Triple-oxygen isotope (δ¹⁸O and $\mathrm{\Delta }\prime$¹⁷O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past.Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which may rapidly exchange oxygen atoms with surrounding water.Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (¹⁸αand ¹⁷α) between water and each individual sulfoxyanion.Despite this importance, there currently exist only limited experimental ¹⁸αdata and no ¹⁷αestimates due to the difficulty of isolating and analyzing short-lived intermediate species.To address this, we theoretically estimate ¹⁸αand ¹⁷αfor a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate species—using quantum computational chemistry.We determine fractionation factors for sulfoxyanion “water droplets” using the B3LYP/6-31G+(d,p) method; we additionally calculate higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) scaling factors and we qualitatively estimate the importance of anharmonic zero-point energy (ZPE) corrections using a suite of gaseous sulfoxy compounds.Methodological scaling factors greatly impact ¹⁸αpredictions, whereas ZPE corrections are likely small (i.e., $⩽1$‰) at Earth-surface temperatures; existing experimental data best agree with ¹⁸αpredictions when including redox state-specific CCSD/aug-cc-pVTZ scaling factors.Theoretical pH- and temperature-specific bulk-solution (i.e., abundance-weighted average of all species) ¹⁸αvalues yield root-mean-square errors for sulfate/water, sulfite/water, and thiosulfate/water equilibrium of 4.5‰ ($n=18$ experimental conditions), 3.7‰ ($n=27$), and 2.2‰ ($n=3$), respectively.However, sulfate- and sulfite-system agreement improves considerably when comparing experimental results only to SO₃(OH)^-/H₂O (RMSE = 1.6‰) and SO₂(OH)^-/H₂O (RMSE = 2.2‰) predictions, rather than bulk solutions.This is particularly true for the sulfite system at high and low pH, when SO₂(OH)^- is not the dominant species.We discuss potential experimental and theoretical biases that may lead to this apparent improvement.By combining ¹⁸αand ¹⁷αpredictions, we additionally estimate that sulfate, sulfite, sulfoxylate, and thiosulfate species can exhibit$\mathrm{\Delta }\prime$¹⁷O values as much as 0.199‰, 0.205‰, 0.101‰, and 0.186‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = $0.5305$). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.

三氧同位素 ( δ ¹⁸ O 和$\mathrm{\Delta }\text{'}$¹⁷ O) 硫酸盐分析正成为评估几种生物和非生物硫循环过程的常用工具，无论是在今天还是在过去的地质过程中。多步硫氧化还原反应通常涉及中间亚硫酸阴离子，例如亚硫酸盐、次硫酸盐和硫代硫酸盐，这可以与周围的水快速交换氧原子。因此，基于过程的重建需要了解水和每个单独的亚砜之间的平衡氧同位素分馏因子（¹⁸ α和¹⁷ α）。尽管如此重要，但目前仅存在有限的实验性¹⁸ α数据没有¹⁷ α由于难以分离和分析短寿命中间物种而导致的估计。为了解决这个问题，我们使用量子计算化学从理论上估计了一组含硫氧阴离子（包括几种硫酸盐、亚硫酸盐、次硫酸盐和硫代硫酸盐物种）的¹⁸ α和¹⁷ α 。我们使用 B3LYP/6-31G+(d,p) 方法确定亚砜“水滴”的分馏因子；我们还计算了高阶方法（CCSD/aug-cc-pVTZ 和 MP2/aug-cc-pVTZ）比例因子，并且我们使用一组气态亚砜化合物定性地估计了非谐波零点能量 (ZPE) 校正的重要性。方法学比例因子对¹⁸ α的影响很大预测，而 ZPE 修正可能很小（即，$⩽1$‰) 在地球表面温度下；当包括特定于氧化还原状态的 CCSD/aug-cc-pVTZ 比例因子时，现有实验数据最符合¹⁸ α预测。理论 pH 和温度特定的本体溶液（即，所有物种的丰度加权平均值） ¹⁸ α值产生硫酸盐/水、亚硫酸盐/水和硫代硫酸盐/水平衡的均方根误差为 4.5‰ ($n=18$实验条件), 3.7‰ ($n=27$), 和 2.2‰ ($n=3$)，然而，当仅将实验结果与 SO ₃ (OH) ^- /H ₂ O (RMSE = 1.6‰) 和 SO ₂ (OH) ^- /H ₂ O (RMSE = 2.2‰) 预测，而不是本体溶液。当 SO ₂ (OH) ^-不是主要物种时，这对于高 pH 值和低 pH 值的亚硫酸盐体系尤其如此。我们讨论可能导致这种情况的潜在实验和理论偏差明显改善。通过组合¹⁸ α和¹⁷ α预测，我们还估计硫酸盐、亚硫酸盐、次硫酸盐和硫代硫酸盐物种可以表现出$\mathrm{\Delta }\text{'}$¹⁷ O 值比地球表面温度下的平衡水低 0.199‰、0.205‰、0.101‰ 和 0.186‰（参考线斜率 =$0.5305$）。这一理论框架为解释几个硫循环过程的实验和观察三氧同位素结果提供了基础，这些过程包括黄铁矿氧化、微生物代谢（例如硫酸盐还原、硫代硫酸盐歧化）和热液硬石膏沉淀。我们用几个例子来强调这一点。