Gf. Camoin et al., Nature and environmental significance of microbialites in Quaternary reefs: the Tahiti paradox, SEDIMENT GE, 126(1-4), 1999, pp. 271
Drill cores from the barrier reef-edge of Tahiti exhibit 85-93-m-thick cora
lgal 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 colo
nies of Porites at the base overlain by a robust-branching community (Acrop
ora gr. danai/robusta) heavily encrusted by coralline algae, sessile vermet
id gastropods and arborescent foraminifers, which grew at depths less than
6 m. Microbialites generally form the last stage of encrustation of coral c
olonies, 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. Microbiali
tes include laminated crusts and clotted micritic masses, commonly associat
ed in compound crusts, probably reflecting differences in the composition o
f the involved biological communities and in biomineralization processes th
at 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 fractio
nation and are close to those expected for calcitic cements precipitated at
equilibrium with normal seawater. The relative scarcity of extraneous part
icles in the Tahiti crusts suggests that sediment trapping was much less im
portant than calcification of organic mucilage associated with living or de
caying organisms, and in-place, microbially mediated, precipitation of micr
ite. 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 t
hese microenvironments and represent the first 'donors' of the organic subs
trates, which may correspond to the mineralizing matrix after recycling by
the microbial biofilms. Microbial organisms (especially bacteria and possib
le nannobacteria) seemingly played a major I role in transformation process
es and in carbonate precipitation via the bacterial degradation of organic
matter; they underwent also individual processes of biomineralization, Besi
des the overall decrease in light and energy conditions reflecting progress
ive burial by coral growth, the widespread development of microbialites wit
hin 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 niche
s of the reef framework ceased about 6000 years ago when the sea level appr
oached its present position. (C) 1999 Elsevier Science B.V. All rights rese
rved.