Paper No. 214-5
Presentation Time: 9:25 AM
STABLE CA AND SR ISOTOPE MULTI-PROXY PROVIDES EVIDENCE FOR PRIMARY SIGNAL PRESERVATION IN MARINOAN CAP CARBONATES
In the 1980s, Knoll and colleagues first recognized large secular variations of δ13C values in upper Proterozoic marine carbonate rocks. These pioneering observations helped generate the hypothesis that the Earth experienced at least two global-scale glaciations during the Neoproterozoic (aka Snowball Earth events). Carbonate sequences (“cap carbonates”) immediately overlie the glacial deposits and occur worldwide. Their origin remains highly controversial. Here, we apply the novel δ44/40Ca-δ88/86Sr multi-proxy to two Marinoan (ca. 635 Ma) cap carbonate sequences from Namibia and find that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an offshore section, dolostone δ44/40Ca and δ88/86Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater. Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates correlate with elevated 87Sr/86Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. A nearshore dolostone shows greater effects from water-mass mixing, but still reveals that precipitation rates increased up-section. Overlying limestones show the greatest Ca and Sr contributions from seawater. Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects in multiple rock records, including Marinoan cap carbonates and Cretaceous OAE1a shallow-water carbonates, provides a basis for developing the δ44/40Ca-δ88/86Sr multi-proxy as an indicator of carbonate saturation state and pCO2.