Abstract

The sulphur and oxygen isotope composition of sulphate is a very powerful tool. It has been used successfully to identify sulphur sources (e.g. sulphates and sulphides), to elucidate processes that produce sulphate (i.e. oxidation of sulphur compounds) and to quantify their respective contribution.

In contrast to this story of success, our understanding of what processes result in the expression of sulphur and oxygen isotope fractionation is still very limited. For example, we know that dissimilatory sulphate reduction (DSR) causes sulphur isotope fractionation as well as oxygen isotope exchange between sulphate and water. In recent years it has become evident that reversibility of the enzymatic steps of DSR dominates the expression of these isotope effects (e.g. Brunner et al., 2005). However, we have only just started to explore what combined sulphur and oxygen isotope effects can be attributed to DSR , and in what instances we have to assume that other processes, i.e. oxidative sulphur cycling, influence net isotope fractionation as well.

On the scale of global biogeochemical cycles, it appears that water is the major source for oxygen in sulphate. To date, it is unresolved why the oxygen isotope composition of sulphate on a perspective of millions of years (e.g. Turchyn and Schrag, 2006) does not appear to follow the secular changes in the oxygen isotope composition of water (e.g. Veizer et al., 1999).

My presentation introduces the concept of reversibility of enzymatic steps in microbial pathways. I will address the opportunities that arise when investigating sulphur and oxygen isotope effects in sulphur cycling in the light of this framework. The talk concludes with a discussion of the question: are we missing an important source for oxygen in sulphate when studying global biogeochemical sulphur cycling?

References

Brunner B., et al. (2005) A model for oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate reduction processes. Geochimica et Cosmochimica Acta, 69, 4773-4785.

Turchyn A. V., Schrag D.P. (2006) Cenozoic evolution of the sulfur cycle; Insight from oxygen isotopes in marine sulfate. Earth and Planetary Science Letters, 241, 763–779.

Veizer J., et al. (1999) 87Sr/86Sr, d13C and d18O evolution of Phanerozoic seawater. Chemical Geology, 161, 59–88.