PALEOCEANOGRAPHY OF THE EARLY ZECHSTEIN SEA DURING KUPFERSCHIEFER DEPOSITION IN THE LOWER RHINE BASIN (GERMANY) - A REAPPRAISAL FROM STABLE-ISOTOPE AND ORGANIC GEOCHEMICAL INVESTIGATIONS

In the study area of the Lower Rhine Basin the Permian Kupferschiefer was deposited in a shallow water environment. Water exchange from this marginal basin with the Zechstein Sea was restricted by palaeohighs. The burial depth of the shale did not exceed 1000 m during the deposit ional history of most parts of the basin. Maturation of the organic ma tter was governed only by the geothermal gradient not exceeding 68 deg rees C/km during the Late Carboniferous. Due to minor thermal alterati on of organic matter, diagenetic effects on the biomarker composition and the light stable isotope ratios (C, H, O, N) of organic matter and carbonates are of minor importance. In the present study, isotopic an d organic geochemical data are interpreted with respect to palaeoceano graphic aspects. The data provide information about salinity variation s, the nature of organisms living in the water column and the importan ce of bacterial activity in the sediment during deposition. From the d egree of methylation of 2-methyl-2-trimethyl-tridecylchromans (MTTC) a n euhaline to mesohaline (30-40 parts per thousand) sea water salinity during Kupferschiefer sedimentation can be inferred. The high abundan ce of biomarkers derived from green/purple sulphur bacteria suggests H 2S saturation of the bottom waters and maximum water depths below 100 m in the basin because these organisms live near the boundary between the photic zone and the anoxic (euxinic) bottom water at a depth of 10 -30 m. Primary production in the upper water column was dominated by p hotosynthetic cyanobacteria or green algae. In the sediment, sulphate reduction occurred due to the availability of abundant sulphate and or ganic detritus from the overlying water column. Furthermore, methanoge nesis was active mostly during early Kupferschiefer deposition. This i s reflected by the light carbon isotopic composition of organic matter originating from recycling of CO, generated by methane-oxidizing bact eria in the water column. Saccate pollen are the only morphologically preserved organic matter in the sediment. (C) 1997 Elsevier Science B. V.