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Source apportionment of methane escaping the subsea permafrost system in the outer Eurasian Arctic Shelf
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2021 (English)In: Proceedings of the National Academy of Sciences, Vol. 118, no 10Article in journal (Refereed) Published
Abstract [en]

Extensive release of methane from sediments of the world’s largest continental shelf, the East Siberian Arctic Ocean (ESAO), is one of the few Earth system processes that can cause a net transfer of carbon from land/ocean to the atmosphere and thus amplify global warming on the timescale of this century. An important gap in our current knowledge concerns the contributions of different subsea pools to the observed methane releases. This knowledge is a prerequisite to robust predictions on how these releases will develop in the future. Triple-isotope–based fingerprinting of the origin of the highly elevated ESAO methane levels points to a limited contribution from shallow microbial sources and instead a dominating contribution from a deep thermogenic pool.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.

Place, publisher, year, edition, pages
2021. Vol. 118, no 10
National Category
Earth and Related Environmental Sciences
Research subject
SWEDARCTIC 2014, SWERUS-C3
Identifiers
URN: urn:nbn:se:polar:diva-8673DOI: 10.1073/pnas.2019672118OAI: oai:DiVA.org:polar-8673DiVA, id: diva2:1548243
Note

All data used in this study are publicly available at the Stockholm University Bolin Centre for Climate Research Database (https://doi.org/10.17043/swerus-2014-methane-isotopes).

Available from: 2021-04-29 Created: 2021-04-29 Last updated: 2021-04-29Bibliographically approved

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