Secular Variations In the Major Ion Chemistry of Seawater: State of the Art

Global seawater has undergone significant oscillations in major element chemistry over the Phanerozoic Eon, including changes in the Mg/Ca ratio that influenced the evolution of shell building organisms and the first appearances of mineralization in the Early Cambrian. These long-term (100-200 My) variations in Phanerozoic seawater chemistry are synchronous with oscillations in global sea level, greenhouse-icehouse climates, and the primary mineralogies of marine limestones and evaporites because they all appear to be fundamentally controlled by global volcanicity. Modeling of Phanerozoic seawater major ion chemistry can be reasonably achieved using variable rates of input of river water inflow and midocean ridge seawater brines that have circulated through ridge axes and flanks. Although data are scarce, it appears from seawater inclusions trapped in unrecrystallized marine calcite cements that the salinity of paleoseawater has not changed significantly in the Phanerozoic.

Possible variations in the major ion chemistry and salinity of Proterozoic and Archean seawater chemistry are far less well known. Direct measurements of ancient evaporated Pre-Phanerozoic seawater in fluid inclusions can only be done to 830 Ma, the oldest known primary marine halite. Evidence for low sulfate concentrations is growing, and it has been suggested that the salinity of Archean seawater was double the present value of 35‰. Identification of crystal pseudomorphs of aragonite, gypsum, halite, and nahcolite (NaHCO3) can help identify primary mineralogies and mineral precipitation sequences in Proterozoic and Archean marine chemical sediments, and shed light on the major ion chemistry of the early oceans.