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Sulfur and oxygen isotope data vary in a laterally systematic manner across the sabkha at Abu Dhabi. At least three different mechanisms are required to explain the variation of ^dgr34S and ^dgr18O of sulfate in evaporite minerals and interstitial brine. Gypsum is precipitating in the seaward zone, and extensive postcrystallization exchange maintains isotopic equilibrium between bulk gypsum and dissolved sulfate. Anhydrite is the dominant sulfate mineral in the central zone where bacterial sulfate reduction results in heavier ^dgr34S and ^dgr18O values. In the landward zone both minerals and brine reflect mixing with the isotopically distinct sulfate of continental brines. Laterally systematic isotopic variation supports Patters n's chemical discrimination between seaward, mixed and landward hydrologic regimes, and is consistent with vertical variation of ^dgr34S demonstrated by Butler et al for this regressive sedimentary sequence. Gypsum-anhydrite transitions do not have significant effects on the isotopic composition of sulfate. Some ancient marine evaporite formations consisting mainly of dolomite and anhydrite may have formed in environments similar to the seaward and central zones of the Abu Dhabi sabkha; considerable isotopic variation would probably accompany subaerial deposition of laterally extensive marine evaporites.
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