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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-264
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-264
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 08 Jul 2019

Submitted as: research article | 08 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Assessing the potential for non-turbulent methane escape from the East Siberian Arctic Shelf

Matteo Puglini1,2, Victor Brovkin1, Pierre Regnier2, and Sandra Arndt2 Matteo Puglini et al.
  • 1Land in the Earth System, Max Planck Institute for Meteorology, Hamburg, Germany
  • 2BGeosys, Department Geoscience, Environment & Society (DGES), Université Libre de Bruxelles, Brussels, Belgium

Abstract. East Siberian Arctic Shelf (ESAS) hosts large, yet poorly quantified reservoirs of subsea permafrost and associated gas hydrates. It has been suggested the global-warming induced thawing and dissociation of these reservoirs is currently releasing methane to the shallow shelf ocean and ultimately the atmosphere. However, the exact contribution of permafrost thaw and methane gas hydrate destabilization to benthic methane efflux from the warming shelf and ultimately methane-climate feedbacks remains controversial. A major unknown is the fate of permafrost and/or gas hydrate-derived methane as it migrates towards the sediment-water interface. In marine sediments, (an)aerobic oxidation reactions generally act as extremely efficient biofilters that often consume close to 100 % of the upward migrating methane. However, it has been shown that a number of environmental conditions can reduce the efficiency of this biofilter, thus allowing methane to escape to the overlying ocean. Here, we used a reaction-transport model to assess the efficiency of the benthic methane filter and, thus, the potential for permafrost and/or gas hydrate derived methane to escape shelf sediments under a wide range of environmental conditions encountered on East Siberian Arctic Shelf. Results of an extensive sensitivity analysis show that, under steady state conditions, anaerobic oxidation of methane (AOM) acts as an efficient biofilter that prevents the escape of dissolved methane from shelf sediments for a wide range of environmental conditions. Yet, high CH4 escape comparable to fluxes reported from mud-volcanoes is simulated for rapidly accumulating (sedimentation rate > 0.7 cm yr−1) and/or active (active fluid flow > 6 cm yr−1) sediments and can be further enhanced by mid-range organic matter reactivity and/or intense local transport processes, such as bioirrigation. In active settings, high non-turbulent methane escape of up to 19 μmolCH4 cm−2 yr−1 can also occur during a transient, multi-decadal period following the sudden onset of CH4 flux triggered by, for instance, permafrost thaw or hydrate destabilization. This window of opportunity arises due to the time needed by the microbial community to build up an efficient AOM biofilter. In contrast, seasonal variations in environmental conditions (e.g. bottom water SO42−, CH4 flux) exert a negligible effect on CH4 efflux through the sediment-water interface. Our results indicate that present and future methane efflux from ESAS sediments is mainly supported by methane gas and non-turbulent CH4 efflux from rapidly accumulating and/or active sediments (e.g. coastal settings, portions close to river mouths or submarine slumps). In particular active sites on the ESAS may release methane in response to the onset or increase of permafrost thawing or CH4 gas hydrate destabilization rates. Model results also reveal that AOM generally acts as an efficient biofilter for upward migrating CH4 under environmental conditions that are representative for the present-day ESAS with potentially important, yet unquantified implications for the Arctic ocean's alkalinity budget and, thus, CO2 fluxes. The results of the model sensitivity study are used as a quantitative framework to derive first-order estimates of non-turbulent, benthic methane efflux from the Laptev Sea. We find that, under present day conditions, AOM is an efficient biofilter and non-turbulent methane efflux from Laptev Sea sediments does not exceed 1 GgCH4 yr−1. As a consequence, we state that previously published estimates of fluxes from ESAS water into atmosphere cannot be supported by non-turbulent methane escape from the sediments, but require the build-up and preferential escape of benthic methane gas from the sediments to the atmosphere that matches or even exceeds such estimated fluxes.

Matteo Puglini et al.
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Status: final response (author comments only)
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Matteo Puglini et al.
Matteo Puglini et al.
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Short summary
A reaction-transport model to assess the potential non-turbulent methane flux from the East Siberian Arctic sediments to water column is here applied. We identify that anaerobic oxidation of methane (AOM) is an efficient filter except for high values of sedimentation rate and advective flow, which enable considerable non-turbulent steady-state methane fluxes. Significant transient methane fluxes can also occur during the building up phase of AOM-performing biomass microbial community.
A reaction-transport model to assess the potential non-turbulent methane flux from the East...
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