A model for calcium, magnesium and sulfate in seawater over Phanerozoic time

Abstract

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 by H 2 CO 3 . 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 sulfur gases derived from the metamorphic/volcanic decomposition of pyrite and CaSO 4 . Calcium is removed from seawater as sedimentary carbonates and sulfates, SO 4 is removed as sedimentary pyrite and CaSO 4 , and Mg is removed by exchange for Ca via basalt-seawater reaction and/or dolomitization (plus minor burial in sediments with pyrite as anoxic dolomite). By adjusting starting conditions modeling results can be found that agree well with results of studies of salt fluid inclusions for variations of Ca and SO 4 . This is especially true of the timing when Ca was more abundant in seawater than SO 4 and vice versa. General 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 perspective on why oceanic Ca, Mg and SO 4 have varied over Phanerozoic time.