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

Drill cores from the barrier 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 shallowing-upward succession with assemblages of branching or massive colonies of Porites at the base overlain by a robust-branching community (Acropora gr. danai/robusta) 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 80% 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 reflecting 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 parts per thousand delta(13)C and -0.86 to +0.86 parts per thousand delta(18)C) are typical for a non-enzymatic fractionation and are close to those expected for calcitic cements precipitated at equilibrium with normal seawater. The relative scarcity of extraneous particles in the Tahiti crusts suggests that sediment trapping was much less important than calcification of organic mucilage associated with living or decaying organisms, and in-place, microbially mediated, precipitation of micrite. The biochemical composition of the crusts is especially characterized by concentrations of aspartic (Asp) and glutamic (Glu) acids of 10 to 15%; abundant hydroxyptoline indicates that metazoans may have proliferated in these microenvironments and represent the first 'donors' of the organic substrates, which may correspond to the mineralizing matrix after recycling by the microbial biofilms. Microbial organisms (especially bacteria and possible nannobacteria) seemingly played a major I role in transformation processes and in carbonate precipitation via the bacterial degradation of organic matter; they underwent also individual processes of biomineralization, Besides the overall decrease in light and energy conditions reflecting progressive burial by coral growth, the widespread development of microbialites within the reef framework may be related to increased alkalinity and nutrient availability in interstitial waters due to terrestrial groundwater seepage and periodic runoffs. The development of microbialites in the cryptic niches of the reef framework ceased about 6000 years ago when the sea level approached its present position. (C) 1999 Elsevier Science B.V. All rights reserved.