C43A-0783
Triple-Isotope-Based Source Apportionment of Methane in Waters of the Outer Laptev Sea

Thursday, 17 December 2015
Poster Hall (Moscone South)
Julia Steinbach1, Henry Holmstrand1, Kseniia Shcherbakova2, Denis Kosmach2, Célia Julia Sapart3, Elena Panova4, Volker Bruchert5, Igor Peter Semiletov6, Natalia E Shakhova6 and Orjan Gustafsson1, (1)Stockholm University, Stockholm, Sweden, (2)Pacific Oceanological Institute FEB RAS, Vladivostok, Russia, (3)Université Libre de Bruxelles, Brussel, Belgium, (4)Tomsk Polytechnic University, Tomsk, Russia, (5)Stockholm University, Geological Sciences and Bolin Centre for Climate Research, Stockholm, Sweden, (6)University of Alaska Fairbanks, Fairbanks, AK, United States
Abstract:
The East Siberian Arctic Shelf (ESAS) hosts large methane (CH4) deposits in the form of gas hydrates, gas pockets within subsea permafrost and in deeper reservoirs. For over a decade, annual expeditions have documented methane releases into the water column in large parts of this region. However, there is still insufficient knowledge of which pools are releasing the methane – one important piece of information required for a deeper understanding of the system and towards predicting future releases. A powerful tool for quantifying the relative contribution of releases from different sources is multi-dimensional isotope analysis of CH4 in the water column. Using the full triple isotope characterization (δ13C-CH4, δD-CH4, Δ14C-CH4) allows deconvolution of CH4 sources between thermogenic and biogenic origins and of different reservoir ages.

During the SWERUS-C3 expedition in summer 2014 we investigated the distribution of dissolved methane and its isotope signatures in the water column along the outer ESAS and its adjacent slope and ridges. Over 1500 samples for CH4 concentration from high-resolution vertical profiles (up to 12 depths) were analyzed onboard; 900 samples were taken for stable isotopes analysis; and a total of 80 larger water samples were taken at selected locations for Δ14C-CH4 analysis. CH4 was extracted from these samples onboard using custom-built CH4-stripping systems and stored in absorbent traps for further processing and radiocarbon analysis onshore.

One focus was to probe CH4 sources in seep areas, identified by a combination of geophysical features in the sediment, observation of bubbles in the water column and measured CH4 profiles. Here we present triple isotope data from inside and outside seep areas of the outer Laptev Sea and source-apportionment for this region. Dissolved methane concentration there reached up to 1400nM. Stable isotopes results in near bottom waters are rather enriched: Initial results show -142 to -133‰ vs SMOW for δD-CH4 and -63 to -33 ‰ vs VPDB for δ13C-CH4, plus δ13C-CH4 values of up to + 22‰ at two stations.

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