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

Abstract

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 (CH 4 ) clumped isotope (Δ 18 ) and 14 C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ 18 values are negatively correlated with CH 4 production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH 4 Δ 18 and the fraction modern carbon. These correlations imply that CH 4 derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ 18 values, are not positively correlated with CH 4 flux estimates, highlighting the likely importance of environmental variables other than CH 4 production rates in controlling ebullition fluxes.