AN OS ISOTOPE PROFILE FROM THE MID-DARRIWILIAN-LOWERMOST SANDBIAN (ORDOVICIAN) ALMELUND SHALE, SWEDEN

Ordovician (485.4 – 443.8 Ma) strata record biotic diversification pulses of marine fauna and the growth of the Gondwanan ice sheets, which culminated in the latest Ordovician glaciation and mass extinction. These phenomena were set against the closure of the Iapetus Ocean, onset of the Appalachian-Caledonian orogeny, and large fluctuations in the isotopic composition of seawater. Particularly, the Ordovician Sr isotope record hosts a large downturn (~0.7090 – 0.7079) with an inflection in the Darriwilian Stage. This decline in ⁸⁷Sr/⁸⁶Sr ratios has been explained as the result of a waning of the Pan-African orogeny, the onset of the Taconic orogeny, and fluctuations in seafloor spreading rates resulting in eustatic sea level changes or a non-unique combination of tectonic and sea level variations. Here we present a new high-resolution Os isotope profile from the Almelund shale Formation, Sweden (Darriwilian–Sandbian boundary) to better constrain the timing and magnitude of changes in continental and seafloor weathering prior to the latest Ordovician (Hirnantian) glaciation. With these new data coupled with other radiogenic isotope systems, high-precision radiometric ages and biostratigraphy, we aim to unravel the tectonic drivers of Ordovician environmental change.