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

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

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09 Mar 2021

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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 – 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.

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Claudio Argentino et al.

Interactive discussion

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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

The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2021-58/bg-2021-58-AC1-supplement.pdf

Citation: https://doi.org/10.5194/bg-2021-58-AC1
RC1:
'Comment on bg-2021-58', Anonymous Referee #1, 15 Apr 2021
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.
Citation: https://doi.org/10.5194/bg-2021-58-RC1
- AC2: 'Reply on RC1', Claudio Argentino, 10 May 2021
  
  We thank the reviewer for the useful comments and constructive criticism. In the Barents Sea we generally employ a 6 m-long gravity corer for investigating biogeochemical processes related to methane oxidation. Unfortunately, this length of sediment recovery does not allow us access to the deeper sediments and hindered the description of deeper sulfate profile terminations, and direct interception of the SMTZ in areas located away from the shallow seismic anomaly. The vertical sampling resolution was adequate to describe the sulfate gradient in the uppermost 2-3 m and we found good linearity in all the examined pore water profiles. Very low microbial activity was measured in the upper 2.4 m of sediment at NW Ingøydjupet trough (Nickel et al., 2012) so we assumed that sulfate diffusing in the sediment undergoes little sulfate reduction linked to organic matter oxidation in the upper sediment column and is mainly consumed deeper, within the ZMTZ. For this reason, we linearly extrapolated the sulfate profiles to deeper sediment to estimate the depth of the SMTZ. It is worth mentioning that sediment core 358-GC actually reached the predicted SMTZ (3.5 m) as shown by CH₄ concentration in the gas samples plotted in Fig.2. For the above reasons we considered the sulfate profiles in the deeper sediment not covered by pore water data to approximate a linear shape also in correspondence of cores 354-GC, 355-GC, 356-GC, 361-GC, 362-GC, 363-GC. This approach has been employed in other cases where the core penetration depth did not allow a direct observation of the SMTZ depth (Borowski et al., 1999; Fan et al., 2018; Graves et al., 2017; Mazumdar et al., 2007; Panieri et al., 2016). We do not have other constraints for the deeper sedimentary column to completely rule out the possibility of concave-down trends for the deepest sections. However, a concave-down termination would result in a deeper SMTZ compared to the one calculated from linear regression, further emphasizing the difference in SMTZ depth between cores located above the seismic anomaly and cores away from it, in agreement with our general interpretations. We implicitly excluded concave-up termination of the sulfate profiles as we assumed steady-state conditions. As described in AC1, DIC fluxes entering and leaving the SMTZ can be used to quantify DIC release by OSR and AOM in the SMTZ. We only have pore water samples above the SMTZ and the deep DIC flux component cannot be quantified, nor DIC sequestration by authigenic carbonates. The strongly depleted d¹³C of DIC can be discussed qualitatively but we also propose to enrich manuscript’s discussion by including case scenarios of carbonate precipitation and deep DIC fluxes based on average global models (Akam et al., 2020) to give quantitative constraints (AC1 comment). We agree that datasets including a wide range of parameters such as d¹³C-CH₄ and d¹³C-DIC, ammonium, POC, would allow complete reaction-transport modeling able to calculate depth-integrated reduction rates, methanogenesis rates and pathways, but the simple modeling approach used in our study explicitly focused on processes within the SMTZ. We acknowledge the reviewer for the recommendations and will take them into consideration for future studies.
  The reviewer said: “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.”
  
  Seismic section in Fig. 4 shows high-resolution P-cable 3D seismic data with vertical resolution of ~4 m and horizontal resolution of 6.25 by 6.25 m. Considering these values, we can confidently argue that the top of the seismic anomaly reaches within range of the seafloor (considering the vertical resolution/dominant frequency of the near-seafloor sediment and their velocity). This is in agreement with the methane-rich gas sample collected at the base of core 358-GC and the shallow SMTZs in 358-GC and 359-GC.
  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?
  We thank the reviewer for pointing this out, sulfate analyses were conducted via ion chromatography, we will modify the text accordingly.
  Line 110: What kinds of gas standard were used?
  We used Messer® CANGas calibration gases (specialtygases.messergroup.com). This information will be included in the manuscript.
  Line 137: Hu et al., 2017 and Hu et al., 2010 are led by different authors.
  We thank the reviewer for this observation and corrected the reference.
  
  Best regards,
  Claudio Argentino
  On behalf of the authors
  
  References
  Akam, S. A., Coffin, R. B., Abdulla, H. A. N. and Lyons, T. W.: Dissolved Inorganic Carbon Pump in Methane-Charged Shallow Marine Sediments: State of the Art and New Model Perspectives, Front. Mar. Sci., doi:10.3389/fmars.2020.00206, 2020.
  Borowski, W. S., Paull, C. K. and Ussler, W.: Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: Sensitivity to underlying methane and gas hydrates, Mar. Geol., doi:10.1016/S0025-3227(99)00004-3, 1999.
  Fan, L. F., Lin, S., Hsu, C. W., Tseng, Y. T., Yang, T. F. and Huang, K. M.: Formation and preservation of authigenic pyrite in the methane dominated environment, Deep. Res. Part I Oceanogr. Res. Pap., 138(July), 60–71, doi:10.1016/j.dsr.2018.07.004, 2018.
  Graves, C. A., James, R. H., Sapart, C. J., Stott, A. W., Wright, I. C., Berndt, C., Westbrook, G. K. and Connelly, D. P.: Methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore western Svalbard, Geochim. Cosmochim. Acta, 198, 419–438, doi:10.1016/j.gca.2016.11.015, 2017.
  Mazumdar, A., Paropkari, A. L., Borole, D. V., Rao, B. R., Khadge, N. H., Karisiddaiah, S. M., Kocherla, M. and Joäo, H. M.: Pore-water sulfate concentration profiles of sediment cores from Krishna-Godavari and Goa basins, India, Geochem. J., 41(4), 259–269, doi:10.2343/geochemj.41.259, 2007.
  Nickel, J. C., di Primio, R., Mangelsdorf, K., Stoddart, D. and Kallmeyer, J.: Characterization of microbial activity in pockmark fields of the SW-Barents Sea, Mar. Geol., 332–334, 152–162, doi:10.1016/j.margeo.2012.02.002, 2012.
  Panieri, G., Graves, C. A. and James, R. H.: Paleo-methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western Svalbard, Geochemistry, Geophys. Geosystems, 17(2), 521–537, doi:10.1002/2015GC006153, 2016.
  
  Citation: https://doi.org/10.5194/bg-2021-58-AC2
RC2:
'Comment on bg-2021-58', Anonymous Referee #2, 18 Apr 2021

The work presented by Argentino et al described the concentrations of porewater sulfate, DIC, methane, and carbon isotopic signatures of DIC from eight sediment cores recovered from SW Barents Sea. By applying a two-component box model, they estimated the fractions of organic matter-dependent and methane-dependent sulfate reduction (OSR and AOM, respectively). Though none of their core fully recovered the entire sulfate reduction zone, they showed that AOM only accounts for ca. 2/3 of the total sulfate reduction in the two sediment cores they have a more complete dataset. They further correlated the geochemical findings with a seismic profile and suggested that the locations with higher AOM rates, and thus methane fluxes, may be associated with a shallow gas reservoir.

Overall, many of the conclusions are not supported by the data presented. This is mostly due to the recovery of only the upper sulfate reduction zone from the sediment cores. There is one core which may recover the SMTZ; however, there is no sulfate/DIC data from the second half of the core that can be used to verify their calculation. Before addressing the methane and sulfate dynamics with the porewater profiles, it is crucial to have the porewater composition from the entire sulfate reduction zone (and ideally part of the methanogenesis zone) determined. The downcore gradient of sulfate concentrations may change abruptly with depth, a phenomenon that has been commonly addressed in many locations (Zabel and Schulz 2001; Hensen et al. 2003; Haeckel et al. 2007; Holstein and Wirtz 2010; Hong et al. 2014). The entire flux/rate/fraction calculation done by the authors can be invalid if this is the case.

Similarly, the authors´ conclusion of methane source is based on one single methane carbon isotope analyses (without the hydrogen isotope analysed). They claim that the source of methane, similar to what has been found from a well that is ca. 10 km away, is a mixture of biogenic and thermogenic methane. While I understand it is difficult to provide more data when the methane concentrations are low in all the cores sampled, the authors then shouldn’t make any inference on the source of methane as such an inference could be incorrect when more data become available. At least, the authors can show the detailed composition of headspace gas, which may (or may not) support their suggestion.

In the last figure of the paper (and last sentence in the conclusion), the authors illustrated a fanaticized scenario under future ocean warming. While I understand the authors intended to make the paper more appealing by linking the work to some future scenarios, such a suggestion is entirely irrelevant and not supported by the data at all. I suggest remove all these hand-waving sections from the paper.

Some more detailed comments of mine are included in the pdf file attached.

Citation: https://doi.org/10.5194/bg-2021-58-RC2
- AC3: 'Reply on RC2', Claudio Argentino, 10 May 2021
  
  We thank the reviewer for the useful comments and constructive criticisms, and for providing a pdf file with detailed review. Regarding the lack of deep pore water data, please refer to our answer to Reviewer1 (AC2). At line 170 we described the hydrocarbon composition in gas sample from core 358-GC “dominated by methane, representing 99.8% of the total hydrocarbon gas fraction”. We do not report ethane and propane concentrations as they are too low compared to analytical uncertainty. We are aware of the fact that d²H is fundamental to classify the origin of gas (lines 246-247):“ We could not measure the hydrogen isotopic composition for our gas sample, due to limited sample size, and the determination of the gas source was out of the aim of this study.”. The good homogeneity in source rock and reservoir spatial distributions is at the base of the high prospectivity of the SW Barents Sea, leading to the discovery of major hydrocarbon fields e.g. Goliat, Snøhvit, Albatross. Deeper Triassic and Jurassic reservoirs generally show a thermogenic composition whereas shallower gas accumulations fit in the area of mixed microbial-thermogenic gas. This has been reported from all over the Hammerfest Basin (Rodrigues Duran et al., 2013) and also in well no. 7122-2-1 at the Caurus field which shows a dominant thermogenic signature in Jurassic sediments (avg d¹³C =-45‰; d²H= -218‰) and a mixed composition in shallower Cretaceous accumulations (avg d¹³C =-55‰; d²H= -188‰). High amplitude gas anomalies related to shallow gas pockets are common around the study area (Chand et al., 2009; Tasianas et al., 2018) for which a mixed thermogenic-microbial origin has been proposed. Therefore, the origin of gas causing the bright seismic spot in the study area remains unknown, but it is conceivable to hypothesize that the gas accumulated in the subsurface has a mixed microbial-thermogenic origin.
  This study aims to emphasize the need of region-specific investigations of SMTZ dynamics and associated SO₄ and CH₄ fluxes, for monitoring the effects of contemporary and future climate change on SMTZ depth and predict the expansion of methane seepage areas at continental margins. There is growing awareness in the scientific community regarding the potential increase in methane emissions from shallow continental shelves and coastal environments due to ocean warming and eutrophication (Wallenius et al., 2021). The figure at the end of the manuscript is not in scale (horizontal) and is a simplified scenario for shallow shelves. It is likely if we consider the occurrence of shallow SMTZs located at few tens of cm to few meters below the seafloor (Egger et al., 2018), and the fact the accelerating high-latitude warming trend may cause a shoaling of the SMTZ due to increasing rates of methanogenesis (Borges et al., 2019; Egger et al., 2016; Humborg et al., 2019).
  
  Best regards,
  Claudio Argentino
  On behalf of the authors
  
  References
  Borges, A. V., Royer, C., Martin, J. L., Champenois, W. and Gypens, N.: Response of marine methane dissolved concentrations and emissions in the Southern North Sea to the European 2018 heatwave, Cont. Shelf Res., 190, 104004, doi:10.1016/j.csr.2019.104004, 2019.
  Chand, S., Rise, L., Ottesen, D., Dolan, M. F. J., Bellec, V. and Bøe, R.: Pockmark-like depressions near the Goliat hydrocarbon field, Barents Sea: Morphology and genesis, Mar. Pet. Geol., 26(7), 1035–1042, doi:10.1016/j.marpetgeo.2008.09.002, 2009.
  Egger, M., Lenstra, W., Jong, D., Meysman, F. J. R., Sapart, C. J., Van Der Veen, C., Röckmann, T., Gonzalez, S. and Slomp, C. P.: Rapid sediment accumulation results in high methane effluxes from coastal sediments, PLoS One, 11(8), 1–22, doi:10.1371/journal.pone.0161609, 2016.
  Egger, M., Riedinger, N., Mogollón, J. M. and Jørgensen, B. B.: Global diffusive fluxes of methane in marine sediments, Nat. Geosci., 11(6), 421–425, doi:10.1038/s41561-018-0122-8, 2018.
  Humborg, C., Geibel, M. C., Sun, X., McCrackin, M., Mörth, C.-M., Stranne, C., Jakobsson, M., Gustafsson, B., Sokolov, A., Norkko, A. and Norkko, J.: High Emissions of Carbon Dioxide and Methane From the Coastal Baltic Sea at the End of a Summer Heat Wave, Front. Mar. Sci., 6, doi:10.3389/fmars.2019.00493, 2019.
  Rodrigues Duran, E., di Primio, R., Anka, Z., Stoddart, D. and Horsfield, B.: Petroleum system analysis of the Hammerfest Basin (southwestern Barents Sea): Comparison of basin modelling and geochemical data, Org. Geochem., 63, 105–121, doi:10.1016/j.orggeochem.2013.07.011, 2013.
  Tasianas, A., Bünz, S., Bellwald, B., Hammer, Ø., Planke, S., Lebedeva-Ivanova, N. and Krassakis, P.: High-resolution 3D seismic study of pockmarks and shallow fluid flow systems at the Snøhvit hydrocarbon field in the SW Barents Sea, Mar. Geol., 403(9037), 247–261, doi:10.1016/j.margeo.2018.06.012, 2018.
  Wallenius, A. J., Martins, P. D., Slomp, C. P. and Jetten, M. S. M.: Anthropogenic and Environmental Constraints on the Microbial Methane Cycle in Coastal Sediments, , 12(February), doi:10.3389/fmicb.2021.631621, 2021.
  
  Citation: https://doi.org/10.5194/bg-2021-58-AC3

Interactive discussion

Status: closed

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

The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2021-58/bg-2021-58-AC1-supplement.pdf

Citation: https://doi.org/10.5194/bg-2021-58-AC1
RC1:
'Comment on bg-2021-58', Anonymous Referee #1, 15 Apr 2021
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.
Citation: https://doi.org/10.5194/bg-2021-58-RC1
- AC2: 'Reply on RC1', Claudio Argentino, 10 May 2021
  
  We thank the reviewer for the useful comments and constructive criticism. In the Barents Sea we generally employ a 6 m-long gravity corer for investigating biogeochemical processes related to methane oxidation. Unfortunately, this length of sediment recovery does not allow us access to the deeper sediments and hindered the description of deeper sulfate profile terminations, and direct interception of the SMTZ in areas located away from the shallow seismic anomaly. The vertical sampling resolution was adequate to describe the sulfate gradient in the uppermost 2-3 m and we found good linearity in all the examined pore water profiles. Very low microbial activity was measured in the upper 2.4 m of sediment at NW Ingøydjupet trough (Nickel et al., 2012) so we assumed that sulfate diffusing in the sediment undergoes little sulfate reduction linked to organic matter oxidation in the upper sediment column and is mainly consumed deeper, within the ZMTZ. For this reason, we linearly extrapolated the sulfate profiles to deeper sediment to estimate the depth of the SMTZ. It is worth mentioning that sediment core 358-GC actually reached the predicted SMTZ (3.5 m) as shown by CH₄ concentration in the gas samples plotted in Fig.2. For the above reasons we considered the sulfate profiles in the deeper sediment not covered by pore water data to approximate a linear shape also in correspondence of cores 354-GC, 355-GC, 356-GC, 361-GC, 362-GC, 363-GC. This approach has been employed in other cases where the core penetration depth did not allow a direct observation of the SMTZ depth (Borowski et al., 1999; Fan et al., 2018; Graves et al., 2017; Mazumdar et al., 2007; Panieri et al., 2016). We do not have other constraints for the deeper sedimentary column to completely rule out the possibility of concave-down trends for the deepest sections. However, a concave-down termination would result in a deeper SMTZ compared to the one calculated from linear regression, further emphasizing the difference in SMTZ depth between cores located above the seismic anomaly and cores away from it, in agreement with our general interpretations. We implicitly excluded concave-up termination of the sulfate profiles as we assumed steady-state conditions. As described in AC1, DIC fluxes entering and leaving the SMTZ can be used to quantify DIC release by OSR and AOM in the SMTZ. We only have pore water samples above the SMTZ and the deep DIC flux component cannot be quantified, nor DIC sequestration by authigenic carbonates. The strongly depleted d¹³C of DIC can be discussed qualitatively but we also propose to enrich manuscript’s discussion by including case scenarios of carbonate precipitation and deep DIC fluxes based on average global models (Akam et al., 2020) to give quantitative constraints (AC1 comment). We agree that datasets including a wide range of parameters such as d¹³C-CH₄ and d¹³C-DIC, ammonium, POC, would allow complete reaction-transport modeling able to calculate depth-integrated reduction rates, methanogenesis rates and pathways, but the simple modeling approach used in our study explicitly focused on processes within the SMTZ. We acknowledge the reviewer for the recommendations and will take them into consideration for future studies.
  The reviewer said: “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.”
  
  Seismic section in Fig. 4 shows high-resolution P-cable 3D seismic data with vertical resolution of ~4 m and horizontal resolution of 6.25 by 6.25 m. Considering these values, we can confidently argue that the top of the seismic anomaly reaches within range of the seafloor (considering the vertical resolution/dominant frequency of the near-seafloor sediment and their velocity). This is in agreement with the methane-rich gas sample collected at the base of core 358-GC and the shallow SMTZs in 358-GC and 359-GC.
  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?
  We thank the reviewer for pointing this out, sulfate analyses were conducted via ion chromatography, we will modify the text accordingly.
  Line 110: What kinds of gas standard were used?
  We used Messer® CANGas calibration gases (specialtygases.messergroup.com). This information will be included in the manuscript.
  Line 137: Hu et al., 2017 and Hu et al., 2010 are led by different authors.
  We thank the reviewer for this observation and corrected the reference.
  
  Best regards,
  Claudio Argentino
  On behalf of the authors
  
  References
  Akam, S. A., Coffin, R. B., Abdulla, H. A. N. and Lyons, T. W.: Dissolved Inorganic Carbon Pump in Methane-Charged Shallow Marine Sediments: State of the Art and New Model Perspectives, Front. Mar. Sci., doi:10.3389/fmars.2020.00206, 2020.
  Borowski, W. S., Paull, C. K. and Ussler, W.: Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: Sensitivity to underlying methane and gas hydrates, Mar. Geol., doi:10.1016/S0025-3227(99)00004-3, 1999.
  Fan, L. F., Lin, S., Hsu, C. W., Tseng, Y. T., Yang, T. F. and Huang, K. M.: Formation and preservation of authigenic pyrite in the methane dominated environment, Deep. Res. Part I Oceanogr. Res. Pap., 138(July), 60–71, doi:10.1016/j.dsr.2018.07.004, 2018.
  Graves, C. A., James, R. H., Sapart, C. J., Stott, A. W., Wright, I. C., Berndt, C., Westbrook, G. K. and Connelly, D. P.: Methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore western Svalbard, Geochim. Cosmochim. Acta, 198, 419–438, doi:10.1016/j.gca.2016.11.015, 2017.
  Mazumdar, A., Paropkari, A. L., Borole, D. V., Rao, B. R., Khadge, N. H., Karisiddaiah, S. M., Kocherla, M. and Joäo, H. M.: Pore-water sulfate concentration profiles of sediment cores from Krishna-Godavari and Goa basins, India, Geochem. J., 41(4), 259–269, doi:10.2343/geochemj.41.259, 2007.
  Nickel, J. C., di Primio, R., Mangelsdorf, K., Stoddart, D. and Kallmeyer, J.: Characterization of microbial activity in pockmark fields of the SW-Barents Sea, Mar. Geol., 332–334, 152–162, doi:10.1016/j.margeo.2012.02.002, 2012.
  Panieri, G., Graves, C. A. and James, R. H.: Paleo-methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western Svalbard, Geochemistry, Geophys. Geosystems, 17(2), 521–537, doi:10.1002/2015GC006153, 2016.
  
  Citation: https://doi.org/10.5194/bg-2021-58-AC2
RC2:
'Comment on bg-2021-58', Anonymous Referee #2, 18 Apr 2021

The work presented by Argentino et al described the concentrations of porewater sulfate, DIC, methane, and carbon isotopic signatures of DIC from eight sediment cores recovered from SW Barents Sea. By applying a two-component box model, they estimated the fractions of organic matter-dependent and methane-dependent sulfate reduction (OSR and AOM, respectively). Though none of their core fully recovered the entire sulfate reduction zone, they showed that AOM only accounts for ca. 2/3 of the total sulfate reduction in the two sediment cores they have a more complete dataset. They further correlated the geochemical findings with a seismic profile and suggested that the locations with higher AOM rates, and thus methane fluxes, may be associated with a shallow gas reservoir.

Overall, many of the conclusions are not supported by the data presented. This is mostly due to the recovery of only the upper sulfate reduction zone from the sediment cores. There is one core which may recover the SMTZ; however, there is no sulfate/DIC data from the second half of the core that can be used to verify their calculation. Before addressing the methane and sulfate dynamics with the porewater profiles, it is crucial to have the porewater composition from the entire sulfate reduction zone (and ideally part of the methanogenesis zone) determined. The downcore gradient of sulfate concentrations may change abruptly with depth, a phenomenon that has been commonly addressed in many locations (Zabel and Schulz 2001; Hensen et al. 2003; Haeckel et al. 2007; Holstein and Wirtz 2010; Hong et al. 2014). The entire flux/rate/fraction calculation done by the authors can be invalid if this is the case.

Similarly, the authors´ conclusion of methane source is based on one single methane carbon isotope analyses (without the hydrogen isotope analysed). They claim that the source of methane, similar to what has been found from a well that is ca. 10 km away, is a mixture of biogenic and thermogenic methane. While I understand it is difficult to provide more data when the methane concentrations are low in all the cores sampled, the authors then shouldn’t make any inference on the source of methane as such an inference could be incorrect when more data become available. At least, the authors can show the detailed composition of headspace gas, which may (or may not) support their suggestion.

In the last figure of the paper (and last sentence in the conclusion), the authors illustrated a fanaticized scenario under future ocean warming. While I understand the authors intended to make the paper more appealing by linking the work to some future scenarios, such a suggestion is entirely irrelevant and not supported by the data at all. I suggest remove all these hand-waving sections from the paper.

Some more detailed comments of mine are included in the pdf file attached.

Citation: https://doi.org/10.5194/bg-2021-58-RC2
- AC3: 'Reply on RC2', Claudio Argentino, 10 May 2021
  
  We thank the reviewer for the useful comments and constructive criticisms, and for providing a pdf file with detailed review. Regarding the lack of deep pore water data, please refer to our answer to Reviewer1 (AC2). At line 170 we described the hydrocarbon composition in gas sample from core 358-GC “dominated by methane, representing 99.8% of the total hydrocarbon gas fraction”. We do not report ethane and propane concentrations as they are too low compared to analytical uncertainty. We are aware of the fact that d²H is fundamental to classify the origin of gas (lines 246-247):“ We could not measure the hydrogen isotopic composition for our gas sample, due to limited sample size, and the determination of the gas source was out of the aim of this study.”. The good homogeneity in source rock and reservoir spatial distributions is at the base of the high prospectivity of the SW Barents Sea, leading to the discovery of major hydrocarbon fields e.g. Goliat, Snøhvit, Albatross. Deeper Triassic and Jurassic reservoirs generally show a thermogenic composition whereas shallower gas accumulations fit in the area of mixed microbial-thermogenic gas. This has been reported from all over the Hammerfest Basin (Rodrigues Duran et al., 2013) and also in well no. 7122-2-1 at the Caurus field which shows a dominant thermogenic signature in Jurassic sediments (avg d¹³C =-45‰; d²H= -218‰) and a mixed composition in shallower Cretaceous accumulations (avg d¹³C =-55‰; d²H= -188‰). High amplitude gas anomalies related to shallow gas pockets are common around the study area (Chand et al., 2009; Tasianas et al., 2018) for which a mixed thermogenic-microbial origin has been proposed. Therefore, the origin of gas causing the bright seismic spot in the study area remains unknown, but it is conceivable to hypothesize that the gas accumulated in the subsurface has a mixed microbial-thermogenic origin.
  This study aims to emphasize the need of region-specific investigations of SMTZ dynamics and associated SO₄ and CH₄ fluxes, for monitoring the effects of contemporary and future climate change on SMTZ depth and predict the expansion of methane seepage areas at continental margins. There is growing awareness in the scientific community regarding the potential increase in methane emissions from shallow continental shelves and coastal environments due to ocean warming and eutrophication (Wallenius et al., 2021). The figure at the end of the manuscript is not in scale (horizontal) and is a simplified scenario for shallow shelves. It is likely if we consider the occurrence of shallow SMTZs located at few tens of cm to few meters below the seafloor (Egger et al., 2018), and the fact the accelerating high-latitude warming trend may cause a shoaling of the SMTZ due to increasing rates of methanogenesis (Borges et al., 2019; Egger et al., 2016; Humborg et al., 2019).
  
  Best regards,
  Claudio Argentino
  On behalf of the authors
  
  References
  Borges, A. V., Royer, C., Martin, J. L., Champenois, W. and Gypens, N.: Response of marine methane dissolved concentrations and emissions in the Southern North Sea to the European 2018 heatwave, Cont. Shelf Res., 190, 104004, doi:10.1016/j.csr.2019.104004, 2019.
  Chand, S., Rise, L., Ottesen, D., Dolan, M. F. J., Bellec, V. and Bøe, R.: Pockmark-like depressions near the Goliat hydrocarbon field, Barents Sea: Morphology and genesis, Mar. Pet. Geol., 26(7), 1035–1042, doi:10.1016/j.marpetgeo.2008.09.002, 2009.
  Egger, M., Lenstra, W., Jong, D., Meysman, F. J. R., Sapart, C. J., Van Der Veen, C., Röckmann, T., Gonzalez, S. and Slomp, C. P.: Rapid sediment accumulation results in high methane effluxes from coastal sediments, PLoS One, 11(8), 1–22, doi:10.1371/journal.pone.0161609, 2016.
  Egger, M., Riedinger, N., Mogollón, J. M. and Jørgensen, B. B.: Global diffusive fluxes of methane in marine sediments, Nat. Geosci., 11(6), 421–425, doi:10.1038/s41561-018-0122-8, 2018.
  Humborg, C., Geibel, M. C., Sun, X., McCrackin, M., Mörth, C.-M., Stranne, C., Jakobsson, M., Gustafsson, B., Sokolov, A., Norkko, A. and Norkko, J.: High Emissions of Carbon Dioxide and Methane From the Coastal Baltic Sea at the End of a Summer Heat Wave, Front. Mar. Sci., 6, doi:10.3389/fmars.2019.00493, 2019.
  Rodrigues Duran, E., di Primio, R., Anka, Z., Stoddart, D. and Horsfield, B.: Petroleum system analysis of the Hammerfest Basin (southwestern Barents Sea): Comparison of basin modelling and geochemical data, Org. Geochem., 63, 105–121, doi:10.1016/j.orggeochem.2013.07.011, 2013.
  Tasianas, A., Bünz, S., Bellwald, B., Hammer, Ø., Planke, S., Lebedeva-Ivanova, N. and Krassakis, P.: High-resolution 3D seismic study of pockmarks and shallow fluid flow systems at the Snøhvit hydrocarbon field in the SW Barents Sea, Mar. Geol., 403(9037), 247–261, doi:10.1016/j.margeo.2018.06.012, 2018.
  Wallenius, A. J., Martins, P. D., Slomp, C. P. and Jetten, M. S. M.: Anthropogenic and Environmental Constraints on the Microbial Methane Cycle in Coastal Sediments, , 12(February), doi:10.3389/fmicb.2021.631621, 2021.
  
  Citation: https://doi.org/10.5194/bg-2021-58-AC3

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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