Oxygen and Sulphur Isotope Geochemistry of Marine Evaporites

Abstract

<p>The variation in the ratio of oxygen isotopes in natural sulphate differs fundamentally from that of sulphur because it is linked to several geochemical cycles in addition to the sulphur cycle. Published results indicate that dissolved sulphate and water do not exchange oxygen isotopes to any measurable extent at low temperatures and near-neutral pH. In sulphate reduction however, the intermediate sulphite rapidly exchanges oxygen with water. Appreciable back oxidation of the intermediate sulphite confirms Rees' steady state model for bacterial sulphate reduction. Analogous indirect exchange with water may be affected by the cycling of sulphur by phytoplankton; crude calculations show that the flux of sulphur in the biomass may quantitatively dominate the processes that determine the processes that determine the oxygen del value of sea-water sulphate.</p> <p>The present formation of gypsum and anhydrite on the sabkhas of the Persian Gulf has led to several reinterpretations of the depositional environments of ancient evaporites. Sulphur and oxygen isotope analyses of the evaporite minerals and the interstitial brines from the Abu Dhabi sabkha strongly confirm Patterson's distinction between the seaward, mixed, and landward hydrological regimes. Isotopic equilibrium between gypsum and dissolved sulphate with considerable exchange between early-formed gypsum and subsequent interstitial brines is demonstrated for the seaward zone. Bacterial reduction is significant in the mixed or central zone where little new deposition is occurring. The influx of continental brines in the landward zone is causing a marked shit to lighter sulphur del values. Gypsum-anhydrite transitions do not involve large isotope fractionations. The seaward and central zones constitute an analogue for shallow or subaerial evaporite mineral deposition and it is likely that a substantial range of variation would prevail on the much larger scale of ancient evaporites.</p> <p>Isotopic studies on ancient evaporites have established that the sulphur del value of seawater sulphate has varied more than twenty per mille in the Phanerozoic. The narrow range of individual anhydrite beds like the Midale Evaporite that are considered to have a sabkha origin may be due to postdepositional homogenization. The very large range of sulphur del values, and to a lesser extent, oxygen del values of the Salina Formation is consistent with a sabkha origin on the flanks of the depositional basin. Two near surface gypsum mines in the Salina B Anhydrite had constant del values on a scale of hundreds of meters. Permian evaporites from geographically separate locations fall within a narrow range and coincidentally have the the lightest sulphur and oxygen del values of any geological period. The sulphur isotope analyses reported here conform to the Phanerozoic pattern, and display comparable fluctuations in the Precambrian. It is suggested that the noise on the curve may be primary depositional information. The oxygen isotope analyses do not support suggestions of significant variations over geological time, and limit the possible range of variation to less than three per mile.</p> <p>Experiments on the exchange between gypsum crystals and dissolved sulphate have established an equilibrium isotope fractionation of about 3.5 per mille. Lower values found elsewhere are considered to be a kinetic isotope effect.</p>