Sequential Extraction of Different Sulfur Species from Solid Standard Reference Materials for Sulfur Stable Isotope Analyses

Sulfur isotopic compositions provide valuable information on biogeochemical processes and paleoenvironmental conditions. Sulfur isotope analysis requires the separation of individual sulfur species prior to analysis. Here, we evaluated a sequential extraction procedure for water-soluble sulfate (WSS), carbonate-associated sulfate (CAS), acid-volatile sulfur (AVS), and chromium(II)-reducible sulfur (CRS) using 5 geological reference materials comprising 3 carbonate-dominated materials (NIST1d, JCp-1, and JLs-1) and 2 silicate-dominated materials (JMS-1 and JMS-2). Sulfur concentrations and sulfur isotope ratios (δ³⁴S) of each fraction were determined by isotope ratio mass spectrometry (IR-MS). The δ³⁴S values of WSS were highly reproducible in the carbonate-dominated materials, indicating negligible isotope fractionation during extraction. In JCp-1, CAS and WSS showed indistinguishable δ³⁴S values, indicating minimal isotope fractionation during sulfate incorporation into carbonate. In contrast, CAS in NIST1d and JLs-1 and CRS in JCp-1 and JLs-1 showed isotopic heterogeneity attributable to depositional heterogeneity, diagenetic modification, or uneven pyrite distribution. In the silicate-dominated materials, incomplete sulfur recovery indicated that sulfate hosted by clay minerals or other acid-resistant phases was not fully extracted. Contrasting behaviors of JMS-1 and JMS-2 suggest that sulfate is associated with acid-resistant Fe-bearing clay minerals in JMS-1, whereas it is hosted by readily acid-soluble Fe oxyhydroxides in JMS-2. These results demonstrate that the method provides reliable sulfur isotope data for sulfur species when sulfur concentrations are sufficient, while clarifying its limitations and improving understanding of sulfur occurrence and host phases in geological materials.