Helium‐3 inside Atoll Barrier Reef interstitial water: A clue for geothermal endo‐upwelling

Interstitial waters from boreholes in the reef conglomerate of Tikehau atoll (S.W. Pacific) contain positive anomalous concentrations of dissolved inorganic nutrients compared to adjacent oceanic and lagoonal waters. These anomalies have been interpreted by geothermal circulation of deep oceanic waters penetrating the porous reef carbonates and ascending through the atoll flanks by thermo‐convective advection as already proposed for other atolls. We present here a new strong evidence of this geothermal circulation inside atoll reefs from the record of helium‐3 anomalies in borehole waters of Tikehau atoll. These results bear directly on three controversial aspects of reef history: the efficiency of thermal energy for circulation of reef pore waters, the sources of nutrients to support the net productivity of reef ecosystems, the early diagenesis of reef foundation carbonates.