Stable isotope biogeochemistry of methane formation in profundal sediments of Lake Kinneret (Israel)

Abstract

Methane production in profundal sediments of Lake Kinneret was recently found to be, to a large extent, syntrophically coupled to the oxidation of acetate, an apparently unique feature of CH4 biogeochemistry in lake sediments. Our measurements of the conversion of 14CO2 and of [2-14C]acetate to CH4 at both 15 and 30°C were in agreement with this observation. Measurement of the fraction of 14CO2-derived CH4 together with δ13CH4 and δ13CO2 allowed the calculation of δ13CH4 values originating from either CO2 (δmc) or acetate (δma). Assuming α-values for 13C fractionation during CO2 reduction to CH4 of 1.06-1.07, CH4 production from acetate required a relatively large fractionation factor that increased with depth and resulted in a δma from -50 to -60‰. This result is consistent with syntrophic acetate conversion to CH4 rather than acetoclastic methanogenesis. The apparent fractionation factor (α) between CO2 and CH4 increased with depth from 1.045 to 1.065. The δ13C of sedimentary particulate organic matter increased with depth from -28 to -24‰. Incubation of sediment samples from layers in the upper 5 cm resulted in a steady increase of α, both at 15 and 30°C. Unlike deeper sediment layers, it remained constant, with δ13CH4 decreasing with incubation time. This behavior is indicative of sediments in which methanogenesis is extremely limited by substrate. Collectively, our experiments demonstrate the usefulness of 13C data to elucidate the pathway of CH4 production and confirm the unique biogeochemical features of CH4 biogeochemistry in Lake Kinneret.