C33F-02
The Origin(s) of the Methane Excess in the Shallow East Siberian Arctic Shelf Unraveled with Triple Isotope Analysis

Wednesday, 16 December 2015: 14:10
3007 (Moscone West)
Célia Julia Sapart1,2, Natalia E Shakhova3, Igor Peter Semiletov3, Joachim Jansen4, Denis Kosmach5, Oleg Dudarev5, Sönke Szidat6, Carina van der Veen2, Valentin Sergienko7, Anatoly Salyuk5, Vladimir Tumskoy8, Jean-Louis Tison9, Matthias Egger10 and Thomas Röckmann11, (1)Université Libre de Bruxelles, Brussel, Belgium, (2)Utrecht University, Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht, Netherlands, (3)University of Alaska Fairbanks, Fairbanks, AK, United States, (4)Stockholm University, Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm, Sweden, (5)Pacific Oceanological Institute FEB RAS, Vladivostok, Russia, (6)University of Bern, 7Department of Chemistry and Biochemistry & Oeschger Centre for Climate Change Research, Bern, Switzerland, (7)Russian Academy of Sciences, Institute of Chemistry, Vladivostok, Russia, (8)Lomonosov Moscow State University, Moscow, Russia, (9)Université Libre de Bruxelles, Bruxelles, Belgium, (10)Utrecht University, Department of Earth Sciences - Geochemistry, Utrecht, Netherlands, (11)Utrecht University, Utrecht, Netherlands
Abstract:
Methane (CH4) is a strong greenhouse gas emitted by both human activity and natural processes. Its emissions from vast marine and terrestrial carbon pools in Arctic shallow reservoirs could create a strong positive feedback to climate change as permafrost is thawing. The East Siberian Arctic Shelf (ESAS) overlays large areas of subsea permafrost that is degrading and causing the release of large amount of CH4 originally stored or formed there towards the water column. Large scale CH4 super-saturation has been observed in the ESAS waters, pointing out to leakages of CH4 through the sea floor and possibly to the atmosphere but the origin(s) of this gas is still debated.

Here we present CH4 concentration and isotope data analyzed on gas extracted from sediment and water sampled over the shallow ESAS from 2007-2013. We find the presence of large amount (up to 500μM) of CH4 within the partially thawed subsea permafrost of this region. For all sediment cores, both hydrogen and carbon CH4 isotope data show the presence of highly depleted (in comparison with the atmospheric CH4 isotope signature) CH4, indicative of a microbial origin. Radiocarbon data demonstrates that in most cases the CH4 present in the ESAS sediment is of Pleistocene age or older. These results reveal the presence of CH4 that is not from thermogenic/natural gas (isotopically enriched CH4) origin as it has long been thought, but the results of microbial CH4 formation using as primary substrate old organic matter in the deep subsea permafrost or below.

Our observations show that this CH4 accumulates under the Holocene marine sediment layer (when present) and slowly diffuses towards the sediment surface where it undergoes anaerobic methane oxidation. However, at locations where deep and/or open taliks develop in subsea permafrost, CH4 escapes to the water column as bubble plumes. In that case ebullition processes prevent CH4 anaerobic oxidation to occur in the surface sediment. CH4 will thus migrate rapidly through the water column to partially escape to the atmosphere (ice-free period) or become trapped in or under the sea ice and be released to the atmosphere during the melt season generating a positive radiative feedback.

<< Previous Abstract | Next Abstract >>