The origin of Mediterranean interstitial waters-relics of ancient Miocene brines: A re-evaluation

Re-examination of interstitial waters from sixteen Mediterranean DSDP sites reveals that the high salinity values of these waters (up to 350 g/l) do not reflect dissolution of underlying evaporites, but are rather related to Miocene evaporated sea water trapped in the sediments and modified by diagenetic reactions and advection-diffusion. Only in two shallow sites was the chemical composition of the interstitial waters controlled by evaporite dissolution. The large variations in the chemistry of the interstitial waters among different sites suggests that the Mediterranean was divided into separate sub-basins with different chemical compositions. The composition of interstitial waters records the history of the Mediterranean since Early Miocene as follows: (1) salinization of the western Mediterranean during the Early and Middle Miocene; (2) intrusion of low salinity water in the eastern basin during the Middle Miocene; (3) desiccation by evaporation of the Mediterranean during the Messinian (Late Miocene). Other processes, such as sulphate reduction and dolomitization, took place along the sedimentary section. The diffusion-advection processes in the interstitial water, in addition to smoothing the contrasts within vertical profiles of chemical concentrations, caused an upward shift in the liquid phase profile in respect to the solid phase, changed the vertical trend in the case of an end-member which became exhausted, and brought about vertical changes associated with the porosity structure.