Carbon (δ13C) and Strontium (87Sr/86Sr)- based chemostratigraphy of the Oxfordian series in the Lower Saxony Basin of Northern Germany

Abstract

The Late Jurassic (ca. 161.5-145 Ma) witnessed significant changes in climate, sea-level, and global sedimentation patterns due to the break-up of the supercontinent Pangea. During this time, the Oxfordian interval in particular experienced multiple and pronounced perturbations of the global carbon cycle. To better understand these changes, shoal-water carbonate rocks, which archive important signatures of the biosphere's evolution, can be studied. The Lower Saxony Basin (LSB) was located in the Boreal Realm during the Late Jurassic and was characterized by widespread shallow-marine carbonate deposition. Despite multiple biostratigraphic schemes that have been developed, the stratigraphy and subdivision of the Upper Jurassic rocks of the LSB are still poorly defined. Due to the scarcity of reliable biostratigraphic markers and numerous sedimentary gaps, the stratigraphic age assignment is still uncertain and hampers further studies on a basin-wide scale. In order to reconstruct stratigraphic correlation and paleoclimatic interpretation, three well-accessible sections (Bisperode section, Osterwald section, Langenberg section) and a scientific borehole (Konrad #101 core) were studied within the LSB. A high-resolution carbonate microfacies and sequence stratigraphy are first analyzed, which forms the baseline for the conceptualization of a reef-bearing carbonate ramp model. The supra-regional correlation of the observed sequences is discussed. Chemostratigraphy represents a reliable technique correlation and calibration of biostratigraphic schemes on local and global scales. However, new biocalcite-derived 87Sr/86Sr results from the Oxfordian Korallenoolith Formation in LSB fail to provide an improved age assignment, possibly due to environmental factors such as continental freshwater discharge affecting the seawater 87Sr/86Sr ratios recorded in the studied shell material. In addition, a high-resolution carbonate δ13Ccarb record is presented and compared with other existing records collected from the LSB as well as from Tethyan and proto-Atlantic sites. The δ13Ccarb data enable the refinement of the pre-existing biostratigraphic framework and the establishment of a high-resolution carbon isotope-based correlation scheme. The new framework improves the stratigraphic correlation between the western and eastern parts of the LSB and allows for a better assessment of the impact of carbon cycle disturbances on the shallow-marine carbonate systems in this area. The study provides critical insights into the global pacing of the Oxfordian carbon isotope excursions and the role of environmental factors in shaping them.