Testing the preservation potential of Carbonate-Associated Sulfate in modern marine sediments

Sulfur and oxygen isotopes in sulfate (δ³⁴S_SO4 and δ¹⁸O_SO4) are key to reconstructing the sulfur cycle over geological time. Variations in marine δ³⁴S_SO4 and δ¹⁸O_SO4 reflect the balance between the weathering and burial fluxes of evaporites and pyrite, and thus the oxidative state of Earth's surface. Furthermore, sulfur and oxygen isotopes in sulfate are sensitive to changes in the pathways of microbial sulfate reduction and sulfide oxidation, key metabolisms over Earth's history; this intimately links the sulfur and carbon cycles. Carbonate-associated sulfate (CAS) is the most promising new proxy for reconstructing δ³⁴S_SO4 and δ¹⁸O_SO4 over geological time, because of the ubiquitous presence of carbonates in the rock record. However, the extent to which δ³⁴S_SO4 and δ¹⁸O_SO4 in CAS are affected by carbonate recrystallisation during marine diagenesis is not well constrained. We tested the preservation potential of CAS in four deep-sea sediment cores, all of which have high rates of carbonate recrystallization but also an isotopically evolving pool of pore fluid sulfate. We present δ³⁴S_SO4 and δ¹⁸O_SO4 from bulk carbonate CAS in these four marine locations, along with pore fluid δ³⁴S_SO4 and δ¹⁸O_SO4. Our data suggest minimal sulfate incorporation during carbonate recrystallization at these sites. These results place an upper limit on sulfate incorporation during primary diagenesis and engender a reasonable level of confidence in bulk CAS. However, the temporal variability of δ³⁴S_SO4 and δ¹⁸O_SO4 in our extracted CAS is greater than that from the coeval marine barite isotope record. The disparity between the CAS and marine barite isotope records may highlight the sensitivity of CAS to extraction artefacts - most notably sulfide oxidation and nitrate occlusion.