Géochimie des eaux et des sédiments de quelques lacs volcaniques du Massif Central français

Remerciements

Nous remercions la Station de Biologie de Besse en Chandesse pour son appui technique sur le terrain, S. Krishnaswamy pour les analyses de ²¹⁰Pb, Mme P. Bernard pour les prélévements de précipitation à Egliseneuve d'Entraigues. L'échantillonnage et les analyses chimiques et isotopiques ont été effectués avec l'aide très appréciée de G. Olivier, A. Noir, P. Blanc et S. Chessel du Centre de Recherches Géodynamiques de Thonon-les-Bains. Nous sommes reconnaissants à E. Siwertz pour son aide dans le calcul du modèle, et à P. Jacob pour la communication d'analyses inédites.

Summary

Despite marked similarities of their thermic and morphometric features and of their drainage basins (climate, vegetation, lithology), lakes Pavin, Chauvet, Montcyneire and Godivelle d'en Haut were found to be very different. The mean water residence time has been determined from a model based on tritium measurements in precipitation, brooks, springs and lakes. As applied to the meromictic lake Pavin these measurements showed that the monimolimnion waters (fed by sub-lacustrine springs) were more than 20 years old.

Total mineral content ranges from 11 to 250 mg·1⁻¹ and silica from 0.3 to 50 mg·⁻¹; this variation is not related to climate, lithology, elevation or even water residence time but rather to hydrologic regime (direct rainfall, runoff, surficial basalts drainage, mineral springs inflow). Determination of major elements in lacustrine deposits revealed two main sedimentation origins, shore erosion of basalts and diatomite as presented in lake Godivelle d'en Haut (43% SiO₂ in the ignited residue) and in lake Pavin (89% SiO₂). Phosphorous was the most constant element, closely related with the non-diatomaceous organic matter. Sedimentation rates were determined by use of ²¹⁰Pb and ⁵⁵Fe isotopic techniques. Biogenic silica deposition was estimated as 8 mg SiO₂ · cm⁻² · year⁻¹ for lake Pavin and 4.5 for lake Montcyneire. Consideration of overall limnological system and of quantitative data obtained by isotopic methods, should allow geochemists to determine the relative residence time of various elements, a measure of their reactivity in the system.