Geochemical evidence invokes anoxic deep oceans until the terminal NeoproterozoicB0.55 Ma, despite oxygenation of Earth’s atmosphere nearly 2 Gyr earlier. Marine sedimentsfrom the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters,interspersed with euxinia (anoxic H2S-rich conditions) along productive continental margins.Today, sustained biotic H2S production requires NO3- depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N2-fixers dominate the production inthe photic zone.Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show howthese feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separationbetween two states: photic zone NO3- with denitrification in lower anoxic waters, and N2-fixation-driven production overlying euxinia. Interchange between these states likely explainsthe varying H2S concentration implied by existing data, which persisted until the Neoproterozoicoxygenation event gave rise to modern marine biogeochemistry.
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