The Neoproterozoic Era marks a critical turning point along Earth’s evolutionary trajectory. Sedimentary rocks from this time period record the breakup of a supercontinent, the advent of eukaryotic biomineralisation, the origin of complex multicellularity, and the most significant and enigmatic perturbations to climate and the carbon cycle in Earth’s history. Nevertheless, a lack of constraints on ocean-atmosphere carbon chemistry has left inorganic carbon burial as a critical yet poorly understood factor in regulating Earth’s surface carbon reservoir. In what ways did the pre-skeletal carbonate factory influence the late Proterozoic C-cycle?
This talk will review new field, experimental, and micro-analytical data in an ongoing effort to unravel the dynamics of CaCO3 nucleation and growth from early Neoproterozoic seawater. These data indicate that the marine inorganic carbon reservoir of the early-mid Neoproterozoic was highly unstable. Seawater periodically featured elevated alkalinity in the presence of high atmospheric pCO2, which sustained excessive marine CaCO3 supersaturation (ΩCalcite); but in ~50 Ma, this inorganic carbon reservoir was halved.
This long-term maintenance of high CaCO3 supersaturation in turn requires the presence of kinetic inhibitors in order to have increased the ΩCalcite threshold for CaCO3 nucleation. Experimental observations indicate that of the kinetic inhibitors supplied by Tonian seawater, variations in marine PO4 (at the μmol/kg level) would have most strongly altered the dynamics of inorganic CaCO3 precipitation and fabric development, in turn imparting unique petrographic characteristics to Tonian carbonates. Consistent with this expectation, 31P solid state NMR and synchrotron-based μ-XRF and P-XANES collected from Tonian carbonates show that elevated marine PO4 concentrations, expressed as carbonate fluoroapatite inclusions within CaCO3 and as early diagenetic cements, were associated with carbonate sedimentation across a variety of lithofacies.
If representative of other late Proterozoic carbonate depositional systems, these data suggest that kinetically-controlled shifts in ΩCalcite 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, thus establishing a key impetus for eukaryotic biomineralization. Thus, exploring connections between inorganic CaCO3 precipitation kinetics and the Precambrian carbonate record should continue to offer new insight into C-cycle dynamics before the advent of biological calcification.