Isotopic composition of salt crusts in Saskatchewan, Canada

Abstract

The source of sulfate salts in the Great Plains region of North America is not well understood. Sulfur and oxygen isotope data of sulfate salt efflorescences and other sulfur-bearing species from selected locations in Saskatchewan were used to understand the sources, mechanism of formation and association of these salts with different facies of the sedimentary rocks in the area. The δ³⁴S_SO4² and δ¹⁸O_SO4²-values of the solid salt samples varied widely and ranged from −42.9 to 10.5‰ and −8.3 to +15.1‰, respectively. The δ³⁴S-values of the pyrite, coal, jarosite and gypsum samples also ranged from −39.0 + 4.0‰. The δ¹⁸O-values of the jarosite and gypsum samples varied between −10.3 and +14.4‰, indicating their close relationships with the salt crusts. Oxidation of pyrite and hydrolysis of natrojarosite appear to play a major role in the salt formation. The very low negative δ³⁴S-values associated with Cretaceous marine shales suggest a bacterial SO₄²⁻ reduction during the formation of pyrite at the bottom of the sea (pelagic facies), which once occupied the area. The salts with positive δ¹⁸O-values are associated with Tertiary continental environment and are consistent with the high sulfur isotopic composition of coal samples in this sediment. This suggests that the δ³⁴S-values depend on the type of lithofacies of the sediment and can be utilized to establish the formation conditions of sulfur species. The positive δ¹⁸O-values of majority of the salt crust samples indicate an incorporation of greater amount of atmospheric oxygen into the SO₄²⁻, hence a drier (semi-arid) environment during their formation, than for those salts with negative values, which are likely formed under a more humid environment or regions with high moisture regimes in the landscape. The oxygen isotopic composition of the salt crust from Chaplin Lake showed that incorporation of oxygen from the atmosphere into the sulfate was far more significant than salts from the rest of the study sites.