Journal of Conference Abstracts

Potential and Pitfalls in the Use of Sulphate-Oxygen Isotopic Compositions to Trace Environmental
Sulphur Redox Reactions

Simon H. Bottrell (simon@earth.leeds.ac.uk), Andy Barker & Rob Newton

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K.

Natural variability in stable sulphur isotope ratios can provide an extremely useful indigenous tracer of sources and fates of sulphur in the environment. Because of large isotopic effects associated with transformations of sulphur between redox states, sulphur isotopic studies can provide information on environmental sulphur redox reactions. The isotopic composition of oxygen in sulphate molecules also exhibits significant natural variation related to sulphate source and sulphur redox chemistry, but has been less widely applied as a tool for the study of environmental sulphur redox reactions. This presentation reviews the essential background for interpreting sulphate oxygen isotopic data, and then concentrates on examples demonstrating that sulphate oxygen isotopic data may retain important evidence of redox transformations of sulphur to which sulphur isotopic data are insensitive.

In environmental situations, reduction of sulphate to sulphide is bacterially-mediated and has associated significant isotopic effects for both S and O in sulphate. Sulphur is fractionated by a kinetic mechanism with preferential incorporation of ³²S into the sulphide product; Enrichment factors are commonly in the range of 10 to 70 per mille (e.g. Chambers and Trudinger, 1978). During Biological sulphate reduction, oxygen isotopic exchange between sulphate and environmental water is promoted (Fritz et al. 1989). In most natural situations, sulphate is depleted in ¹⁸O relative to equilibrium with environmental water, so isotopic exchange causes an enrichment in ¹⁸O in the residual sulphate.

In the Mersey Estuary, N.W. England, recharge occurs from the estuary, through estuarine sediments into the underlying Sherwood sandstone aquifer. Groundwaters in the aquifer below the estuary can be sampled from the Mersey Rail Tunnel and are characteristically saline but have undergone chemical modification during their transit through the sediment. One of the most important modifications is a substantial fall in sulphate concentration and in SO₄^2-/Cl^-. However, sulphur isotopic compositions of the sulphate in the groundwaters shows no variation with concentration or SO₄^2-/Cl^-, remaining constant at the marine sulphate value of the estuary waters. These data would militate against bacterial sulphate reduction as the cause of the decrease in sulphate concentration, since BSR carries a diagnostic large sulphur isotope fractionation and as sulphate is consumed the residual sulphate should become strongly enriched in ³⁴S. The sulphate oxygen isotopic compositions do however show a distinction between the estuary waters and the groundwaters, sulphate in the groundwaters being enriched in ¹⁸O. This isotopic effect is consistent with sulphate reduction as the cause of the fall in sulphate concentrations. It appears that cyclic oxidation and reduction of sulphur species in the estuarine sediment can cause a net conversion of sulphate to sulphide fixed in the sediment without a sulphur isotopic effect on the sulphate, whereas the sulphate does become enriched in ¹⁸O. This possibly relates to the different mechanisms of isotopic fractionation for S and O, with S undergoing a kinetic fractionation and O in sulphate being modified by exchange with water. Similar effects were noted by Caron et al. (1986) for redox transformations of sulphate during transit through soils in Canadian catchments. Again there was no net change in sulphate sulphur isotopic composition of sulphate and only changes in the sulphate oxygen isotopic composition recorded the sulphur redox transformations which had occurred.

Clearly there are situations in which sulphur isotopic ratios in sulphate are insensitive to sulphur redox reactions and do not undergo the shifts that simple theory would predict. However, oxygen isotopic ratios in sulphate provide an alternative tracer for redox processes and appear to provide a more robust record of the redox transformations which have occurred in such situations.

References

Caron, F., Tessier, A., Kramer, J.R., Schwartz, H.P. and Rees, C.E. Appl. Geochem. 1, 601-606 (1986).

Chambers, P.A. and Trudinger, L.A. Geomicrobio. J. 1, 249-293 (1978).

Fritz, P., Basharmal, G.M., Drimmie, R.J., Ibsen, J. and Qureshi, R.M. Chem. Geol. 79, 99-105 (1989).