Paleoenvironment of the upper Oligocene Trbovlje coal seam (Slovenia)

摘要

In the Trbovlje region (Slovenia), an upper Oligocene coal seam up to 30 m thick developed in a topogenous mire and evolved within a transgrcssive setting. The lower ash-rich part of the seam (unit Ⅰ) accumulated in a forest swamp strongly influenced by fluvial activity. Palynomorph spectra are dominated by Taxodiaceae-Cupressaceae pollen. Ash yields decrease in the middle (unit Ⅱ) and upper part of the seam (unit Ⅲ), indicating decreasing fluvial activity. An increase in CaO contents reflects increasing amounts of inflowing water from an alkaline lake, which progressively controlled peat hydrology in units Ⅱ and Ⅲ. High sulfur contents in the nonmarine coal are a result of the carbonate-rich water. Abundant pollen and spores from Myricaceae and Polypodiaceae indicate that unit Ⅱ represents a bush moor. During deposition of unit Ⅲ, this environment was replaced by a reed moor. Sapropelic coal (unit Ⅳ) and sapropelic shale (unit Ⅴ) accumulated in a relatively large lake, which was established by another water table rise. The interpreted environmental changes are confirmed by biomarker data. Pristane/phytane ratios reflect variations of the redox conditions within the mire and reach a minimum in the sapropelic unit Ⅳ. The abundances of gymnosperm- and angiosperm-derived terpenoid hydrocarbons testify the predominance of gymnosperms over angiosperms during early peat stages (unit Ⅰ), and increasing proportions of angiosperms during deposition of units Ⅱ and Ⅲ. Highest contents of diterpenoids coincide with maxima in the preservation of plant tissue (TPI) within unit Ⅰ (wet forest swamp), suggesting that TPI is partly controlled by the input of resin-rich, decay-resistant conifers. Sections of the seam characterized by advanced gelification show low steroids/hopanoids ratios. The results imply that gelification of plant tissue is related to increased microbial activity. Evidence is provided that the hopanes could be used as a measure for the activity of aerobic bacteria, whereas for the hop-17(21)-ene a microbial origin from anaerobic (iron-reducing) bacteria is assumed. A general influence of the floral assemblage and of varying contributions of algal biomass on carbon isotopic composition of the organic matter (δ~(13)C = ―24.0 to ―28.1 per thousand) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is assumed to affect the δ~(13)C values of the coal. Fossil wood collected from the seam is identified as derived from gymnosperms, based on the molecular composition of terpenoid hydrocarbons. This chemotaxonomical classification is corroborated by the mean carbon isotopic compositions of the fossil wood (δ~(13)C = -24.0 per thousand) and by the δ~(13)C value (-21.8 per thousand) of the extracted cellulose from one macrofossil. The low cellulose content of the sample (2.7%) together with the slightly higher isotopic difference of 1.6 per thousand between cellulose and fossil wood, compared to that found in modern trees, can most probably be explained by the possible reversal in ~(13)C discrimination for cellulose when compared with wood in highly degraded samples.