Complexity of Saharan paleoclimate reconstruction and implications for modern human migration

Abstract Chronological and geochemical data from scarp and scarp-foot depression sediments (SSDS) have been used for deciphering Saharan paleoclimates and patterns of migration of anatomically modern humans (AMH). U–Th dating of thick accumulations of SSDS indicates prevalent deposition during long-lasting cool glacial periods (marine isotope stage [MIS] 6, 8, 10, and 12) and limited deposition during warm interglacial periods (MIS 5, 7 and 13). In contrast, Quaternary sediments associated with short residence, hydrologic systems in Sinai yielded MIS 2 OSL ages of 27.7 to 10.2 ka. The lack of SSDS of MIS 2 ages and the wide range of warm and cool stages in Eastern Sahara is attributed to: (1) lengthy travel times of groundwater prior to discharge and deposition of SSDS from deep groundwater originating from distant sources, and (2) sampling and dating of SSDS from depressions where natural discharge has been apparently continuous through wet and dry periods. Given that the previously dated depression SSDS could have been deposited during dry periods, and that the ages of SSDS reflect the timing of groundwater discharge rather than the ages of the wet periods during which recharge occurred, we suggest that earlier interpretations of Saharan paleoclimate and AMH migrations that heavily relied on dating of SSDS should be revisited.