Plain Language Summary
Changes in ocean oxygen contents have had major impacts on marine biodiversity since the rise of animals 600 million years ago. Here we use paired iodine concentrations and sulfur isotope data to constrain paleoredox conditions surrounding the Late Ordovician Mass Extinction (LOME; ~445 Ma), the second-largest mass extinction in Earth history, and the only of the “Big 5” that occur during icehouse conditions. Despite previous work, the role of oxygen deficiency during the LOME remains controversial and under constrained. This study is the first multi-basinal and multiproxy datasets to specifically reconstruct local and global marine redox conditions surrounding the two LOME pulses. These results suggest that a unique and vacillating combination of anoxic and euxinic (anoxic and sulfidic water column) marine conditions. Thus, redox variability tied with climatic cooling, and glacioeustacy were potential mechanisms leading to the first mass extinction in the Phanerozoic.