STABLE CARBON AND OXYGEN ISOTOPIC COMPOSITION OF EARLY-HOLOCENE GASTROPODS FROM WADI-MANSURAB, NORTH-CENTRAL SUDAN

The 8500- to 7000-year-old shell-bearing sediments in shallow claypans near Wadi Mansurab in north-central Sudan appear to reflect a widespr ead and much wetter period in northeast Africa. Stable carbon and oxyg en isotope analysis of the Wadi Mansurab gastropods reveals that the s hell carbonate is highly depleted in C-13 and O-18. Th, highly deplete d oxygen isotopic composition is indicative of rainfall derived from a distant oceanic source. The highly negative nature of the oxygen isot opes indicates that the region had significantly less evaporation than today while the extreme variability in isotopic composition (up to 6- 7 parts per thousand PDB) is consistent with a seasonal rainfall regim e, characterized by a high degree of inter-annual variability. The dom inance of semi-aquatic and swamp-dwelling gastropods over truly aquati c species implies that the region was probably a seasonally flooded gr assplain similar to the toich-lands of south-central Sudan today. Our data support the inference that towards 8500-7000 BP there was a stron ger southwest monsoon and an associated northward shift in the summer rainfall zone, which caused the Wadi Mansurab region in north-central Sudan to be seasonally flooded during a wetter and possibly cooler per iod with lower rates of evapotranspiration. From 8500 to 7000 BP, lake levels were high elsewhere in northern Africa, suggesting a regionall y wetter climate at that time.