MODELS FOR THE COUPLED SR-SULFATE BUDGET IN DEEP-SEA CARBONATES

A numerical model for the Sr and sulfate budgets during deposition of deep-sea carbonate sediments was developed and used to fit porewater S r2+ and SO42- data from DSDP Sites 593, 590, 574, 573, 572, 516, and 2 89. The Sr and sulfate budgets are coupled whenever the porewaters bec ome saturated with respect to celestite (SrSO4). Models for the porewa ter Sr and sulfate at Sites 593, 590, 516, and 289 suggest that porewa ter Sr concentrations are controlled by the precipitation of celestite and predict maximum celestite concentrations of the order of 0.02 wt% for sediments at Sites 593, 590, and 289, and trace amounts of celest ite at Site 516. The actual amount of celestite predicted by the model is not well constrained because it depends on the as yet poorly known value of the equilibrium distribution coefficient for Sr between calc ite and porewaters. The fact that celestite has been found in sediment s from Sites 590 and 593 is used to derive an upper bound for the Sr d istribution coefficient, that suggests that all laboratory determined values for the Sr distribution coefficient between calcite and porewat er are too large. The occurrence of celestite at Sites 593, 590, 516, and 289 does not significantly change earlier estimates of the rates o f solution-reprecipitation of calcite and its consequences in terms of shifting the Sr content or Sr isotopic composition of the bulk sedime nt from what the sediment incorporated when originally deposited. The sulfate data at Sites 593 and 590 are best fit by invoking a flow of p orewater within the sediments of the order of 75 m/Ma. The Sr and sulf ate concentrations in porewaters from Sites 572, 573, and 574 never re ach celestite saturation and thus the two budgets are decoupled. Both the Sr and the sulfate data require that all three Sites have abundant unaltered seawater flowing in the underlying basement and can only be modeled if seawater concentrations of Sr and sulfate are imposed as b oundary conditions at both the sediment-water interface and the bottom of the section. Model sulfate profiles for Sites 572, 573, and 574 co nfirm prior estimates of the rate of porewater flow at these Sites tha t were made using Sr data.