A new model for Proterozoic ocean chemistry

There was a significant oxidation of the Earth's surface around 2 billion y ears ago (2 Gyr)(1-4). Direct evidence for this oxidation comes, mostly, fr om geological records of the redox-sensitive elements Fe and U reflecting t he conditions prevailing during weathering(1-3). The oxidation event was pr obably driven by an increased input of oxygen to the atmosphere arising fro m an increased sedimentary burial of organic matter between 2.3 and 2.0 Gyr (5). This episode was postdated by the final large precipitation of banded iron formations around 1.8 Gyr(1,2). It is generally believed that banded i ron formations precipitated from an ocean whose bottom waters contained sig nificant concentrations of dissolved ferrous iron, and that this sedimentat ion process terminated when aerobic bottom waters developed, oxidizing the iron and thus removing it from solution(1,2). In contrast, I argue here tha t anoxic bottom waters probably persisted until well after the deposition o f banded iron formations ceased; I also propose that sulphide, rather than oxygen, was responsible for removing iron from deep ocean water. The sulphu r-isotope record supports this hypothesis as it indicates increasing concen trations of oceanic sulphate, starting around 2.3 Gyr(6), leading to increa sing rates of sulphide production by sulphate reduction. The increase in su lphide production became sufficient, around 1.8 Gyr,to precipitate the tota l flux of iron into the oceans. I suggest that aerobic deep-ocean waters di d not develop until the Neoproterozoic era (1.0 to similar to 0.54 Gyr), in association with a second large oxidation of the Earth's surface. This new model is consistent with the emerging view of Precambrian sulphur geochemi stry and the chemical events leading to the evolution of animals, and it is fully testable by detailed geochemical analyses of preserved deep-water ma rine sediments.