Reconsidering the Hirnantian oceanic anoxic event – exploring the evidence for and implications of a late Ordovician oceanic oxygenation event

The Ordovician-Silurian transition was a time of marked upheaval in global climate, ocean oxygenation and marine biodiversity. Geochemical and sedimentary evidence for anoxia in the late Hirnantian and Rhuddanian has led to the coining of a Hirnantian oceanic anoxic event, linked to globally widespread organic carbon burial and potentially a second pulse of the Late Ordovician Mass Extinction. Evidence from stable uranium isotopes indicates that a late Hirnantian shift towards more expanded anoxic marine conditions was global in nature, supporting widespread sedimentological data. However, how stable was the inferred early-middle Hirnantian background state of near-modern ocean oxygenation? If the late Katian was in fact characterised by a warm climate and expanded anoxic marine conditions (as hinted at by geochemical data), should we actually be reframing our discussion of late Ordovician ocean oxygenation and discussing a late Ordovician oceanic oxygenation event (or OOE) similar to those discussed in the late Neoproterozoic?

 

We integrate an updated synthesis of geochemical data across the Ordovician-Silurian with a new stage-by-stage series of 3D biogeochemical models to provide an updated perspective on the end-Ordovician Earth system. Our intermediate complexity Earth system modelling framework builds on global circulation and long-term carbon cycle modelling by coupling cGENIE to existing SCION and HADCM3L simulations. This enables us to present new reconstructions of 3D ocean biogeochemistry over key intervals of the Phanerozoic, including dissolved oxygen in shelf environments, volume of oxygen minimum zones and seafloor redox. Here, we find that circulation in icehouse climates has a dramatic impact on reconstructed ocean oxygenation. We further integrate this series with a sensitivity analysis interrogating the importance of Earth system boundary conditions to these predictions of non-linear climatic drivers of ocean oxygenation. Finally, we place this experimental analysis in the context of our knowledge of the Ordovician-Silurian Earth system and highlight future directions to reconcile data and model perspectives.