A mass balance of carbon stable isotopes in an organic-rich methane-producing lacustrine sediment (Lake Bled, Slovenia)

The sources of dissolved inorganic carbon (DIC) were determined in subalpine lacustrine high-carbonate (>50%) sediments (Lake Bled, Slovenia) using a combination of chemical and carbon stable isotope analyses together with incubation experiments. The carbon dynamics in the sediment was simulated by a numerical model that accounts for all basic processes controlling the concentration and isotopic composition of DIC. The remineralized carbon (DIC plus CH4) has mean δ13C value of −28.4‰. The dominant carbon isotope effect expressed during early diagenesis of this organic matter is associated with methanogenesis. Methane produced at this site has a measured value of −69‰, while DIC is enriched in 13C (−12.6‰) because of the preferential synthesis of 12CH4. In sediment pore water, the isotopic composition of DIC reflects the combination from degradation of organic carbon using various electron acceptors and methanogenesis. It was found that the major contribution (38–78%) of DIC is derived from methanogenesis. The highest contribution of CaCO3 dissolution was observed in the spring (12%), characterized by lower bottom water temperatures and slower degradation of sedimentary organic matter.