Lake Turkana is an important modern analog to ancient rift environments in east Africa and elsewhere. Carbonate is second in abundance after the detrital silicate fraction of the sediments in the lake. It consists of ostracod tests and a silt-sized micrite. Scanning electron microscopy and X-ray diffraction analysis revealed the micrite to be euhedral crystals of low-Mg calcite about 10 microns in length. Carbon isotope values of the micrite indicate that it forms in the upper water column rather than in the sediments after burial. The oxygen isotope analyses indicate that the micrite could be at isotopic equilibrium with the modern lake waters at temperatures of 36°C. This suggests that the micrite forms either in the surface waters of the open lake or in isolated shallow bays, and is then transported to the deep lake.
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The ~I 3C values suggest that the micrite precipitates inorganically in the surface waters of the lake. Its formation may be enhanced by photosyn-theUc activity. The micrite is in isotopic equilibrium with the lake water if its temperature of formation is higher than the mean water temperature of the lake. This suggests that the micrite precipitates in the super-heated surface waters, perhaps on calm days, or in shallow shoreline pools. Isotopic equ111brium could also be achieved if the isotopic composition of the lake were 1°/oo lower. This is hypothetically possible in the northern part of the north basin during the flood season of the Omo River. Acknoledgemen- tWs e thank the captain and crew of the M/V Halcyon, operated and owned by the Kenyan Department of Fisheries for their valuable assistance in the field, at times under stressful conditions, and R. Leakey, National Museum of Kenya for IogisUcal assistance. K. Ghilardi was invaluable in direcUng the operaUon of the ETH piston corer. T. Ceiling kindly allowed us to use his isotope data of sediment porewater and lake water. D. Van Nieuwenhuise kindly provided ostracod identifications. We thank C. Paull for his careful review o fan earlier draft of this manuscript. Financial support was provided by Project PROBE and two grants to T.C.J. numbered PRF-17428-AC2 from the American Chemical Society and DPE-S542-G-SS-OO from the Agency for International Development. At the time of the Turkana field program, Project PROBE was funded by Amoco, Arco, Conoco, Esso, Marathon, Mobil, Pecten, Penzofl, and Shell IntemaUonal Oil Companies, and by the World Bank.
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