| Abstract | Distinguishing between microbial, abiotic, and thermogenic methane sources is fundamental for understanding Earth’s carbon cycle and for assessing potential biosignatures beyond our planet. Stable carbon isotopic analyses have long been central to geochemical investigations, and extending such approaches to volatile reservoirs on other Solar System bodies remains a major goal of planetary science. However, whether the carbon isotopic composition of methane can be confidently used as a biosignature remains debated. Over the years, numerous diagnostic diagrams have been developed to infer methane origins based on stable isotope measurements and hydrocarbon compositions, yet unambiguous source identification remains challenging. A key limitation is the strong dependence of existing proxies on the isotopic composition of the initial carbon pool, potentially an unknown parameter in many planetary environments. Here, we compiled a dataset comprising 351 microbial natural samples, 94 microbial laboratory experiments, 615 thermogenic natural samples, 6 thermogenic laboratory experiments, 125 abiotic natural samples, and 52 abiotic laboratory experiments, spanning early and recent studies. We apply the same isotopic and compositional proxies used in conventional approaches but also integrate them into combined parameters, such as the difference in carbon isotopic composition between methane and ethane, which normalize isotope signatures and reduce sensitivity to initial carbon isotopic composition. In the resulting diagnostic diagrams, a Bayes classifier provides a statistical framework that quantifies source probabilities and enables robust discrimination between microbial and abiotic methane. The results suggest hydrocarbon geochemistry can be a transferable and rigorous tool for interpreting methane origins across terrestrial and extraterrestrial environments. |
