¹⁴C and Th/U Dating of Pleistocene and Holocene Stromatolites from East African Paleolakes

Abstract

During recent humid episodes, stromatolites were built along paleolake margins, some 60 m above the modern water level of Lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently covering pebbles and boulders eroded from the older ones. The youngest one yielded 14 C ages ranging from approximately 12,000 to 10,000 yr B.P. Their δ 13 C values (≥2.6%) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO 2 , thereby lending credence to the reliability of the 14 C. An initial 230 Th/ 232 Th ratio in the detrital component was determined by Th/U measurements on the 14 C dated stromatolites. Using this value a 230 Th/ 234 U chronology for the older stromatolites was calculated. Ages of ≥240,000 and 135,000 ± 10,000 yr were obtained for the first and second generations, respectively. A humid episode apparently characterized eastern Africa during each glacial-interglacial transition. 18 O and 13 C measurements on stromatolites, when compared to values on modern waters and carbonates, provide paleohydrological information. Long residence time of the paleolake waters and less seasonally contrasted regimes are inferred.