Extreme ocean anoxia during the Late Cryogenian recorded in reefal carbonates of Southern Australia

• Cryogenian dolomite marine cements preserve their parent seawater chemistry.
• Geochemical analysis of cements reveals the structure of a Neoproterozoic ocean.
• Cements reveal a peritidal chemocline over a stratified, ferruginous watermass.
• Seawater chemical and redox gradients are present in shallow water.
• These ferro-sulfidic seas encompass some of the most extreme ocean anoxia recorded.

The Neoproterozoic was a time of great change in the Earth's surface and marine environments, including extensive climate variability, the widespread oxygenation of the oceans and the accompanying rise of animal life. However, the timing of ocean oxygenation remains uncertain, particularly in regard to Cryogenian seas, which were disrupted by large periods of global glaciation. Interglacial Cryogenian reef complexes in the Northern Adelaide Fold Belt of Australia contain abundant primary marine dolomite cements. These cements have well-preserved textural and growth zonation, indicating they preserve their original marine chemistry and can be used as geochemical proxies for Late Cryogenian paleooceanography. Analysis of marine cements from peritidal nearshore facies, shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo-depth. While nearshore cements have low Fe contents and commonly contain iron-oxide inclusions, the shallow and deep cements have very high Fe concentrations. Chalcophile elements (Cu, Cd, Pb, Zn, etc.) are most abundant in the nearshore cements, whereas rare earth elements are found in highest concentrations in the deeper facies cements. Rare earth element profiles are unusual, with shallow and deep facies having convex profiles with negligible Ce/Ce* anomalies and positive Eu/Eu* anomalies.Being constrained by sedimentology, this carbonate geochemistry provides a window into interglacial Cryogenian ocean chemistry and structure. The marine cements reveal pronounced chemical stratification in this Late Cryogenian ocean. A thin veneer of oxic surface waters existed at the ocean surface, in peritidal facies, with increasingly anoxic and Fe-rich seawater at depth. The distribution of strongly chalcophile elements like Cd and Cu across the chemocline suggests that although ferruginous, deeper anoxic waters probably contained some dissolved sulphide. These conditions describe a ferro-sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late Precambrian. A return to Archean-like ocean conditions at this time suggests large-scale disruption of the ocean system during the Neoproterozoic.