Carbonate Sedimentation and Recent Influence of Human Activity on Tarawa Atoll, Republic of Kiribati, South Pacific

Abstract

Tarawa Atoll is a shallow isolated platform rising from the 4000-m-deep sea floor of the western central Pacific Ocean just north of the equator at 1°28' N and 173° 00'E. This study describes the sedimentary environments (ocean reef flat and beach; islets and the channels between them; lagoon reef flat and beach; and the lagoon itself), and reports on texture, skeletal and chemical composition, mineralogy, and grain degradation of the sediment based on 170 samples. The mild climate (mean temperature 29° C), and a hydrodynamic environment resulting from brief, frequent and modest easterly storms, are also reviewed. Experiments and observations undertaken in order to help understand the processes by which the atoll sustains itself are described. Tarawa Atoll was found to consist entirely of skeletal carbonate debris that is being actively generated on the ocean facing-facies along North Tarawa and within the northern portion of the lagoon today. The main types of organisms are corals (41%), foraminifera (27%), molluscs (18%) and Halimeda algal clasts (9%). Foraminifera and algae live in a symbiotic relationship with the corals. The molluscs, Halimeda and most corals are largely aragonite, whereas the foraminifera and echinoids are calcite. The atoll is currently in a state of dynamic balance between the actively growing reef of North Tarawa and a mild wave climate generated by the frequent but brief easterly storms. Biotic carbonate formed on the reef flats is broken down by waves into fine gravel and sand sizes and transported into the lagoon, which is for this reason relatively shallow (average 6 m deep). The waves ensure that mud-size sediment survives only in small protected pockets and deeper parts of the lagoon. Changes observed in the sediment cycle over the last 3 decades have been mainly confined to South Tarawa and include the death of most of the sediment-producing biota (corals, foraminifera, algae) along South Tarawa coasts. Changes to the sediment cycle have also occurred as a consequence of the construction of causeways along South Tarawa, reducing sediment transport into the lagoon and reducing flushing of the lagoon with waters from the open ocean. This has resulted in significant levels of bacterial pollution in the lagoon. These changes are attributed to increased human activity over the last 3 decades as a consequence of a doubling of population (31,300), 85% of which lives on South Tarawa (approximately 15 km2). This concentration is considered to account for the pollution that, along with the reduced circulation in the lagoon, has led to the death of most of the reef-forming biota of South Tarawa, though North Tarawa remains virtually unpolluted. Studies of the type of biota found on Tarawa suggest that if pollution could be reduced and water circulated through the lagoon a healthy reef-building biota could re-establish itself on South Tarawa in a few years.