Decomposition of sedimentary organic matter and methane formation in the recent sediment of Lake Bled (Slovenia)

Abstract

Decomposition of sedimentary organic matter and methane formation in a eutrophic dimictic subalpine lake (Lake Bled, NW Slovenia) were investigated by measurement of in situ concentrations of dissolved inorganic carbon (DIC) in the pore water and its stable carbon isotopic composition, as well as by incubation experiments. The isotopic composition of carbon in the methane produced during incubation experiments was also studied. The ratio between methane produced by acetate fermentation and CO2/H2 reduction was estimated from δ13C-DIC vs. DIC concentration profiles in sediment pore water using a mathematical model developed by LaZerte [LaZerte, 1981. The relationship between dissolved carbon dioxide and its stable isotope ratio in aquatic sediments. Geochim. Cosmochim. Acta 45, 647–656]. It was found that in the spring acetate fermentation is dominant when about 65% of methane is produced by this reaction pathway, whereas in the autumn, over 90% of the methane results from CO2 reduction. δ13C of methane released during a 6-week incubation experiment with sediment taken in the autumn varied between −70 to −85‰ V-PDB, i.e., values typical of methane produced by CO2/H2 reduction, in spite of methane oxidation evident from the low δ13C-CO2 values, thus confirming the results obtained by mathematical modelling.