Paper No. 44-8
Presentation Time: 10:05 AM
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 (δ34Ssw), marine sulfur cycle in paleoceans, as well as chemistratigraphic correlations. However, the stratigraphic and spatial variations of δ34SCAS cast doubt on its veracity. To understand the origin of δ34SCAS oscillation, we studied ribbon rocks of the middle Cambrian Xuzhuang and Zhangxia formations in North China and measured sulfur isotopic compositions of CAS (δ34SCAS) 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 δ34SCAS values but lower CAS contents than the marlstone layers. Such differences cannot be explained by rapid oscillation of δ34Ssw, 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 δ34SCAS 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 δ34SCAS 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.