Triple-isotopic analyses pinpoint microbial methane release from subsea permafrost in the inner Laptev Sea

Abstract Elevated methane concentrations are observed in the shallow water column above subsea permafrost on the East Siberian Arctic Shelf, including the inner Laptev Sea. The subsea source of this methane is poorly understood, yet crucial for predicting future methane emissions. Here, we combine analyses of dissolved methane concentrations, triple-isotopic fingerprinting, and Bayesian Markov Chain Monte Carlo statistics to constrain the source of high-concentration methane sampled during four expeditions, 2016-2020. In contrast to earlier findings of predominantly thermogenic methane release in the outer Laptev Sea, our results for the major methane release region of the inner Laptev Sea suggest that old microbial methane (radiocarbon age >48000 y BP) is being released from preformed methane pools stored within the subsea permafrost system. This observation reveals that several sources contribute to the elevated methane on the shelf and thus the necessity to consider a diversity of sources when estimating future methane release trajectories.