Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation

Geochemical redox proxies indicate that seafloor anoxia occurred during the latest Ordovician glacial maximum, coincident with second pulse of Late mass extinction. However, expanded in a climate strikingly contrasts warming-associated Mesozoic anoxic events and raises questions as to both causal mechanism ocean deoxygenation its relationship Here we firstly report iodine-to-calcium ratio (I/Ca) data document increased upper-ocean oxygenation despite concurrent expansion anoxia. We then resolve these apparently conflicting observations well their global by means series Earth system model simulations. Applying available (Hirnantian) sea-surface temperature estimates from oxygen isotope studies constraints, alongside our I/Ca data, leads us identify scenario which Hirnantian conditions permit spread oxygenation. Our simulated reorganization circulation, reduced importance northern-sourced waters poorer ventilated deoxygenated deep has parallels Pleistocene state transitions Atlantic meridional overturning (despite very different continental configuration) suggests no simple predictable between past may exist. Reorganized circulation cooling altered through water column, provoking new look at extinction mechanism, according reconstructions using proxy modelling.