Abstract

Phosphatic limestones and associated soils occur on each of the nine islands of Tuvalu, central Pacific. Deposits range from gram-sized crusts to >500,000 metric tons. Carbonate-hydroxylapatite (dahllite) forms multilaminar botryoidal crustose cement around calcareous bioclasts which it sometimes replaces. Consolidated rock occurs as hardpan with unconsolidated phosphatic layers above and below. The genetic relationship of phosphatic limestone to unconsolidated phosphatic sediments and to phosphatic soil is unclear. Phosphatization has occurred either continuously or episodically within the vadose zone for at least 2,000 yrs, while relative sea level has fluctuated. The present deposits are exhumed accumulations of apatite formed at earlier, higher, Holocene sea levels, with phosphorous derived from possibly both terrestrial and marine organisms, including birds, plants, and degradation of the bioclastic calcareous substrate. Humic complexes could enable phosphate to enter the lensoid fresh water table. Degradation of these humic complexes would liberate free HPO (super -2) 4 with subsequent precipitation of apatite under suitable conditions of alkalinity and carbonate and magnesium activity. Pisonia grandis should not be regarded as a geobotanical indicator of phosphate deposits in present-day Tuvalu.

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