QUATERNARY RESEARCH 38, 205-226 (1992) Chronology and Paleohydrology of Late Quaternary High Lake Levels in the Manyara Basin (Tanzania) from Isotopic Data (“0, 13C, 14C, Th/U) on Fossil Stromatolites JOEL CASANOVA*~ AND CLAUDE HILLAIRE-MARCEL? *BRGM, Department of Geochemistry, BP 6009, O&am, 45060, France; and fGEOTOP, UniversitC du QuCbec ri Montrkal, P.O. Box 8888, Station A, MontrCal, Quebec, H3C 3P8, Canada Received May 8, 1991 Superimposed phases of stromatolite buildup are observed on the eastern margin of Lake Man- yara and depict a paleoshoreline at about 20 m above modem lake level. Radiocarbon and Th/U measurements permitted the dating of the last two phases of stromatolite formation at ca. 90,000 yr and between 27,000 and 23,000 yr B.P., respectively. The Th/U chronology is based on the decay of a strong 23@Th-excess (over 234U) inherited with the detrital particles cemented into the stro- matolites. The various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels. This indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation. A comparison with similar stroma- tolitic units from the nearby Lake Natron-Lake Magadi basin shows that such conditions occurred during only a few of the late Quatemary humid episodes known in eastern Africa and that they are different in each basin. Stromatolites do not necessarily represent all high lake levels that Lake Manyara experienced during the late Pleistocene and Holocene. 0 1992 University of Washington. INTRODUCTION Beyond the last 30,000 to 40,000 yr (i.e., beyond the i4C method limit), the chronol- ogy of hydrologic change in intertropical Africa has yet to be documented. In West Africa, recent uranium-series dating of ground- or surface-water carbonates (cal- cretes, nodules, cements, and shelly depos- its) provides a few chronological data on some of the main paleoclimatic events of late Pleistocene age (Causse ef al., 1988, 1989; Szabo et al., 1989). In East Africa, where siliciclastic sedimentation domi- nates, the paucity of suitable materials re- stricts the setting-up of a Th/U chronology. In recent papers (Hillaire-Marcel et al., 1986; Hillaire-Marcel and Casanova, 1987), however, it was demonstrated that paleo- ecological and isotopic analysis of lacus- trine stromatolites could provide precise data on the occurrence and amplitude of paleoclimatic fluctuations in the East Afii- can Rift system. When present, stromato- lites are a precise indicator of high lake- level limits; their morphology and struc- ture, in relation to their paleobathymetric distribution, provide an insight into the pa- leoenvironments of the time (Casanova, 1986a,b); and their isotopic composition (“C, ‘*O) allows the assessment of paleo- hydrological conditions, with reference to modern hydrology. Finally, 14C and Th/U data on such carbonates may give an access to time and therefore to the age of the cor- responding climatic episodes. In this article, new isotopic data on the hydrology and paleohydrology of the Lake Manyara basin are presented. They com- plement previous studies (e.g., Hillaire- Marcel and Casanova, 1987) on lakes Na- tron and Magadi, to the north, and provide additional information on late Pleistocene hydrologic changes in the eastern branch of the Rift. LAKE MANYARA Lake Manyara (altitude 960 m) is located in the southernmost basin of the eastern 205 0033-5894192 $5.00 Copyright 0 1992 by the University of Washington. All rights of reproduction in any form reserved.