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X-WR-CALNAME;VALUE=TEXT:Professor Nicholas Tosca (University of Oxford) "The Tonian carbonate factory and the long-term evolution of the Precambrian CaCO3 cycle"
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SUMMARY:Professor Nicholas Tosca (University of Oxford) "The Tonian carbonate factory and the long-term evolution of the Precambrian CaCO3 cycle"
DESCRIPTION:<p>	Abstract</p><p>	 </p><p>	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?</p><p>	 </p><p>	This talk will review new field, experimental, and micro-analytical data in an ongoing effort to unravel the dynamics of CaCO<sub>3</sub> 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 <em>p</em>CO<sub>2</sub>, which sustained excessive marine CaCO<sub>3</sub> supersaturation (Ω<sub>Calcite</sub>); but in ~50 Ma, this inorganic carbon reservoir was halved.</p><p>	 </p><p>	This long-term maintenance of high CaCO<sub>3</sub> supersaturation in turn requires the presence of kinetic inhibitors in order to have increased the Ω<sub>Calcite</sub> threshold for CaCO<sub>3</sub> nucleation. Experimental observations indicate that of the kinetic inhibitors supplied by Tonian seawater, variations in marine PO<sub>4</sub> (at the μmol/kg level) would have most strongly altered the dynamics of inorganic CaCO<sub>3</sub> precipitation and fabric development, in turn imparting unique petrographic characteristics to Tonian carbonates. Consistent with this expectation, <sup>31</sup>P solid state NMR and synchrotron-based μ-XRF and P-XANES collected from Tonian carbonates show that elevated marine PO<sub>4 </sub>concentrations, expressed as carbonate fluoroapatite inclusions within CaCO<sub>3</sub> and as early diagenetic cements, were associated with carbonate sedimentation across a variety of lithofacies. </p><p>	 </p><p>	If representative of other late Proterozoic carbonate depositional systems, these data suggest that kinetically-controlled shifts in Ω<sub>Calcite</sub> may have: (1) influenced the partitioning of carbon burial between organic and inorganic forms; (2) impacted atmospheric <em>p</em>CO<sub>2 </sub>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 CaCO<sub>3 </sub>precipitation kinetics and the Precambrian carbonate record should continue to offer new insight into C-cycle dynamics before the advent of biological calcification. </p><p>	 </p>
LOCATION:Haller Hall, Geological Museum, 24 Oxford Street
STATUS:CONFIRMED
DTSTART:20190515T200000Z
DTEND:20190515T213000Z
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