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 (δ13C) 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, CO2 and CH4 were the major gas components with small amounts of N2. The carbon isotope values of CO2 (δ13CCO2) and CH4 (δ13CCH4) 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 CO2, while CH4 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 (δ13C values up to 16.7‰) indicate that microbial methanogenesis was active during formation of the basin. The δ13CDIC 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 δ13CDIC values (from − 9.1 to 0.2‰) suggestive of degradation of organic matter and enrichment via microbial reduction of CO2.
COBISS.SI-ID: 24995623
This paper presents results of macroscopic, microscopic, chemical, and isotopic investigations of 12 samples of dark grey to black coloured interbeds occurring within Upper Permian and Triassic lime-, dolo- and marlstones in an area of Outer Dinarides between Ljubljana and Bloke in Central Slovenia. An additional sample is anthracite of the Carnian age from the Orle locality. Concentration of Corg in four samples is below 1 %, and in seven samples it varies between 1 and 2.3 %. Only in one sample, in the black Carnian limestone of the Lesno Brdo area, it is somewhat higher than 5 %. The highest Corg. content, 30.61 %, was analysed in the Orle anthracite. Chemical analysis of major elements (as oxides) showed that four samples (No. 6, 10, 11, 12) are clearly siliciclastic mudrocks, with 65-80 % SiO2+Al2O3. Three samples (No. 4, 8, 9) are typical calcite rich – dolomite poor rocks, with high loss on ignition (LOI about 40 %) derived from calcite decomposition. Four samples are calcite – dolomite characterized rocks with LOI values 34–43 %. One sample, from the Slugovo quarry, is composed of quartz, dolomite and calcite. The anthracite sample from Orle has inorganic matter composed almost exclusively of SiO2+Al2O3 (clays), and some iron and sulphur which form pyrite. Isotopic composition of the calcite carbon ranges from δ13CCaCO3 -5.7 to 1.9 ‰, whereas isotopic composition of the organic carbon varies between δ13Corg. -34.7 and -21.6 ‰. The most negative δ13Corg. value of -34.7 ‰ was analysed in a sample, which is the most organic-rich limestone. Isotopic investigations of nitrogen, expressed by δ15N values, also did not express notable differences in respect to lithology. They vary between 4.6 and 9.1 ‰. Microscopy of polished surface samples showed clearly fine grained siliciclastic, carbonate and coal composition of the treated rocks.
COBISS.SI-ID: 25927207