Gaining insight into Clippertonʹs lagoon hydrology using tritium

Historical descriptions of the Clipperton lagoon appear to converge on the fact that it became isolated from the surrounding ocean around 1858. Since then, because of the high precipitation rate which largely exceeds evaporation in this region of the eastern tropical Pacific, a brackish lens has formed on top of the saline oceanic waters. In 1980, literature data show that the thickness of this water body was reaching 14 m. During the 2005 Etienneʹs Clipperton expedition, we collected lagoon water on two vertical profiles. Salinity, δ18O and tritium analyses were performed on these samples with the objective of gaining further insight into the lagoon hydrology and age of the deep waters. The upper 15 m were characterized by low salinities (5.4 ± 0.2), and δ18O and tritium values typical of local precipitation. At depth, waters had salinity and δ18O similar to oceanic surface waters but with low tritium concentrations, hence pointing to quite isolated waters representing a remnant of marine waters when the lagoon was still communicating with the ocean. At lagoon closure, the excess of precipitation over evaporation raised the lagoon level, thus creating a hydraulic pressure head which favored salt expulsion through the permeable walls of the atoll. A simple geohydrological modeling of this salt expulsion process based on Darcyʹs law describes reasonably well the time-evolution of the brackish lens. Tritium is used to discuss the main physical processes potentially involved in the slow ventilation of the halocline and deep saline layer, including vertical diffusion, sinking of salty surface water intrusions and deep horizontal exchange through fissures in the limestone. These different mechanisms give reasonable results, which are all compatible with available salinity and isotopic data (δ18O and tritium), and therefore are all plausible candidates. Unfortunately, the lack of a detailed description of the vertical tritium profile in the halocline, between −15 m and −20 m, precludes any further quantification of their respective role in the ventilation process.