The oxygen isotope equilibrium fractionation between sulfite species and water

Abstract

Sulfite is an important sulfoxy intermediate in oxidative and reductive sulfur cycling in the marine and terrestrial environment. Different aqueous sulfite species exist, such as dissolved sulfur dioxide (SO2), bisulfite (HSO3−), pyrosulfite (S2O52−) and sulfite sensu stricto (SO32−), whereas their relative abundance in solution depends on the concentration and the pH. Conversion of one species into another is rapid and involves in many cases incorporation of oxygen from, or release of oxygen to, water (e.g. SO2+H2O↔HSO3−+H+), resulting in rapid oxygen isotope exchange between sulfite species and water. Consequently, the oxygen isotope composition of sulfite is strongly influenced by the oxygen isotope composition of water. Since sulfate does not exchange oxygen isotopes with water under most earth surface conditions, it can preserve the sulfite oxygen isotope signature that it inherits via oxidative and reductive sulfur cycling. Therefore, interpretation of δ18OSO42- values strongly hinges on the oxygen isotope equilibrium fractionation between sulfite and water which is poorly constrained. This is in large part due to technical difficulties in extraction of sulfite from solution for oxygen isotope analysis.To overcome these challenges, anoxic isotope equilibration experiments were performed with dissolved sodium sulfite in solutions with distinct oxygen isotope signatures. Sulfite was precipitated using two different agents, barium chloride and silver nitrate. The experiments were performed at 22°C and varying pH of 1.5, 6.3, 6.6, and 9.7 to investigate how changes in sulfite speciation affect the oxygen isotope equilibrium fractionation between sulfite and water.From the experiments at pH 1.5 where SO2 is the dominant sulfite species, a rough estimate of 37.0‰ was determined for the oxygen isotope equilibrium fractionation factor between aqueous SO2 and water (εSO2↔H2OEQ). The oxygen isotope equilibrium fractionation between the aqueous phases is much larger than the known oxygen isotope equilibrium fractionation between gaseous SO2 and water vapor, probably because of a stronger association with water molecules. At pH values of 6.3–9.7 a more firm estimate for the oxygen isotope equilibrium fractionation between HSO3−, SO32− and water (εSO32-↔H2OEQ) of 15.2±0.7‰ was obtained.Our results provide new insights into the oxygen isotope fractionation during reductive and oxidative sulfur cycling. They demonstrate that isotope exchange between sulfite and water during dissimilatory sulfate reduction (DSR) alone is too small to be responsible for the apparent oxygen isotope equilibrium fractionation between sulfate and water mediated by DSR. Our estimates also provide a basis for tracing and quantifying the transformation of sulfoxy intermediates during the oxidation of reduced sulfur compounds to sulfate.