Isotopic evidence of the pivotal role of sulfite oxidation in shaping the oxygen isotope signature of sulfate

Abstract

The oxygen isotope composition of sulfate serves as an archive of oxidative sulfur cycling. Studies on the aerobic oxidation of reduced sulfur compounds showed discrepancies in the relative incorporation of oxygen from dissolved molecular oxygen (O2) and water (H2O) into newly formed sulfate, which likely result from slight differences in the production and consumption rate of sulfoxy intermediates that exchange oxygen isotopes with water. Sulfite is often considered the final sulfoxy intermediate in the oxidation of reduced sulfur compounds to sulfate and its residence time strongly affects the oxygen isotope signature of produced sulfate. However, data on the oxygen isotope signature of sulfate derived from sulfite oxidation are scarce.We determined the oxygen isotope effects of abiotic oxidation of sulfite with O2 or ferric iron (Fe3+) under different pH conditions (pH1, 4.9 and 13.3). These parameters impact the relative contribution of oxygen from H2O and O2 to the produced sulfate, and control the competition between the rates of oxygen isotope exchange between sulfite and water and the sulfite oxidation. There is a striking overlap in the range of oxygen isotope offsets between sulfate and water from our experiments at different chemical conditions (Δ18OSO4−H2O from 5.9‰ for anaerobic oxidation with Fe3+ up to 17.6‰ for oxidation at low pH with O2 as sole oxidant, respectively) with the variations in the oxygen isotope composition of sulfate derived from oxidative processes in the environment. This implies that oxygen isotope effects during sulfite oxidation largely control the isotope signature of sulfate derived from the oxidation of sulfur compounds. However, our results also show that preexisting non-equilibrium isotope signatures of sulfite are likely partially preserved in the final sulfate product under most environmental conditions. Our study furthermore provides a mechanistic explanation for positive isotope offsets between the oxygen isotope composition of sulfate and water observed in anoxic pyrite oxidation experiments with Fe3+ as the sole oxidizing agent. This apparently inverse isotope effect is caused by the interplay between sulfite-water oxygen exchange and normal kinetic isotope fractionation effects during sulfite oxidation, the former driving the isotope composition towards the isotope equilibrium fractionation between sulfite and water, inducing a positive offset whereas the latter induces a negative offset.