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

Citation
C. Caudwell et al., Lamination of swampy-rivulets Rivularia haematites stromatolites in a temperate climate, SEDIMENT GE, 143(1-2), 2001, pp. 125-147
Citations number
57
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
SEDIMENTARY GEOLOGY
ISSN journal
0037-0738 → ACNP
Volume
143
Issue
1-2
Year of publication
2001
Pages
125 - 147
Database
ISI
SICI code
0037-0738(20010815)143:1-2<125:LOSRHS>2.0.ZU;2-J
Abstract
Swampy-rivulets Rivularia haematites (D.C.) Agardh stromatolites were colle cted from the same site on the Plateau de Langres (Chaugey, France) over a seven year period. Specimens were studied in thin sections, in culture medi a under varying conditions of temperature and illumination, and by scanning electron microscope (SEM) in order to investigate the processes of calcifi cation and lamination. New SEM observations confirm the polycrystalline structure of the 'lamellae ' reported by the authors and show that the crystals composing these lamell ae are elongate and aligned in the same direction as the filaments linked b y them. The lamellae were also isolated in transverse sections under SEM an d found to correspond to what under the light microscope appear to be 'hone ycomb' structures. The diameter of alveoli can be the same as that of a 'fa lse branching'; their walls, formed by the outer sheath, have a dense netwo rk of microfibrils. These observations confirm the formation of calcitic la mellae at the false branching points and in contact with the microfibrils o f the outer sheath. Confirmation of earlier hypotheses was sought through observation in thin s ection of the stromatolites collected over seven years. Micritic dark lamin ae are thought to form in three stages: (i) formation of a dark lamina by f ormation of a false branching zone in the wet season, (ii) initial calcific ation in this zone as microsparitic and sparitic lamellae during even a sho rt 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 rat e of R. haematites. The thin section may display either a single dark lamin a for two years' growth, or a dark lamina thicker than the annual growth ra te. Examination of local meteorological data accounts for the first phenome non by a wet spell of more than 13 months and the second by a warm, dry per iod of one month. It may therefore take several years for a couplet to form as growth of the cyanobacterial colony is influenced by climatic variation s. The different calcification phenomena were also correlated with dry spel ls. These interpretations are consistent with results obtained by experimen ts. (C) 2001 Elsevier Science B.V. All rights reserved.