The origin of methane in the East Siberian Arctic Shelf unraveled with triple isotope analysis

Célia J Sapart,Carina Van Der Veen,Sönke Szidat,Matthias Egger,Valentine Sergienko,Anatoly Salyuk,Jean-Louis Tison,Thomas Röckmann,Denis Kosmach,Joachim Jansen,Oleg Dudarev,Natalia Shakhova,Vladimir Tumskoy,Igor Semiletov

doi:10.5194/bg-14-2283-2017

Abstract

Abstract. The Arctic Ocean, especially the East Siberian Arctic Shelf (ESAS), has been proposed as a significant source of methane that might play an increasingly important role in the future. However, the underlying processes of formation, removal and transport associated with such emissions are to date strongly debated. CH4 concentration and triple isotope composition were analyzed on gas extracted from sediment and water sampled at numerous locations on the shallow ESAS from 2007 to 2013. We find high concentrations (up to 500 µM) of CH4 in the pore water of the partially thawed subsea permafrost of this region. For all sediment cores, both hydrogen and carbon isotope data reveal the predominant occurrence of CH4 that is not of thermogenic origin as it has long been thought, but resultant from microbial CH4 formation. At some locations, meltwater from buried meteoric ice and/or old organic matter preserved in the subsea permafrost were used as substrates. Radiocarbon data demonstrate that the CH4 present in the ESAS sediment is of Pleistocene age or older, but a small contribution of highly 14C-enriched CH4, from unknown origin, prohibits precise age determination for one sediment core and in the water column. Our sediment data suggest that at locations where bubble plumes have been observed, CH4 can escape anaerobic oxidation in the surface sediment.

Highlights

The Arctic subsea permafrost harbors a very large active carbon pool of similar size to the terrestrial Siberian permafrost reservoir (Shakhova et al, 2010a)
CH4 concentration and triple isotope composition were analyzed on gas extracted from sediment and water sampled at numerous locations on the shallow East Siberian Arctic Shelf (ESAS) from 2007 to 2013
Radiocarbon data demonstrate that the CH4 present in the ESAS sediment is of Pleistocene age or older, but a small contribution of highly 14C-enriched CH4, from unknown origin, prohibits precise age determination for one sediment core and in the water column

Summary

Introduction

The Arctic subsea permafrost harbors a very large active carbon pool of similar size to the terrestrial Siberian permafrost reservoir (Shakhova et al, 2010a). The formerly terrestrial permafrost has been continuously exposed to increasing seawater temperature, salt and anoxic conditions (Dmitrenko et al, 2011; Nicolsky et al, 2012), allowing the remobilization of carbon from the Pleistocene reservoirs. The four suggested mechanisms controlling the release of Pleistocene carbon to the ESAS are the deepening of the permafrost level, gas hydrate degradation, coastal erosion and riverine discharge Holoceneage carbon originating mainly from coastal erosion and riverine discharge (Charkin et al, 2011; Semiletov et al, 2012; Karlsson et al, 2011, 2016) has accumulated on the ESAS and overlays the Pleistocene age sediment (Vonk et al, 2012, 2014; Feng et al, 2013)

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Conclusion