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摘要
Chromium isotopes are powerful in recording redox fluctuations in marine environments, potentially tracing past atmospheric oxygenation and continental weathering. We present delta Cr-53 signals with Sr-87/Sr-86, delta C-13 and delta O-18, and element abundances in carbonates of two shallow marine successions of the Otavi Group, Namibia, stretching over four stages from preglacial setting to retreat of the late Neoproterozoic Ghaub glaciation (similar to 635 Ma). The fluctuating but generally heavy delta Cr-53 signals indicate oxygenated shallow seawater prior to the glaciation (stage 1). Sustained high delta C-13(carb) signatures at that time suggest organic matter formed a greater proportion of buried carbon, which implies a net increase in oxidant flux to the surface. The delta O-18 signals fluctuate, with a tendency towards lighter values approaching the Ghaub Fm. Correlating delta Cr-53 and delta C-13(carb) values indicate disruptions affecting the carbon cycle and oxygen availability (stage 2). Prior to the Ghaub diamictite, a sequence of Sr-87/Sr-86 in the range typical of late Neoproterozoic seawater occurs with high Sr concentrations, low Mn/Sr and low Mg/Ca. Enhanced detrital input after the Ghaub glaciation (stage 3) indicates elevated weathering rates likely accompanied by enhanced nutrient influx. The delta Cr-53-delta C-13(carb) patterns are resolved in a two-step recovery towards the late glacial aftermath (stage 4), which is characterized by values similar to those of the early preglacial interval. There, a sequence of postglacial limestones also records typical late Neoproterozoic seawater Sr-87/Sr-86 ratios. The carbonate delta Cr-53 signatures at the base of the postglacial sequence are characterized by values even below the range of bulk silicate Earth (BSE). We hypothesize that this is due to (i) redox cycling of Cr in seawater, e.g. by (partial) reduction of Cr(VI) during microbial-mediated cap dolostone deposition and/or at the redox boundary of redox-stratified seawater and/or due to (ii) increased contribution of negatively fractionated Cr associated with an enhanced input of detrital-derived Cr from postglacial weathering and/or subaqueous hydrothermal influx. Previously it was demonstrated that carbonate delta Cr-53 signatures trace changes in continental weathering and provide valuable indication of fluctuating seawater redox conditions prior to and after one of the major syn-Marinoan glaciations. Furthermore, changes in delta Cr-53 values might scale with shifts in bioproductivity.
- 出版日期2017-9
