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] and their absence has raised questions about whether the lake actually existed at such large sizes.[17] 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.[18] Alluvial fans have been identified on the northwestern shores.[19]

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.[20] The lake probably resembled present day Lake Chad.[21] 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.[22]

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.[23]

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).[24][25]

This runoff reached the lake through various wadis,[23] many of which entered the lake from the north.[26] The Ennedi was critical for the water balance of Lake Ptolemy.[27] The various water systems aided in the propagation of plants.[28] To the northwest the Lake Ptolemy drainage system was bordered by northward flowing drainage and to the northeast by northeastward draining systems.[29]

The presence of Asphataria indicates that Lake Ptolemy was a freshwater lake,[15] although with occasional brackish phases.[30] 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.[31] A connection between Lake Ptolemy and Wadi Howar is possible,[4] but not proven.[32] 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.[32] Plant species documented from Lake Ptolemy include Acacia and Tamarix species, as well as Balanitos aegyptiaca and Capparis decidua.[23] Reed vegetation formed on the southern and western shores of the lake.[18] 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.[33]

Ostracods found in the lake include Candonopsis, Cyprideis, Cypridopsis, Cyprilla, Darwinula, Herpetocypris and Limnocytherae.[30]

About 18 fish species existed in Lake Ptolemy,[21] 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.[23] Ungulates and cane rats lived around the lake.[34]

The south shore of Lake Ptolemy could have been inhabited by neolithic pastoralists, and possibly elephants were present as well.[35] In addition, many human artifacts have been found in the region surrounding the former lake.[36]

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.[37] No lake deposits are found dating back to the late Pleistocene; climate at that point was as dry as the present day.[38]

Already around 9,180 ± 185 years before present did Lake Ptolemy exist as a freshwater lake.[39] A temporary lowstand is dated to 7,470 ± 100 and 8,100 ± 80 years before present and was associated with strong trophic growth.[40] Lowstands in the lake levels allowed land animals to reach the interior of the lake basin.[34]

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.[26] Other dates from the northern reaches are 7,900 – 6,400 years before present, and 9,250 – 3,800 years before present.[39] 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.[26] No actual dryings are preserved in the fossil data.[41]

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.[40] Deflation has removed the youngest deposits, thus the exact time when the lake disappeared is not known.[34]

Relationship

Lake Ptolemy is related to the Nubian Sandstone Aquifer; in simulations maximum water levels in the aquifer reached the surface of the lake,[42] and about 3 cubic kilometres (0.72 cu mi) of water from the lake entered the aquifer every year.[43] The lake further aided in the interchange between Lake Chad and Nile species.[5]

References

  1. 1 2 3 4 5 Pachur & Altmann 2006, p. 205.
  2. 1 2 Gossel, Ebraheem & Wycisk 2004, p. 705.
  3. 1 2 Elsheikh, Abdelsalam & Mickus 2011, p. 82.
  4. 1 2 Pachur & Altmann 2006, p. 219.
  5. 1 2 3 Pachur & Altmann 2006, p. 35.
  6. ↑ Hoelzmann et al. 2001, p. 193.
  7. ↑ Pachur & Altmann 2006, p. 226.
  8. ↑ Hoelzmann et al. 2001, p. 213.
  9. 1 2 Elsheikh, Abdelsalam & Mickus 2011, p. 83.
  10. ↑ Pachur & Altmann 2006, p. 221.
  11. ↑ Ghoneim & El-Baz 2007, pp. 5008, 5009.
  12. ↑ Ghoneim & El-Baz 2007, p. 5001.
  13. ↑ Ghoneim & El-Baz 2007, p. 5013.
  14. 1 2 Ghoneim & El-Baz 2007, p. 5014.
  15. 1 2 3 4 5 Pachur & Altmann 2006, p. 207.
  16. 1 2 Pachur & Altmann 2006, p. 216.
  17. ↑ Quade, J.; Dente, E.; Armon, M.; Dor, Y. Ben; Morin, E.; Adam, O.; Enzel, Y. (2018). "Megalakes in the Sahara? A Review". Quaternary Research: 9–11. doi:10.1017/qua.2018.46. ISSN 0033-5894.
  18. 1 2 Pachur & Altmann 2006, p. 212.
  19. ↑ Ghoneim & El-Baz 2007, p. 5010.
  20. ↑ Elsheikh, Abdelsalam & Mickus 2011, p. 86.
  21. 1 2 Pachur & Altmann 2006, p. 36.
  22. ↑ Pachur & Altmann 2006, p. 294.
  23. 1 2 3 4 Pachur & Altmann 2006, p. 206.
  24. ↑ Hoelzmann et al. 2001, p. 214.
  25. ↑ Ghoneim & El-Baz 2007, p. 5005.
  26. 1 2 3 Pachur & Altmann 2006, p. 208.
  27. ↑ Pachur & Altmann 2006, p. 230.
  28. ↑ Pachur & Altmann 2006, p. 224.
  29. ↑ Elsheikh, Abdelsalam & Mickus 2011, p. 84.
  30. 1 2 Pachur & Altmann 2006, p. 465.
  31. ↑ Pachur & Altmann 2006, p. 236.
  32. 1 2 Pachur & Altmann 2006, p. 218.
  33. ↑ Pachur & Altmann 2006, p. 220.
  34. 1 2 3 Pachur & Altmann 2006, p. 228.
  35. ↑ Pachur & Altmann 2006, p. 44.
  36. ↑ Pachur & Altmann 2006, p. 231.
  37. ↑ Pachur & Altmann 2006, p. 223,224.
  38. ↑ Pachur & Altmann 2006, pp. 227, 228.
  39. 1 2 Pachur & Altmann 2006, p. 209.
  40. 1 2 Pachur & Altmann 2006, p. 210.
  41. ↑ Pachur & Altmann 2006, p. 468.
  42. ↑ Gossel, Ebraheem & Wycisk 2004, p. 708.
  43. ↑ Pachur & Altmann 2006, p. 229.

Sources

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  • Ghoneim, E.; El-Baz, F. (2007-11-20). "DEM‐optical‐radar data integration for palaeohydrological mapping in the northern Darfur, Sudan: implication for groundwater exploration". International Journal of Remote Sensing. 28 (22): 5001–5018. Bibcode:2007IJRS...28.5001G. doi:10.1080/01431160701266818. ISSN 0143-1161.
  • Gossel, W.; Ebraheem, A. M.; Wycisk, P. (2004-12-01). "A very large scale GIS-based groundwater flow model for the Nubian sandstone aquifer in Eastern Sahara (Egypt, northern Sudan and eastern Libya)". Hydrogeology Journal. 12 (6): 698–713. Bibcode:2004HydJ...12..698G. doi:10.1007/s10040-004-0379-4. ISSN 1431-2174.
  • Hoelzmann, Philipp; Keding, Birgit; Berke, Hubert; Kröpelin, Stefan; Kruse, Hans-Joachim (2001-05-15). "Environmental change and archaeology: lake evolution and human occupation in the Eastern Sahara during the Holocene". Palaeogeography, Palaeoclimatology, Palaeoecology. 169 (3–4): 193–217. Bibcode:2001PPP...169..193H. doi:10.1016/S0031-0182(01)00211-5.
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