Kinetic controls on the early Neoproterozoic carbonate factory
- Nicholas Tosca
University of Cambridge, UK

Geologists have long hypothesized that the distinctive styles of nucleation and crystal growth preserved in Precambrian rocks reflect long-term maintenance of CaCO3 supersaturation, but the specific kinetic inhibitors that may have facilitated these conditions remain poorly constrained. In this talk I will discuss new experimental and analytical approaches to understanding the dynamics of CaCO3 formation from early Neoproterozoic seawater. Our experimental data show that of the kinetic inhibitors supplied by early Neoproterozoic seawater, variations in marine PO4 (at the μmol/kg level) would have strongly altered the dynamics of inorganic CaCO3 precipitation and fabric development, in turn imparting unique petrographic characteristics to early Neoproterozoic carbonate sediments. Consistent with this hypothesis, solid-state NMR and synchrotron-based μ-XRF and P-XANES collected from early Neoproterozoic carbonates show that elevated marine PO4 concentrations, expressed as lattice-bound phosphate, carbonate fluoroapatite inclusions and as discrete phosphatic early diagenetic cements, were associated with carbonate sedimentation across a variety of lithofacies. Together, these data suggest that kinetically-controlled shifts in early Neoproterozoic marine carbonate chemistry may have: (1) influenced the partitioning of carbon burial between organic and inorganic forms; (2) impacted atmospheric pCO2 and climatic stability; (3) imposed environmental pressure on eukaryotes to control unwanted calcification, potentially establishing a key impetus for eukaryotic biomineralization.

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