Redox fluctuations in the Early Ordovician oceans: an insight from chromium stable isotopes
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Chromium (Cr) stable isotopes are a useful tracer of changes in redox conditions because changes in its oxidation state are accompanied by an isotopic fractionation. Recent co-precipitation experiments have shown that Cr(VI) is incorporated into the calcite lattice, suggesting that carbonates are a potential sink for dissolved Cr in the ocean and could effectively capture the Cr isotope composition of ambient seawater. Thus, the Cr isotopic composition (δ53Cr) of carbonates has the potential to record changes in the oxygenation state of ancient oceans. To test the potential of this system, we measured δ53Cr values of limestones and dolostones from an Early Ordovician marine carbonate platform (Cerro La Silla section of the Precordillera, San Juan, Argentina). An increasing number of studies support the hypothesis that the Early Ordovician oceans were stratified with respect to oxygen and that the global ocean experienced multiple episodes of anoxic expansion and contraction, making this time period ideal for testing the usefulness of the Cr isotope system. The most prominent feature of the Cerro La Silla Cr isotope profile is a positive excursion (of ~+0.5‰) during the Late Tremadocian, accompanied by exceptionally low Cr concentrations (< 0.05 mg/kg) at the peak of the excursion. Initial 87Sr/86Sr ratios remain relatively stable (0.70897 ± 0.00001) over this time period, indicating that this excursion is not due to major changes in terrestrial runoff or hydrothermal input into the global ocean. It is likely, then, that the excursion reflects changes in redox conditions, with the most tenable cause being reductive drawdown of dissolved Cr(VI). The lack of major perturbations in carbon and sulphur isotope trends over the Early Ordovician, however, indicates that the Tremadocian Cr isotope positive excursion was not in response to changes in the redox state of the global ocean, but instead is a perturbation in response to changes in local redox conditions.
|Number of pages
|Published - 2017
- Carbonates, Chromium isotopes, Ordovician oceans, Redox proxy