A 25-million year record of organo-facies evolution in latest Triassic–early Jurassic coastal-deltaic to offshore environments in NE-Germany

This study applies bulk and molecular-geochemical techniques at high stratigraphic resolution to the Rhaetian–Toarcian sediment core Barth 10/65 (NE Germany), recording the evolution of a marginal marine depositional environment from a coastal-deltaic to a prodelta-offshore setting that occurred in response to Late Triassic to Early Jurassic paleogeographic and environmental change. Data allow discussing trends in organo-facies and organic carbon (OC) burial over a timespan of about 25 Myr, which has hardly been carried out in any study to date. In addition, molecular-geochemical proxies are applied here to OC-lean oxic strata, which are generally considered unsuitable for the preservation of organic matter and are therefore usually excluded from organic-geochemical studies. However, such deposits represent the majority of the rock record and thus reflect common marine conditions. Results of this study show that the distribution patterns of aquatic algal- and land plant-derived n -alkanes provide detailed insights into sources and preservation of different organic matter types. On the contrary, the widely applied pristane/phytane ratio fails in predicting the redox regime, pointing to the partly incomplete knowledge about the preservation and transformation of these biomolecules in the geosphere. Land plant organic matter dominates throughout most of the section, whereby abundances are related to changes in sea-level, hydrological cycling and fluvial runoff. Substantial amounts of well-preserved aquatic organic matter solely occur in the Posidonia Shale Formation, marking the Toarcian Oceanic Anoxic Event as a major Jurassic organic carbon burial event.