Proposed origin of aragonite Bahaman and some Pleistocene marine ooids involving bacteria, nannobacteria(?), and biofilms

• Autores: Charles F. Kahle
• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 22, Nº. 1, 2007, págs. 10-22
• Idioma: inglés
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• Resumen

  • The vast majority of laminae in the cortex of modern Bahaman ooids and ooids in the Pleistocene Miami Limestone in the Florida Keys are aragonite. The aragonite has two forms: rod-shaped batons (0.5-3.0 µm long and 0.17-0.7 µm in diameter), and nannoballs (0.025-0.3 µm in diameter). Nannoballs are always subordinate to batons and are much more common in Bahaman ooids than in ooids in the Miami Limestone.

    Variations in the size and shape of known rod-shaped bacteria correspond exactly or very closely to variations in the size and shape of batons. Accordingly, batons are interpreted as calcified rod-shaped bacteria The intimate association of batons and nannoballs suggests a formational link. Aggregates of nannoballs and aggregates of nannoballs and batons probably originated via the complete decay of original batons in the former cases, and the partial decay of original batons in the latter case. Nannoballs not in contact with one another may be bacterial spores that become separated from gram-positive bacterial vegetative cells as a result of a lack of nutrients or some other from of environmental pressure.

    Calcification of rod-shaped bacteria probably involved aragonite formation in the bulk phase external to individual bacteria. It is proposed that calcification of individual bacteria took place in biofilms that were adsorbed to the surface of a nucleus (such as a fecal pellet) and then to successive laminae as the ooids grew in size. Each lamina would initially, therefore, represent a single biofilm and, ultimately, a layer of calcified rod-shaped bacteria. Calcification of bacteria in biofilms probably took place rapidly so that successive biofilms were formed on mostly solid surfaces. Thus, the ooids lithified from the inside out.

    It is hypothesized that ooids stop growing as a result of termination of a nutrient supply necessary to create biofilms on the surface of developing ooids. The most likely cause is burial of older ooids by younger ooids so that biofilms in older ooids no longer have access to a nutrient source in the water column.