Shorelines in the Sahara: geomorphological evidence for an enhanced monsoon from palaeolake Megachad

Abstract

The Sahara Desert is the most extensive desert on Earth but during the Holocene it was home to some of the largest freshwater lakes on Earth; of these, palaeolake Megachad was the biggest. Landsat TM images and Shuttle Radar Topography Mission (SRTM) digital topographic data reveal numerous shorelines around palaeolake Megachad. At its peak sometime before 7000 years ago the lake was over 173 m deep with an area of at least 400 000 km 2 , bigger than the Caspian Sea, the biggest lake on Earth today. The morphology of the shorelines indicates two dominant winds, one northeasterly that is consistent with the present-day winds in the region. The other originated from the southwest. We attribute it to an enhanced monsoon caused by a precessionally driven increase in Northern Hemisphere insolation. Subsequent desiccation of the palaeolake is recorded by numerous regressive shorelines in the Sahara Desert.

Human Y chromosome haplogroup R-V88: a paternal genetic record of early mid Holocene trans-Saharan connections and the spread of Chadic languages

“…47 Indeed, geomorphological evidence 4 from the paleolakes that existed in the Sahara during the mid-Holocene indicates that these lakes could have covered an area as large as about 10% of the Sahara, providing an important corridor for human migrations across the region. 5 In summary, our data indicate a significant male contribution from northern Africa (and ultimately Asia) to the gene pool of the central Sahel. The trans-Saharan population movements resulting in this genetic pattern would seem to mirror the spread of the proto-Chadic languages, and most likely took place during the early mid Holocene, a period when giant paleolakes may have provided a corridor for human migrations across what is now the Sahara desert.…”

“…3 At the same time, the Sahara was home to giant lakes, 4 the largest of which, the paleolake Megachad, may have possibly covered an area of at least 400 000 km 2 , more than the Caspian Sea, the biggest lake on earth today. 5 This greening scenario was interrupted by a number of arid episodes, and at about 5-6 kya, the region experienced a rapid onset of dryer conditions. These marked the beginning of a shift towards permanent aridity, with variations in the distribution and timing of these changes between the eastern and central/western Sahara.…”

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

“…The latter was the largest freshwater lake in Africa and probably the largest pluvial lake on Earth. During the Early to mid-Holocene wet phase, Lake Mega-Chad attained an area of 361,000 km 2 (16)(17)(18) and a depth of up to 160 m (Fig. 1).…”

Ancient watercourses and biogeography of the Sahara explain the peopling of the desert

“…Furthermore, arid plant taxa are found continuously through Atlantic Ocean cores between 23°and 27°north for the last 250 ka suggesting a longlasting Saharan arid belt at this latitude (8). In contrast, a green Sahara route has been proposed whereby this currently hyperarid region could have been habitable during "green" periods of greater humidity (9,10). This humid period was first suggested by Duveyrier (11) to explain the existence of the Nile crocodiles in isolated Saharan oases and has periodically been promoted since, notably by Dumont (12) who noted the trans-Saharan distribution of numerous aquatic animal species.…”