Local δ34S Variability in ~580 MA Carbonates of the Clemente Formation, Caborca, Mexico: Implications for Neoproterozoic Marine Sulfate

Many δ34S records have been produced from carbonate-associated sulfate (CAS) in order to understand the oxidation state of the Neoproterozoic oceans. Here, a well-defined stratigraphic interval containing the Wonoka-Shuram (W-S) δ13C excursion was analyzed to explore the sulfur isotope record. First, the local δ34S record from multiple, closely spaced sections near Caborca, Mexico, was examined to explore potential local heterogeneities, and then these were compared to more distant sections from Death Valley, Oman and S. China. In the Caborca region, the inception and isotopic minimum of the W-S excursion is located in the Clemente Formation and coincides with a locally extensive oolite marker bed. Five sections over ~25 km of lateral distance exhibit significant variability in δ34SCAS (range from +18.6 to +27.6% VCDT) and severe variability in CAS concentration (range from <30 to >1,200 ppm). Unlike the δ34SCAS values, CAS concentrations show strong negative correlation with Mn/Sr ratios and Fecarb concentrations, consistent with CAS removal upon meteoric recrystallization. When compared to coeval W-S strata in Death Valley and Oman, these three locations record a decrease in δ34SCAS and an increase in CAS concentration, but the absolute values, the detailed isotope profiles, and the magnitude of the changes are distinct from one another. South China, on the other hand does not exhibit a decrease in δ34SCAS, or a relative increase in CAS concentration in the W-S inception facies. The variability in isotopic compositions likely developed from either local δ34Ssulfate heterogeneity or non-meteoric diagenetic overprinting. Whereas Neoproterozoic oceanic heterogeneity in δ34Ssulfate has been proposed, it has not been reported on such a local scale. In addition, the existence of variability among the Caborca sections in which strata were all deposited at the same depth indicates that heterogeneity could have occurred laterally as well as vertically (as in a stratified water column). This degree of disagreement in δ34SCAS has not as of yet been explicitly addressed, however it must be considered in order to rigorously understand the chemistry of Neoproterozoic oceans.