INVESTIGATING REDOX DYNAMICS ACROSS THE CAMBRIAN-ORDOVICIAN BOUNDARY: A DEEP-WATER PERSPECTIVE FROM BALTICA

After the Cambrian explosion, animal diversity plateaued in the late Cambrian through Early Ordovician prior to the most profound radiation, the Great Ordovician Biodiversification Event (GOBE). This interval of stagnant biodiversity is characterized by episodic marine extinctions at biomere boundaries potentially linked to instability in the redox landscape of the late Cambrian–Early Ordovician oceans. The TOCE (Top of Cambrian Excursion, also known as the HERB event) is a negative excursion that marks a notable baseline shift between the isotopically more positive Cambrian carbon cycle and the more negative Ordovician carbon cycle and may represent an important shift to more globally oxidizing marine conditions. This study proposes an expansion of oxidizing conditions as a potential mechanism for the TOCE from a deep shelf/upper slope black shale sequence from the present-day southern margin of Baltica. It will utilize paired stable organic-carbon and pyrite-sulfur isotopes (δ¹³C_org and δ³⁴S_pyr) as well as iron speciation, molybdenum and vanadium concentrations ([Mo] and [V],) and thallium isotopic analysis (ε²⁰⁵Tl) to elucidate changing redox conditions. The δ¹³C_org and δ³⁴S_pyr will be used as proxies for local changes in the burial of organic matter and pyrite, respectively, while iron speciation will provide insight into the local redox conditions. Trace metal concentrations have the potential to elucidate global redox dynamics across the Cambrian-Ordovician boundary, while Tl isotopes fingerprint changes in global manganese oxide burial – a mineral that responds to initial changes in oxygenation. This study will test the hypothesis that there was a major redox transition between the mostly reducing Cambrian ocean to a more oxygenated state in the Ordovician which may have fostered an increase in marine diversity later in the Ordovician. With the data collected, this study will greatly improve the understanding of the late Cambrian–Early Ordovician oceans and how changing redox states affected cycling of bioessential nutrients and its impact on global marine biodiversity.