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Article in Proceedings of the National Academy of Sciences – PNAS

Source apportionment of methane escaping the subsea permafrost system in the outer Eurasian Arctic Shelf

Julia Steinbacha,b,c, Henry Holmstranda,c, Kseniia Shcherbakovad, Denis Kosmachd, Volker Brüchertb,c, Natalia Shakhovae,f,g, Anatoly Salyukd, Célia J. Saparth,i, Denis Chernykhd, Riko Noormetsj, Igor Semiletovd,e,f, and Örjan Gustafssona,c

aDepartment of Environmental Science, Stockholm University, 106 91 Stockholm, Sweden
bDepartment of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden
cBolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden
dV.I. Il’ichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, 690041, Russia
eInstitute of Ecology, Higher School of Economics, Moscow, 101000, Russia
fInternational Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK 99775
gScientific Centre Moscow State University-Geophysics, Moscow, 119991, Russia
hLaboratoire de Glaciologie, Université Libre de Bruxelles, 1050 Brussels, Belgium
iInstitute for Marine and Atmospheric Research, Utrecht University, 3584 CC Utrecht, The Netherlands
jDepartment of Arctic Geology, University Centre Svalbard, Longyearbyen, N-9171, Norway

https://www.pnas.org/content/118/10/e2019672118

Abstract
The East Siberian Arctic Shelf holds large amounts of inundated carbon and methane (CH4). Holocene warming by overlying seawater, recently fortified by anthropogenic warming, has caused thawing of the underlying subsea permafrost. Despite extensive observations of elevated seawater CH4 in the past decades, relative contributions from different subsea compartments such as early diagenesis, subsea permafrost, methane hydrates, and underlying thermogenic/ free gas to these methane releases remain elusive. Dissolved methane concentrations observed in the Laptev Sea ranged from 3 to 1,500 nM (median 151 nM; oversaturation by ∼3,800%). Methane stable isotopic composition showed strong vertical and horizontal gradients with source signatures for two seepage areas of δ13C-CH4 = (−42.6 ± 0.5)/(−55.0 ± 0.5) ‰ and δD-CH4 = (−136.8 ± 8.0)/(−158.1 ± 5.5) ‰, suggesting a thermogenic/natural gas source. Increasingly enriched δ13C-CH4 and δD-CH4 at distance from the seeps indicated methane oxidation. The Δ14C-CH4 signal was strongly depleted (i.e., old) near the seeps (−993 ± 19/−1050 ± 89‰). Hence, all three isotope systems are consistent with methane release from an old, deep, and likely thermogenic pool to the outer Laptev Sea. This knowledge of what subsea sources are contributing to the observed methane release is a prerequisite to predictions on how these emissions will increase over coming decades and centuries.