Triple-isotopic source fingerprinting of dissolved and bubble-held methane across the East Siberian Arctic Shelf Seas

The shallow East Siberian Arctic Shelf (ESAS) is the World’s largest shelf sea system and overlies a complex sedimentary drape that includes thawing subsea permafrost, methane hydrates, and gas and oil reservoirs. Uncertain estimates suggest that the ESAS releases as much methane to the atmosphere as the rest of the World Ocean; yet the relative contributions from different sources are poorly constrained—a prerequisite for anticipating future release trajectories. Here, multi-year source-diagnostic triple-isotopic compositions (δ¹³C, δ²H, and Δ¹⁴C) of seawater-dissolved and ebullitive methane show that methane contributions vary greatly across the ESAS, with the subsea permafrost-associated biogenic methane pools only standing for one-tenth (Outer Laptev Sea), three-tenths (East Siberian Sea), and six-tenths (Inner Laptev Sea) of the total methane releases. For the East Siberian Sea and the Outer Laptev Sea, distinct fossil gas seeps of different origins were identified. Multi-year constancy in each regime’s isotopic fingerprints of ebullitive and dissolved methane and concentration patterns suggests that bubble dissolution is the primary source of elevated methane levels below and above the pycnocline. Furthermore, the high methane concentrations in bubbles reaching the sea surface (80±22%) indicate direct release of methane from the seabed into the atmosphere via ebullition, thereby going past potential microbial degradation. While it is complicated to include both ebullition and the diversity of methane sources in methane budgets, it appears critical for predicting methane release trajectories in the ESAS region and, consequently, their contribution to the increasing atmospheric methane pool.