Sulfate-induced isotopic variation in biogenic methane from a tropical swamp without anaerobic methane oxidation

The oxidative consumption of methane (CH4) generally proceeds with a significant isotope fractionation, and isotopic variations in CH4 observed in sulfate-containing anaerobic sediments have often been interpreted as an indicator of anaerobic methane oxidation at the expense of sulfate. However, we found variations in δ13C value of CH4 depending on sulfate availability in tropical swamp sediments, in which no anaerobic CH4 oxidation was detected. In one sediment, the range of δ13C variation due to sulfate was as large as 20‰. The variations in δ13C of decomposed organic matter and CO2 failed to explain the variation in CH4 δ¹³C. We postulate a syntrophic linkage between sulfate-reducing and methanogenic bacteria via acetate as a mechanism of the observed δ'13C variation. This revised version was published online in July 2006 with corrections to the Cover Date.