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.