![]() The reef islands appear to commence formation about 4,500 years BP with the oldest radiocarbon ages located about three quarters of the width across reef islands of Tabiteuea and Marenanuka from the ocean side.įossil ages from Tabiteuea, Marenanuka and Notoue becoming progressively younger towards the ocean side indicate a pattern of oceanward accretion. A program of paired AMS and amino-acid racemisation (AAR) dating was initiated to determine whether this technique could be used to augment radiocarbon analysis. The evolution of three reef islands was investigated by dating individual foraminifera, Amphistegina, using radiocarbon dating by Accelerator Mass Spectrometry (AMS). Only two reef islands show a more concave form with a significant proportion of low-lying areas. Ten reef islands, typically the smallest are classified as platforms, with little variation in elevation across them. Most reef islands (24 of 36) show an S-shaped form with a large proportion of the reef island being in the lowest elevation range. Based on the distribution of elevation classes, the hypsometry of reef islands has been examined. The lagoonward ridge is generally 1.5 to 2.0 m above MSL. More than 50% of the land area lies below 2.4 m above MSL. Many reef islands have a low-lying central depression. Most reef islands show a morphology that comprises an oceanward ridge that is generally 2 to 3 m, and in some places greater than 4 m, above mean sea level (MSL). Using geographical information systems (GIS), it is estimated that approximately 60 million m 3 of sediment has accumulated on the reef islands since the mid Holocene. The topography of the reef islands was described, based on a digital terrain model (DTM) derived from photogrammetry. To achieve this aim, the following tasks were carried out: investigation of the topography and the accumulated volumes of sediment sequestered on the reef islands, reconstruction of the evolution of selected reef islands and determination of whether or not rates of sediment supply have been reduced and, examination of historical shoreline changes. This thesis aims to examine the morphological development of these reef islands over a range of time scales. The reef islands around Tarawa Atoll are composed of unconsolidated carbonate sediments, particularly the tests of benthic foraminifera. ![]() Low-lying reef islands formed on the rim of atolls in Kiribati appear threatened by the impacts of anticipated sea-level rise. ![]()
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