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.
