Carbon isotopic composition of early-diagenetic 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.