干旱区生态环境知识资源中心

An Atlantic source of precipitation can be inferred from stable isotopic data (H and O) for fossil groundwaters and uraniumseries-dated carbonate spring deposits from eases in the Western Desert of Egypt, In the context of available stable isotopic data for fossil groundwaters throughout North Africa, the observed isotopic depletions (delta D -72 to -81 parts per thousand; delta(18)O -10.6 to -11.5 parts per thousand) of fossil (greater than or equal to 32,000 yr B.P.) groundwaters from the Nubian aquifer are best explained by progressive condensation of water vapor from paleowesterly wet oceanic air masses that traveled across North Africa and operated at least as far back as 450,000 yr before the present, The values of delta(18)O (17.1 to 25.9 parts per thousand) for 45,000- to >450,000-yr-old tufas and vein-filling calcite deposits from the Kharga and Farafra Gases are consistent with deposition from groundwaters having oxygen isotopic compositions similar to those of fossil groundwaters sampled recently at these locations. (C) 1997 University of Washington.