Geographic variability in freshwater methane hydrogen isotope ratios and its implications for global isotopic source signatures

Douglas, Peter M. J.; Stratigopoulos, Emerald; Park, Sanga; Phan, Dawson

doi:https://doi.org/10.5194/bg-18-3505-2021

Articles | Volume 18, issue 11

https://doi.org/10.5194/bg-18-3505-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-18-3505-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 11

Research article

|

11 Jun 2021

Research article |

| 11 Jun 2021

Geographic variability in freshwater methane hydrogen isotope ratios and its implications for global isotopic source signatures

Peter M. J. Douglas, Emerald Stratigopoulos, Sanga Park, and Dawson Phan

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Final revised paper (published on 11 Jun 2021)
Supplement to the final revised paper
Preprint (discussion started on 13 Nov 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of Douglas et al', Susan Waldron, 06 Dec 2020
- AC1: 'Reply on RC1', Peter Douglas, 15 Feb 2021
RC2: 'Review of "Global geographic variability in freshwater methane hydrogen isotope ratios and its implications for emissions source apportionment and microbial biogeochemistry"', Anonymous Referee #2, 14 Dec 2020
- AC2: 'Reply on RC2', Peter Douglas, 15 Feb 2021
RC3: 'Review of bg-2020-410', Edward Hornibrook, 25 Dec 2020
- AC3: 'Reply on RC3', Peter Douglas, 15 Feb 2021

Peer-review completion

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

ED: Reconsider after major revisions (22 Feb 2021) by Caroline P. Slomp

AR by Peter Douglas on behalf of the Authors (30 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Apr 2021) by Caroline P. Slomp

RR by Anonymous Referee #2 (23 Apr 2021)

Suggestions for revision or reasons for rejection

Second review of “Global geographic variability in freshwater methane hydrogen isotope ratios and its implications for emissions source apportionment and microbial biogeochemistry” by Douglas et al.

The authors have incorporated the numerous suggestions that were raised from the different reviewers in considerable detail. The results are now discussed in a more balanced way regarding previous publications. It is not an easy paper to digest, as each section has its own in-depth discussion, but this reflects the complexity of the subject. Otherwise, the presentation quality is high in terms of structure and clarity of language. The core of the paper is the use of statistical tools to detect correlations. It is a very useful exercise even if it turns out that often no clear relations can be found. This is not always surprising given the rather limited number of datapoints and the many parameters that are involved. In my opinion, the discussion sometimes goes a bit too far in such cases, trying to find out which parameter would cause which effect. Such discussions could be reduced, which would also reduce redundancies between different sections. In some cases, possible signals are mentioned, but then it is stated in the next sentence that the effect is not significant. This could also be skipped, but I leave these points for consideration of the authors. Otherwise, I only have a few minor comments.

Specific comments:
Abstract L23 ff: Related to my general comment, would you really expect to find one dominant process that explains the residual variability? I think it is not really surprising that there is not a clear result, given the vast differences in the conditions in the various datasets. This could be left out

L58 ff: The methods represented by the references also include isotope ratio mass spectrometry.

L104: subscript 4

L119: Remove: “The isotope notation used in this study is briefly introduced here.” This is standard notation, not only used in this study

Table 1 and Section 3.8: Although the authors did not agree with my comment on the small errors of the d13C signatures of the fossil fuel sources, I still think it is valid. I acknowledge that the estimates come from published literature, but I am still skeptical. Given that the reported signatures span a range of about 50 ‰, the fluxes are uncertain, emissions and signatures can change over time, etc., I wonder whether the flux weighted average signature is really known to ±0.5 ‰. I think this could be mentioned as additional uncertainty, in particular since there is a clear discrepancy between the bottom-up and top-down estimates for d13C.

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RR by Edward Hornibrook (26 Apr 2021)

ED: Publish subject to minor revisions (review by editor) (08 May 2021) by Caroline P. Slomp

AR by Peter Douglas on behalf of the Authors (17 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (18 May 2021) by Caroline P. Slomp

AR by Peter Douglas on behalf of the Authors (18 May 2021) Author's response Manuscript

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Short summary

Hydrogen isotopes could be a useful tool to help resolve the geographic distribution of methane emissions from freshwater environments. We analyzed an expanded global dataset of freshwater methane hydrogen isotope ratios and found significant geographic variation linked to water isotopic composition. This geographic variability could be used to resolve changing methane fluxes from freshwater environments and provide more accurate estimates of the relative balance of global methane sources.