Late Pleistocene desiccation of Lake Tana,source of the Blue Nile |
| |
| Institution: | 1. Large Lakes Observatory, University of Minnesota Duluth, Duluth, MN 55812, USA;2. Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812, USA;3. Dept. of Geology & Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260, USA;4. Dept. of Biology, Duke University, Durham, NC 27708, USA;5. WA-Organic and Isotopic Geochemistry Centre, Dept. of Chemistry, Curtin University, Perth, Australia;6. NIOZ Royal Netherlands Institute for Sea Research, Dept. of Marine Organic Biogeochemistry, The Netherlands;1. Institute for Earth and Environmental Sciences, University of Potsdam, Karl-Liebknecht-Str. 24, Am Mühlenberg 3, D-14476 Potsdam, Germany;2. DFG Graduate School on Shaping the Earth''s Surface in a Variable Environment, University of Potsdam, Karl-Liebknecht-Str. 24, 14476 Potsdam, Germany;3. Senckenberg Forschungsinstitut und Naturkundemuseum Frankfurt, Senckenberganlage 25, 60325 Frankfurt, Germany;4. Geology Department, University of Nairobi, P.O. BOX 30197, Nairobi, Kenya |
| |
| Abstract: | High-resolution seismic data from Lake Tana, the source of the Blue Nile in northern Ethiopia, reveal a deep sedimentary sequence divided by four strong reflectors. Data from nearshore cores show that the uppermost strong reflector represents a stiff silt unit, interpreted as a desiccation surface. Channel cuts in this surface, bordered by levee-like structures, are apparent in the seismic data from near the lake margin, suggesting fluvial downcutting and over-bank deposition during seasonal flood events. Periphytic diatoms and peat at the base of a core from the deepest part of the lake overlie compacted sediments, indicating that desiccation was followed by development of shallow-water environments and papyrus swamp in the central basin between 16,700 and 15,100 cal BP. As the lake level rose, open-water evaporation from the closed lake caused it to become slightly saline, as indicated by halophytic diatoms. An abrupt return to freshwater conditions occurred at 14,750 cal BP, when the lake overflowed into the Blue Nile. Further reflection surfaces with downcut structures are identifiable in seismic images of the overlying sediments, suggesting at least two lesser lake-level falls, tentatively dated to about 12,000 and 8000 cal BP. Since Lake Victoria, the source of the White Nile, was also dry until 15,000 cal BP, and did not reach overflow until 14,500 cal BP, the entire Nile system must have been reduced to intermittent seasonal flow until about 14,500 cal BP, when baseflow was re-established with almost simultaneous overflow of the headwater lakes of both the White and Blue Nile rivers. Desiccation of the Nile sources coincides with Heinrich event 1, when cessation of northward heat transport from the tropical Atlantic disrupted the Atlantic monsoon, causing drought in north tropical Africa. The strong reflectors at deeper levels in the seismic sequence of Lake Tana may represent earlier desiccation events, possibly contemporaneous with previous Late Pleistocene Heinrich events. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
