  The Neoproterozoic Earth experienced at least two global-scale glaciations
termed Snowball Earth events. 'Cap carbonates' were widely deposited after the
events, but controversy surrounds their origin. Here, we apply the novel
$\delta^{44/40}$Ca-$\delta^{88/86}$Sr multi-proxy to two Marinoan (ca. 635 Ma)
cap carbonate sequences from Namibia and show that the rocks archive primary
environmental signals deriving from a combination of seawater-glacial meltwater
mixing and kinetic isotope effects. In an outer platform section, dolostone
$\delta^{44/40}$Ca and $\delta^{88/86}$Sr 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. An inner-platform 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 the rock record provides a basis for
developing the $\delta^{44/40}$Ca-$\delta^{88/86}$Sr multi-proxy as an
indicator of saturation state and $p$CO$_2$.
