Nature and environmental significance of microbialites in Quaternary reefs: the Tahiti paradox

Abstract

Drill cores from the bamer reef-edge of Tahiti exhibit 85-93-m-thick coralgal sequences recording at least 13,500 years of continuous reef growth in optimal environmental conditions. The cored reef sequences form an overall shallowingupward succession with assemblages of branching or massive colonies of Porires at the base overlain by a robust-branching community (Acropom gr. dmmi/robirstn) heavily encrusted by coralline algae, sessile vermetid gastropods and arborescent foraminifers, which grew at depths less than 6 m. Microbialites generally form the last stage of encrustation of coral colonies, or more commonly, of related encrusting organisms, thus appearing as a major structural component of the reef sequence where they may locally form 809 of the rock by volume. They developed in an open cavity system of the reef framework with freely circulating normal-marine water. Microbialites include laminated crusts and clotted micritic masses, commonly associated in compound crusts, probably feflecting differences in the composition of the involved biological communities and in biomineralization processes that controlled the accretion of the crusts. The isotopic composition of the microbialites (+2.05 to +3.92% I they underwent also indivitlual processes of biominrlali2ation. Besicles the overall decrease in lizht and enqy conditions rttlecting progressive burial by conil growth, the widespread development of microbialites within the reef framework may be related to increased alkalinity and niitrient availability in interstitial waters due to temstrial groundwwr seepage and puiodic runoffs. The tlevelopment of micrubialites in the cryptic niches of the reef framework ceased about 6000 years