Chronologie et paléohydrologie des hauts niveaux quaternaires du bassin Natron-Magadi (Tanzanie-Kenya) d'après la composition isotopique (18O, 13C, 14C, U/Th) des stromatolites littoraux. Chronology and paleohydrology of former lake levels in the Natron-Magadi basin (Tanzania-Kenya) from isotopic composition (18O, 13C, 14C, U/Th) of shoreline stromatolites

Résumé En Fr

The Lake Natron-Lake Magadi basin has been occupied by extensive lakes on several occasions during the late Pliocene and Pleistocene times. During the most recent lacustrine episodes, stromatolites were built at the lake margins, some 50 m above the modern water level. Three distinct phases of encrustation were identified by detailed morphologic and pétrographie examination of these algal limestones. The more recent phases frequently cover pebbles and boulders eroded from the older phases. More than 30¹⁴C measurements on the third generation yielded ages ranging from approximately 12,000 to 10,000 yrs B.P. Their ¹³C values (≥ 2.6 ‰) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO₂, thereby lending credence to the reliability of the ¹⁴C ages. I80 and 13C measurements on these algal limestones when compared to values on modern waters and carbonates, give some indication about the paleohydrology. Long residence time of the paleolake waters and less seasonally contrasted regimes have been inferred for the corresponding humid episodes. The ²³⁰Th/²³²Th ratio of the detrital component was determined by Th/U measurements on the recent stromatolites. Using this values we then calculated a ²³⁰Th/²³⁴U chronology for the older stromatolites. Age ≥240,000 yrs and 135,000 ± 10,000 yrs were obtained respectively for the first and second generations. A humid episode seems therefore to characterize eastern Africa during each glacial-interglacial transition period.

Trois générations de stromatolites marquent l'extension récente de paléolacs dans le bassin Natron-Magadi, à la frontière tanzano-kenyenne. Leurs teneurs en ¹⁸O et en ¹³C, relativement élevées, reflètent un long temps de résidence de l'eau des paléolacs favorable à l'établissement d'un équilibre entre le CO₂ atmosphérique et le carbone inorganique dissous malgré les forts apports de carbone profond lié au volcanisme carbonatitique régional. De ce fait, les âges ¹⁴C obtenus sur les stromatolites les plus récents (12 00 à 10 000 B.P.) sont considérés comme représentatifs. Les stromatolites plus anciens ont donné des âges U/Th de 135 000 ans et de ca. 240 000 ans. Un épisode humide semble donc caractériser l'Afrique de l'Est au cours de chaque transition entre les périodes glaciaires et interglaciaires.