Depth of the water column in relation to carbon isotope ratios in methane in freshwater sediments

Abstract

Methane was collected from the surficial sedimentary layer (0 to about 20 cm) in 45 freshwater lakes in Poland. Sampling was also carried out at various depths of the overlying water column (0.15 to 12 m) between noon and early afternoon, on a seasonal basis, between 1992 and 1996. A positive correlation between the depth of the lake water, the d13 C (CH4) value (from ca. -l.4 to -2.3% per l m depth) and the wider d13 C(CH4) variation over the thermocline are probably due to: the time of sinking of organic particles resulting in more intense acetate fermentation in shallower portions of the lake; the temperature variation; differences in the precursors of methane, the diffusion effect, and an increase of bioavailable DIC (dissolved inorganic carbon) at greater depths. Non-seasonal variation of isotope ratios in methane and the depth-isotope ratio correlation show that the lake system is in dynamic equilibrium on a scale of hours and days. Therefore, earlier models of methanogenesis relating and the atmospheric methane isotopic budget, proposed by other authors and based on sampling of methane from sediments, need to be revised. Moreover, d13 C (CH4) values higher during seasonal overturn (mixing of benthic and surface waters) than during stagnation, have been observed. This is probably due to the fact that during overturn periods some organic compounds (methane precursors) and methane in the surficial part of sediments, are oxidized with a resalting kinetic isotope effect. It is proposed that oxidation of methane and other organic compounds during seasonal overturn may be responsible for post-depositional lamination observed as pale (autumn overturn) and dark (summer organic-rich unoxic stagnation) millimetres-thick layers in freshwater lake sediments.