Dating lacustrine episodes in the eastern Sahara by the epimerization of isoleucine in ostrich eggshells

The eggshell of the African ostrich, Struthio camelus, closely approximates a closed system for the retention of indigenous proteinaceous residues. Epimerization of the protein amino acid isoleucine follows linear first-order kinetics in laboratory simulations nearly to racemic equilibrium, and the variation in D/L ratio within a single fragment, or between fragments of the same age, is significantly less than in other carbonate systems. These observations suggest that the extent of isoleucine epimerization (aIle/Ile ratio) in ostrich eggshell offers the potential for high-resolution geochronology of Quaternary deposits. From the simulation experiments, and dated early Holocene samples for which we have in situ mean annual sediment temperature measurements, Arrhenius parameters have been calculated; the activation energy is 30.33 kcal mol-1, similar to that of other carbonate systems. We have measured the aIle/Ile ratio in ostrich eggshell associated with lacustrine episodes at Bir Tarfawi and Bir Sahara East, two depressions in what is currently the hyperarid eastern Sahara. The ratios can be used directly to indicate qualitatively the time represented by each series of lake sediment, and to correlate disjunct lacustrine deposits within and between the basins. Uranium-series disequilibrium dating of algal mats contained within some of the lake beds indicate that a major wet interval occurred about 130 ka ago. Using the U-series date for calibration, the amino acid ratios are used to date the most recent lacustrine interval to about 100 ka B.P., and two older intervals, one about 200 ?? 25 ka B.P., and an older interval that occurred prior to 250 ka ago. ?? 1991.