Methane Formation in a Remote Mountain Lake (Lake Planina, NW Slovenia)

摘要

An incubation experiment was performed with the sediment from a eutrophic mountain Lake Planina (NW Slovenia). In the experiment chemical and stable C and H isotope analyses were used to determine the processes of CH4 formation. The isotopic composition of dissolved inorganic carbon, delta C-13(DIC), indicates three different processes occurring during incubation: (a) oxic degradation of organic matter (OM), (b) transition between oxic degradation and methanogenesis (transition) and (c) methanogenesis. During oxic degradation of OM delta C-13(DIC) values decreased from -7.9 parts per thousand to -13.5%parts per thousand and stayed relatively constant during transition with mean delta C-13(DIC) value of -12.6 +/- 0.6 parts per thousand. After 56 days the delta C-13(DIC) value reached -7.6 parts per thousand and did not change much until the end of incubation. C-13 enrichment coincided with the formation of CH4, which started to increase at the same time. The methane produced had a measured average delta C-13(CH4) value of -70.1 +/- 1.1 parts per thousand and delta D-CH4 value of -208 +/- 10 parts per thousand indicating the formation of methane through bacterial CO2 reduction. The flux-weighted C-13/C-12 content of DIC and CH4 is equivalent to the isotopic composition of the deposited metabolizable organic carbon and plankton. Thus, at least at the surface of the sediment, oxidized carbon reflects the reactivity of planktonic OM, since the delta C-13 of remineralized carbon was estimated to be -40.1 parts per thousand. A lower delta C-13(rem) value of -48.1 parts per thousand was calculated during methanogenesis due to selective lipid degradation of planktonic origin and/or to delta C-13 depleted microbial lipid degradation. According to FTIR and solid state C-13 NMR measurements of lake sedimentary organic matter, the most degradable components are polysaccharides and aromatic/olefinic organic compounds, while carbonyl and amide structures as well as and alkyl components seem more resistant. The contribution of the dissolution of calcite to DIC decreased during the incubation from 20% during the oxic degradation of OM to 7% during methanogenesis.