Lake Ptolemy is a former lake in Sudan. This lake formed during the Holocene in the Darfur region, during a time when the monsoon over Africa was stronger. The existence of the lake is dated between about 9,100–2,400 years before present. This lake could have reached a surface area of 30,750 square kilometres (11,870 sq mi), larger than present-day Lake Erie, although estimates of its size vary and it might have been much smaller. The shorelines in some places, insofar as they are recognizable, feature riparian landscapes and reeds. The lake was a freshwater lake replenished by groundwater and runoff from neighbouring mountains and might itself have been the source for the Nubian Sandstone Aquifer System. The lake featured a diverse ecosystem with a number of species, and possibly facilitated the spread of species between the Nile and Lake Chad.
Name and research history The lake's deposits were first recognized in 1985–1986, but the "chelonide marshes" or "turtle swamp" of Ptolemy almost certainly refer to the former lake. It is also known as "West Nubian lake", "West Nubian Paleolake" and "Northern Darfur Megalake". "Ptolemy lake archipelago" refers to dune fields that were periodically submerged along the eastern shores, forming archipelagos. The lake name was first mentioned in a 1858 map, but its existence became clear only in 1980–1982.
Geomorphology
Context During the early and middle Holocene, large lakes such as Lake Chad and Lake Ptolemy developed within the Sahara and river systems such as Wadi Howar flowed, although it is not clear if they were flowing through a still desertic landscape. 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. Today the eastern Sahara is among the driest locations on Earth as it is far removed from oceanic moisture sources.
Lake Lake Ptolemy is located on the territory of present-day Sudan. 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); 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); 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. 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; in the former case the lake may have occupied an area of no less than 17,864 square kilometres (6,897 sq mi). There is also evidence of shorelines at altitudes of 570–576 metres (1,870–1,890 ft) above sea level; 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. Such a size is comparable to Canada's largest lake, the Great Bear Lake, and larger than Lake Erie. It would have been up to 83 metres (272 ft) deep. Lower lake stages might have reached 565 metres (1,854 ft) and 560 metres (1,840 ft) elevation, and there is fossil evidence that episodes of shallow water level occurred in the lake. The lake floor in its southern and western sectors reaches 549 metres (1,801 ft) elevation above sea level. Shorelines developed on Lake Ptolemy's northern margin and the lake submerged two tributary valleys there; the development of dune fields on the western shore makes identification of the shores there difficult and their absence has raised questions about whether the lake actually existed at such large sizes. 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, and alluvial fans have been identified on the northwestern shores. Chalks formed in Lake Ptolemy generated yardangs, 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. The lake existed in the area of present-day Wadi Howar in the Darfur Basin. Today the oases of Oyo, Bidi and Nukheila are located on the lake-floor that Lake Ptolemy occupied at maximum highstand. The lake probably resembled present day Lake Chad.
Hydrology The lake was nourished by runoff from the Ennedi, Erdi Ma and part of the Kufrah Depression, as well as groundwater; at least one site of the lake floor shows evidence of the release of pressurized water and evidence of higher groundwater levels is widespread in the eastern Sahara. 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). This runoff reached the lake through various wadis, many of which entered Lake Ptolemy from the north, such as Wadi Fesh-Fesh. The Ennedi was critical for the water balance of lake, with three tributaries joining it from there. To the northwest the Lake Ptolemy drainage system was bordered by areas that drained north, while to the northeast drainage was northeastward. Unlike Lake Chad/Lake Megachad, Lake Ptolemy was not nourished by rivers from the humid and semi-humid tropics, but solely from regional catchments. The presence of Asphataria indicates that Lake Ptolemy was a freshwater lake especially close to its inflows, although with occasional brackish phases. Precipitation at the time was about 300 millimetres (12 in) per year. 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. A connection between Lake Ptolemy and Wadi Howar which drains to the Nile is possible. but not proven. At water levels of 577–583 metres (1,893–1,913 ft) Lake Ptolemy would overflow into Wadi Arid.
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