Paleohydrology of Lake Victoria, East Africa, inferred from O-18/O-16 ratios in sediment cellulose

delta(18)O values of aquatic cellulose preserved in a 7.6 m sediment core from near the center of Lake Victoria, East Africa, record a 13 000 yr hydrological history of the basin. Relative to the present value (+3.4 parts per thousand), highly elevated lake-water delta(18)O values of +8 parts per thousand+/-2 parts per thousand from 13 200 to 7250 C-14 yr B.P. indicate closed-basin conditions and precipitation:evaporation ratios much smaller then at present, An abrupt drop in the lake-water delta(18)O to 0 parts per thousand +/-1 parts per thousand marks the rapid transition to an open basin between 7250 and 7150 C-14 yr B.P. From 7140 to 5400 C-14 yr B.P. increasing lake-water delta(18)O values from 0 parts per thousand +/- 1 parts per thousand to +5 parts per thousand +/- 1.5 parts per thousand are proposed to reflect outlet downcutting and a likely decrease in relative humidity, These data have unexpectedly shifted our view of the timing of changes in the Lake Victoria water balance and imply a period of at least 10 000 years (17 000 to 7000 C-14 yr B.P.) when the Lake Victoria basin did not contribute waters to the main Nile River.