Abstract:
High-resolution organic-geochemical data were collected from a composite section of the Lower Toarcian Posidonia Shale from the SW-German Dotternhausen area. Special emphasis was given to detailed investigation of organic matter composition by biomarker and stable-isotope analysis. The interdisciplinary approach combined geochemical, organic-geochemical, palaeoecological and sedimentological results of the Posidonia Shale. The subject of this study was to reconstruct the coupling of climatic and oceanographic factors to oxygen availability and thus the living conditions at the sediment/water interface and in the closely overlying waterbody of the epicontinental depositional environment suitable for black shale formation. The sequence stratigraphic development of the sea-level curve in the Posidonia Shale of SW-Germany, which can be derived from the C27/C29-sterane ratios as well as from sedimentological and palaeoecological observations, closely corresponds to changes of oxygen availability in the Posidonia shale sea. Redox and alkalinity parameters show a trend which corresponds to the 3rd order sea-level curve and exhibit a good correlation with the subordinated 4th order cycles. Long chain isoprenoids provide first evidence for archaea beside sulfate reducing bacteria in the Posidonia Shale of SW-Germany. Consortia of methane oxidizing and sulfate reducing bacteria within the range of the anaerobic methane oxidation zone caused an increase of alkalinity in the pore water, which resulted in carbonate precipitation. Light d13Ccarb values approaching -11 ‰ in the ‘Unterer Stein’ (exaratum Subzone) prove that isotopically very light CO2 derived from microbial methane oxidation was incorporated into diagenetic carbonates. Apart from the sea-level changes, further controlling factors for the depositional environment are palaeogeography and climate. A trade-wind/monsoon circulation system caused a seasonal change of summer monsoon rains and winter dry periods.