Sulfate reduction and anaerobic oxidation of methane in sediments of
the South-Western Barents Sea

Argentino, Claudio; Waghorn, Kate Alyse; Bünz, Stefan; Panieri, Giuliana

doi:https://doi.org/10.5194/bg-2021-58

Preprints

https://doi.org/10.5194/bg-2021-58

Preprints

09 Mar 2021

Review status: this preprint is currently under review for the journal BG.

Sulfate reduction and anaerobic oxidation of methane in sediments of the South-Western Barents Sea

Claudio Argentino, Kate Alyse Waghorn, Stefan Bünz, and Giuliana Panieri Claudio Argentino et al.

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CAGE - Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geosciences, UiT The Arctic University of Norway, 9037 Tromsø, Norway

Received: 02 Mar 2021 – Accepted for review: 08 Mar 2021 – Discussion started: 09 Mar 2021

Abstract. Anaerobic oxidation of methane (AOM) in marine sediments strongly limits the amount of gas reaching the water column and the atmosphere but its efficiency in counteracting future methane emissions at continental margins remains unclear. Small shifts in methane fluxes due to gas hydrate and submarine permafrost destabilization or enhanced methanogenesis in warming Arctic continental shelves may cause the redox boundary in which AOM occurs, known as Sulfate-Methane Transition Zone (SMTZ), to move closer to seafloor, with potential gas release to bottom waters. Here, we investigated the geochemical composition of pore water (SO₄²⁻ and DIC concentration, δ¹³C_DIC) and gas (CH₄, δ¹³C_CH4) in eight gravity cores collected from Ingøydjupet trough, South-Western Barents Sea. Our results show a remarkable variability in SMTZ depth, ranging from 3.5 m to 29.2 m, and that all methane is efficiently consumed by AOM within the sediment. From linear fitting of the sulfate concentration profiles, we calculated diffusive sulfate fluxes ranging from 1.5 nmol cm⁻² d⁻¹ to 12.0 nmol cm⁻² d⁻¹. AOM rates obtained for two cores using mixing models are 6.5 nmol cm⁻² d⁻¹ and 6.7 nmol cm⁻² d⁻¹ and account for only 64 % and 56 % of total sulfate reduction at the SMTZ (SRR_tot), respectively. The remaining 36 % and 44 % SRR_tot correspond to organoclastic sulfate reduction with rates of 3.7 nmol cm⁻² d⁻¹ and 5.3 nmol cm⁻² d⁻¹. The shallowest SMTZs (< 5 m) and largest SRR_tot rates are associated with a shallow subsurface accumulation of gas visible in seismic data, highlighting how small changes in sulfate reduction rates linked to subsurface methane gradients resulted in vertical shifts in SMTZ position of > 20 m. This study provides new insights into the dynamic and biogeochemistry of the SMTZ in marine sediments of continental margins and may help predict the response of the microbial methane filter to future increase in methane fluxes due to ocean warming.

Claudio Argentino et al.

Status: open (until 24 Apr 2021)

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AC1: 'Comment on bg-2021-58; DIC fluxes at the SMTZ and estimations of OSR and AOM rates', Claudio Argentino, 21 Mar 2021 reply

The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2021-58/bg-2021-58-AC1-supplement.pdf
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Citation: https://doi.org/10.5194/bg-2021-58-AC1
RC1:
'Comment on bg-2021-58', Anonymous Referee #1, 15 Apr 2021 reply
Using “linear” porewater sulfate gradients to estimate methane fluxes has been frequently reported from areas with high methane fluxes especially in the past few decades. The approach has raised uncertainties due to sulfate consumption by OSR and AOM reactions. Therefore, some studies have expanded the diffusive flux calculations on depth profiles of DIC concentrations and stable carbon isotopes of DIC (e.g., Wehrmann et al. 2011, Chemical Geology; Burdige and Komada, 2011, L&O) or have applied a reaction-transport model to simulate sulfate and methane depth profiles (e.g., Wallmann et al., 2006, GCA; Dale et al., 2019, GCA) in order to get better estimations on OSR and AOM processes. The controls on δ¹³C-DIC and δ¹³C-CH₄ depth profiles with special focuses on the SMTZ were also examined in recent years (e.g., Burdige et al., 2016, JMR; Chuang et al., 2019, GCA; Meister et al., 2019 JMS). The relevant literature reviews are largely lack in this study, hence, which results in the lack of novel aspects. Despite the authors have been aware of some references in the supplement for their δ¹³C-DIC and DIC calculations, they don’t have data to provide direct constraints on their flux estimations. The study presented by Argentino et al. needs more consolidated works on the introduction, methods, results and discussion, so that I couldn’t recommend the publication in the journal of Biogeosciences.

Comments:

None of the sediment cores collected in this study reached the SMTZ. The limited data (only sulfate) can’t support the estimation of the depth of SMTZ. Many studies have shown that sulfate might stay constant in the deeper depths. The authors should provide other valid data to support their arguments (e.g., sediment cores reaching SMTZ and showing varying depth of the SMTZ and data to constrain OSR and AOM such as depth profiles of NH₄⁺, TA, H₂S, Ca²⁺, DIC, TOC, δ¹³C-CH₄, δ¹³C-DIC etc.).

Eq. 3 used in other studies such as by Martin et al. (2000, GCA) and Hu and Burdige (2007, GCA) is to estimate the amount of calcite dissolution adding to the porewater. The fact is that δ¹³C-DIC in the porewater is not only affected by SOC source and AOM reaction but also SOC degradation, carbonate mineral dissolution and precipitation etc. Therefore, the estimated OSR and AOM rates are not valid, despite the authors have additional information on δ¹³C-DIC calculations in the supplement which have no data reaching SMTZ to support their arguments.

The scale of seismic profile is different from the length of sediment cores. Gas migrations can also be controlled by the tectonic structure beneath the coring sites. Therefore, gas accumulation shown in the seismic profile doesn’t mean gas exist in the cored sediments.

Line 95: Sulfate analysis needs some more detailed information on analysis details and analytical quality. Why was sulfate measured by ICP-OES? How did authors separate other sulfur species? If the authors assume that all the sulfur species in the porewater measured by ICP-OES is only sulfate. This may overestimate sulfate fluxes. Or This may imply no H₂S production through OSR and AOM which is in contradiction to their flux calculations.

Line 110: What kinds of gas standard were used?

Line 137: Hu et al., 2017 and Hu et al., 2010 are led by different authors.

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Citation: https://doi.org/10.5194/bg-2021-58-RC1

Claudio Argentino et al.

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Short summary

We investigated sulfate and methane cycling in sediments of the SW Barents Sea associated with a shallow gas accumulation. The depth of the sulfate-methane transition zone ranges between 3.5 m and 29.2 m, and all methane is consumed within the sediment. Results from this study are important to better understand the dynamic of the sulfate-methane transition and to predict its response to future scenarios of increasing methane fluxes in Arctic continental shelves affected by ocean warming.


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