CARBONATE ASSOCIATED SULFATE (CAS) COMPOSITION OF CAMBRIAN RIBBON ROCKS AND RETHINKING THE VERACITY OF CAS METHOD

Carbonate associated sulfate (CAS) is widely used to reconstruct seawater sulfate isotopic composition (δ³⁴S_sw), marine sulfur cycle in paleoceans, as well as chemistratigraphic correlations. However, the stratigraphic and spatial variations of δ³⁴S_CAS cast doubt on its veracity. To understand the origin of δ³⁴S_CAS oscillation, we studied ribbon rocks of the middle Cambrian Xuzhuang and Zhangxia formations in North China and measured sulfur isotopic compositions of CAS (δ³⁴S_CAS) extracted from ribbon rock samples. Ribbon rocks consist of alternating limestone and marlstone layers, which precipitate in different environments and have distinct diagenetic history. The limestone layers have consistently higher δ³⁴S_CAS values but lower CAS contents than the marlstone layers. Such differences cannot be explained by rapid oscillation of δ³⁴S_sw, because deposition of a limestone-marlstone couplet takes hundreds to thousands of years. Although authigenic carbonate precipitation within the microbial sulfate reduction zone may explain the data, this model requires more than half of limestone be authigenic, which is inconsistent with low Mn and Fe contents in the limestone. With the consideration of seafloor precipitation of Paleozoic carbonates and potential influence of benthic flux from porewater, low δ³⁴S_CAS of the marlstone might result from more severe influence of benthic flux, which delivers Fe²⁺, Mn²⁺, and ³⁴S-depleted sulfate (originating from H₂S oxidation) from porewater to seawater. This model is consistent with higher Fe and Mn contents and lower sedimentation rate of the marlstone. Finally, our study suggests that δ³⁴S_CAS may not necessarily record δ³⁴S of sea water, even diagenetic alteration can be excluded. Only carbonate with low Mn and Fe contents might be suitable for CAS analyses.