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 widespread and much wetter period in northeast Africa. Stable carbon and oxygen isotope analysis of the Wadi Mansurab gastropods reveals that the shell carbonate is highly depleted in C-13 and O-18. Th, highly depleted oxygen isotopic composition is indicative of rainfall derived from a distant oceanic source. The highly negative nature of the oxygen isotopes 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 regime, characterized by a high degree of inter-annual variability. The dominance of semi-aquatic and swamp-dwelling gastropods over truly aquatic species implies that the region was probably a seasonally flooded grassplain similar to the toich-lands of south-central Sudan today. Our data support the inference that towards 8500-7000 BP there was a stronger 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 period with lower rates of evapotranspiration. From 8500 to 7000 BP, lake levels were high elsewhere in northern Africa, suggesting a regionally wetter climate at that time.