Geochemical Records Reveal Protracted and Differential Marine Redox Change Associated With Late Ordovician Climate and Mass Extinctions

The Ordovician (Hirnantian; 445 Ma) hosts the second most severe mass extinction in Earth history, coinciding with Gondwanan glaciation and increased geochemical evidence for marine anoxia. It remains unclear whether cooling, expanded oxygen deficiency, or a combination drove Late Mass Extinction (LOME). Here, we present combined iodine sulfur isotope data from three globally distributed carbonate successions to constrain changes local global redox conditions. Iodine records suggest locally anoxic conditions were potentially pervasive on shallow shelves, while isotopes reduction euxinic (anoxic sulfidic) Katian sulfate-sulfur show large negative excursion that initiated during elevated sea level continued through peak Hirnantian glaciation. Geochemical box modeling suggests of decreasing pyrite burial increasing weathering are required drive observed suggesting ∼3% decrease seafloor euxinia Ordovician. datasets provide further this trend was followed by increases which coincided eustatic sea-level rise subsequent deglaciation late Hirnantian. A persistence shelf anoxia against backdrop waning then waxing linked two LOME pulses. These results place important constraints throughout non-sulfidic shelfal anoxia—along glacioeustatic climatic cooling—were environmental stressors worsened fauna, resulting second-largest history only example an icehouse climate.