Isolation of Methane from Ambient Water and Preparation for Source-Diagnostic Natural Abundance Radiocarbon Analysis

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.