title: Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean.
abstract: Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic ~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 H(2)S-rich conditions) along productive continental margins. Today, sustained biotic H(2)S production requires NO(3)(-) 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 NO(3)(-) with denitrification in lower anoxic waters, and N(2)-fixation-driven production overlying euxinia. Interchange between these states likely explains the varying H(2)S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.
authors: Earth System Science Group, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4PS, UK. r.a.boyle@exeter.ac.uk
authors: R A
authors: RA
authors: Boyle
authors: J R
authors: JR
authors: Clark
authors: S W
authors: SW
authors: Poulton
authors: G
authors: G
authors: Shields-Zhou
authors: D E
authors: DE
authors: Canfield
authors: T M
authors: TM
authors: Lenton
mesh_terms: D005246:Feedback; D008962:Models, Theoretical; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000032:analysis; D058458:Nitrogen Cycle; D009586:Nitrogen Fixation; D009587:Nitrogen Isotopes; D009792:Oceans and Seas; D010100:Oxygen / Q000032:analysis*; D010758:Phosphorus / Q000032:analysis; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D013997:Time Factors
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D010758:Phosphorus; D009584:Nitrogen; D010100:Oxygen