Marine cements reveal the structure of an anoxic, ferruginous Neoproterozoic ocean

Neoproterozoic oceans provided the setting for the rise of animals, yet little is known of their chemical composition. Marine carbonates from the Cryogenian Oodnaminta Reef Complex, South Australia, reveal the chemical structure of a Neoproterozoic ocean. Pseudo-depth profiles from shallow- to deep-water reef facies have been constructed from geochemical and sedimentological analysis of marine cements. Evidence suggests that under a peritidal oxic–anoxic chemocline, the water column was largely anoxic, strongly ferruginous and had a chemistry profoundly different from that of modern seawater. These geochemical data suggest early Archaean-like conditions for this late Cryogenian ocean, posing problems for metazoan evolution in extremely anoxic conditions.