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Diagenèse précoce en domaine lacustre : étude des composés minéraux et organiques des sédiments récents du lac d'Aydat (Puy de Dôme, France).

Abstract : Aydat is a small eutrophic and dimictic lake of the French Massif-Central, in a volcanic environment. The study of early diagenesis of current (traps) and recent (cores) sediments of the lake, has been carried out according to the classical methods of sedimentology, inorganic geochemistry, as well as by the analysis of major organic constituents : the polysaccharides. Current sediments have been collected monthly, from October 1995 till September 1996, thanks to 3 traps placed at different depths in the water column near the centre of the lake. The 50 cm long sedimentary cores have been taken at the vertical of the traps. The inorganic fraction of the sediments is dominated by authigenic particles. The opal-silica of diatoms, representing over 80% of the sediments, contribute directly to the sedimentary record. In contrast, authigenic minerals, such as calcite and oxi-hydroxides of iron and manganese, are strongly involved in the biogeochemical dynamics of the lake, mainly governed by the redox processes induced by the biological activity. The behaviour of the most mobile elements of the bottom sediments, whatever they are soluble, exchangeable and/or sorb, has been further assessed by sequential chemical extractions. All the geochemical data obtained on the solid phase of the sediments, combined with the analyses of the elements in solution performed by the team of the Laboratoire de Géochimie des Eaux (Université Paris VII), has allowed to establish an annual budget of chemical elements in lake Aydat. This budget has mainly allowed to identify 5 different behaviours of the various steady elements during the whole year and has also allowed to estimate the intensity of the recycling of reactive elements such as Fe, Mn, Co, Ba and As, in the lacustrine reservoir. The dynamics of the organic matter, that is essentially governed by the lacustrine geochemical processes via the biological activity, has been followed thanks to the analysis of the polysaccharides, majors ubiquist compounds of the vegetable reign that are easily biodegraded. Despite an important autochtonous organic matter flux in Aydat lake, the neutral sugar composition does not reflect the primary plancktonic production (diatoms, cyanobacteria), but rather that of the microbial flora. The intense bacterial activity within the water column and especially at the water-sediment interface, marked by the abundance of rhamnose, a deoxyhexose, prevents sugars to participate significantly to the sedimentary record. Thus, these biological polymers contribute via bacteria to diagenetic processes, and therefore to support the biogeochemical cycles in which the most reactive mineral elements are periodically involved.
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Submitted on : Wednesday, January 15, 2003 - 10:47:45 AM
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  • HAL Id : tel-00002273, version 1

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Sylvie Ogier. Diagenèse précoce en domaine lacustre : étude des composés minéraux et organiques des sédiments récents du lac d'Aydat (Puy de Dôme, France).. Géologie appliquée. Université d'Orléans, 1999. Français. ⟨tel-00002273⟩

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