Boyle, R. A.3; Clark, J. R.4; Poulton, Simon W.5; Shields-Zhou, G.6; Canfield, Donald Eugene7; Lenton, T. M.3
1 Nordic Center for Earth Evolution (NordCEE), Department of Biology, Faculty of Science, SDU2 Department of Biology, Faculty of Science, SDU3 Univ Exeter, Coll Life & Environm Sci, Earth Syst Sci Grp, Exeter EX4 4PS, Devon4 Plymouth Marine Lab, Plymouth PL1 3DH, Devon5 University of Leeds6 UCL, Fac Maths Phys Sci, Dept Earth Sci, London WC1E 6BT7 Nordic Center for Earth Evolution (NordCEE), Department of Biology, Faculty of Science, SDU
Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic similar to 0.55 Ma, despite oxygenation of Earth's atmosphere nearly 2 Gyr earlier. Marine sediments from 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 N-2-fixers dominate the production in the photic zone. Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO3- with denitrification in lower anoxic waters, and N-2-fixation- driven production overlying euxinia. Interchange between these states likely explains the varying H2S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.