Oxygenation of Ediacaran Ocean recorded by iron isotopes

Abstract

The increase in atmospheric oxygen during the late Neoproterozoic Era (ca. 800–542 Ma) may have stimulated the oxy- genation of the deep oceans and the evolution of macroscopic multicellular organisms. However, the mechanism and magni- tude of Neoproterozoic oxygenation remain uncertain. We present Fe isotopes, Fe species and other geochemical data for two sections of the Doushantuo Formation (ca. 635–551 Ma) deposited after the Nantuo glacial episode in the Yangtze Gorge area, South China. It is highlighted that highly positive d 56 Fe values reflect a lower oxidation rate of Fe(II) aq under ferrugi- nous conditions, and in turn near zero d 56 Fe values indicate oxidizing conditions. Our study suggests that during the depo- sition of the bottom of Member II of the Doushantuo Formation the shallow seawater was oxic, but the deep water was characterized by ferruginous conditions, which is consistent with a redox chemical stratification model. Subsequent anoxic conditions under shallow seawater, represented by positive d 56 Fe and negative d 13 C carb excursions, should be ascribed to the upwelling of Fe(II) aq and dissolved organic carbon (DOC)-rich anoxic deep seawater. The oxidation of Fe (II) aq and DOC-rich anoxic deep seawater upon mixing with oxic shallow water provides an innovative explanation for the well-known negative d 13 C carb excursions (ENC2) and positive d 56 Fe excursions in the middle of Doushantuo Formation. Meanwhile, the upwelling Fe (II) aq - and DOC-rich anoxic deep seawater could increase photosynthetic activity. The balance between oxygen consumption and production was most important criteria for the oxygenation of Early Ediacaran Ocean and diversity of eukaryotic organisms.