Carbon cycling in the Pliocene Velenje Coal Basin, Slovenia, inferred from stable carbon isotopes

Stable isotopes of carbon were used to trace organic and inorganic carbon cycles and biogeochemical processes, especially methanogenesis within different geologic substrates of the Pliocene lignite-bearing Velenje Basin in northern Slovenia. Lithotypes of lignite, coalbed gases, calcified woods (xylites), carbonate-rich sediments, and groundwaters were investigated. Carbon isotope (δ ¹³C) values of the different lignite lithotypes ranged from -28.1 to -23.0%, with the variability likely a function of the original isotopic heterogeneity of the source plant materials and subsequent biogeochemical processes (i.e. gelification, fusinitization, mineralization of organic matter) during the early stage of biomass accumulation and diagenesis. In the lignite seam, CO ₂ and CH ₄ were the major gas components with small amounts of N ₂. The carbon isotope values of CO ₂ (δ ¹³C _CO2) and CH ₄ (δ ¹³C _CH4) were highly variable, ranging from -9.7 to 0.6% and -70.5 to -34.2%, respectively. Carbon dioxide is likely sourced from a mixture of in situ microbial activity and external CO ₂, while CH ₄ is dominantly sourced from microbial methanogenesis, with possible addition of thermogenic gas from deeper formations, and the influence of microbial oxidation of methane. Calcified xylites enriched with ¹³C (δ ¹³C values up to 16.7%) indicate that microbial methanogenesis was active during formation of the basin. The δ ¹³C _DIC values (from -17.4 to -3.2%) of groundwaters recharging the basin from the Triassic aquifer are consistent with degradation of organic matter and dissolution of dolomite. Groundwaters from the Pliocene sandy and Lithotamnium carbonate aquifers have δ ¹³C _DIC values (from -9.1 to 0.2%) suggestive of degradation of organic matter and enrichment via microbial reduction of CO ₂.