Spatial pattern of marine oxygenation set by tectonic and ecological drivers over the Phanerozoic

Marine redox conditions (that is, oxygen levels) impact a wide array of biogeochemical cycles, but the main controls of marine redox since the start of the Phanerozoic about 538 million years ago are not well established. Here we combine supervised machine learning with shale-hosted trace metal concentrations to reconstruct a near-continuous record of redox conditions in major marine depositional settings. We find synchronously opposite redox changes in upper ocean versus deep shelf and (semi-)restricted basin settings ('redox anticouples', nomen novum ) in several multi-million-year intervals, which can be used to track the positions of oxygen-minimum zones and the primary locations of organic burial through time. These changes coincided with biological innovations that altered large-scale oxidant-reductant fluxes (mid-Palaeozoic spread of land plants; Mesozoic plankton revolution) and tectonic upheavals that regulated sea-level elevation (Pangaea amalgamation and break-up). We find that the pre-Devonian deep shelf was buffered at a largely anoxic state probably by dissolved organic matter, switched to a transitional state during the Devonian–Carboniferous interval characterized by the inception of persistent oxygen-minimum zones and subsequently shifted to a redox regime featuring thin oxygen-minimum zones ballasted probably by particulate organic matter. Deep shelf redox changes are correlated with background extinction rates of marine animals, and mass extinctions during major redox transitions generally were more severe.