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

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 CH 4 biogeochemistry in lake sediments. Our measurements of the conversion of 14 CO 2 and of [2‐ 14 C]acetate to CH 4 at both 15 and 30°C were in agreement with this observation. Measurement of the fraction of 14 CO 2 ‐derived CH 4 together with δ 13 CH 4 and δ 13 CO 2 allowed the calculation of δ 13 CH 4 values originating from either CO 2 ( δmc) or acetate ( δma). Assuming α‐values for 13 C fractionation during CO 2 reduction to CH 4 of 1.06–1.07, CH 4 production from acetate required a relatively large fractionation factor that increased with depth and resulted in a dma from ‐50 to ‐60‰. This result is consistent with syntrophic acetate conversion to CH 4 rather than acetoclastic methanogenesis. The apparent fractionation factor ( α) between CO 2 and CH 4 increased with depth from 1.045 to 1.065. The δ 13 C 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 δ 13 CH 4 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 13 C data to elucidate the pathway of CH 4 production and confirm the unique biogeochemical features of CH 4 biogeochemistry in Lake Kinneret.