Rhenium‑osmium geochronology of the Toarcian Posidonia Shale, SW Germany

Abstract

Black shale samples from sedimentary layers below and between the Unterer Stein, Oberer Stein, Inoceramus Bank, and Nagelkalk horizons from the Dormettingen quarry, SW Germany were analysed for their Re and Os isotope composition and content. The ~12-m-thick sedimentary sequence of the Posidonia Shale Formation, composed of multiple layers of black shale and intercalated limestone beds, was deposited during the Early Jurassic (Toarcian), a time during which black shale sedimentation was ubiquitous in Western Europe during a widespread oceanic anoxic event. Both the marl and shale layers beneath the black shales and adjacent to the Oberer Stein limestone layer show signs of bioturbation. The excellent biostratigraphic constraints and the high Re and Os contents (24–290 ppb and 290–1050 ppt, respectively) of the Dormettingen shales thus make these samples ideal for testing the influence of bioturbation on the Re-Os isotope system, as well as refining age constraints for the Toarcian OAE and potential triggers for this event. The calculated isochron age for the Dormettingen shales is 183.0 ± 2.0 Ma, with a low initial 187Os/188Osi of 0.377 ± 0.065, indicating a Pliensbachian to Toarcian age. This age is in agreement with published U-Pb and Re-Os ages for other Lower Jurassic sites, astronomical constraints, and biostratigraphic correlations, suggesting limited disturbance of the isotope system by minor degrees of biological activity. The low 187Os/188Osi is consistent with significant influx of mantle-derived material into the Toarcian ocean, likely from weathering of Karoo-Ferrar Large Igneous Province basalts.