Experimental determination of equilibrium fractionation of triple oxygen isotopes between dissolved sulfite species and water

Abstract

Sulfite, a key intermediate sulfoxyanion in both the reductive and oxidative sulfur cycles, rapidly exchanges oxygen isotopes with ambient water under circumneutral to acidic conditions. Equilibrium oxygen isotope fractionation factors (¹⁸α and ¹⁷α) between sulfite and water are therefore critical for interpreting the triple oxygen isotope composition of sulfate and for constraining sulfur cycling. However, equilibrium ¹⁸α values remain poorly constrained, with significant discrepancies among experimental and theoretical estimates, largely due to experimental challenges in determining sulfite oxygen isotope compositions and uncertainties in theoretical calibrations. Moreover, equilibrium ¹⁷α values have previously been derived only through theoretical calculation. In this study, we applied a pH-shifting technique to eliminate kinetic isotope effects during sulfite precipitation. We also employed a recently developed high-temperature reduction–discharge–CO₂/O₂ isotope exchange technique to minimize the influence of sulfite hygroscopicity and precisely measured the triple oxygen isotope composition of sulfite equilibrated with water across a pH range of 4.60 to 8.89 and temperature range of 12 to 55 °C. Our results show a consistent, monotonic dependence of oxygen isotope fractionation between bulk sulfite (encompassing all S(IV)-oxyanions, including SO₃^2– and the bisulfite isomers (HS)O₃^– and SO₂(OH)^–) and water on both pH and temperature. We attribute the former to pH-controlled speciation of dissolved sulfite. From our data, we derived equilibrium fractionation factors for oxygen isotopes between bisulfite and water, and between sulfite (SO₃^2–) and water:

1000ln¹⁸α_{bisulfite–H2O} = (7.06 ± 1.06) × 10³/T – 8.80 ± 3.49

1000ln¹⁸α_{sulfite–H2O} = (6.59 ± 1.32) × 10³/T – 12.56 ± 4.34

The associated mass-dependent fractionation exponents (θ = ln¹⁷α/ln¹⁸α) are temperature-independent within our studied range, with values of 0.5202 ± 0.0003 for bisulfite and 0.5155 ± 0.0008 for sulfite. These imply that the Δ′¹⁷O values of bisulfite and sulfite are offset by -0.152‰ and −0.143‰, respectively, from that of ambient water (assuming a reference slope of 0.5305) at 25 °C. Our findings offer new insights into the isotope fractionations associated with microbial sulfate reduction and pyrite oxidation. The observed pH-dependent variations in sulfate δ¹⁸O signatures during abiotic aerobic pyrite weathering may reflect oxidation of distinct sulfite species under variable pH conditions. Furthermore, the isotope fractionation observed between residual sulfate and water during microbial sulfate reduction suggests a preferential reoxidation of (HS)O₃^– to sulfate.