Tour of Ordovician Paleoredox Conditions

After decades of research biogeochemists have identified several intervals of variable marine oxygenation throughout Earth's history. These fluctuations in oxygenation have been proposed to directly correlate to changes in ancient biodiversity, and potentially be primary drivers for these observed changes in faunal richness. The Ordovician was a dynamic time in terms of marine biodiversity, as it hosts the largest increase in biodiversity events in the Phanerozoic, which occurs in the Early to Middle Ordovician commonly referred to as the Great Ordovician Biodiversification Event (GOBE). This major marine radiation is subsequently terminated by the second largest mass extinction in Earth's history that occurs in the Late Ordovician, commonly referred to as the Late Ordovician Mass Extinction (LOME). While the primary causal mechanisms for both events remain under debate, an emerging field of study has associated an increase in marine oxygenation with the GOBE, while a decrease in marine oxygenation has been proposed for the LOME. Using a suite of paleoredox proxies, we present new geochemical datasets surrounding key intervals of the GOBE and LOME to elucidate the paleoredox landscape surrounding these events and link the known changes in biodiversity with fluctuations in marine oxygenation. These paleoredox proxies include local proxies that have previously been shown to constrain basinal changes in redox, which allows the use of global proxies to identify global changes in redox which might influence global biodiversity. These local redox proxies include manganese concentrations, iodine-to-calcium ratios, iron speciation, and pyrite sulfur isotopic compositions, while the global redox proxies include thallium isotopic compositions, vanadium, uranium and molybdenum concentrations, and carbonate-associated sulfate isotopic compositions. The major increase in faunal richness associated with the GOBE is thought to begin after the Cambrian‒Ordovician boundary, however paleoredox conditions surrounding this interval of time is largely under constrained. Currently a singular paleoredox dataset has been published to constrain paleoredox conditions across this boundary, however this dataset is limited in stratigraphic range, and is not tied to changes in biodiversity. New geochemical datasets from the Baltic Basin presented here from modern day southern Sweden (Scania) provide additional context for paleoredox conditions during the initiation of the GOBE. A progressive increase in oxygenation succeeding the Cambrian‒Ordovician boundary is interpreted based on trace metal dynamics and thallium isotopic compositions, which may be providing enhanced metabolic efficiency and new niche space due to newly oxygenated shelf areas leading into the later Ordovician. The Middle‒Late Ordovician is associated with peak faunal diversity which has been attributed to enhanced marine oxygenation. Similarly with the Cambrian‒Ordovician boundary, very few studies have provided direct paleoredox evidence for a progressive increase in marine oxygenation, and thus redox conditions surrounding this integral period of diversification. New geochemical datasets from the Baltic Basin show a long-term increase in marine oxygenation throughout this interval though novel thallium isotopic compositions. The primary causal mechanisms for the LOME remains unclear, however there is growing evidence that anoxia is playing a larger role than initially thought. Classically, the LOME occurs between two extinction pulses, the first being attributed to climatic cooling and subsequent expansion of Gondwanan glaciation which resulted in widespread habitat loss, while the second pulse is associated with climatic warming, deglaciation, and an expansion of globally reducing conditions. New geochemical evidence from three carbonate successions derived from hydrographically dispersed basins show evidence of persistent reducing conditions throughout the Late Ordovician though low iodine-to-calcium ratios, while carbonate-associated sulfate isotopic suggest an overall reduction of global pyrite burial. Lastly, new thallium isotopic compositions from two Upper Ordovician basins of predominantly organic-rich shales identify highly variable global burial of manganese oxides which is intimately linked to marine (de)oxygenation and the two extinction pulses of the LOME. These thallium isotopic trends show that there were two major fluctuations between oxygenated and reducing conditions, which suggests that dynamic changes in oxygenation levels is a primary control on marine biodiversity, rather than the magnitude of reducing conditions.