Laetitia Guibourdenche’s PhD

The formation of the Mediterranean Salt Giant about 5.5 million years ago remains poorly understood. This thick layer of salt is mainly found in the form of halite-and-gypsum deposit in the deep basins of the Mediterranean Sea.  But some of these evaporites -gypsum in most of the cases-have been formed in peripheral basins of the Mediterranean Sea that were hydrologically separated from the main Mediterranean basin.

Theses gypsum outcroups, that are easily accessible on the field, pose questions with regards to the geochemical processes responsible for their formation. Indeed, in the simplest case, gypsum (Ca.SO₄.H₂O) forms by seawater evaporation that concentrates all dissolved salts, including Ca²⁺ and SO₄^2-. But recent studies have shown that some gypsum deposits bear low-salinity fluid inclusion indicative of precipitation from a water body less saline than modern seawater. Isotopic analyses of the gypsum water molecules support this result and show that the water from which the gypsum formed had experienced limited evaporative concentration.

This implies that sulphate and calcium ions haven’t been concentrated by evaporation but by addition. With that in mind, I am currently focusing on the multiple sulfur isotopic composition (³²S, ³³S,³⁴S and ³⁶S) of the sulphate ion of the gypsum.  Theses analyses will allow us to understand whether the sulphate ion composing gypsum was brought by rivers or if it was produced by biogeochemical processes.   I will also analyse the oxygen (¹⁷O and ¹⁸O) and hydrogen (D) isotopic composition of structural-bound water in gypsum to derive information on the state of the hydrological cycle during gypsum deposition.