Phosphatic limestones and related soils occur on eight of the nine islands of Tuvalu, central Pacific Ocean. Phosphatic limestones and related soils occur on at least eight of the nine islands of Tuvalu, central Pacific Ocean. The term phosphorite (cf. McConnell, 1950; American Geological Institute, 1974) is inappropriate for most of the Tuvalu phosphate-bearing rocks.
The descriptions of the islands presented below rely heavily on the McLean7 maps and data for the seven islands. Holthus, Hosking and Woodroffe (1986a) mapped approximately 20 ha of phosphatic soils covering 5.5% of the atoll's land area. Both extend west of the pulaka (taro) area across the inner slopes of the coastal ridge and the edges of the central basin.
Based on information provided by residents, McLean et al., (1986a) mapped, but did not visit, an area of phosphatic soils northeast of the village where small specimens of Pisonia were also believed to be growing. The first occurs near the center of the island about midway between the two lagoons. PisonialHernandia broadleaf forest covers most of the area which also includes part of the Agricultural Department experimental station.
The result was in stark contrast to the still damaged appearance and agriculture of the other islands (eg Luomala, 195 1; Tudor, 1966). The western side of the atoll consists of a bare reef flat, 600-1000 m wide, and exposed at low tide. McLean, Holthus, Hosking, Woodroffe, and Hawke (1986b) observed that the most mature soils in Nui—including the phosphatized horizons—occur in the central flat and surrounding ridge areas of the larger islets.
In total, these workers mapped 3.07 ha of phosphatic soil in Nui, representing 0.87% of the land area. The total land area of 331 ha comprises 37 separate islands with a narrow and almost continuous strip of land forming the south-eastern side of the atoll. Funafuti is the largest atoll of the group, being roughly pear-shaped, with the narrow end pointing south (Fig. 7).
Phosphate rock is no more than a thin coating around calcareous bioclasts. The western and southern half of the island is a more or less featureless plain bounded by a coastal ridge. A solid sheet of hardened phosphatic limestone, up to 20 cm thick, often occurs a few centimeters below the surface, and is also exposed in considerable parts of the island where the topsoil has been removed.
The intensity of the staining decreases with depth until unaltered parent carbonate sand is reached.
GEOBOTANY
He drew attention to the humus build-up and acidic nature of the subsoil. The contribution of birds to the phosphorus supply of deposits is tacitly assumed by most writers, while they may have been major donors in Tuvalu's past, no present or former bird colonies have been identified with any known phosphate deposits . No phosphatization was found to have occurred under any of the existing bird colonies in the group.
Polypodium, Ochrosia and various elements of the Barringtonia Formation, of which Pisonia is a part, are also commonly present. Not enough detailed field studies have been carried out to provide a complete picture of the relationship between phosphate deposits in the geology of the Tuvalu Islands, but some general conclusions can be made. The apparently different descriptions of the petrography of individual deposits represent no more than different sections of the diverse, phosphatized sediments of the atoll seen by different workers during their various visits, e.g.
While the size of the various deposits varies greatly, phosphatization has been more extensive than surface exposures indicate. Interruption of deposits occurs through root-braiding (e.g. Nui, Funafuti, Nukulaelae, Niulakita) and human activities (e.g. Funafuti, Vaitupu) and domesticated animals, especially pigs (e.g. Amatuku). The width and depth of the solid panel in Tuvalu probably reflects the fluctuation of the vadose environment within the substratum.
The height of the vadose zone varies with the thickness of the underlying sediment, and it can be observed that the thickness of the hard plate often varies with the thickness of the cemented clastics (eg Niulakita.). The mineralogy of the deposits, the source of phosphate, and the mechanism by which it accumulates in the vadose zone are discussed elsewhere (e.g., Rodgers, 1989a,b). Phosphorus is believed to have come from the surface by terrestrial and marine organisms, including birds, plants, and the degradation of calcareous substrate biominerals.
Transport from the surface to the vadose zone, through the high pH soils of the islands, probably took place via humus complexes (cf. Fosberg, 1957). Precipitation of phosphorus as insoluble apatite in the vadose zone would effectively remove the element from further immediate participation in the atoll's biogeochemical cycle, thereby preventing eutrophication of the atoll's groundwater as progressive amounts of phosphorus circulate through them. Dr. Alex Ritchie and Carol Cantrell of the Ausmlian Museum furnished residences conducive to study.
University of Auckland Education and Leave and Research Committees provided funding to enable this work to be carried out. In: Brocher, T.M., ed., Exploring the Northern Frontier of Melanesia: Circum-Pacijic Council for Energy and Mineral Resources Earth Science Series 3:67-76.
