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.

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.

A key challenge in climate change research is apportioning the greenhouse gas methane (CH4) between various natural and anthropogenic sources. Isotopic source fingerprinting of CH4 releases, particularly with radiocarbon analysis, is a promising approach. Here, we establish an analytical protocol for preparing CH4 from seawater and other aqueous matrices for high-precision natural abundance radiocarbon measurement. Methane is stripped from water in the optionally field-operated system (STRIPS), followed by shore-based purification and conversion to carbon dioxide (CO2) in the CH4 Isotope Preparation System (CHIPS) to allow Accelerator Mass Spectrometry analysis. The blank (±1σ) of the combined STRIPS and CHIPS is low (0.67 ± 0.12 μg C), allowing natural sample sizes down to 10 μg C-CH4 (i.e., 30 L samples of 40 nM CH4). The full-system yield is >90% for both CH4-spiked seawater and ambient samples from CH4 hotspots in the Baltic Sea and the Arctic Ocean. Furthermore, the radiocarbon isotope signal of CH4 remains constant through the multistage processing in the STRIPS and the CHIPS. The developed method thus allows for in-field sampling and sample size reduction followed by precise and CH4-specific radiocarbon analysis. This enables powerful source apportionment of CH4 emitted from aquatic systems from the tropics to the polar regions.