KOPARA IN POLYNESIAN ATOLLS - EARLY STAGES OF FORMATION OF CALCAREOUSSTROMATOLITES

Widespread modem calcareous stromatolites known as kopara in the verna cular language on the Tuamotu Archipelago have been found accreting on the bottom of shallow (generally < 1 m depth) lakes located on the ri ms of these French Polynesian atolls. They are flat, gelatinous sedime nts, several tens of centimetres thick, with lamination comprised of v ertically alternating red organic-rich, and white carbonate-rich, mill imetre-thick laminae. They originate from benthic microbial communitie s mainly composed of cyanobacteria, generally dominated by the genus P hormidium. The living cyanobacteria occupy only the top 1-3 cm of the deposit, the red organic matter below being made of remnants of the de ad microbes, essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three-dimensional network enclosing p ores ranging from a few tens of angstroms to a few micrometres wide. T he carbonates are predominantly high-Mg calcite (9-19 mole% MgCO3) whi ch precipitated as micron-size bunches, within the pores, on walls of the organic network. It is sometimes accompanied by aragonite (less th an 22% of the total mineral fraction when present), and lesser high-Mg calcite, allochthonous bioclasts, which occasionally form up to few c entimetres-thick detrital intercalations. The pore waters are variable mixtures of freshwater (rain or ground water) and sea water from the ocean or lagoon which are feeding waters of the lakes. Their chemistry deviate from a straight mixture gradient with respect to Ca2+, Mg2+, and alkalinity under the influence of local calcification and bacteria l processes (e.g. ammonia production, sulphate reduction) within the s ediments. It is hypothesized that the calcification is initiated at ca rboxylic sites on the walls of the polysaccharide network, the pores a cting as confining organic compartments with increased internal supers aturation. Lamination is suggested to be due to the combined effects o f the stratification of the microbial activity of the deposits, and al ternations of fresh and saline periods within the lakes. Lacustrine en vironments on oceanic atolls are found to be important habitats of mod ern microbialite formation.