Atoll reef-island formation and response to sea-level change: West Island,Cocos (Keeling) Islands

Reef islands around the margin of coral atolls generally comprise unconsoli dated Holocene sands and gravels, overlying a reef flat or cemented conglom erate platform. Such islands have accreted within the last 3000-4000 years, since sea level has reached a level close to present and the reef flat and conglomerate platform have formed. Island morphology consists of an oceanw ard ridge, a less distinct lagoonward ridge, and low-lying central depressi on. Several alternative models of how such reef islands might have develope d are examined in relation to chronology and sediment provenance, particula rly in the context of the Cocos (Keeling) Islands where this issue has been debated since Darwin visited the atoll. Which of these models appears most appropriate for an elongate reef island on the atoll margin is assessed us ing conventional radiocarbon dating of coral shingle and accelerator mass s pectrometry (AMS) radiocarbon dating of individual sand grains from pits ac ross West Island. The dating results suggest that both coral clasts and ind ividual grains of various components are generally reliable and replicable indicators of the chronology of island accumulation, implying rapid transpo rt of skeletal material, after death of the contributing organisms, across the reef fiat zone, and relatively little reworking. The central part of We st Island appears to have formed first, with oceanward accretion up until a bout 2000 years BP. Gradual oceanward accretion with lesser lagoonward exte nsion has continued beyond 2000 years BP at the northern and southern ends of the island, and a sequence of lagoonward recurving spits has formed adja cent to the inter-island passage at the southern end of the island. Radioca rbon dating of fossil microatolls indicates a gradual fall of sea level ove r this period which appears to have had Little effect on the pattern of isl and accretion. (C) 1999 Elsevier Science B.V. All rights reserved.