Friday, October 14, 2016

Evidence of Fluctuating Paleoatmospheric Oxygen Levels Before and After the Cryogenian Marinoan (Snowball Earth) Glaciations


Rodler et al


Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmospheric oxygenation and continental weathering thus facilitating the reconstruction of the redox state of ancient seawater. We use the δ53Cr signature coupled with REE+Y patterns and redox-sensitive trace elements to monitor environmental changes recorded by marine carbonates of the Otavi Group, Namibia. These carbonates were deposited in a platform and foreslope setting in subtropical latitudes during the Neoproterozoic and comprise the transition from a marine depositional setting through glaciation into a postglacial environment in four stages. Preglacial carbonates (Stage 1) yield positively fractionated δ53Cr values, increased U and Mn concentrations, indicative of mobilization during oxidative terrestrial weathering and stabilization in oxic surface waters. Carbonates deposited just before the Ghaub diamictites (Stage 2) record δ53Cr values (>+0.4 ‰) comparable to modern seawater and negative Ce anomalies (∼0.7) characteristic for oxygenated seawater. We interpret this as a pulse of intense oxidative weathering shortly before the advance of the glaciers. Marginal shale contamination persists in carbonates of both sections and is slightly elevated during the glacial aftermath; Cr is vulnerable towards detrital contamination. Early postglacial cap dolostones (Stage 3) were influenced by enhanced detrital contamination potentially supplied by freshwater particulate load, which was then drastically reduced in the overlying postglacial limestones in the upper Maieberg Fm (Stage 4) where near-preglacial δ53Cr values are reached again. REE+Y patterns along with Eu and Ce anomalies record a transformation from a marine, slightly anoxic and stratified water column with distal hydrothermal influence to a freshwater-influenced depositional environment with decreased hydrothermal activity and fluctuating oxic surface water conditions after glacial retreat. Here, we demonstrate that carbonate δ53Cr signatures are sensitive to changes in continental weathering balanced between detrital contamination and oxidative weathering on land and are capable of tracing fluctuating redox conditions prior and after one of the major syn-Marinoan glaciations.

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