Widespread iron-rich conditions in the mid-Proterozoic ocean.

The chemical composition of the ocean changed markedly with the oxidation of the Earth’s surface1, and this process has profoundly influenced the evolutionary and ecological history of life2,3. The early Earth was characterized by a reducing ocean–atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radi- ation of animals. The redox characteristics of surface environments during Earth’s middle age (1.8–1billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean2,3. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe21-rich) conditions were both spatially and tem- porally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal dis- tribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.