Carbon isotope biosignatures use delta-13C ratios in CH4 vs CO2 as a life-detection marker. On Earth, methanogenic archaea produce strongly 12C-enriched methane, creating a measurable isotopic offset from abiotic CO2.
Higgins et al. (2026) show that abiotic isotope fractionation on enceladus introduces enough uncertainty to prevent definitive biosphere inference from hypothetical delta-13C measurements. Their modular process-based model tracks isotope enrichment factors across influx/outflux modules, demonstrating that the abiotic-baseline must be constrained before carbon isotopes can serve as reliable biosignatures on ocean-worlds. See src-ocean-worlds-biosignature-framework-2026-05.
This complements the surface-flux-inference approach (Wogan et al.) which uses photochemical-climate model inversion to estimate surface CH4 fluxes. See src-biosignature-gas-flux-inference-2026-04.