Pore water geochemistry and mixing processes within the Tahiti barrier reef

Abstract

We report a multi-parameter study of hydrological and geochemical tracers in a deep borehole (150m) of the Tahiti barrier reef. The principal objective was to characterize the origin of the reef interstitial waters and their exchange and mixing patterns with the surrounding ocean waters. The measurements show that the hydrological and geochemical properties within the borehole at any given level are distinct from the oceanic composition. The vertical distribution of the various tracers displays a coherent picture and identifies two major sources for the reef interstitial waters: first, a deep Pacific water recharge from at least 350 m depth and second, Pacific surface waters that penetrate from the top and the upper section of the reef flanks. The deep Pacific waters are thought to infiltrate at the periphery of the volcanic edifice. They move upward through the basaltic basement and on rising are further mixed, inside the carbonate pile, with the Pacific surface waters.