Inorganic geochemistry of the German ‘Posidonia Shale’: palaeoenvironmental consequences

Abstract

Abstract 42 samples of the SW and NW German ‘Posidonia Shale’ have been analysed for their major element and sulphur isotopic composition, as well as the trace metals Ag, As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sr, V, and Zn. Several of these trace metals are found enriched in the TOC-rich ‘bituminous’ layers. A comparison of the enrichment of specific metals in the Toarcian shales with black shales from the Cenomanian/Turonian anoxic event and Recent TOC-rich sediments from the Black Sea and the Gulf of California reveals a strong similarity of the ‘Posidonia Shale’ with Black Sea sapropels. The origin of the trace metals found enriched in this rock type predominantly seems to be fluvial input. Seawater alone fails to explain the high Co, Mn, and Pb concentrations of the ‘Posidonia Shale’. The environment of deposition therefore seems to be characterized by less saline surface waters overlying intermittently anoxic, more saline water masses of this epicontinental sea. Anoxic conditions must have persisted during the deposition of some bituminous layers and probably during the ‘falciferum’ Subzone.