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

Abstract

Carbonate associated sulfate (CAS) is widely used to reconstruct the deep time seawater sulfate sulfur isotopic composition (δ34Ssw) and marine sulfur cycle. However, δ34SCAS normally shows substantial stratigraphic and spatial variations, casting doubt on its validity in recording δ34Ssw. To understand the origin of δ34SCAS oscillation, we measured sulfur isotopic compositions of CAS (δ34SCAS) extracted from ribbon rock samples collected from the middle Cambrian Xuzhuang and Zhangxia formations in North China. The limestone layers have consistently higher δ34SCAS values but lower CAS contents than the marlstone layers. Sulfate oxygen isotope data (δ18OCAS, Δ17OCAS) 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 δ34SCAS values in marlstone and limestone layers may reflect two predominant processes. Low δ34SCAS of the marlstone might mainly result from the influence of benthic H2S flux. Oxidation of H2S in the seafloor would generate 34S-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 δ34SCAS and lower CAS contents. Finally, our study suggests that δ34SCAS record δ34Ssw only when the influence of both benthic flux and early diagenesis was insignificant.