Colloquium: R. Burton Thomas

Abstract: Carbon isotopic mass balance approaches are commonly used to trace carbon flow in biogeochemical systems.  Unfortunately, isotopic heterogeneity within organic compounds may obscure important source-sink relationships in natural systems. Northern peatlands are one such system where the processes that produce methane from organic matter cannot be constrained by compound-specific isotope analysis alone. Since northern wetlands represent a sensitive lever in the climate system on glacial-interglacial timescales, elucidating the pathways of methane production and consumption directly impacts interpretation of both past and future climate change feedbacks. Here I report on the measurement and interpretation of the intramolecular difference in carbon isotopes of the two carbon groups present in acetate, a key intermediate in the methane cycle. These results indicate that in northern peats the decomposition of organic matter produces acetate from microbial fermentation, but surprisingly, 1/3 of the acetate pool is derived from microbial CO₂ reduction. This process generates anomalously depleted isotopic compositions for acetate that are passed on to the methane that then escapes to the atmosphere. These data suggest that estimates of the methane source to the atmosphere based solely on gas isotopic data are flawed and that the assumed dominant methane production pathway is absent. I conclude by presenting a testable hypothesis for the biogeochemical role of this acetate production pathway in the maintenance of a permanently acidic peatland.

R. Burton Thomas
Research Hydrologist, National Research Program, U.S. Geological Survey