The geochemical role of atoll phosphates on Vaitupu, Nukulaelae, Funafuti and neighbouring low islands of Tuvalu, central Pacific

Abstract

Bioclastic limestones cemented by dahllite, carbonate hydroxy-apatite, admixed with organic mucilage, are a normal aspect of low island geology, occurring on at least eight of the nine islands of the central Pacific archipelago of Tuvalu. Analyses of rocks, associated soils, groundwaters, and vegetation from three islands show concentrations of phosphorus range from 102 μg/g in the fresh groundwater, through 102–104 μg/g in the calcareous substrate and terrestrial flora, to 104 μg/g in terrestrial fauna. The phosphatic limestones have concentrations two orders of magnitude greater than in the non-phosphatized substrate. Precipitation of apatite occurs in the vadose zone following transport of phosphorus from the surface as a humate complex, and in response to prevailing bicarbonate alkalinity, high calcium concentration, net ionic strength and moderate pH. Formation of mineral phosphate provides a stable geochemical sink for excess phosphorus cycling through the atoll’s ecosystem, immobilizing the element and effectively removing it from further participation in an island’s geochemical cycle. A combination of physical and chemical factors may control deposition of the apatite, including climate, sea level changes, and composition of the substrate and its connate waters.