Carbon Isotopic Composition Of Earlydiagenetic Methane: Variations With Sediments Depth

We demonstrate the annual cycle of methane in fresh sediments of two lakes - Moszne (E Poland) and Skrzynka (W Poland). The vertical gradient in δ¹³C(CH₄) values varied widely from about -4.5‰/(-1 m) in late summer 1993 to about +2.5‰/(-1 m) in late winter, in the uppermost sediment profiles of about 3-meters in length. These vertical variations apparently are not due to oxidation or temperature changes, but rather to the higher gradient of the downward decrease of production rates via the acetic acid fermen-tation pathway rather than via the CO₂-H₂ pathway. The production of methane and δ¹³C(CH₄) values are the highest during summer while the lowest during winter, reflect-ed especially during surface sampling. The downward gradient of δ¹³C in winter, late autumn and, at greater depths, in late summer results from isotope enrichment of the residual pool of precursors of methane, predominantly CO₂.