Coupling between sulfur recycling and syndepositional carbonate dissolution: evidence from oxygen and sulfur isotope composition of pore water sulfate, South Florida Platform, U.S.A.

Abstract

Sulfur cycling in Fe-poor, organic-rich shelf carbonates, known to have rapid rates of SO4−2 reduction, remains poorly studied despite the volumetric significance of shelf deposits in modern and ancient carbon budgets. We investigated sulfur cycling in modern carbonates of the Florida Platform from end-member depositional environments (muddy sands from the Atlantic reef tract and finer-grained mudbank and island flank deposits from Florida Bay). Relations between pore water chemistry (SO4−2, ΣCO2, Ca−2/Cl−) and oxygen and sulfur stable isotope compositions of SO4−2 require direct coupling between sulfur redox cycling and syndepositional carbonate dissolution.Oxygen isotope compositions of pore water sulfate were remarkably shifted away from the established value for marine SO4−2 (+9.5‰), despite near normal SO4−2/Cl− ratios. Chemical evolution was least in reef tract pore waters and greatest in Florida Bay. Relative to overlying seawater, mudbank sediments exhibited sulfate depletion, with δ18OSO4 and δ34SSO4 values both increasing by about 7‰. More bioturbated island flank sediments, colonized by Thalassia grass, had a 5‰ increase in δ18OSO4, variable δ34SSO4 values (+17.7 to +23.3‰) and exceptionally high Ca+2/Cl− ratios. The large excess of Ca+2 (up to 1.7 mM) requires a much larger acid source than the amounts derived from utilization of dissolved O2 (∼0.3 mM) and small degrees of net SO4−2 reduction (<0.5 mM reduced).A conceptual model was constructed using chemical and isotopic data on natural pore waters and on sulfate isotope fractionation factors obtained from sediment incubation experiments. The model outputs show that pore water compositions can be explained by a redox cycle where microbial SO4−2 reduction is followed by very efficient H2S oxidation, thus maintaining virtually invariant SO4−2/Cl− ratios. The enhanced O2 transport may be driven by associated marine grass rhizome systems and microbial communities established in bioturbated sediments. The net result of the cycle is that the rate of sulfide oxidation, which is largely balanced by the rate of microbial sulfate reduction, is stoichiometrically related to the rate of carbonate dissolution. This is consistent with previously reported rates of carbonate dissolution (∼400 μmol/cm2-yr) and average rates of sulfate reduction (∼200 μmol/cm2-yr) from the Florida Platform and a 2:1 stoichiometry.