Spatial variability in oceanic redox structure 1.8 billion years ago

The evolution of ocean chemistry during the Proterozoic eon (2.5-0.542 billion years ago) is thought to have played a central role in both the timing and rate of eukaryote evolution(1,2). The timing of the deposition of iron formations implies that, early in the Earth's history, oceans were predominantly anoxic and rich in dissolved iron(3). However, global deposition of iron formations ceased about 1.84 billion years ago. This termination indicates a major upheaval in ocean chemistry(4), but the precise nature of this change remains debated(5-8). Here we use iron and sulphur systematics to reconstruct oceanic redox conditions from the 1.88- to 1.83-billion-year-old Animikie group from the Superior region, North America. We find that surface waters were oxygenated, whereas at mid-depths, anoxic and sulphidic (euxinic) conditions extended over 100 km from the palaeoshoreline. The spatial extent of euxinia varied through time, but deep ocean waters remained rich in dissolved iron. Widespread euxinia along continental margins would have removed dissolved iron from the water column through the precipitation of pyrite, which would have reduced the supply of dissolved iron and resulted in the global cessation of the deposition of 'Superior-type' iron formations. We suggest that incursions of sulphide from the mid-depths into overlying oxygenated surface waters may have placed severe constraints on eukaryotic evolution.