Lake Ptolemy

During the early and middle Holocene, large lakes such as Lake Chad and Lake Ptolemy developed within the Sahara[6] and river systems such as Wadi Howar flowed, although it is not clear if they were flowing through a still desertic landscape.[7] 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.[8] Today the eastern Sahara is among the driest locations on Earth[3] as it is far removed from oceanic moisture sources.[7]

Lake Ptolemy is located on the territory of present-day Sudan.[9] Estimates of its size have changed as the quality of regional maps changed: At first, it was believed to have reached surface areas of about 27,000 square kilometres (10,000 sq mi);[10] 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);[11] 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.[12]

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;[13] in the former case the lake may have occupied an area of no less than 17,864 square kilometres (6,897 sq mi).[14] There is also evidence of shorelines at altitudes of 570–576 metres (1,870–1,890 ft) above sea level;[15] if they reflect the lake reaching 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.[12] Such a size is comparable to Canada's largest lake, the Great Bear Lake,[16] and larger than Lake Erie. It would have been up to 83 metres (272 ft) deep.[17] Lower lake stages might have reached 565 metres (1,854 ft) and 560 metres (1,840 ft) elevation,[18] and there is fossil evidence that episodes of shallow water level occurred in the lake.[19] The lake floor in its southern and western sectors reaches 549 metres (1,801 ft) elevation above sea level.[20]

Shorelines developed on Lake Ptolemy's northern margin[21] and the lake submerged two tributary valleys there;[22] the development of dune fields on the western shore makes identification of the shores there difficult[20] and their absence has raised questions about whether the lake actually existed at such large sizes.[23] 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,[24] and alluvial fans have been identified on the northwestern shores.[25] Chalks formed in Lake Ptolemy generated yardangs,[2] 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.[26]

The lake existed in the area of present-day Wadi Howar[9] in the Darfur Basin.[6] Today the oases of Oyo, Bidi and Nukheila are located on the lake-floor that Lake Ptolemy occupied at maximum highstand.[27] The lake probably resembled present day Lake Chad.[28]

The lake was nourished by runoff from the Ennedi, Erdi Ma and part of the Kufrah Depression, as well as groundwater;[2] at least one site of the lake floor shows evidence of the release of pressurized water[21] and evidence of higher groundwater levels is widespread in the eastern Sahara.[7] 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).[29][30]

This runoff reached the lake through various wadis,[26] many of which entered Lake Ptolemy from the north,[31] such as Wadi Fesh-Fesh.[32] The Ennedi was critical for the water balance of lake,[33] with three tributaries joining it from there.[34] To the northwest the Lake Ptolemy drainage system was bordered by areas that drained north, while to the northeast drainage was northeastward.[35] Unlike Lake Chad/Lake Megachad, Lake Ptolemy was not nourished by rivers from the humid and semi-humid tropics, but solely from regional catchments.[5]

The presence of Asphataria indicates that Lake Ptolemy was a freshwater lake[21] especially close to its inflows,[36] although with occasional brackish phases.[37] Precipitation at the time was about 300 millimetres (12 in) per year.[38]

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.[39] A connection between Lake Ptolemy and Wadi Howar which drains to the Nile is possible.[4][40] but not proven.[41] At water levels of 577–583 metres (1,893–1,913 ft) Lake Ptolemy would overflow into Wadi Arid.[18]