Coupled δ13C and 87Sr/86Sr chemostratigraphy of Kimmeridgian shoal-water deposits: A new composite record from the Lower Saxony Basin, Germany

Correlation of shallow-marine carbonate deposits with their coeval pelagic counterparts is often hampered by the lack of open-marine stratigraphic marker fossils and by the restrictions of regional biostratigraphic schemes. These difficulties can partly be overcome by the use of coupled δ13C and 87Sr/86Sr chemostratigraphy based on pristine shell calcite and bulk rock material. Here, a new high-resolution chemostratigraphic framework covering Kimmeridgian strata is presented based on three stratigraphic sections located in the Subboreal Lower Saxony Basin (LSB) of Northern Germany. Combined with sedimentological data, the bulk rock δ13C signal is critically evaluated for both diagenetic alteration and local environmental effects. In general, the δ13C signatures are considered to predominantly record the global marine signal. Diagenetic and facies-related discrepancies are restricted to dolomitic supratidal back ramp and intertidal back ramp deposits, respectively. Strontium-isotope stratigraphic data, obtained from diagenetically screened low-Mg calcite shells, refine the existing ostracod biostratigraphic scheme in the Lower Saxony Basin (LSB) and contribute to the existing global strontium-isotope dataset of the Kimmeridgian. These new results confirm the potential of well-preserved low-Mg calcite from shallow-marine settings to preserve a global marine Sr-isotope signal. Calibrated by the integrated biostratigraphic and strontium-isotope results, a high-resolution composite δ13C record for Kimmeridgian shoal-water deposits is established. Comparison of the new composite δ13C curve from the Subboreal Lower Saxony Basin (LSB) with existing Kimmeridgian records from the peri-Tethyan and Western Tethyan realms provides new insights into the long-term global carbon cycle during the Kimmeridgian.