Year

2012

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Low-lying reef islands formed on the rim of atolls in Kiribati appear threatened by the impacts of anticipated sea-level rise. The reef islands around Tarawa Atoll are composed of unconsolidated carbonate sediments, particularly the tests of benthic foraminifera. This thesis aims to examine the morphological development of these reef islands over a range of time scales. 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.

The topography of the reef islands was described, based on a digital terrain model (DTM) derived from photogrammetry. Using geographical information systems (GIS), it is estimated that approximately 60 million m3 of sediment has accumulated on the reef islands since the mid Holocene. 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). Many reef islands have a low-lying central depression. More than 50% of the land area lies below 2.4 m above MSL. The lagoonward ridge is generally 1.5 to 2.0 m above MSL. Based on the distribution of elevation classes, the hypsometry of reef islands has been examined. 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. Ten reef islands, typically the smallest are classified as platforms, with little variation in elevation across them. Only two reef islands show a more concave form with a significant proportion of low-lying areas.

The evolution of three reef islands was investigated by dating individual foraminifera, Amphistegina, using radiocarbon dating by Accelerator Mass Spectrometry (AMS). 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 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.

Fossil ages from Tabiteuea, Marenanuka and Notoue becoming progressively younger towards the ocean side indicate a pattern of oceanward accretion. More recent accretion has occurred locally, particularly on the ends of the reef islands and in small embayments.

Estimated AAR ages of individual Amphistegina show a range that is broadly consistent with the AMS age trend. Decreasing order of relative fossil ages is as follows: a cluster of fossils obtained from on land pits, lagoon beaches, ocean beaches and younger ages for live foraminifera. Fossils obtained from beaches of reef islands show mixed ages. South Tarawa fossils appear relatively old as they are continuously reworked. Live foraminifera are found on the ocean reef flats of North Tarawa but appear scarce in South Tarawa. It seems likely that the supply of modern foraminifera will continue in North Tarawa, however, it appears limited in South Tarawa.

Historical changes to reef-island morphology were determined on a majority of reef islands. Reef-island area and shoreline changes were examined over 30 years by comparing the shoreline positions derived from 1968 and 1998 aerial photography using the Digital Shoreline Analysis System. Analysis of the area change shows that Tarawa Atoll has substantially increased in size by more than 450 ha, largely driven by development pressures and increasing population on South Tarawa. Reclamations have been directly associated with this net increase, contributing a significant amount of more than 360 ha. Widespread erosion and high average accretion rates appear to be related to numerous reclamations of varying sizes spread along South Tarawa’s shorelines. Conversely, reef islands in North Tarawa appear to be predominantly stable with localised shoreline changes observed in areas such as embayments, sand spits, beaches close to blocked channels and beaches facing inter-island channels. Factors affecting changes on North and South Tarawa differ. Shoreline changes on North Tarawa are largely driven by natural factors, whereas shorelines in South Tarawa are predominantly influenced by human activity and seasonal variability associated with El Niño Southern Oscillation (ENSO), which affects the longshore sediment transport.

In the future, reef islands of North Tarawa are anticipated to be stable and more resilient to sea-level rise unless population density increases and conditions do not favour the growth of foraminifera. However, serious concerns are raised for the future of South Tarawa reef islands, as evidence shows that widespread erosion along the ocean and lagoon shorelines is primarily related to the modification of shorelines due to increasing development and population pressures. With sediment supply appearing to be limited due to the scarcity of live foraminifera, it is anticipated that these reef islands will experience increased erosion and be at risk from inundation.

Share

COinS