The major-ion evolution of seawater: Fluid inclusion evidence from terminal Proterozoic, Early Cambrian, Silurian, and Tertiary marine halites

This study uses Environmental Scanning Electron Microscope analyses of primary fluid inclusions from marine halites and geochemical modeling to determine the major-ion composition of terminal Proterozoic, Early Cambrian, Silurian, and Tertiary seawaters. The oceans of the Early Cambrian and Silurian had lower concentrations of Mg2+, Na+, and SO ₄2−, and much higher concentrations of Ca 2+ relative to the ocean's present-day composition. Furthermore, Early Cambrian and Silurian seawater had Ca2+ in excess of SO ₄2−. Evaporation of these modeled seawater compositions produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The Mg 2+/Ca2+ ratios of Early Cambrian and Silurian seawaters were ∼1, lower than the present-day seawater value of 5.2. Seawaters with Mg2+/Ca2+ <2 facilitate the precipitation of low-magnesian calcite (mol% Mg <4) marine ooids and submarine carbonate cements whereas seawaters with Mg2+/Ca2+ >2 (e.g. modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg2+/Ca2+ ratio of seawater, not the PCO ₂ of the atmosphere. Terminal Proterozoic and Tertiary seawaters were qualitatively similar to modern seawater, in that SO₄2− was more abundant that Ca2+. However, these seawaters had lower concentrations of Mg2+ and SO₄ 2−, relative to modern seawater, and slightly higher concentrations of K+. The fluid inclusions from Tertiary marine halites indicate that the concentrations of K+ and Ca2+ have decreased and Na+, Mg2+ and SO₄ 2− have increased in seawater over the past ∼35 Ma. The modeled compositions of terminal Proterozoic and Early Cambrian seawaters indicate a three-fold increase of seawater Ca2+ concentrations during the Early Cambrian (544–515 Ma), which produced chemical conditions conducive for biomineralization and may have triggered the Cambrian Explosion of marine shell-building organisms. This increase in seawater Ca2+ coincided with a short-lived drop in marine 87Sr/ 86Sr during the breakup of Rodinia and subsequent formation of Gondwanaland, which suggests a link between the advent of biocalcification, increased global tectonic activity, and hydrothermal mid-ocean ridge brine production. This Early Cambrian surge in oceanic Ca2+ was likely the first such increase following the rise of metazoans.