Interpretation of oxygen and sulfur isotopes from dissolved sulfates in tills of southern Alberta, Canada

Abstract

Tills of the Interior Plains Region of southern Alberta, Canada, consist of an upper sulfate‐rich weathered zone and a lower sulfate‐deficient nonweathered zone. The δ18O of aqueous sulfate from the nonweathered zone was enriched over the δ18O of the weathered zone ; however, no statistical difference was observed in the δ34S of the aqueous sulfate from the two zones . The δ18O and δ34S data from the aqueous sulfate provide independent support for an earlier finding that most sulfate in the weathered zone is derived from the oxidation of reduced sulfur. The )δ18O of aqueous sulfate from the nonweathered zone is different from that of the associated groundwaters , whereas the δ18O of aqueous sulfate from the weathered zone is similar to that of the groundwater . These data are not consistent with equilibrium isotopic exchange between the aqueous sulfate and the groundwater and suggest that the rate of equilibration is very slow because the groundwater and sulfate in the nonweathered zone have been in contact for up to 37,000 years. Isotopic balance calculations imply that most of the oxygen in the sulfate produced by the oxidation of reduced sulfur in the weathered zone is incorporated from the associated water molecules.