The origin of gypsum in Vertisols in New Caledonia determined by isotopic characteristics of sulphur

Gypsum crystals occur in Vertisols on the west coast of New Caledonia. These soils are derived from the weathering of basic rocks that contain no gypsum. The sulphur isotopic ratio 34S/32S was used as tracer to identify the origin of the gypsum. A total of 26 measurements of 1 3 ~ ~ s were made on gypsum crystals from the Vertisols and compared to those of continental sulphates and to marine gypsum in tidal flats. The S34S value of the crystals located in New Caledonian Vertisols decreases with increasing distance from the coast. This suggests that the gypsum in the Vertisols comes from two sources: an oceanic source where the major part of the sulphur is introduced by atmospheric precipitation, and a continental source where sulphides disseminated in various subsurface deposits are oxidized. Gypsum from atmospheric precipitation penetrates the Vertisols through vertical cracks that probably formed in dry seasons during the last glacial period. 1