Kopara in Polynesian atolls: early stages of formation of calcareous stromatolites

Widespread modern calcareous stromatolites known as kopara in the vernacular language on the Tuamotu Archipelago have been found accreting on the bottom of shallow (generally < 1 m depth) lakes located on the rims of these French Polynesian atolls. They are flat, gelatinous sediments, several tens of centimetres thick, with lamination comprised of vertically alternating red organic-rich, and white carbonate-rich, millimetre-thick laminae. They originate from benthic microbial communities mainly composed of cyanobacteria, generally dominated by the genus Phormidium. The living cyanobacteria occupy only the top 1–3 cm of the deposit, the red organic matter below being made of remnants of the dead microbes, essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three-dimensional network enclosing pores ranging from a few tens of ångstroms to a few micrometres wide. The carbonates are predominantly high-Mg calcite (9–19 mole% MgCO 3) which precipitated as micron-size bunches, within the pores, on walls of the organic network. It is sometimes accompanied by aragonite (less than 22% of the total mineral fraction when present), and lesser high-Mg calcite, allochthonous bioclasts, which occasionally form up to few centimetres-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 Ca 2+, Mg 2+, and alkalinity under the influence of local calcification and bacterial processes (e.g. ammonia production, sulphate reduction) within the sediments. It is hypothesized that the calcification is initiated at carboxylic sites on the walls of the polysaccharide network, the pores acting as confining organic compartments with increased internal supersaturation. Lamination is suggested to be due to the combined effects of the stratification of the microbial activity of the deposits, and alternations of fresh and saline periods within the lakes. Lacustrine environments on oceanic atolls are found to be important habitats of modern microbialite formation.