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RESEARCH PRODUCT

Lamination of swampy-rivulets Rivularia haematites stromatolites in a temperate climate

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Abstract Swampy-rivulets Rivularia haematites (D.C.) Agardh stromatolites were collected from the same site on the Plateau de Langres (Chaugey, France) over a seven year period. Specimens were studied in thin sections, in culture media under varying conditions of temperature and illumination, and by scanning electron microscope (SEM) in order to investigate the processes of calcification and lamination. New SEM observations confirm the polycrystalline structure of the ‘lamellae’ reported by the authors and show that the crystals composing these lamellae are elongate and aligned in the same direction as the filaments linked by them. The lamellae were also isolated in transverse sections under SEM and found to correspond to what under the light microscope appear to be ‘honeycomb’ structures. The diameter of alveoli can be the same as that of a ‘false branching’; their walls, formed by the outer sheath, have a dense network of microfibrils. These observations confirm the formation of calcitic lamellae at the false branching points and in contact with the microfibrils of the outer sheath. Confirmation of earlier hypotheses was sought through observation in thin section of the stromatolites collected over seven years. Micritic dark laminae are thought to form in three stages: (i) formation of a dark lamina by formation of a false branching zone in the wet season, (ii) initial calcification in this zone as microsparitic and sparitic lamellae during even a short dry spell, and (iii) subsequent bacterial micritization of the lamellae during an extended warm, dry season. The sparitic light laminae form around the hair-bearing regions. Lamination was first considered with regard to the annual radial growth rate of R. haematites. The thin section may display either a single dark lamina for two years’ growth, or a dark lamina thicker than the annual growth rate. Examination of local meteorological data accounts for the first phenomenon by a wet spell of more than 13 months and the second by a warm, dry period of one month. It may therefore take several years for a couplet to form as growth of the cyanobacterial colony is influenced by climatic variations. The different calcification phenomena were also correlated with dry spells. These interpretations are consistent with results obtained by experiments.