Stable isotopes of carbon (12C and 13C) and sulphur (32S and 34S) are fractionated during some of the transformations between reservoirs in the global carbon and sulphur cycles. The main processes controlling the isotopic composition of Earth surface reservoirs are, for carbon, photosynthesis, respiration and equilibration between oceanic dissolved inorganic carbon and atmospheric CO2, and, for sulphur, bacterial sulphate reduction and sulphide reoxidation. Thus the different reservoirs of carbon and sulphur in the global cycles attain characteristic isotopic compositions that can be used as a ‘fingerprint’ for different carbon and sulphur sources and to track changes in the balance of carbon and sulphur stored in different reservoirs. These isotopic signals can be used both to examine contemporary anthropogenic impacts on the cycling of carbon and sulphur and, where isotopic compositions are faithfully preserved in ancient sediments, to examine historical changes in global geochemical cycles through geological time. A number of case studies are discussed to illustrate how isotopic techniques represent one of the most powerful and important tools available in the study of both the causes and affects of ancient and modern global environmental change.

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