SULFUR ISOTOPIC VARIABILITY OF CAS ALONG AN EARLY TRIASSIC ENVIRONMENTAL TRANSECT

The S isotopic composition of carbonate associated sulfate (CAS) has been studied in marine limestones and dolostones of all ages and has been used to interpret global redox conditions in ancient marine environments. However, while CAS in recent sediments appears to record the S isotopic composition of seawater sulfate, environmental and diagenetic gradients in the S isotopic composition of CAS in ancient sediments is largely unexplored. Here we present the results of a petrographic, stable isotopic and trace element geochemical analysis of limestone, dolostone and gypsum formed along an Early Triassic environmental transect in the western USA.

Within-section δ³⁴S varied up to 7‰ in limestone samples, with lower δ³⁴S values corresponding to higher Mn/Sr. Notably, δ³⁴S values of dolomitic samples were up to 5‰ lower than limestone samples from the same section. The largest variation between sections was ~13‰, with higher δ³⁴S values in the most offshore samples exhibiting the lowest Mn/Sr, and lower δ³⁴S values in the most nearshore samples with the highest Mn/Sr. δ³⁴S values varied logarithmically with Ca/Mg, suggesting diagenetic effects associated with dolomitization. The δ³⁴S values of gypsum exhibiting Spathian ⁸⁷Sr/⁸⁶Sr was within 1‰ of closely associated limestones and dolostones with low Mn/Sr. Gypsum samples with more radiogenic ⁸⁷Sr/⁸⁶Sr values showed similar δ³⁴S values to dolostones with high Mn/Sr.

The results suggest significant variation in the S isotopic composition of CAS with environment and diagenetic grade. The decrease in δ³⁴S with increased Mn/Sr may have resulted from proximity to terrestrial meteoric sources, perhaps with isotopically depleted S from the oxidative weathering of shales and organic-rich soils. Dolomitizing fluids were also characterized by lower δ³⁴S values that were incorporated as CAS at the time of dolomitization. Such trends would not be apparent without a detailed, basin scale analysis; thus studies based on isolated sections without detailed environmental analysis may be problematic. Although CAS chemostratigraphy is a useful tool in understanding ancient environments, CAS samples should undergo trace element and petrographic analysis to understand potential environmental and diagenetic variations.