Paleoenvironmental implications from biomarker and stable isotope investigations on the Pliocene Velenje lignite seam (Slovenia)

Abstract

A Pliocene lignite seam up to 160 m thick occurs in the Velenje basin (Slovenia). The seam originated in a topogenous mire and evolved within a non-marine, transgressive setting. Differences in soluble organic matter yield and hydrocarbon content of borehole samples from the lignite are related to differences in the composition of free lipids of microbial origin and/or hydrocarbons derived from the biogeochemical degradation of plant tissue. Variations of the redox conditions within the mire are reflected by pristane/phytane ratios. The abundance of terpenoid biomarkers indicates the predominance of gymnosperms over angiosperms, which is consistent with palynomorphic spectra dominated by pollen of the Sequoia-Taxodium-Metasequoia plant community rather than by angiosperms. Evidence is also provided that the content of land plant derived biomarkers and the preservation of plant tissue is controlled by the input of resin-rich, decay-resistant conifers. Sections of the seam characterized by a high degree of gelification of humotelinite (gelification index) show high contents of hop-17(21)-ene but low hopane concentrations. The results suggest that the gelification of plant tissue may be governed by the activity of anaerobic rather than aerobic bacteria. Despite the minor variation in the proportions of gymnosperms versus angiosperms in the peat-forming vegetation, a general influence of the floral assemblage on carbon isotopic composition of the coals (δ 13C=−25.3 to −27.0‰) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is assumed to affect the δ 13C value of the lignite. Petrographic and geochemical data of gelified and ungelified fossil wood provide evidence that gelification may be governed by microorganisms (e.g. anaerobic bacteria) different from those responsible for decreasing cellulose contents during early diagenetic, aerobic degradation of wood. Based on the molecular composition of terpenoid biomarkers, the wood fragments are identified as derived from gymnosperms. The relative proportions of saturated versus aromatic hydrocarbon fractions of fossil gymnosperms display a general tendency towards lower values in gelified wood remains. This indicates that bacteria involved in gelification of plant tissue may also be involved in aromatisation of diterpenoid hydrocarbons. The chemotaxonomical classification of the macrofossils as gymnosperms is corroborated by the mean carbon isotopic compositions of the macrofossils (δ 13C=−24.5‰) and the extracted cellulose (δ 13C=−22.0‰). The higher isotopic difference of about 2.5‰ between cellulose and fossil wood, compared to that found in modern trees, can most probably be explained by the smaller effect of 13C discrimination for cellulose when compared with wood during decomposition [Chem. Geol. 158 (1999) 121]. Compared with the coals, the δ 13C values of wood and extracted cellulose are affected to a minor extent by microbial activity.