Iron Cycling and Redox Evolution in the Precambrian

The chemical composition of the ocean changed dramatically with the oxidation of the Earth's surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth is generally thought to have been characterized by a reducing ocean-atmosphere system, while the Phanerozoic Eon (<542 million years ago) is known for a stably oxygenated biosphere conducive to the radiation of animals. However, the exact redox landscape of the Earth's oceans is poorly constrained. Similarly, the redox characteristics of surface environments during Earth's middle age (1.8 to 1 billion years ago) are poorly known. I will provide several lines of evidence that even the mixed layer of the surfical ocean was anoxic in the Archean. Oxygen must have been restricted to very localized regions of the regions--`oxygen oases'. Further, I will present evidence that anoxic and Fe(II)-rich conditions were both spatially and temporally extensive across diverse paleogeographic settings in the mid-Proterozoic ocean. This is a new view of Proterozoic redox conditions; it has been commonly assumed for the last decades that the mid-Proterozoic was home to a globally sulfidic (euxinic) or oxic deep ocean. Lastly, I will explore the underlying controls behind the transition from the reducing state that characterized most of the Precambrian to the oxidized state typical of the Phanerozoic. More specifically, I propose based, on a new record of how marine phosphate concentrations have changed through time, that a spike in phosphate levels in the ocean in late Precambrian was key in causing the switch to a more oxidizing ocean atmosphere system.