Analysis of isotope-transfer kinetics during sulfate reduction by dextrose under hydrothermal conditions

Abstract

Sulfur isotopes are fractionated during the reduction of sulfate by dextrose under hydrothermal conditions. The changes in the isotopic composition of sulfate during the reaction have been previously interpreted to reflect the kinetic isotope effect associated with the initial reduction reaction involving sulfate. By a separate pathway, isotope exchange between sulfate and newly formed sulfide may also affect the isotopic composition of sulfate. Computer simulations of the isotopic evolution of the experiments were pursued in this study to determine whether sulfate-sulfide isotopic exchange influenced the isotopic composition of sulfate and whether the reported kinetic isotope effects represent the kinetic isotope effects due solely to sulfate reduction. Numerical solutions of the rate laws governing the isotopic species show that a sufficiently fast isotope-exchange reaction influences the isotopic composition of sulfate. Comparison of the observed rates of sulfate reduction with the expected rates of sulfate-sulfide isotope exchange under the experimental conditions suggests that the exchange reaction largely determines the isotopic composition of sulfate in the experiments. Therefore, the kinetic isotope effects calculated previously are different from, and probably larger than, the true kinetic isotope effects associated with the reduction of sulfate by dextrose.