Sequential extraction of carbonate-associated sulfate from calcite and dolomite in carbonate rocks

Carbonate-associated sulfate (CAS)—trace SO42- incorporated into carbonate mineral lattices by substituting for CO32—serves as a valuable geochemical proxy for reconstructing seawater chemistry, redox conditions, microbial sulfate reduction activity, and diagenetic sequences in ancient marine environments, cold seeps, and gas hydrate reservoirs. However, conventional CAS extraction methods, which typically use HCl to digest bulk carbonate samples, yield mixed isotopic and concentration signals from coexisting calcite and dolomite. Therefore, it is difficult to precisely distinguish the sulfur isotopes of CAS in the original sedimentary environment from that resulting from secondary alteration. In order to isolate calcite-hosted CAS (calcite-CAS) from dolomite-hosted CAS (dolomite-CAS), we evaluated six acetic acid leaching conditions in sequence (0.1-0.5 mol/L, 1-2hours). The results indicate that treatment with 0.1 mol/L acetic acid for 1 hour effectively releases calcite-CAS while suppressing dolomite dissolution. When applied to natural carbonate outcrop samples, this approach reveals that the δ³4S values of calcite-CAS are consistently ~8.22‰ higher than those of coexisting dolomite-CAS. Notably, the lowest measured δ³4SCalcite-CAS values are 2-5‰ higher than previously reported bulk CAS data, suggesting that earlier records were biased by the inclusion of dolomite-CAS. We therefore recommend the 0.1 mol/L acetic acid leaching protocol (1 hour) for extracting calcite-CAS from dolomite-bearing carbonate rocks, as this enables more accurate reconstruction of depositional and diagenetic conditions.