A MODEL FOR CALCIUM, MAGNESIUM AND SULFATE IN SEAWATER OVER PHANEROZOIC TIME

A theoretical model has been constructed for calculating the concentrations of dissolved calcium, magnesium and sulfate in seawater over the past 550 million years (Phanerozoic time). It is demonstrated that the weathering of Ca carbonates and silicates by carbonic acid can be generally ignored in the modeling. A previous model of the carbon cycle is used to guide input to the oceans of Mg from carbonate and silicate weathering. Similarly a previous model of the sulfur cycle is used to guide input fluxes to the ocean by calcium sulfate weathering and the weathering of Ca and Mg carbonates and silicates by sulfuric acid. The sulfuric acid is derived from the oxidation of pyrite. during weathering plus the oxidation of reduced sulfur gases derived from the metamorphic/volcanic decomposition of pyrite.. Calcium is removed from seawater as sedimentary carbonates and sulfates, SO4 is removed as sedimentary pyrite and CaSO4, and Mg is removed by exchange for Ca via basalt-seawater reaction or dolomitization.

Modeling results show good agreement with results of studies of salt fluid inclusions for variations of Ca and SO4. This is especially true of the timing when Ca was more abundant in seawater than SO4 and vice versa. Agreement was also found with fluid inclusion results for changes in the Mg/Ca ratio thereby distinguishing “calcite seas” from “aragonite seas”. Because of this overall agreement the modeling offers a new explanation of why oceanic Ca, Mg and SO4 have varied over Phanerozoic time.