NEW CONSTRAINTS FOR THE OXIDIZATION OF DEEP OCEANS

It remains largely unknown when the ancient deep oceans reached their present oxygenated state. In one model the anoxic Archean oceans became progressively oxidized until they passed the anoxic-oxic threshold at ~1.8 Ga. A more protracted changeover is envisaged by others, one in which a transitional ocean state developed for most of the Proterozoic Eon. This transitional state involved oceans that were oxic near surface and either euxinic (anoxic and sulfidic) or suboxic at depth. The number of known Proterozoic deepwater successions makes choosing any one model in favor of another difficult at present. Compounding the problem of sparse stratigraphic data, is the consideration of whether paleoredox proxies are representative of local or global conditions. Here we report the presence of primary hematite within distal shelf to pelagic pink limestone of the Pranhita-Gordavari Valley and Chhattisgarth basins of India. These successions, which were deposited in a basin open to global circulation, serve as a vital window into late Mesoprotorezoic ocean chemistry. Our results, together with independent geochemical data, suggest that an oxygenated deepwater column existed before 1180 Ma. This has important implications for both the availability of nutrients and biological development in the run-up to the Ediacaran Period.