Heterogeneous CO<sub>2</sub> and CH<sub>4</sub> content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

Pain, Andrea J.; Martin, Jonathan B.; Martin, Ellen E.; Rennermalm, Åsa K.; Rahman, Shaily

doi:https://doi.org/10.5194/tc-15-1627-2021

Articles | Volume 15, issue 3

https://doi.org/10.5194/tc-15-1627-2021

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

https://doi.org/10.5194/tc-15-1627-2021

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

Articles | Volume 15, issue 3

Research article

|

01 Apr 2021

Research article |

| 01 Apr 2021

Heterogeneous CO₂ and CH₄ content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

Andrea J. Pain, Jonathan B. Martin, Ellen E. Martin, Åsa K. Rennermalm, and Shaily Rahman

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Final revised paper (published on 01 Apr 2021)
Supplement to the final revised paper
Preprint (discussion started on 03 Jul 2020)

Interactive discussion

Status: closed

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

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- Supplement

SC1: 'Geography and Place Names', Joseph Graly, 15 Jul 2020
- AC3: 'Response to comment', Andrea Pain, 22 Sep 2020
RC1: 'Review for tc-2020-155', Anonymous Referee #1, 29 Jul 2020
- AC1: 'Response to reviewer comments', Andrea Pain, 22 Sep 2020
RC2: 'Review of ‘Heterogenous CO2 and CH4 content of glacial meltwater of the Greenland Ice Sheet and implications for subglacial carbon processes’', Marek Stibal, 31 Jul 2020
- AC2: 'Response to reviewer comments', Andrea Pain, 22 Sep 2020

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (20 Oct 2020) by Elizabeth Bagshaw

AR by Andrea Pain on behalf of the Authors (31 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 Jan 2021) by Elizabeth Bagshaw

RR by Anonymous Referee #1 (22 Jan 2021)

RR by Marek Stibal (05 Feb 2021)

Suggestions for revision or reasons for rejection

The authors have addressed most comments, and I am especially happy to see the Watson river discharge data replaced with the better fitting AK4 dataset; I think this has made the ms much more robust. Some specific comments, especially in the introduction, have not been addressed though, and are again listed below. Other minor issues have emerged that require some attention and should be addressed before the study can be published.

Title – correct to ‘heterogeneous’
Abstract – verb still missing in sentence at lines 20-22

Intro
57 and elsewhere please specify if Graly et al 2017a or b
62-64 relevant work should be cited here, eg the recent review by Wadham et al (2019)
70 Musilova et al (2017) did not study subglacial microbial activity; this reference is irrelevant here

Methods
91 typo in ‘the Qinnguata Kuussua’
97 mentioning the Isunnguata catchment size is irrelevant and potentially misleading here and should be removed

102 The estimate of the Russell Glacier catchment size of 300 km2 has remained in the text. This is questionable and I strongly advise the authors to remove it or to add this is probably exaggerated (van de Wal and Russell 1994).

Results
277-278 The correlations between Sub-IS discharge and δ13CCH4, εC, and fox are weak and the data points certainly do not suggest linear relationships (Fig 5bcd). The relevant discussion should be toned down accordingly (see below).

Discussion
337-339 This explanation is confused. Methane production requires a very negative redox potential and any external terminal electron acceptors brought in by meltwater (O2, NO3-) would inhibit it and lead to CH4 oxidation instead. The EAs for methanogenesis are either CO2 (for the hydrogenotrophic pathway) or acetate (acetoclastic methanogenesis is a disproportionation). Please remove or rephrase this.

354-356 If CH4 is stored under the ice sheet no fractionation is likely to occur. Therefore, it is impossible to decide whether the released CH4 comes from active methanogenesis or old reservoirs, based only on δ13CCH4. Please remove or rephrase this.

367 While I agree it is unlikely that the extent of outgassing would vary significantly between sampling times, it may be affected by discharge due to changes in turbulent flow. Please change to ‘outgassing would not fully explain temporal differences’.

372-374 This paragraph is based on the weak correlations shown in Fig 5 (see above) and should be toned down.

392, 415 please remove the sermiat/glacier pleonasms
423-8 unfinished sentences, please rephrase

491-498 More recent and appropriate literature should be referred to. For example, microbial sulphide and thiosulphate oxidation in the subglacial environment have been quantified by Boyd et al 2014 and Harrold et al 2016, respectively; the presence of growth substrates as a factor for CH4 production in subglacial samples has been shown experimentally by Stibal et al 2012; Wadham et al 2010 provided a thorough overview of subglacial weathering and the role of microbial processes in it.

References
Boyd ES, Hamilton TL, Having JR, Skidmore ML, Shock EL (2014) Chemolithotrophic primary production in a subglacial ecosystem. Appl Environ Microbiol 80:6146–6153
Harrold ZR, Skidmore ML, Hamilton TL, Desch L, Amada K, van Gelder W, Glover K, Roden EE, Boyd ES (2016) Aerobic and anaerobic thiosulfate oxidation by a cold-adapted, subglacial chemoautotroph. Appl Environ Microbiol 82:1486 –1495
Musilova M, Tranter M, Wadham J, Telling J, Tedstone A, Anesio AM (2017) Microbially driven export of labile organic carbon from the Greenland ice sheet. Nat Geosci 10:360–365
Stibal M, Wadham JL, Lis GP, Telling J, Pancost RD, Dubnick A, Sharp MJ, Lawson EC, Butler CEH, Hasan F, Tranter M, Anesio AM (2012) Methanogenic potential of Arctic and Antarctic subglacial environments with contrasting organic carbon sources. Glob Change Biol 18:3332–3345
van de Wal RSW, Russell AJ (1994) A comparison of energy balance calculations, measured ablation and meltwater runoff near Søndre Strømfjord, West Greenland. Glob Planet Change 9:29-38
Wadham JL, Tranter M, Skidmore M, Hodson AJ, Priscu J, Lyons WB, Sharp M, Wynn P, Jackson M (2010) Biogeochemical weathering under ice: Size matters. Glob Biogeochem Cycles 24:GB3025
Wadham JL, Hawkings JR, Tarasov L, Gregoire LJ, Spencer RGM, Gutjahr M, Ridgwell A, Kohfeld KE (2019) Ice sheets matter for the global carbon cycle. Nat Commun 10:3567

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ED: Publish subject to minor revisions (review by editor) (18 Feb 2021) by Elizabeth Bagshaw

AR by Andrea Pain on behalf of the Authors (22 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (24 Feb 2021) by Elizabeth Bagshaw

Short summary

The greenhouse gases (GHGs) methane and carbon dioxide can be produced or consumed by geochemical processes under the Greenland Ice Sheet (GrIS). Chemical signatures and concentrations of GHGs in GrIS discharge show that organic matter remineralization produces GHGs in some locations, but mineral weathering dominates and consumes CO₂ in other locations. Local processes will therefore determine whether melting of the GrIS is a positive or negative feedback on climate change driven by GHG forcing.

Heterogeneous CO2 and CH4 content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

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Heterogeneous CO₂ and CH₄ content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes