Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs

Gastropod shells and bulk sedimentary carbonate deposits found in palaeolake sediments in the presently hyperarid regions of NW Sudan provide proxy materials for the evaluation of the vastly different and wetter climatic conditions that prevailed for several thousand years in that region at the beginning of the Holocene. Oxygen and carbon stable isotope ratio measurements on these shells and carbonates suggest that the African Summer Monsoon provided extensive rainfall up to 800 km further north than at present, creating substantial lakes and refilling the deep aquifers of the region with isotopically depleted water up to 21°N. Variations in stable isotope ratios, as one proceeds upwards through the sediments, indicate that the wettest phase occured about 9000 years B.P., and that a considerably drier period began after about 5600 years B.P., after which the record is obliterated by decreasing rainfall and subsequent deflation of the sediments. During the wet phase, large quantities of isotopically depleted (light) moisture — corroborating the convective origin of the rainfall — were brought to the Eastern Sahara by intensified monsoonal rains. Variations in the oxygen isotope ratios during the growth of individual shells demonstrate that considerable seasonality existed in yearly rainfall.