Geographic variability in freshwater methane hydrogen isotope ratios and its implications for global isotopic source signatures

Abstract. There is growing interest in developing spatially resolved methane (CH4) isotopic source signatures to aid geographic attribution of CH4 emissions. hydrogen isotope measurements (?2H–CH4) have the potential be a powerful tool for differentiation emissions from freshwater environments, as well other microbial sources. This because ?2H–CH4 values are partially dependent on ?2H environmental water (?2H–H2O), which exhibits large and well-characterized spatial variability globally. We refined existing global relationship between ?2H–H2O by compiling more extensive dataset including wetlands, inland waters, rice paddies, comprising total 129 different sites, compared these with estimates ?2H–H2O, ?13C-CH4 ?13C–CO2 measurements. found that explain approximately 42 % observed variation ?2H–CH4, flatter slope than previous studies. The inferred vs. regression not sensitive using either modelled precipitation or measured predictor variable. similar observations incubation experiments but pure culture experiments. result consistent suggestions acetate, controlled important determining ?2H–CH4. leads significant differences distribution northern high latitudes (60–90? N), relative regions. estimate flux-weighted ?310 ± 15 ‰, higher most estimates. Comparison ?13C both CO2 implies residual complex interactions oxidation, dominant pathway methanogenesis, potentially biogeochemical variables. observe significantly greater values, corrected waters suggest this difference caused prevalent oxidation waters. used expanded calculate bottom-up includes Our (?278 ‰) approach, enriched signature derived our dataset. However, it agreement top-down based atmospheric estimated sink fractionations. In contrast lower estimates, partly lack data C4-plant-dominated ecosystems. general, we find there particular need constrain low-latitude