediment"', Anonymous Referee #2, 11 May 2017
- AC2: 'Response to referee 2', Volker Brüchert, 02 Jul 2017

Peer-review completion

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

ED: Reconsider after major revisions (07 Jul 2017) by Francien Peterse

AR by Volker Brüchert on behalf of the Authors (22 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Aug 2017) by Francien Peterse

RR by Anonymous Referee #2 (08 Sep 2017)

Suggestions for revision or reasons for rejection

The revised version of the manuscript “Carbon mineralitazion in the Laptev and East Siberian Sea shelf and slope sediment” by Brüchert and co-workers was substantially improved in terms of structure and clarity in comparison to the original submission. The authors considered several of my comments but there are still several points that should be addressed before publication.

The assessment of the carbon burial efficiency (section 4.2) is still difficult to follow since the authors assumptions are not clearly described. The burial efficiency of terrestrial organic carbon seems to be derived from total carbon accumulation rates and sulfate reduction rates. But how is this possible if total organic carbon contains both marine and terrestrial carbon? From the authors response to my remarks to the original submission I understand that they assume that only marine organic matter decomposition is responsible for oxygen consumption. This is most likely not the case, as the authors are aware. This is yet a quite long paper and the authors might think about shortening the manuscript by omitting sections that are based on questionable assumptions.

The discussion still starts with the presentation of new results.

The numbering of the figures should be according to their reference in the text.

The conclusion may substantially be shortened. Currently it is rather an extension of the discussion than a summary of the main findings of the presented work.

Specific comments:
Line 47ff: This statement is still not supported by the cited Hugelius et al. (2014) paper and the statement in the current form is still wrong. The authors should be aware of the difference between permafrost carbon (carbon in permafrost) and carbon in the permafrost region which comprises also carbon stored in non-permafrost deposits. According to Hugelius et al. (2014) 800 Pg C are stored in the permafrost, which is by definition frozen ground for more than 2 years. The phrase “perennially frozen permafrost” in the authors response is a tautology since permafrost is by definition perennially frozen. According to Hugelius et al. (2014) about 500 Pg is stored in the permafrost region in seasonally or perennially unfrozen material (eg. active layer or river and lake sediments) which is not permafrost. The carbon in this non-permafrost pool contributes to the current carbon cycle and this carbon is the source for the terrestrial carbon in the investigated sediments. If the authors insist to refer to the number of 1100-1500 Pg given in the Hugelius paper they should refer to carbon in the permafrost region, as does the cited paper. The permafrost carbon pool, to which the authors still refer, is 500 Pg smaller, which is a significant number.

Line 56: Again, please clarify what is meant by “qualitative … rates”. A rate is a quantitative measure.

Line 175: DIC comprises beside dissolved CO2 also bicarbonate and carbonate. Please clarify if only dissolved CO2 was considered or total DIC.

Line 275: … proportion of mineralized terrestrial…

Line 345: The authors should comment on station 63 where the rates measured with the two methods deviate by a factor of 15.

Line 366: The Fe concentration at 11 cm seems above 300µM, please clarify.

Line 458ff: I suggest that these data should be presented in the results section with a reference to the respective figure before discussing them. The discussion should not start with the presentation of new results. Furthermore, the authors should give the statistical tests used for the correlation analysis. Since reduced sulfur oxidation is responsible for only a minor part of oxygen consumption, the authors might think about other reasons for the observed correlation, e.g. burial of labile organic matter as discussed in lines 544-548.

Line 463: The slope of the regression line in Fig. 9 referred to in the preceding sentence is 5.5 not 6.1. It is unclear which “data set” is referred to here, since the authors presented a lot of interesting data. Please clarify. The regression analyses discussed here was not presented, and it’s difficult to follow the discussion of data that are not presented. Please clearly present data before they are discussed.

Line 513: Please explain DOC and POC

Line 523: Please explain BBL

Lines 563. I still believe that the δ13C values of the bulk organic carbon of the investigated sediments are needed here to give the reader the chance to evaluate the modelled results presented by the authors. It is unclear to me why the authors refuse to present the bulk δ13C results so the reader can compare measured data with the results from the authors model. The authors state that they do not wish to reiterate published data in the discussion but to my opinion, an important aim of the discussion is to compare the presented data in the context of previously published data and the authors do so in other sections of the discussion. Knowing that lowest δ13C-values in the sediments of the investigation sites are at about -26‰ is important to evaluate the presented results. A δ13C-value of -35.8‰ for remineralized DIC is hardly explainable if no methane is involved (as the authors assume) and if the bulk organic carbon has values above -26‰. Hence, the authors might also include a sentence on the limitation of their modelling approach.

Lines 582 – 585: It is unclear how a δ13C value of -24‰ for marine organic matter is deduced from the reported Alling et al. (2012) values of -2 to -4 for the DIC. Has Alling et al. (2012) also measured δ13C values for the marine POC and which values did they find? Please clarify.

Lines 620-624: Could the authors please give the rates of Boetius and Damm (1988) here because most readers will not know them by heart.

Line 633: The authors might use statistical tests to test for significant differences between these datasets.

Line 635: East Siberian…

Lines 661 – 663: It is unclear to me how the burial efficiency of terrestrial organic carbon can be estimated by using the accumulation rates of total organic carbon comprised of both terrestrial and marine organic carbon. Please clarify. Furthermore, please explain why only sulfate-reduction was considered and not total anoxic degradation rates.

Lines 664 – 667: This assumption of the approach seems to be refuted by the oxygen consumption rates (see discussions in lines 688 – 694). Please clarify.

Line 667 – 668: Please explain why these numbers are substantially different than in the original manuscript (91±6% and 94±4% in contrast to 69±28% and 79±6% in the original submission).

Line 731: The section numbering needs to be revised and there is no section 4.3

Line 759: The numbers given in the manuscript are above 90%.

1128: Table 4

References:
Alling, V., Porcelli, D., Mörth, C. M., Anderson, L. G., Sanchez-Garcia, L., Gustafsson, Ö., Andersson, P. S., and Humborg, C., 2012. Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC, Geochimica et Cosmochimica Acta, 95, 143-159.

Hugelius, G., Strauss, J., Zubrzycki, S., Harden, J.W., Schuur, E.A.G., Ping, C.-L., Schirrmeister, L., Grosse, G., Michaelson, G.J., Koven, C.D., O'Donnell, J.A., Elberling, B., Mishra, U., Camill, P., Yu, Z., Palmtag, J., Kuhry, P., 2014. Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps. Biogeosciences 11, 6573-6593.

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ED: Reconsider after major revisions (21 Sep 2017) by Francien Peterse

AR by Volker Brüchert on behalf of the Authors (02 Nov 2017) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (14 Nov 2017) by Francien Peterse

Dear Volker Brüchert and co-workers,

Thank you for submitting a revised version of your manuscript. You have provided responses to most of the comments raised by the reviewer, but there are still a few issues I’d like you to address pending acceptance of your work.

Yours sincerely,
Francien Peterse

In general:
- Results, discussion, and conclusions are still mixed at times. See detailed comments below.
- Provide p-values when stating that something is significantly higher/lower/different.
- Provide references to figures/tables to support statements made in the discussion, especially section 4.1.
- The conclusions are still mostly an extension of the discussion. Please limit the conclusion to actual facts. Based on the abstract I only expect statements on spatial O2 uptake rates, terrestrial vs marine OC contributions to the sediment, and amounts of C respired. Either move the discussion parts to the relevant sections in the discussion, or simply delete them when they are a mere repetition.

Specific/textual comments:
L48: add ‘carbon’ after 1500 Pg
L72: delete one of the ‘rates’. Either the first time (…directly measure carbon mineralization rates…) or the second time (…directly measured rates of carbon mineralization…)
L172&173: replace CO2 with DIC
L274-277: Rephrase this sentence to remove the (partial?) repetition. Not sure what you are trying to say here.
L279: refer to the figure with the Keeling plots.
L444-460: This is interpretation/discussion of the data, and should thus not be part of the results section. It is also unclear what the exact point is of this paragraph?
L441: ‘At this station…’ which station is this?
L467: representative for what?
L510ff: This part is hard to follow. Can we see this slope in one of the figures, or is the data in a table? If so, add a referral.
Section 4.1: refer to relevant figures when discussing your data.
L623: remove ‘rates’.
L623: …9B), except for on the shelf, where reduction rates are up to a factor 15 lower.
L629: delete ‘2)’
L629: provide p-value when using significantly, or adjust phrasing (‘probably significantly’), and provide a reference to support the statement made in this sentence.
L630 and L635 and L650: again, provide p-value when using significant.
L699: Refer to the correct section
L704-711: add O2 or C to Tg/y
L704: delete respectively

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AR by Volker Brüchert on behalf of the Authors (01 Dec 2017) Author's response Manuscript

ED: Publish as is (11 Dec 2017) by Francien Peterse

AR by Volker Brüchert on behalf of the Authors (11 Dec 2017) Author's response Manuscript

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Short summary

We determined the aerobic and anaerobic degradation rates of land- and marine-derived organic material in East Siberian shelf sediment. Marine plankton-derived organic carbon was the main source for the oxic dissolved carbon dioxide production, whereas terrestrial organic material significantly contributed to the production of carbon dioxide under anoxic conditions. Our direct degradation rate measurements provide new constraints for the present-day Arctic marine carbon budget.