Paper No. 277-4
Presentation Time: 8:50 AM
CA AND MG ISOTOPE RECORD OF THE DESCENT AND ESCAPE OF THE END-CRYOGENIAN GLACIATION
Extreme negative carbon isotope (δ13C) excursions below the canonical mantle value are found globally in carbonate rocks from the Neoproterozoic Era. One of the more spectacular is the Trezona excursion that pre-dates the end-Cryogenian glaciation and contains a steady decline of 16-18‰ δ13C. In the glacial aftermath, the enigmatic cap-carbonate sequence reaches -5‰ δ13C. Despite the geologic association, the mechanistic relationship between these negative excursions and the glacial sequence remains unclear. In the modern icehouse, glacio-eustatic change has been linked to increased fluid flow through carbonate platforms potentially driving extensive early diagenesis in carbonate sediments . Here we present a simple diagenetic dissolution-reprecipitation model that demonstrates the isotopic and mineralogical changes associated with early diagenesis and dolomitization. We use the modern seawater composition as boundary conditions to model calcium (δ44Ca) and magnesium (δ26Mg) isotope measurements from a substantial new data set from the Bahama Banks . Extending this model, we explore the potential diagenetic boundary conditions of end-Cryogenian carbonate sediments constrained by 450 new δ44Ca and 200 δ26Mg measurements. We find that the nadir of the Trezona excursion corresponds with high Sr concentrations and light δ44Ca values, suggesting a primary aragonitic origin . The geochemical fingerprint of these sediments is preserved through closed-system diagenesis where the altering fluid is rock buffered . These results suggest that the Trezona nadir records the ancient δ13C of seawater from restricted aragonite producing settings prior to the end-Cryogenian glaciation. Furthermore, the cap carbonate sequence and the recovery of the Trezona excursion demonstrate similar correlations between δ44Ca and δ26Mg perhaps indicating open-system diagenetic resetting by a fluid of similar chemical composition.
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