Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes

Abstract

The release of long‐stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CH4) clumped isotope (Δ18) and 14C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ18 values are negatively correlated with CH4 production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH4 Δ18 and the fraction modern carbon. These correlations imply that CH4 derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ18 values, are not positively correlated with CH4 flux estimates, highlighting the likely importance of environmental variables other than CH4 production rates in controlling ebullition fluxes.

Clumped isotopologue fractionation by microbial cultures performing the anaerobic oxidation of methane

“…The kinetic isotopologue signals from surface methane are consistent with those from laboratory culture experiments, in which methanogens consistently produced methane with non-equilibrium isotopologue compositions (Wang et al, 2015;Douglas et al, 2017Douglas et al, 2020Gruen et al, 2018;Stolper et al, 2015). Previous studies have also investigated the clumped methane isotopologue fractionation during sink reactions, including aerobic methane oxidation (Wang et al, 2016) and gas phase methane oxidation by OH and Cl radicals (Whitehill et al, 2017;Joelsson et al, 2014Joelsson et al, , 2016.…”

“…The ln(γAOM) values from this study (-4.8 to -12.8 ‰, Table 2) and experimentally derived ln(γmtg) values of -3.8 to +2.3 ‰ for hydrogenotrophic methanogenesis (Gruen et al, 2018;Douglas et al, 2020) yield the range of the Δ 13 CH3D value from +1.0 to +15.1 ‰. Most environmental data show Δ 13 CH3D values smaller than ca.…”

“…Studies thus far have shown that the clumped isotopologue composition of methane from subsurface environments (e.g., marine sediment pore water, natural gas deposits) often yields reasonable formation temperatures, whereas methane sampled from surface environments (e.g., ruminants, lakes, and swamps) is characterized by clear kinetic signals that yield apparent clumped isotopologue temperatures much higher than the environmental temperatures (Ash et al, 2019, Douglas et al, 2017, 2020Giunta et al, 2019;Stolper et al, 2014bStolper et al, , 2015Wang et al, 2015;Young et al, 2017).…”

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

“…Based on this, we estimate a typical error for δ 2 H-CH 4 data of less than 3 ‰. Studies where data were derived from graphs are identified in Supplement Table S1 (Douglas et al, 2020b).…”

Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation

“…Although microbial methane is often in clumped isotope disequilibrium Wang et al, 2015;Douglas et al, 2016;Young et al, 2017), microbial methane found in marine and/or subsurface environments is usually in or close to equilibrium. It has been proposed that slow anaerobic coupled methanogenesis and methanotrophy in nature under low thermodynamic drive (e.g., H 2 supply) could drive methane to internal isotopic equilibrium at its environmental temperature (e.g., Gruen et al, 2018;Giunta et al, 2019;Douglas et al, 2020). Besides that, microbes degrading C 2 -C 4 short alkanes have been found in the Gulf of Mexico sediments (Kniemeyer et al, 2007;Laso-Pérez et al, 2016;Chen et al, 2019).…”