Abstract:
Major ion compositions of primary fluid inclusions from terminal Proterozoic (ca. 544 Ma) and Early Cambrian (ca. 515 Ma) marine halites indicate that seawater Ca{sup 2+} concentrations increased approximately threefold during the Early Cambrian. The timing of this shift in seawater chemistry broadly coincides with the 'Cambrian explosion,' a brief drop in marine {sup 87}Sr/{sup 86}Sr values, and an increase in tectonic activity, suggesting a link between the advent of biocalcification, hydrothermal mid-ocean-ridge brine production, and the composition of seawater. The Early Cambrian surge in oceanic [Ca{sup 2+}] was likely the first such increase following the rise of metazoans and may have spurred evolutionary changes in marine biota.