HR: 0800h
AN: PP31A-1518    [Abstracts]
TI: Paleohydrology of a Pleistocene Pluvial Lake in the Western Desert of Egypt
AU: * Kieniewicz, J
EM: kieniewicz@levee.wustl.edu
AF: Washington University in St Louis, Campus Box 1169, One Brookings Dr., St Louis, MO 63117
AU: Smith, J R
EM: jensmith@wustl.edu
AF: Washington University in St Louis, Campus Box 1169, One Brookings Dr., St Louis, MO 63117
AB: Stable isotope and minor element analyses of authigenic calcite silts and gastropods from a small (~3-4 km²) paleolake at Wadi Midauwara in Kharga Oasis provide evidence for the prolonged presence of perennial fresh surface water in the currently hyper-arid Western Desert of Egypt during the ~130 ka pluvial event. While sequential chemical analyses of the freshwater gastropod Melanoides tuberculata provide a seasonal record of lake water chemistry, analyses of the silts present an integrated centennial- to millennial-scale archive of longer term climatic shifts. Thus, climatic variation can be examined over a variety of temporal scales. Stable isotope analyses of the low-Mg calcite silts imply precipitation in equilibrium with water isotopically equivalent to `fossil' groundwater at Kharga Oasis (~-11‰_SMOW), and with a dissolved inorganic carbon reservoir exhibiting little influence of respired carbon (average: -0.5‰_PDB). While about 3‰ and 1.5‰ variability occurs in δ{¹⁸O and δ¹³C}, respectively, the isotopes do not covary, suggesting that the lacustrine environment at Wadi Midauwara may have been a hydrologically open system. This is supported by sequential intra-shell stable isotope and minor element analyses of Melanoides tuberculata. Stable isotope variability in δ¹⁸O can be accounted for by seasonal changes in water temperature coupled with minor fluctuations in water salinity. The Mg/Ca and Sr/Ca ratios of calcite from the lacustrine silts both decrease upwards through the stratigraphic profile, and weakly covary with the δ¹⁸O values for the silts. Mg/Ca and Sr/Ca ratios likely reflect the residence time of the water in the local limestone aquifer, which is driven principally by the amount of local recharge. Thus the decrease probably indicates a gradual decrease in the residence time of local water in the limestone aquifer, and implies increasing rainfall. Surprisingly, the Ba/Ca values for the silts do not covary with the Mg/Ca and Sr/Ca. Because Ba and Sr behave similarly in karstic systems, and are also incorporated similarly into the calcite lattice, the lack of covariance of Ba/Ca with Mg/Ca and Sr/Ca was unexpected. As barite cements are not uncommon in the sandstones of the Nubian aquifer, the Ba/Ca ratio of lacustrine calcite may actually be recording the relative contribution of Ba-rich Nubian aquifer water to a lake that was principally fed by local (Sr- and Mg-rich) limestone aquifers as well as by surficial drainage from limestone-floored watersheds. Wadi Midauwara, like many other foci of spring activity within the Western Desert, is located along a fault which may have provided for artesian discharge from the Nubian aquifer at relatively high elevations above the oasis floor. The climate of the pluvial events of the Sahara has been hypothesized to have been dominated by the Atlantic monsoon, with possible minor input from the Indian Ocean Monsoon. The apparent presence of perennial fresh water at Midauwara is contrary to what would be expected for ponds, fed only by seasonal rainfall. If the Western Desert during the ~130 ka pluvial event received significant summer and winter rains, similar to modern equatorial Africa, the seasonal variation in rainfall source should have been recorded as substantial variation in the δ¹⁸O of lake water, a variation which we do not observe. It is more likely, then, that perennial groundwater inflow derived from springs along the Libyan Plateau escarpment, as well as, perhaps, from the Nubian aquifer, maintained the lake.
DE: 1051 Sedimentary geochemistry
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1637 Regional climate change
DE: 1845 Limnology (0458, 4239, 4942)
DE: 9305 Africa
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005