Abstract
During the Quaternary period, the eastern Sahara's hydroclimate oscillated between wet and dry intervals. These oscillations caused drastic changes in precipitation rates, often associated with ancient human migrations. In particular, significant migration of riparian populations from the Nile Valley to the west and northwestward of the Sahara occurred during the African Humid Period (AHP), an episode of increased monsoons, which characterized North Africa in response to increasing insolation. Several fossil rivers, now preserved as ridges throughout southern Egypt due to their floodplains' deflation, contain archeological artifacts and thus represent a potentially important record of fluvial activity during this episode of past human dynamics and environmental change. Here we present 14C and Optically Stimulated Luminescence (OSL) ages of sediments preserved in these palaeorivers, which cluster within the AHP and are thus consistent with increased fluvial activity during this distinct humid period. Palaeohydraulic reconstructions based on grain size, channel geometry, and drainage area suggest typical precipitation intensities of 55–80 mm/h during sediment transport events. Given previous annual rainfall estimates, these hydrologic conditions may have lasted, or occurred, during the AHP up to 3–4 times more frequently than before and after this period. Such intense fluvial activity is consistent with monsoon intensification and may have rendered the area inhospitable for human settlements, congruent with population migration out of the Nile Valley during the AHP. These findings highlight links between past human ecodynamics and environmental signals, providing a concrete narrative of human population response to warming with potential echo in the current situation.
Highlights
A number of uncertainties remain on the regional extent of this wet period across North Africa, as well as on its timing and magnitude, notably because of evidence of low lake and river discharge levels during this period (Gasse et al, 2008; and review in Williams, 2019a, 2019b, pp. 107e126), multiple palaeo-climatic proxies broadly converge toward estimates of annual precipitation rates of 300e920 mm/yr in the eastern Sahara (Ritchie and Haynes, 1987; Hoelzmann et al, 2000; Tierney et al, 2017)
To further test how plausible the hypothesis is that environmental perturbations during the African Humid Period (AHP) drove human migration away from the Nile Valley, this work aims to estimate the palaeo-rainfall intensities involved in transforming the Nile Valley into an inhospitable region
When coupled with estimates of precipitation from palaeo-hydraulic reconstructions and calculated sedimentation rates, our study enables a quantitative assessment of rainfall perturbations during late-Quaternary climate oscillations in the Sahara, during the AHP
Summary
Late Quaternary climate oscillations in the eastern Sahara region have been identified, based on palaeo-discharge and sediment-load estimates of the Nile River, cave speleothems, dust fluxes, pollen records, groundwater-fed deposits, ancient watercourses, archeological evidence, and abrupt fluctuations of lake levels and lacustrine deposits (Fig. 1A; Foucault and Stanley, 1989; Hoelzmann et al, 2000; Schuster et al, 2005; Hamdan and Brook, 2015; Hoffmann et al, 2016; Drake et al, 2011; Williams et al, 2015; Macklin et al, 2013, 2015; Woodward et al, 2015; Abotalib et al, 2016; Palchan and Torfstien, 2019; Manning and Timpson, 2014; Pausata et al, 2020) These proxies suggest that at least four humid periods may have punctuated the past ca. To further test how plausible the hypothesis is that environmental perturbations during the AHP drove human migration away from the Nile Valley, this work aims to estimate the palaeo-rainfall intensities involved in transforming the Nile Valley into an inhospitable region
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