Atmospheric methane is rising rapidly due to a combination of anthropogenic and natural methane emissions. Uncertainties in the contributions of especially natural sources are high, while these emissions may alter in the near future due to climate change-induced feedback mechanisms. Permafrost regions are considered a natural source of methane, and thawing permafrost and associated greenhouse gas emissions represent a potential biogeochemical tipping point for climate change.

About one-seventh of the world’s permafrost area is situated below sea level, predominantly on the East Siberian Arctic Shelf (ESAS). The ESAS sea region is the world’s largest shelf sea system and was formed by sea level rise during the last deglaciation. This shelf seabed hosts large yet uncertain amounts of organic matter and methane in different deposits. While elevated methane concentrations have been observed in this region for more than two decades, the methane source and magnitude of methane emissions have been poorly constrained. Uncertainty in the methane sources limits the ability to estimate the magnitude of methane releases and expand these into future projections.

This PhD thesis focuses mainly on the knowledge gap on methane sources. The sedimentary drape contains fossil gas reservoirs, methane hydrates, preformed methane trapped by permafrost, and organic matter in frozen, thawed, and recently accumulated sediments that can be degraded to methane. The dual stable isotopic composition of methane (δ¹³C_CH4 and δ²H_CH4) is indicative of its formation pathway and potentially of partial degradation. Methane’s radiocarbon content constrains the age of the methane precursor. Therefore, triple-isotopic analyses of both seawater-dissolved and ebullitive methane were used to quantify the relative contributions of different methane sources to the observed elevated methane levels in both phases. To this end, a new preparation method for radiocarbon analysis of aqueous methane was developed. A second part of the thesis focuses on the fate of methane in both bubbles and seawater.

It was found that multiple methane sources contribute to the elevated methane concentrations across the ESAS, while at all hotspots, methane was dominantly old (¹⁴C_age > 48000 y before present). Microbial methane from subsea permafrost environments was the major methane source at methane hotspots in the inner Laptev Sea. In the East Siberian Sea and the outer Laptev Sea, fossil gas seeps of different origins were identified. The isotopic fingerprints of both dissolved and ebullitive methane in surface and bottom waters were similar and persistent over multiple years. In combination with concentration patterns, it was inferred that ebullition is an important source of methane to the water column. In the outer Laptev Sea, methane is oxidized in sub-pycnocline waters to measurable extents. In the inner Laptev Sea, no direct indication of strong methane oxidation was found. The high methane concentrations measured for surface bubbles (80±22%) show that methane is also directly transported from the seabed to the atmosphere. This ebullitive flux bypasses microbial degradation in both sediments and seawater. The implication of these results is that both ebullition and the multitude of methane sources across the ESAS need to be incorporated into future modeling efforts and included in methane release estimates for the ESAS region.