Figure 4: Redox conditions of the atmosphere and oceans.
(a) During the second Palaeoproterozoic glaciation, the atmospheric O2 levels are likely to have been low. It is considered that the shallow-marine environments were also reducing during the glaciation. (b) In the glacial aftermath, the atmospheric O2 levels would have become high, sufficient to deliver radiogenic Os to the ocean. The redox-sensitive elements on continents, such as Os, Re and Mo, would have been delivered to the ocean under oxidizing atmospheric conditions and then would have been reduced and deposited in shallow-marine environments, leading to enrichments in the sediments.
