THE UTILITY OF MULTI-LITHOLOGY, MULTI-PROXY APPROACHES TO UNRAVELING LOCAL-TO-GLOBAL DEEP-TIME BIOGEOCHEMICAL EVENTS: A HOLISTIC CASE STUDY OF LINKS BETWEEN MIDDLE ORDOVICIAN BIODIVERSIFICATION AND MARINE OXYGENATION

The oxygen content of ancient oceans has been postulated as a major controlling factor for biodiversity throughout Earth’s history. One of the most notable increases in marine biodiversity occurs within the Ordovician Period (~487–443 million years ago), commonly referred to as the Great Ordovician Biodiversification Event or Ordovician Radiation. Some intervals within the Ordovician were associated with unusually prominent rises in taxonomic richness, the reasons for which remain debated. Multiple causal factors have resulted in this long-term adaptive radiation, but until very recently direct links to marine oxygen levels remained poorly characterized. Here we utilize new multi-proxy datasets from the paleocontinent of Baltica generated from carbonate-dominated and black shale-dominated successions. Specifically, we present an expansive multi-site iodine-to-calcium (I/Ca) record from Lower–Middle Ordovician marine carbonates in Baltoscandia that provides a detailed account of the spatiotemporal development of oxygen conditions across this paleoshelf. We integrate these carbonate platform data with an outer shelf/upper slope thallium (Tl) isotope dataset (global ocean redox proxy) that was locally anoxic throughout the Middle Ordovician. The I/Ca data record a progressive oxygenation of regional seafloor environments, with well-oxygenated waters sourced from the palaeoequator-ward Iapetus Ocean and peak I/Ca values coinciding with remarkable biodiversity increases and ecosystem reorganizations. Concurrently, our Tl isotopic dataset from local reducing sediments suggests a global ocean backdrop of a protracted increase in marine oxygenation through the Middle to Late Ordovician. The collective data illustrate a complex picture wherein dominant influences of the Ordovician Radiation likely varied between places and times, thus highlighting the importance of local–regional scale studies tied to the diversity records. Lastly, connecting local–regional–global scale proxies across different lithologies and different environments provides a more nuanced and holistic perspective on major biotic events in Earth’s history.