Widespread iron-rich conditions in the mid-Proterozoic ocean

The search for Proterozoic iron The ancient oceans are thought to have been ferruginous (anoxic, with abundant Fe2+), until much of the ocean's dissolved iron content was dumped to form strata known as banded iron formations (BIFs), about 1.8 billion years ago. The prevailing view has been that increasing levels of hydrogen sulphide titrated this iron out of solution, and that mid-Proterozoic oceans were euxinic (sulphur-rich and anoxic; the atmosphere was weakly oxygenated). Recently, it has been proposed that iron-rich conditions persisted beyond BIF deposition, but the supporting evidence is slight, not least because there is a one-billion-year gap in the geological record. Planavsky et al. fill this gap, examining the iron reactivity of rock samples from this period to determine the gross chemistry of the ocean in which the rocks were formed. They show that mid-Proterozoic oceans were indeed ferruginous, thereby calling into question widespread interpretations of what BIFs tell us about their environment.