Paleogeography and paleoecology of the upper Miocene Zillingdorf lignite deposit (Austria)

The Zillingdorf deposit formed during the late Miocene filling of Lake Pannon and contains Austria's largest lignite reserves. Two lignite seams are present and developed within frequently flooded, low-lying mires in near shore environments. High sulphur contents are due to the influence of the brackish water body of Lake Pannon. During peat accumulation a transgression forced the NW-SE trending shoreline northeastwards. Differences in soluble organic matter (SOM) yield and hydrocarbon content of borehole samples and woody macrofossils are related to differences in the content and composition of free lipids of microbial origin and/or hydrocarbons derived from the biogeochemical degradation of plant tissue. Variations of pristane/phytane ratios are interpreted to reflect differences in the redox conditions of the mire. Peatification in an acidic and aerobic environment is further reflected by the predominance of aromatic over saturated hydrocarbons, the presence of an intense complex mixture in the GC traces due to biodegradation processes, high ratios for diasterenes relative to sterenes, and high concentration ratios of hopanes to hop-17(21)-ene of the respective samples. Gelification of plant tissue is governed by microbial activity, as indicated by the positive relationship between gelification index and hopanoids concentration. The composition of terpenoid biomarkers indicates the predominance of gymnosperms over angiosperms and increasing proportions of angiosperms in the peat-forming vegetation with decreasing depth in the upper scam. From the sesqui- and diterpenoids present in the lignite and fossil wood remnants, a predominant role of species of the Coniferales families Cupressaceae/Taxodiaceae are concluded. The preservation of plant tissue is governed by the presence/absence of decay-resistant gymnosperms. A general influence of the floral assemblage on the isotopic composition of organic carbon of the lignite (delta C-13=-27.2 to -24.6 parts per thousand) is proposed. Decomposition of plant tissue and biogeochemical carbon cycling is assumed to further affect the delta C-13 values of the lignite. Carbon isotope data of fossil wood remnants are consistent with their chemotaxonomical classification as gymnosperms (delta C-13 between -22.5 parts per thousand and -24.9 parts per thousand) and angiosperms (delta C-13 between -25.5 parts per thousand and -26.6 parts per thousand), respectively. The delta C-13 data of the extracted cellulose reveal clear differences between fossil wood from gymnosperms (average delta C-13=-20.3 parts per thousand) and angiosperms (mean delta C-13=-22.9 parts per thousand). The higher isotopic difference of about 3.5 parts per thousand between cellulose and fossil wood, compared to that found in modern trees, is explained by the smaller effect of C-13 discrimination for cellulose when compared with wood during decomposition. (c) 2006 Elsevier B.V. All rights reserved

Relations of petrographical and geochemical parameters in the middle Miocene Lavanttal lignite (Austria)

Samples from two lignite seams (Lower Seam, Upper Seam) of the Lavanttal basin (Austria) and additional xylite were investigated for variations in maceral composition, petrography-based facies indicators, bulk geochemical parameters, and molecular composition of hydrocarbons. Both seams originated in a topogenous mire and evolved within a transgressive setting. The final drowning of the mire is indicated by sapropelic shales. Whereas the sapropelic shale overlying the Lower Seam was deposited in a freshwater lake, the sapropelic shale above the Upper Seam represents a brackish lake.Numerous relationships are found between petrography-based facies indicators and the geochemical composition of organic matter. The contents of macerals of the liptinite group are positively correlated with soluble organic matter (SOM) yields and hydrogen index (HI). Consistent with maceral composition and high HI values, enhanced proportions of short-chain n-alkanes, which are predominantly found in algae and microorganisms, are obtained from samples of the sapropelic shales. The final drowning of the mire is reflected by decreasing pristane/phytane ratios, due to the rise in (ground)water table and the establishment of anaerobic conditions, as well as by decreasing ratios of diasterenes/sterenes, indicating increasing pH values in the mire. The degree of gelification of plant tissue (gelification index) is governed by the microbial activity in the mire, as indicated by the hopanes concentration. The differences in floral assemblage during the formation of the Lavanttal lignite seams are reflected by major differences in tissue preservation. Preservation of plant tissue (TPI) in the Lavanttal lignite is obviously controlled by the presence/absence of decay-resistant gymnosperms in the peat-forming vegetation, and additionally influenced by the relative contribution of wood to coal formation. The results provide evidence that valuable information for coal facies characterization could be obtained by petrography-based and geochemical facies indicators. An influence of the floral assemblage (gymnosperms/angiosperms ratio) and of the contribution of algal biomass on carbon isotopic composition of the organic matter (delta C-13 = -24.2 to -28.6%.) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is suggested to affect the delta C-13 values of the coal, The chemotaxonomical classification of the xylites as gymnosperm remnants, based on the molecular composition of terpenoid biomarkers, is corroborated by the carbon isotopic composition of the xylites (mean delta C-13 = -24.1%) and the extracted cellulose (mean delta C-13 = -20.2%). The higher isotopic difference of about 3.9%. between cellulose and total organic carbon of the xylites, compared to the difference between cellulose and wood found in modem trees, is explained by the smaller effect of decomposition on delta C-13 of cellulose. (C) 2006 Elsevier B.V All rights reserved