Local δ~(34)S variability in ～580Ma carbonates of northwestern Mexico and the Neoproterozoic marine sulfate reservoir

摘要

Many δ~(34)S records have been produced from carbonate-associated sulfate (CAS) in order to understand the oxidation state of the Neoproterozoic oceans, but interregional correlation is complicated by the absence of robust chronostratigraphic markers. Here, a globally correlatable stratigraphic interval containing the Wonoka-Shuram (W-S) δ~(13)C excursion was analyzed to explore variability in the sulfur isotope record. In the excursion-containing units, the local δ~(34)S record from multiple, closely spaced sections in Sonora, Mexico, was examined to explore potential heterogeneities, and then these were compared to more distant sections elsewhere.In Sonora, the inception and isotopic minimum of the W-S excursion is located in the Clemente Formation and coincides with an extensive oolite marker bed. Five sections containing this marker bed span ～25km of lateral distance, exhibit significant variability in δ~(34)S_(CAS) (range: +18.6 to +27.6‰ VCDT) and high variability in CAS concentration (range: <30 to >1200ppm). Unlike the δ~(34)S_(CAS) values, CAS concentrations show strong negative correlation with Mn/Sr ratios and Fe_(carb) concentrations, consistent with CAS removal upon diagenetic recrystallization and an absence of an accompanying sulfur isotopic fractionation. Indeed, samples containing low CAS concentrations exhibit petrographic characteristics consistent with diagenetic recrystallization including fabric destructive neomorphism and relatively coarse-crystalline textures.Coeval W-S strata in Death Valley, Oman and the least altered Sonora samples record a decrease in δ~(34)S_(CAS) and an increase in CAS concentration; however, the magnitude of the changes are distinct, perhaps reflecting differential response to a transient oxidation event. In contrast, time equivalent facies of South China do not exhibit a decrease in δ~(34)S_(CAS) nor a relative increase in CAS concentrations. The variability in geochemical characteristics likely developed from local and/or regional marine δ34Ssulfate heterogeneity.Although Neoproterozoic oceanic heterogeneity in δ~(34)S_(sulfate) has been proposed, it has not been reported on such close spatial scales. In addition, the existence of variability among the Sonora sections in which strata were all deposited at similar depth indicates that heterogeneity could have occurred laterally. Finally, these findings demonstrate that while CAS concentrations may be drastically affected by diagenesis, δ~(34)S_(CAS) may retain primary signatures.