Stable isotope biogeochemistry of methane formation in profundal sediments of Lake Kinneret (Israel)
B. Nüsslein, W. Eckert, R. Conrad
Limnol. Oceanogr., 48(4), 2003, 1439–1446

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 ¹³CH₄ and ¹³CO₂ allowed the calculation of ¹³CH₄ values originating from either CO₂ ( mc) or acetate ( ma). Assuming -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 dma from -50 to -60‰. This result is consistent with syntrophic acetate conversion to CH₄ rather than acetoclastic methanogenesis. The apparent fractionation factor ( ) between CO₂ and CH₄ increased with depth from 1.045 to 1.065. The ¹³C 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 , both at 15 and 30°C. Unlike deeper sediment layers, it remained constant, with ¹³CH₄ 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.