Lake Ptolemy

Today the eastern Sahara is among the driest locations on Earth (Elsheikh, Abdelsalam & Mickus 2011, p. 82) as it is far removed from oceanic moisture sources. (Pachur 1997, p. 228) However, during the early and middle Holocene, large lakes such as Lake Chad and Lake Ptolemy developed within the Sahara (Pachur & Altmann 2006, p. 35) and river systems such as Wadi Howar flowed, although it is not clear if they were flowing through a still desertic landscape. (Pachur 1997, p. 228) The formation of these paleolakes was ultimately linked to a stronger African monsoon caused by a higher axial tilt and the perihelion of Earth coinciding with late July and thus the monsoon season. (Hoelzmann et al. 2001, p. 193)

Lake Ptolemy was originally believed to have reached surface areas of about 27,000 square kilometres (10,000 sq mi); (Pachur & Altmann 2006, p. 226) later research on the basis of more reliable elevation maps suggested that it was no larger than 5,330 square kilometres (2,060 sq mi). (Hoelzmann et al. 2001, p. 213) Later still, newer maps indicated larger surface areas of 8,133 square kilometres (3,140 sq mi) and 11,230 square kilometres (4,340 sq mi) and a volume of 372 cubic kilometres (89 cu mi) and 547 cubic kilometres (131 cu mi), respectively. (Elsheikh, Abdelsalam & Mickus 2011, p. 83)

Water depths reached 15 metres (49 ft). Depending on the location, evidence for water levels of 550 metres (1,800 ft) or even 555 metres (1,821 ft) above sea level have been found; (Pachur & Altmann 2006, p. 221) in the former case the lake may have occupied an area of no less than 17,864 square kilometres (6,897 sq mi). (Pachur 1997, p. 240) There is also evidence of shorelines at altitudes of 570–576 metres (1,870–1,890 ft) above sea level; (Ghoneim & El-Baz 2007, pp. 5008, 5009) if they reflect a lake stand at that altitude, this stand would reach a surface of 30,750 square kilometres (11,870 sq mi) and a volume of 2,530 cubic kilometres (610 cu mi) at that stage. (Elsheikh, Abdelsalam & Mickus 2011, p. 83) Such a size is comparable to Canada's largest lake, the Great Bear Lake, (Ghoneim & El-Baz 2007, p. 5001) and larger than Lake Erie. It would have been up to 83 metres (272 ft) deep. (Ghoneim & El-Baz 2007, p. 5013) Lower lake stages might have reached 565 metres (1,854 ft) and 560 metres (1,840 ft) elevation, (Ghoneim & El-Baz 2007, p. 5014) and there is fossil evidence that shallow water episodes occurred in the lake. (Pachur 1997, p. 231) The lake floor in its southern and western reaches reaches 549 metres (1,801 ft) elevation above sea level. (Pachur & Altmann 2006, p. 216)

Shorelines developed on Lake Ptolemy's northern margin (Pachur & Altmann 2006, p. 207) and the lake submerged two tributary valleys there; (Pachur 1997, p. 238) the development of dune fields on the western shore makes identification of the shores there difficult (Pachur & Altmann 2006, p. 216) and their absence has raised questions about whether the lake actually existed at such large sizes.[1] The shores on the southern and western side developed a riparian zone with vegetation and irregular lakefloor. River deltas formed where wadis entered Lake Ptolemy, (Pachur & Altmann 2006, p. 212) and alluvial fans have been identified on the northwestern shores. (Ghoneim & El-Baz 2007, p. 5010) Chalks formed in Lake Ptolemy generated yardangs, (Pachur & Altmann 2006, p. 205) and aragonite, calcite and goethite formed deposits in the adjacent desert, often in swampy areas. Tufa pinnacles formed in the lake, and upon drying playa deposits were left behind. (Pachur & Altmann 2006, p. 206)

The lake existed in the area of present-day Wadi Howar (Gossel, Ebraheem & Wycisk 2004, p. 705) in the Darfur Basin. (Pachur & Altmann 2006, p. 35) Today the oases of Oyo, Bidi and Nukheila are located on the lakefloor that Lake Ptolemy occupied at maximum highstand. (Pachur 1997, p. 230) The lake probably resembled present day Lake Chad. (Pachur & Altmann 2006, p. 36)

The lake was nourished by runoff from the Ennedi, Erdi Ma and part of the Kufrah Depression, as well as groundwater; (Pachur & Altmann 2006, p. 205) at least one site of the lake floor shows evidence of the release of pressurized water (Pachur & Altmann 2006, p. 207) and evidence of higher groundwater levels is widespread in the eastern Sahara. (Pachur 1997, p. 228) The catchment of the lake covers a surface area of 78,000 square kilometres (30,000 sq mi), with later estimates of 128,802 square kilometres (49,731 sq mi). (Hoelzmann et al. 2001, p. 214) (Ghoneim & El-Baz 2007, p. 5005)

This runoff reached the lake through various wadis, (Pachur & Altmann 2006, p. 206) many of which entered Lake Ptolemy from the north, (Pachur & Altmann 2006, p. 208) such as Wadi Fesh-Fesh. (Pachur 1997, p. 239) The Ennedi was critical for the water balance of lake. (Pachur & Altmann 2006, p. 230) To the northwest the Lake Ptolemy drainage system was bordered by northward flowing drainage and to the northeast by northeastward draining systems. (Elsheikh, Abdelsalam & Mickus 2011, p. 84) Unlike Lake Chad/Lake Megachad, Lake Ptolemy was not nourished by rivers from the humid and semi-humid tropics, but solely from regional catchments. (Pachur 1997, p. 229)

The presence of Asphataria indicates that Lake Ptolemy was a freshwater lake (Pachur & Altmann 2006, p. 207) especially close to its inflows, (Pachur 1997, p. 235) although with occasional brackish phases. (Pachur & Altmann 2006, p. 465) Precipitation at the time was about 300 millimetres (12 in) per year. (Pachur & Altmann 2006, p. 222)

At a water level of 550 metres (1,800 ft) Lake Ptolemy would have been connected with a paleodrainage system belonging to the Abyad Plateau. (Pachur & Altmann 2006, p. 236) A connection between Lake Ptolemy and Wadi Howar which drains to the Nile is possible, (Pachur & Altmann 2006, p. 219) (Pachur 1997, p. 233) but not proven. (Pachur & Altmann 2006, p. 218) At water levels of 577–583 metres (1,893–1,913 ft) Lake Ptolemy would overflow into Wadi Arid. (Ghoneim & El-Baz 2007, p. 5014)