Sedimentary carbonates as isotopic marker horizons at Lake Turkana, Kenya

Abstract

Summary The Plio-Pleistocene sediments of the Koobi Fora region of Lake Turkana contain a variety of carbonate horizons: molluscs, stromatolites, indurated calcitic sediments and concretions. The stable oxygen isotope ratios of these carbonates are largely unchanged since deposition, and constitute a record of environmental conditions at the time of deposition. Two factors influence these isotope ratios: (1) temperature, which produces more negative δ 18 O values with increasing temperature; and (2) evaporative concentration of the heavier oxygen isotope in the rainfall and body of the lake, which makes for more positive values of δ 18 O under arid conditions. The major change in climate which took place at about the time of deposition of the KBS Tuff is isotopically plain in most sections, but stratigraphic control is not sufficient to correlate other significant changes. Nevertheless, the general trends in environmental change during the deposition of any particular section of sediments can be clearly perceived provided there are enough carbonate horizons. Gastropods provide the least equivocal evidence for environmental conditions, and a survey of the entire continent of Africa for stable isotope ratios in contemporary gastropods has been conducted to provide a database for comparison of environmental conditions in the fossil gastropods.