Recent technological advances providing high precision measurements of clumped isotopes have the potential to give further insights into the tracing of sources and their geological or biological processes of formation. Here we report a summary of our results from multiple studies conducted at UMD using methane isotopologue data (¹³CH₄,¹²CH₃D, ¹³CH₃D, and ¹²CH₂D₂) for air collected from urban, wetland and farm sites located in Maryland, as well as CH₄samples from incubations, chambers, natural gas, wood burning, and bubbles collected from a mesohaline tidal wetland. Overall, CH₄ from urban air samples displayed Δ¹²CH₂D₂ values of +46 to +50‰, which is lower than prior predicted enrichment by published theoretical models and reflects sources with negative Δ¹²CH₂D₂. Consideration of these data in the context of models of atmospheric CH₄ point to negative δ values for the total source composition with negative Δ¹²CH₂D₂. The negative Δ¹²CH₂D₂ of the source indicates a greater sensitivity of atmospheric Δ¹²CH₂D₂ to variations in the proportions of biogenic to thermogenic sources than previously thought. Also, the degree of enrichment of Δ¹²CH₂D₂of air samples collected at emission sites show variations with the location and time of collection, suggesting an ongoing mixing process between the methanogenic source with a characteristically low D/H and atmospheric methane. The mixing behavior is strong in the air samples collected from emission sites, providing a way to fingerprint local contributions to CH₄. Finally, CH₄ bubbles collected from various zones along the tidal channel within the wetland site show an oxidation signature of clumped isotopologues in all locations. The degree of oxidation changed with temperature and other environmental factors known to impact microbial communities during the growing season. Our measurements suggest that CH₄ clumped isotopesshow promise as complementary tracers of atmospheric CH₄ sources and sinks, and provide complementary insights into atmospheric CH₄ dynamics. Also, seasonal oxidation signatures can be observed and traced. Therefore, clumped isotopes of CH₄ can provide a method for tracking microbial CH₄ production and consumption and the seasonal variations in microbial activities. This could allow fingerprinting of microbial wetland contributions to the air.

Patrick Megonigal

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