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 [1]. 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 [2]. 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 [3]. The geochemical fingerprint of these sediments is preserved through closed-system diagenesis where the altering fluid is rock buffered [4]. 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.

1. Henderson, G.M., et al. EPSL 1999 169 (1-2) p. 99-111
2. Higgins. J.A., et al. Unpubl.
3. Husson, J.M., et al., GCA, 2015. 160 p 243-266
4. Blättler, C. L. et al., EPSL, 2015. 419 p 32-42.