Holocene growth of a mid-Pacific atoll: Tarawa, Kiribati

Abstract

Cores from ten holes, drilled to a maximum depth of 30 m, on Tarawa atoll in the central Pacific have been utilised in a study of the Holocene development of the atoll. Four dominant lithologies, in descending order, are cay rock, unconsolidated sediment, corals and leached limestone. Petrographic and radiometric age analyses indicate that the Holocene reef has developed on a previous (last interglacial) reef; the latter shows the effects of both vadose and phreatic freshwater diagenesis. Hydrological investigations beneath the present islands indicate the presence of freshwater lenses up to 29 m thick; the modern lenses are unrelated to freshwater diagenetic imprints preserved within the limestones. Vertical accretion rates of 5–8 m/1000 years for the Holocene reef section on Tarawa are significantly higher than rates measured for other Pacific atolls. The dated coral sequences suggest a more rapid rate of sea level rise during the early Holocene, and a relatively earlier stabilisation of sea level than has been suggested previously.