OCEAN REDOX CONTROL ON NUTRIENT CYCLING IN THE EARLY NEOPROTEROZOIC

Multiple independent lines of evidence suggest that the mid-Proterozoic ocean was dominantly anoxic, with expansive euxinia along productive continental margins and in epicontinental seas, overlying deeper waters that were ferruginous. In contrast, geochemical analyses of later Neoproterozoic marine successions suggest that euxinic environments became much rarer, and instead the global ocean was dominantly ferruginous, prior to the onset of deep ocean oxygenation ~580 million years ago. However, a major gap exists in the record of ocean redox conditions in the early Neoproterozoic, and it is currently not clear whether conditions at this time were similar to the mid-Proterozoic or later Neoproterozoic. To address this, we have focused on reconstructing ocean redox conditions in the ~1.0 Ga Liulaobei, Jiuliqiao and Sidingshan Formations of the Huainan and Feishui Groups, Huainan region, north China craton. These sedimentary successions begin with relatively deep-water continental slope deposits, but shallow upwards to intertidal stromatolitic dolomites, and are thus ideal for identifying potential spatial variability in ocean redox conditions. However, we find no evidence for euxinic water column conditions at any point in the succession, and instead the water column was anoxic and ferruginous throughout this region, suggesting that the early Neoproterozoic was likely characterised by conditions similar to the later Neoproterozoic. In addition, we have reconstructed nutrient cycling at this time, based on detailed analyses of P speciation. In contrast to recent reports, our data suggest that the early Neoproterozoic ocean was highly oligotrophic, due to uptake of P in association with Fe minerals and subsequent sequestration in deposited sediments.