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

Julia Steinbach,Anatoly Salyuk,Örjan Gustafsson,Igor Semiletov,Natalia Shakhova,Riko Noormets,Volker Brüchert,Denis Chernykh,Henry Holmstrand,Denis Kosmach,Célia J Sapart,Kseniia Shcherbakova

doi:10.1073/pnas.2019672118

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.

Highlights

The East Siberian Arctic Shelf holds large amounts of inundated carbon and methane (CH4)
Conceptual development and modeling have predicted that warming of the East Siberian Arctic Shelf (ESAS) system by a combination of geothermal heat and climate-driven Holocene heat flux from overlying seawater, recently further enhanced by Anthropocene warming, may lead to thawing of subsea permafrost [6, 7]
In addition to mobilization of the carbon/ methane stored within the subsea permafrost, its degradation can lead to the formation of pathways for gaseous methane from underlying reservoirs, allowing further methane release to the overlying water column [3, 9]

Summary

Introduction

The East Siberian Arctic Shelf holds large amounts of inundated carbon and methane (CH4). Methane concentrations and isotopic signatures in ESAS seawater are generally influenced by a mixture of sources and degradation processes (SI Appendix, text S1 and Fig. S1).

Results

Conclusion