Sulfur and oxygen isotope characterization of an Eocene playa deposit, northern High Plains, and rainwater sulfate, Baton Rouge, USA

Abstract

This thesis is composed of two independent chapters which are linked through the application of multiple stable isotope measurements of sulfate. Chapter 1 compares two Cenozoic playa deposits in the northern High Plains. Anomalous playa sulfate was discovered at Scotts Bluff which is speculated to have originated via the oxidation of volcanically emitted reduced sulfur gases. A question arises, then, is a volcanic origin of sulfate common for other Cenozoic playa deposits in the region. A complete stable isotope characterization was performed on the gypsum sulfate collected at Wolf Butte (nearby apparent playa setting) to determine if the sulfate has a similar origin to that in the Scotts Bluff playa. No17O anomaly was found in the sulfate at Wolf Butte and the δ34S values range from +12.2 to +14.8‰. Potential sources of sulfate at Wolf Butte include weathering of Cretaceous shale and leaching of volcanic ashes. It is concluded that Scotts Bluff and Wolf Butte represent two different playa gypsum deposits that existed during the Cenozoic in the High Plains. Chapter 2 looks into atmospheric chemical reactions that form tropospheric sulfate in Baton Rouge area. Multiple stable isotope compositions of atmospheric sulfate in La Jolla, California (Lee 2001) revealed competing oxidation pathways exist between ozone (O3) and hydrogen peroxide (H2O2). To elucidate the source and origin of atmospheric sulfate in Baton Rouge air shed, oxygen and sulfur isotopic compositions were analyzed. The outcome of this survey found that the sulfate isotopic composition data range from 0.25 to 1.43‰ and 11.8 to 19.3‰ for ∆17O and δ18O, respectively, and -1.4 to 3.8‰ for δ34S. High ∆17O values were not favored in the winter season as was seen in the study preformed in California (Lee 2001). The sulfate δ34S appear to be slightly depleted with respect to those in La Jolla rainwater and may be due to a predominately anthropogenic (δ34S~0‰) rather than marine (δ34S~+9.9‰ ±6‰) source of sulfate. Also, sulfate concentrations in rainwater were more than twice that measured in two nearby Louisiana monitoring stations, which may be due to the sampling site’s close proximity to oil refinery facilities.