PALEOENVIRONMENTAL IMPLICATIONS OF THE ISOTOPE GEOCHEMISTRY AND GRANULOMETRY OF QUATERNARY ALLUVIAL SEDIMENTS AND ASSOCIATED PALEOSOLS FROM SAI ISLAND, SUDAN

δ¹⁸O and δ¹³C analysis of carbonate nodules found in alluvial paleosols on Sai, an alluvial island within the Nile River in northern Sudan, reveals depleted values consistent with pedogenic activity during the last pluvial (humid) phase in the currently hyperarid region. Stratigraphic position of the paleosols and associated archaeological material suggests soil formation was concurrent with the most recent regional humid phase between ~9500 and ~4000BP. The most depleted ¹⁸O values observed suggest that the source of at least some of the water contributing to the formation of these carbonates was meteoric in origin, subject to little evaporative enrichment, and therefore locally sourced. Correlation of δ¹³C values of surface carbonate samples with SRTM-derived topographic data suggest that these paleosols have undergone erosion since their initial formation. Unusual trends in δ¹³C with respect to depth in some locations (δ¹³C does not decrease with depth as is typical) suggest multiple buried surfaces that were once active, and therefore provide evidence for multiple episodes of deposition caused by channel migration or flooding. Significant variations (up to ~10‰) in δ¹⁸O of pedogenic carbonates over kilometer scale areas suggest that water was subject to evaporative enrichment either as it flowed downhill from a meteoric source or as it moved to more distal parts of the floodplain of a paleochannel. Additionally, grain size analysis of certain alluvial sediments on the island reveal variations in sorting and grain size suggestive of higher flow regimes at other times in the Quaternary than those represented by relatively recent overbank deposits, and may comprise channel rather than overbank facies, indicating past channel migration. Sai contains a rich archaeological record dating from the Pleistocene to the present, and the results described here provide insight into the history of the regional climate and local landscape over the times of various occupations.