Seawater chemistry and the advent of biocalcification

Major ion compositions of primary fluid inclusions from terminal Proterozoic (ca. 544 Ma) and Early Cambrian (ca. 515 Ma) marine halites indicate that seawater Ca²⁺ 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 ⁸⁷Sr/⁸⁶Sr 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 [Ca²⁺] was likely the first such increase following the rise of metazoans and may have spurred evolutionary changes in marine biota.