Sulfur and oxygen isotopic compositions of carbonate associated sulfate (CAS) of Cambrian ribbon rocks: Implications for the constraints on using CAS to reconstruct seawater sulfate sulfur isotopic compositions

Carbonate associated sulfate (CAS) is widely used to reconstruct the deep time seawater sulfate sulfur isotopic composition (delta S-34(sw)) and marine sulfur cycle. However, delta S-34(CAS) normally shows substantial stratigraphic and spatial variations, casting doubt on its validity in recording delta S-34(sw). To understand the origin of delta S-34(CAS) oscil-lation, we measured sulfur isotopic compositions of CAS (delta S-34(CAS)) extracted from ribbon rock samples collected from the middle Cambrian Xuzhuang and Zhangxia formations in North China. The limestone layers have consistently higher delta S-34(CAS) values but lower CAS contents than the marlstone layers. Sulfate oxygen isotope data (delta O-18(CAS), Delta O-17(CAS)) suggest that such isotopic differences can neither be explained by contamination of secondary atmospheric sulfate (SAS) nor oxidation of pyrite. Instead, the different trends of modification on delta S-34(CAS) values in marlstone and limestone layers may reflect two predominant processes. Low delta S-34(CAS) of the marlstone might mainly result from the influence of benthic H2S flux. Oxidation of H2S in the seafloor would generate S-34-depleted sulfate that is subsequently taken in carbonate precipitated in the seafloor. In contrast, the limestone might have precipitated in more oxic seafloor and was mainly sourced from seawater, with less influence of benthic flux. The diagenetic dissolution and reprecipitation of limestone within MSR zone accounts for the higher delta S-34(CAS) and lower CAS contents. Finally, our study suggests that delta S-34(CAS) record delta S-34(sw) only when the influence of both benthic flux and early diagenesis was insignificant.

Automated summary

Carbonate associated sulfate (CAS) is widely used to study the ancient seawater sulfur isotopic composition, but the variations in δS-34(CAS) values suggest different processes in marlstone and limestone layers. The lower δS-34(CAS) in marlstone may be influenced by benthic flux, while the higher values in limestone could be due to diagenetic processes. The study concludes that δS-34(CAS) may accurately reflect seawater sulfur isotopic composition when benthic flux and early diagenesis are not significant.