Environmental and diagenetic variations in carbonate associated sulfate: An investigation of CAS in the Lower Triassic of the western USA

Abstract

An integrated stable isotope, elemental and petrographic analysis of Early Triassic (Spathian) carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in δ 34S composition of carbonate associated sulfate (CAS). The variations in the δ 34S of CAS are strongly correlated with the Ca/Mg composition of carbonates, suggesting that the variations are driven by the degree of dolomitization. The δ 34S of dolostones and evaporites are similar to one another and exhibit lower δ 34S values than limestones from all localities. Three hypotheses may explain the differences in δ 34S between proximal dolostones/evaporites and inner/middle shelf limestones: (1) limestones experienced anaerobic sulfate reduction and subsequent incorporation of 34S-enriched sulfate into CAS during diagenesis, while dolostones did not—this is unlikely because of the lack of correlation between δ 34S CAS and TOC, as well as other indicators of diagenesis, (2) dolomitization controlled the δ 34S CAS in proximal paleoenvironments, where the source of the 34S depleted fluids was either continentally-derived or the result of Rayleigh distillation during evaporite formation, and (3) a δ 34S depth gradient existed during the Early Triassic such that limestones formed in distal waters are more enriched in 34S versus evaporites and dolostones formed in proximal settings—we do not favor this hypothesis because the strong correlation between Ca/Mg and δ 34S CAS implies that dolomitization controls the δ 34S CAS in these samples. Results from subtidal, well-preserved (non-dolomitized) limestones suggest that the δ 34S of Spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone.