Holocene palaeoclimates in northwestern Sudan: stable isotope studies on molluscs

Gastropod shells and bulk sedimentary carbonate deposits found in palaeolak e sediments in the presently hyperarid regions of NW Sudan provide proxy ma terials for the evaluation of the vastly different and wetter climatic cond itions that prevailed for several thousand years in that region at the begi nning of the Holocene. Oxygen and carbon stable isotope ratio measurements on these shells and carbonates suggest that the African Summer Monsoon prov ided extensive rainfall up to 800 km further north than at present, creatin g substantial lakes and refilling the deep aquifers of the region with isot opically depleted water up to 21 degreesN. Variations in stable isotope rat ios, as one proceeds upwards through the sediments, indicate that the wette st phase occured about 9000 years B.P., and that a considerably drier perio d began after about 5600 years B.P., after which the record is obliterated by decreasing rainfall and subsequent deflation of the sediments. During th e wet phase, large quantities of isotopically depleted (light) moisture - c orroborating the convective origin of the rainfall - were brought to the Ea stern Sahara by intensified monsoonal rains. Variations in the oxygen isoto pe ratios during the growth of individual shells demonstrate that considera ble seasonality existed in yearly rainfall. (C) 2000 Elsevier Science B.V. All rights reserved.