Isolation of\nMethane from Ambient Water and Preparation\nfor Source-Diagnostic Natural Abundance Radiocarbon Analysis

A key challenge in climate change research is apportioning\nthe\ngreenhouse gas methane (CH<sub>4</sub>) between various natural and\nanthropogenic sources. Isotopic source fingerprinting of CH<sub>4</sub> releases, particularly with radiocarbon analysis, is a promising\napproach. Here, we establish an analytical protocol for preparing\nCH<sub>4</sub> from seawater and other aqueous matrices for high-precision\nnatural abundance radiocarbon measurement. Methane is stripped from\nwater in the optionally field-operated system (STRIPS), followed by\nshore-based purification and conversion to carbon dioxide (CO<sub>2</sub>) in the CH<sub>4</sub> Isotope Preparation System (CHIPS)\nto allow Accelerator Mass Spectrometry analysis. The blank (±1σ)\nof the combined STRIPS and CHIPS is low (0.67 ± 0.12 μg\nC), allowing natural sample sizes down to 10 μg C–CH<sub>4</sub> (i.e., 30 L samples of 40 nM CH<sub>4</sub>). The full-system\nyield is >90% for both CH<sub>4</sub>-spiked seawater and ambient\nsamples from CH<sub>4</sub> hotspots in the Baltic Sea and the Arctic\nOcean. Furthermore, the radiocarbon isotope signal of CH<sub>4</sub> remains constant through the multistage processing in the STRIPS\nand the CHIPS. The developed method thus allows for in-field sampling\nand sample size reduction followed by precise and CH<sub>4</sub>-specific\nradiocarbon analysis. This enables powerful source apportionment of\nCH<sub>4</sub> emitted from aquatic systems from the tropics to the\npolar regions.