Environmental change and carbon cycling during the Early Jurassic: A multi-proxy study on the Posidonienschiefer of NW Germany

The Early Jurassic epoch was marked by widespread marine anoxia and euxinia, in re- sponse to climatic and environmental change associated with perturbations to the global carbon cycle. This project uses a multi-proxy approach to reconstruct the depositional environment and geochemical evolution of the Lower Saxony Basin (LSB) during the for- mation of the organic-rich Posidonienschiefer Formation, commonly associated with the Toarcian Oceanic Anoxic Event (T-OAE). High-resolution carbon isotope stratigraphy from Core A drilled in the LSB, used as a reference core, in combination with low-resolution data from two other cores, allows identification of the T-OAE, and shows that the event is recorded at the base of the formation, and indicates that the onset of organic-rich sedimentation coincided with the carbon cycle perturbation and climatic changes. The carbon isotope dataset of Core A is calibrated with new calcareous nannofossil biostrati- graphic data, allowing the assignment of nannofossil zones from the Late Pliensbachian NJT5a zone to the Middle/Late Toarcian NJT8a zone. Combination of carbon isotope stratigraphy and biostratigraphy shows that the Posidonienschiefer Formation was de- posited during the Early Toarcian to the Middle/Late Toarcian with most of the formation deposited after the T-OAE. Organic geochemical data (Rock-Eval pyrolysis and biomark- ers) show that the Posidonienschiefer formed in a basin wherein persistent euxinic condi- tions reached the photic zone, caused by high primary productivity, dominated by marine algae and salinity-driven stratification, that commenced during the T-OAE. Euxinic con- ditions are confirmed by Fe speciation data showing that euxinic conditions developed in the LSB during the Early Toarcian and during the deposition of the Posidonienschiefer Formation. Molybdenum concentration further shows that basin geochemistry was modu- lated by local hydrography and relative sea-level change, causing isolation and extended deep-water renewal times. This led to the basin remaining prone to organic matter preser- vation during the Middle/Late Toarcian. Automated mineralogy using scanning electron microscopy is widely used in the minerals industry and other research fields. This study provides a review of possible applications for paleoenvironmental studies of mudrocks. A method to prepare samples as well as new mineralogical data are shown, including quantitative data on pyrite framboid distribution indicating euxinic conditions during de- 3 position of the Posidonienschiefer, and mineralogy changes across the T-OAE. The re- lationship of the T-OAE and large-scale volcanism of the Karoo Ferrar Large Igneous Province is explored through sedimentary Hg concentration data. The data showing in- creasing mercury concentrations starting during the latest Pliensbachian, possibly related to the Pliensbachian-Toarcian boundary perturbation, and the highest concentrations in the sediments deposited during the T-OAE. The formation of several epicontinental basins with organic matter rich sediments during the Toarcian, such as the LSB, led to high car- bon burial and resulted in efficient removal from the atmosphere, thus creating a negative feedback mechanism during the long recovery phase of the T-OAE. Calculations show that as much as 2000 Gt of C was buried in the LSB and adjacent basins during an interval of 2 Myr