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

Abstract

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, δ 13 CDIC, 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 δ 13 CDIC values decreased from −7.9‰ to −13.5‰, and stayed relatively constant during “transition” with mean δ 13 CDIC value of −12.6 ± 0.6‰. After 56 days the δ 13 CDIC value reached −7.6‰ and did not change much until the end of incubation. 13 C enrichment coincided with the formation of CH4, which started to increase at the same time. The methane produced had a measured average δ 13 C CH4 value of −70.1 ± 1.1‰ and δD CH4 value of −208 ± 10‰ indicating the formation of methane through bacterial CO 2 reduction. The flux-weighted 13 C/ 12 C 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 δ 13 C of remineralized carbon was estimated to be −40.1‰. A lower δ 13 Crem value of −48.1‰ was calculated during methanogenesis due to selective lipid degradation of planktonic origin and/or to δ 13 C depleted microbial lipid degradation. According to FTIR and solid state 13 C 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.