Oscillations in Phanerozoic seawater chemistry: Evidence from fluid inclusions

Systematic changes in the chemistry of evaporated seawater contained in pri mary fluid inclusions in marine halites indicate that seawater chemistry ha s fluctuated during the Phanerozoic. The fluctuations are in phase with osc illations in seafloor spreading rates, volcanism, global sea level, and the primary mineralogies of marine limestones and evaporites. The data suggest that seawater had high Mg2+/Ca2+ ratios (>2.5) and relatively high Na+ con centrations during the Late Precambrian [544 to 543 million years ago (Ma)] , Permian (258 to 251 Ma), and Tertiary through the present (40 to 0 Ma), w hen aragonite and MgSO4 salts were the dominant marine precipitates. Conver sely, seawater had low Mg2+/Ca2+ ratios (<2.3) and relatively low Na+ conce ntrations during the Cambrian (540 to 520 Ma), Silurian (440 to 418 Ma), an d Cretaceous (124 to 94 Ma), when calcite was the dominant nonskeletal carb onate and K-, Mg-, and Ca-bearing chloride salts, were the only potash evap orites.