Growth and Calcification of Calothrix — Dominated Oncolites from Northern England
Oncolite growth rates were measured using a new technique over a period of 622 days in a shallow north Yorkshire stream. Deposition was seasonally dependent and significantly correlated to water temperature with maximum radial rates of around 1 μ m/day recorded during the summer. During winter, rates were negligible and slight corrosion may have occurred. The annual radial growth was 143± 98 μ m.
The surface layers of actively accreting oncolites were colonized by a range of microorganisms but the cyanobacterium Calothrix parietina Thuret. was dominant. No seasonal differences in the microbial flora were detected though the deposits showed marked concentric banding which correlated significantly with the estimated annual growth rate. Observations with the scanning electron microscope showed that Calothrix calcification is confined to the mucilaginous sheath and consists of unoriented crystals of micrite. The fine structure of modern and subfossil oncolites, collected from a nearby site, is compared, and the value of oncolites as paleoenvironmental indicators is assessed in the light of the results obtained.
KeywordsMicrobial Flora Radial Growth Rate Algal Layer Calcify Tube Mucilaginous Sheath
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