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

Julia Steinbach^a,b,c, Henry Holmstrand^a,c, Kseniia Shcherbakova^d, Denis Kosmach^d, Volker Brüchert^b,c, Natalia Shakhova^e,f,g, Anatoly Salyuk^d, Célia J. Sapart^h,i, Denis Chernykh^d, Riko Noormets^j, Igor Semiletov^d,e,f, and Örjan Gustafsson^a,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.