Right: Submarine methane cold seep, Eel River Basin, California USA. Source methane is thermogenic characterized by light δ¹³C values up to -27 ‰. Variable contributions by more depleted gas hydrates and a local methane pool with a biogenic signature down to -67 ‰. ¹³C values from ANME biomass is significantly ¹³C depleted (as low as -96 ‰). δ¹³C values from authigenic aragonite are significantly more depleted than that of normal marine carbonate indicating in situ mineralization of CO₂ produced via AOM. Data from (Orphan et al. 2004). Center: Wolf Spring, Axel Heiberg Island, Nunavut, Canada. Source methane has an isotopic signature indicative of a predominately thermogenic source (δ¹³CH₄ -38 ‰). Data from (Niederberger et al. 2010). δ¹³C measurements from biomass collected at depth is currently planned. Representative δ¹³CH₄ for background atmosphere taken from ~ 7 km altitude during a stratospheric balloon flight launched from Kiruna, Sweden sampling Arctic air mass during the Arctic summer (Röckmann et al. 2011). Left: Methane evolved from mudstones in Gale Crater, Mars. There are two main methane sources, background seepage and periodic plumes, contributing to the methane pool in the near surface atmosphere with unknown δ¹³C values. Recently, ¹³CH₄ was measured from a mudstone collected in Gale crater with an extremely wide range of values (House et al. 2022). These highly ¹³C-depleted values are reminiscent of the authigenic carbonates produced via mineralization of biogenically produced CO₂ during AOM in submarine methane seeps. While the carbon isotopic reservoirs on Mars are not well constrained, on Earth highly ¹³C depleted value are consistent with methane derived through microbial methanogenesis. All δ¹³C values compared to V-PDB.