Stable Isotopes of Salts in Some Acid Sulfate Soils of North America
- G. N. Dowuona,
- A. R. Mermut and
- H. R. Krouse
Sulfur and O isotope data were used to elucidate the origin of soluble sulfate salts in soils and surficial sediments at three sites in Saskatchewan, Canada (Arborfield, Cedoux, and Trossachs), and two sites in the USA (Charles County, Maryland, and Hettinger, North Dakota). Isotopic data supported the previously documented hypothesis that pyrite oxidation and subsequent hydrolysis of natrojarosite in marine and continental sediments are the dominant mechanisms for the source of sulfate salts including gypsum in the areas studied. Evidence for this conclusion is that all the sulfide- and sulfate-bearing minerals (pyrite, natrojarosite, and gypsum) from the three marine Arborfield, Cedoux (Cretaceous), and Maryland (Eocene) sites were highly depleted in 34S with δ34S values between −29.9 and −39.0‰. The LiCl-extractable (soluble) sulfate-S and insoluble S, containing mainly inorganic S, had also δ34S values within the same range (−40.8 to −31.2‰). Corresponding minerals and other S species in the Trossachs and Hettinger sites, both occurring in the nonmarine (Tertiary continental) sediments, had higher but still negative δ34S (SO4) values between −5.0 and −14.0‰, indicating that the initial sulfide was relatively eariched in 34S. Gypsiferous evaporites or brines cannot be counted as possible sources of salts for all the sites studied, because of their high positive δ34S values. The δ18O (SO4) values of natrojarosite and gypsum from the Eocene marine sediment in Maryland were 10.5 and 10.3‰ and jarosite from Tertiary Trossachs was 14.3‰. These high values suggest a greater incorporation of molecular O (δ180 = 23‰) into the SO4 during sulfide oxidation at these two sites. The relatively high δ18O (SO4) value of 14.3‰ in the jarosite from Trossachs suggests that O2 from the air is the only source of sulfate O and supports the validity of KF (O2) (kinetic isotope fractionation factor for the incorporation of O2 from air into the sulfate) = −8.7 under natural conditions. This high δ18O value also indicates that the O isotope exchange between water and sulfate is indeed very low.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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