$\delta^{44/40}$Ca-$\delta^{88/86}$Sr multi-proxy constrains primary origin of Marinoan cap carbonates

TL;DR

This study introduces a novel multi-proxy approach using calcium and strontium isotopes to trace the primary environmental signals and formation processes of Marinoan cap carbonates, shedding light on post-glacial ocean chemistry.

Contribution

The paper presents the first application of the $oldsymbol{ extdelta^{44/40}Ca}$-$oldsymbol{ extdelta^{88/86}Sr}$ multi-proxy to Marinoan cap carbonates, revealing their formation from seawater-meltwater mixing and kinetic effects.

Findings

Cap carbonates record seawater and meltwater mixing signals.

Kinetic isotope effects indicate precipitation rates and saturation states.

Stratigraphic variations reflect changes in environmental conditions during deglaciation.

Abstract

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$_2$.