Refining the δ34S and δ18O Values of Sulphate in Ancient Oceans

Abstract

The isotopic composition of ocean sulphate is essential in the reconstruction of the global sulphur cycle throughout the Phanerozoic. Holser and Kaplan (1966) and Claypool et al. (1980) have established the evolution of the 834S and 8lap values of marine sulphate deposits. Since the residence time of sulphate in the ocean is larger by several orders of magnitude than the time to achieve complete mixing, homogeneous 834S values are expected, However, the sulphates of the same age, and also from the same basin, may show no constant 834S and 81So values, but vary with ranges as large as 10%0 (Claypool et al., 1980). These variations lead to the main question still unsolved: what was exactly the isotopic composition of seawater sulphate at this particular time?. Several processes taking place within the sedimentary basin have been invoked to account for the variation of the isotope values: 1) fractionation as sulphate precipitates; 2) bacterial sulphate reduction; 3) mixing with sulphate of a different age. Holser and Kaplan (1966) and Claypool et al. (1980)propose a compensated mean of the measured 83~S and 8180 values as the best estimate of the isotope composition of seawater sulphate of a particular time. In the present contribution we propose a method to identify the different processes affecting the isotope composition of sulphate within the sedimentary basin. It is based on the systematic analysis of the 834S and 8180 values of sulphates sampled throughout a complete evaporite sequence. A computer program has been used to predict the evaporation paths in different scenarios. The comparison of the predictions with the observed isotope compositions has been used to constrain the processes taking place in the basin and to infer the isotope composition of sulphate dissolved in the parental seawater. Method