Palaeoceanography of the early Zechstein Sea during Kupferschiefer deposition in the Lower Rhine Basin (Germany): A reappraisal from stable isotope and organic geochemical investigations

Abstract

Abstract 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 depositional history of most parts of the basin. Maturation of the organic matter was governed only by the geothermal gradient not exceeding 68°C/km during the Late Carboniferous. Due to minor thermal alteration 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 and organic geochemical data are interpreted with respect to palaeoceanographic aspects. The data provide information about salinity variations, the nature of organisms living in the water column and the importance of bacterial activity in the sediment during deposition. From the degree of methylation of 2-methyl-2-trimethyl-tridecylchromans (MTTC) an euhaline to mesohaline (30–40‰) sea water salinity during Kupferschiefer sedimentation can be inferred. The high abundance of biomarkers derived from green/purple sulphur bacteria suggests H2S 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 photosynthetic cyanobacteria or green algae. In the sediment, sulphate reduction occurred due to the availability of abundant sulphate and organic detritus from the overlying water column. Furthermore, methanogenesis was active mostly during early Kupferschiefer deposition. This is reflected by the light carbon isotopic composition of organic matter originating from recycling of CO2 generated by methane-oxidizing bacteria in the water column. Saccate pollen are the only morphologically preserved organic matter in the sediment.