The dolomite problem revisited

A reassessment of the abundance of dolomite in carbonate sediments has conf irmed that carbonates deposited during the past 150 Ma contain, on average, less dolomite than Proterozoic and Paleozoic carbonates. The lower dolomit e content of the more recent carbonate sediments results from the increase in the deposition of CaCO3 in deep-sea sediments, and to the difficulty of dolomitizing deep-sea CaCO3 by reaction with cold, unevaporated seawater. T he decrease in the rate of dolomite formation during the past 150 Ma has le d to an increase in the output of oceanic Mg+2 by the reaction of seawater with clay minerals anti with ocean-floor basalts. The increase in the outpu t of marine Mg+2 into these reservoirs has been brought about by an increas e in the Mg+2 concentration of seawater. During the past 40 Ma, the concent ration of Mg+2 in seawater has probably increased by similar to 18 mmol/kg, and probably has been accompanied by an equimolar increase in the concentr ation of SO4-2.