Application of the δ44/40Ca-δ88/86Sr multi-proxy to Namibian Marinoan cap carbonates

Several studies have exploited the stable calcium (Ca) and strontium (Sr) isotope compositions of marine carbonate rocks to investigate ancient carbon cycle dynamics, seawater geochemistry, controls on production preservation. However, two proxies rarely been applied together. Based knowledge achieved date, δ44/40Ca-δ88/86Sr multi-proxy can distinguish signatures imparted by mass-dependent fractionation reservoir mixing, including early diagenesis shifts in isotopic composition seawater. In detail, represents a five-isotope system because determination δ88/86Sr values requires analysis fractionation-corrected 87Sr/86Sr ratios, which provide additional constraints mixing. Here, we apply novel Marinoan (ca. 635 Ma) “cap carbonate” sequences from Namibia. Cap carbonates were widely deposited after Neoproterozoic Snowball Earth Event, but controversy surrounds their origin. We find that archive primary environmental signals deriving combination seawater-glacial meltwater mixing kinetic effects. an outer platform section, dolostone δ44/40Ca define line predicted for fractionation. This mostly precipitated meltwater. Moreover, stratigraphically higher samples exhibiting fastest precipitation rates display elevated consistent with long-held expectations rapid deglacial weathering pulse forced cap formation. An inner-platform shows greater effects water-mass still reveals increased up-section. Overlying limestones show Ca Sr contributions Amplification local coastal processes during global ice sheet collapse offers simple sufficient proposition explain heterogeneity carbonates. Detection provides evidence predominantly rock-buffered further basis developing as indicator saturation state pCO2.