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

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.

Methane release from carbonate rock formations in the Siberian permafrost area during and after the 2020 heat wave

“…For the subsea permafrost in the East Siberian Arctic Shelf, it was argued that thawing can make the permafrost layer permeable for gas stored as hydrates or as free gas within the permafrost layer and also for subpermafrost gas (5). Isotopic signatures of methane released in the East Siberian Arctic Shelf are consistent with an origin as old, deep, and likely thermogenic methane (6).…”

Integrated airborne investigation of the air composition over the Russian sector of the Arctic

“…Extra methane may come from the decomposition of gas hydrates on the ocean floor. This phenomenon was observed, in particular, during measurements from sea vessels (Sapart et al, 2017;Shakhova et al, 2010;Steinbach et al, 2021). The scale of the threat from the decomposition of gas hy-B.…”

Conflicting estimates of natural geologic methane emissions

“…Utilizing Russian studies (e.g., Shakova et al, 2014) to achieve better spatial coverage of shallow portions of the ESAS not studied in Thornton et al (2016; leads to an ESAS total CH 4 emission estimate of approximately 4.65 Tg yr -1 , a value also reported in Thornton et al (2020). Shakhova et al (2014) reported 17 Tg yr -1 as their best estimate (no error bars) for the ESAS and 9 (2.87-18.46) Tg yr -1 from bubbling hot spots alone. Although obviously far larger than other studies, these have not been directly refuted in the literature.…”

“…A major example of this contrast is found with the recent estimates of CH 4 emissions from the ESAS (the Laptev, East Siberian, and part of the Chukchi seas; Figure 2b). ESAS emissions of approximately 3 Tg yr -1 CH 4 (Thornton et al, 2020) include 14 C-depleted Pleistocene microbial and older thermogenic gas (Cramer and Franke, 2005;Berchet et al, 2020;Steinbach et al, 2021). The gas seeps within the Laptev Sea have been suggested to stem from an active, subsurface petroleum system based on measurements of CH 4 , ethane, and propane adsorbed on the surface sediments in the seep regions, along with both geochemical (isotopic compatibility with source rock maturity) and geophysical (seismic reflection) data (Cramer and Franke, 2005).…”