Kopara deposits in French Polynesia: formation, microstructure, biochemical features. - INSU - Institut national des sciences de l'Univers Access content directly
Conference Papers Year : 2004

Kopara deposits in French Polynesia: formation, microstructure, biochemical features.

Abstract

In French Polynesia, thick cyanobacterial mats (« kopara ») inhabit shallow water ponds of atoll rims. Calcium carbonate (Mg-calcite or more rarely aragonite) intensively precipitates in these mats, exceptionally in a close relationship with living cyanobacterial filaments (biomineralization) or, in the most usual case, within microalveoles of the structured, 3D network that derives from the degradation of cyanobacterial sheaths (organomineralization). CryoSEM allows the fine scale localization of mineralization sites in both types, and it demontrates the causual relationship between CaCO3 precipitation and cyanobacteria, either under control of their metabolism or induced by organic substrate and solute properties within the degrading network. Amino-acid compositions are analyzed separately in the cyanobacterially derived organic matter and in macromolecules dissolved in pore waters, both being considered as potentially involved in organomineralization processes, and compared with compositions of intramineral (soluble and insoluble) organic matrices. The influence of macromolecules dissolved in pore waters on CaCO3 precipitation is also tested in experimental conditions. The extracted natural macromolecules definitely control the rate of in vitro calcium carbonate precipitation: they exert an inhibiting effect, which strength depends on the type of layer – carbonate-rich, or carbonate-poor – out of which they have been extracted. The quality (with respect to their aspartic acid contents) of soluble protein-rich macromolecules present in the pore waters in each layer plays a key role in active calcification processes. These results may help (at least partly) to understand the alternating highly- and poorly carbonated character of the successive layers in a laminated kopara deposit from Rangiroa (Tuamotu archipelago).

Domains

Geochemistry
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Dates and versions

hal-00091766 , version 1 (07-09-2006)

Identifiers

  • HAL Id : hal-00091766 , version 1

Cite

Pascale Gautret, Christian Défarge, Jean Trichet. Kopara deposits in French Polynesia: formation, microstructure, biochemical features.. International workshop « Microbialites and microbial communities in sedimentary systems, 2004, Paris, France. pp.57. ⟨hal-00091766⟩
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