Occurence of phosphate rock and associated soils in Tuvalu, Central Pacific

Abstract

Phosphatic limestones and associated soils occur on eight of the nine islands of Tuvalu, central Pacific. Deposits range from gram-size to >500,000 tons. Carbonate hydroxyapatite, dahllite, forms crustose cement about calcareous bioclasts which it sometimes replaces. Precise genetic relationship of rock to soil is unclear. Consolidated rock occurs as hardpan within phosphatic soil profiles, with unconsolidated phosphatic layers above and below. Phosphatization has occurred either as a continuous or episodic process within the vadose zone for at least 4000 years. Present outcrops are exhumed accumulations of apatite formed in vadose zones corresponding to earlier, higher sea levels. A geobotanical relationship between the Tuvalu phosphate deposits and Pisonia grandis can not be sustained on present evidence and the tree should not be regarded as a geobotanical indicator today.