Heterogeneous redox evolution of the Meso-Neoproterozoic ocean: Insights from eastern China

Abstract

Redox-sensitive element (RSE) compositions in marine sedimentary archives have been important in improving our understanding of redox history, especially in the Meso-Neoproterozoic ocean, where early biological innovations took place before the Cambrian Explosion. However, despite increasing availability of RSE data that represent this long time interval, temporal comparisons are difficult, and thus provide poor constraints on the dramatic changes in atmosphere-ocean chemical settings. Here we analyzed carbonate rocks and black shales from the North China and Yangtze platforms (eastern China) for trace elements and isotopic compositions (δ13C, δ34S and 87Sr/86Sr). These data were combined with previously published RSE data to allow establishment of successive elemental chemostratigraphies from sequences of both black shale and carbonate RSE archives, thus yielding direct insights into the role of marine RSE cycling. Generally low Th/RSE ratios in relatively pure carbonate rocks (Al < 0.35%, Th < 0.5 μg/g) and black shales likely indicate enriched RSEs in oxidized shallow conditions, consistent with a more oxidized Neoproterozoic ocean or confined water body. The chemostratigraphy of RSEs not only shows enrichment in Neoproterozoic marine sediments, but temporally variable trends in the Meso-Neoproterozoic. A mass balance model based on modern marine budgets demonstrates that even a small oxidation event within the dominantly anoxic setting could be an important factor for increasing dissolved RSE reservoirs, and that observed heterogeneous elemental records may represent the amplified results of a fluctuating palaeoredox (oxic-anoxic) structure with pulsed marine oxidation. Meso-Neoproterozoic marine redox heterogeneity in eastern China suggests that increases in free oxygen were potentially related to: 1) supercontinent cycles, 2) enhanced weathering inputs with high 87Sr/86Sr ratios (> 0.705 at ca. <1.4–1.1 Ga), and 3) previously reported biological processes as emphasized by biological C-S cycles. Thus, pulsed oxic conditions may have provided sequential opportunities for biological innovation.