Stable carbon isotope data of enhanced dissolved methane in the East Siberian Arctic Shelf region

During the past two decades, extensive ebullition and elevated methane concentrations in both seawater and atmosphere have been observed in East Siberian Arctic Shelf (ESAS) region. The relative contribution of the potential sources to these enhanced methane concentrations and the related release processes are yet poorly understood. The sources of the observed methane could be recently microbially produced methane from organic material in Holocene sediments or thawing subsea permafrost, or preformed methane released from subsea permafrost, destabilizing methane hydrates or thermogenic gas reservoirs. We here use the ¹³C content of methane toward separating the contribution of thermogenic and microbial sources to methane-enriched bottom waters collected during four expeditions across the East Siberian Arctic Shelf (n = 181).

Our data suggest variability in methane sources between methane hotspots in three different regions of the ESAS, which are separated by large spatial scales (500-900 km). For the outer Laptev Sea, the average, 10^th and 90^th percentile δ¹³C values of near-bottom water methane were -44‰, -54‰ and -35‰, which suggests a dominant thermogenic source (expeditions in 2014, 2016, 2018 and 2020). For the inner Laptev Sea, the average, 10^th and 90^th percentile δ¹³C values of near-bottom water methane were -69‰, -77‰ and -58‰, which suggests a dominant microbial source (expeditions 2016, 2018, 2020). For the East Siberian Sea, samples of the years 2014, 2016 and 2020 have been analysed and the pattern is less consistent in time, where bottom water samples from 2014 are more enriched in ¹³C (δ¹³C average of -41‰) compared to the later years 2016 and 2020 (δ¹³C averages of -65‰ and -57‰). The differences between the three regions suggest that the dominant sources of the methane releases are different in these regions and likely reflect differences both in subsea compartments and processes forcing the current releases.