Lake Ptolemy

Coordinates: 19°30′N 26°00′E / 19.5°N 26°E / 19.5; 26^[1] Lake Ptolemy is a former lake in Sudan.^[2] 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. The lake was a freshwater lake replenished by groundwater and runoff from neighbouring mountains and might itself have been the source for Nubian Sandstone Aquifer. 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

The lake is also known as "West Nubian lake",^[1] "West Nubian Paleolake" and "Northern Darfur Megalake".^[3] "Ptolemy lake archipelago" refers to dune fields that were periodically submerged along the eastern shores, forming archipelagoes.^[4]

Geomorphology

Context

Today the eastern Sahara is among the driest locations on Earth.^[3] During the early and middle Holocene, large lakes such as Lake Chad and Lake Ptolemy developed within the Sahara.^[5] The formation of these paleolakes is 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.^[6]

Lake

Lake Ptolemy was originally believed to have reached surface areas of about 27,000 square kilometres (10,000 sq mi);^[7] later research on the basis of more reliable elevation maps indicated that it was no larger than 5,330 square kilometres (2,060 sq mi).^[8] Later, 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.^[9]

Water depths reached 15 metres (49 ft). Depending on the location, water levels of 550 metres (1,800 ft) or even 555 metres (1,821 ft) above sea level have been found.^[10] Evidence of shorelines at altitudes of 570–576 metres (1,870–1,890 ft) has been found;^[11] if a shoreline existed at that altitude, the lake 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.^[9] Such a size is comparable to Canada's largest lake, the Great Bear Lake,^[12] and larger than Lake Erie. It would have been up to 83 metres (272 ft) deep.^[13] Lower lake stages might have reached 565 metres (1,854 ft) and 560 metres (1,840 ft) elevation.^[14]

Shorelines developed on its northern margin;^[15] the development of dune fields on the western shore makes identification of the shores difficult.^[16] River deltas formed where wadis entered the lake. The shores on the southern and western side developed a riparian zone with vegetation and irregular lakefloor.^[17] Alluvial fans have been identified on the northwestern shores.^[18]

The lake existed in the area of present-day Wadi Howar,^[2] in the Darfur Basin.^[5] The oases of Atrun and Nukheila exist today on the former bed of Lake Ptolemy.^[19] The lake probably resembled present day Lake Chad.^[20] The lake floor in its southern and western reaches 549 metres (1,801 ft) elevation above sea level.^[16] The lake basin was probably formed before the Holocene by deflation.^[21]

Chalks formed in the lake generated yardangs,^[1] 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.^[22]

Hydrology

The lake was nourished by runoff from the Ennedi, Erdi Ma and part of the Kufrah Depression, as well as groundwater;^[1] at least one site of the lake floor shows evidence of pressurized water being released.^[15] The catchment of the lake reached a surface of 78,000 square kilometres (30,000 sq mi); later estimates are 128,802 square kilometres (49,731 sq mi).^[23][24]

This runoff reached the lake through various wadis,^[22] many of which entered the lake from the north.^[25] The Ennedi was critical for the water balance of Lake Ptolemy.^[26] The various water systems aided in the propagation of plants.^[27] To the northwest the Lake Ptolemy drainage system was bordered by northward flowing drainage and to the northeast by northeastward draining systems.^[28]

The presence of Asphataria indicates that Lake Ptolemy was a freshwater lake,^[15] although with occasional brackish phases.^[29] 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.^[30] A connection between Lake Ptolemy and Wadi Howar is possible,^[4] but not proven.^[31] At water levels of 577–583 metres (1,893–1,913 ft) Lake Ptolemy would overflow into Wadi Arid.^[14]

Biology

Lake Ptolemy featured a diverse ecosystem.^[31] Plant species documented from Lake Ptolemy include Acacia and Tamarix species, as well as Balanitos aegyptiaca and Capparis decidua.^[22] Reed vegetation formed on the southern and western shores of the lake.^[17] The existence of Typha suggests that shallow lake phases occurred.^[15] Stromatolites also formed on the lake shores and together with limnites are used to delimit the lake surface.^[32]

Ostracods found in the lake include Candonopsis, Cyprideis, Cypridopsis, Cyprilla, Darwinula, Herpetocypris and Limnocytherae.^[29]

About 18 fish species existed in Lake Ptolemy,^[20] such as Clarias lazera, Lates niloticus and Synodontis.^[15] Likewise, fossils of water tortoises and hippopotamus were found on the area of the former lake. The existence of marsh animals in the region was already reported in a map of 1858.^[1] Further animals documented in fossils include the Nile crocodile and pelomedusidae and trionychidae species.^[22] Ungulates and cane rats lived around the lake.^[33]

The south shore of Lake Ptolemy could have been inhabited by neolithic pastoralists, and possibly elephants were present as well.^[34] In addition, many human artifacts have been found in the region surrounding the former lake .^[35]

Lake chronology

During the Pleistocene, a "Lake Sidiq" formed in the area of northern Lake Ptolemy. It has been dated at 21,600 ± 600 years before present.^[36] No lake deposits are found dating back to the late Pleistocene; climate at that point was as dry as present-day.^[37]

Already around 9,180 ± 185 years before present did Lake Ptolemy exist as a freshwater lake.^[38] A temporary lowstand is dated to 7,470 ± 100 and 8,100 ± 80 years before present and was associated with strong trophic growth.^[39] Lowstands in the lake levels allowed land animals to reach the interior of the lake basin.^[33]

Radiocarbon dating of chalks in a wadi that entered the lake from the north has yielded ages for a highstand of 6,680 ± 135 and 6,810 ± 70 years before present.^[25] Other dates from the northern reaches are 7,900 – 6,400 years before present, and 9,250 – 3,800 years before present.^[38] Dates obtained from fish fossils in the northern reaches are 2,360 ± 65 and 3,285 ± 70 years before present. This was a time where lake levels were less stable.^[25] No actual dryings are preserved in the fossil data.^[40]

Wadis flowing into the lake were transporting water as late as 3,300 – 2,900 and 3,300 – 2,400 years before present on the southern and northern side, respectively. During its drying, the lake split into separate pools.^[39] Deflation has removed the youngest deposits, thus the exact time when the lake disappeared is not known.^[33]

Relationship

Lake Ptolemy is related to the Nubian Sandstone Aquifer; in simulations maximum water levels in the aquifer reached the surface of the lake,^[41] and about 3 cubic kilometres (0.72 cu mi) of water from the lake entered the aquifer every year.^[42] The lake further aided in the interchange between Lake Chad and Nile species.^[5]

References

Sources