The Middle Pleistocene Al Mahrúqah Formation in the Murzuq Basin,northern Sahara,Libya evidence for orbitally-forced humid episodes during the last 500,000 years |
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| Authors: | Mebus A Geyh Friedhelm Thiedig |
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| Institution: | 1. Leibniz Institute of Applied Geosciences (GGA), Hannover, Germany;2. Geological Institute of the University Münster, Germany;1. Geological Institute, ETH Zürich, Switzerland;2. Physics Institute, University of Bern, Switzerland;3. Oeschger Centre for Climate Change Research, University of Bern, Switzerland;4. Institute of Geology, University of Innsbruck, Austria;5. Institute of Applied Physics, University of Bern, Switzerland;6. EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Switzerland;7. Institute of Geochemistry and Petrology, ETH Zürich, Switzerland;8. Department of Earth and Atmospheric Sciences, Georgia Institute of Technology, USA;9. Division of Geological and Planetary Sciences, California Institute of Technology, USA;10. Department of Archaeology, Centre for Past Climate Change, School of Archaeology, Geography and Environmental Science, University of Reading, United Kingdom;1. School of Geography and Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK;2. Long Term Ecology Laboratory, Department of Zoology, University of Oxford, UK;3. Department of Environmental and Geographical Science, University of Cape Town, South Africa;1. PACEA, Université de Bordeaux, Bâtiment B19, Avenue des Facultés, 33405 Talence Cedex, France;2. Department of Geography, King''s College London, K4U.06 Strand Campus, London WC2R 2LS, United Kingdom;3. School of Archaeology, Research Laboratory for Archaeology and the History of Art, University of Oxford, 28 New Barnett House, Little Clarendon Street, Oxford OX1 2HU, United Kingdom;1. Departamento de Geología, Universidad de Salamanca, 37008, Salamanca, Spain;2. Centro de Ciências do Mar (CCMAR), Universidade do Algarve, Campus de Gambelas, 8005-139, Faro, Portugal;3. Divisão de Geologia e Georecursos Marinhos, Instituto Português do Mar e da Atmosfera (IPMA), Av. Doutor Alfredo Magalhães Ramalho 6, 1495-165, Algés, Portugal;4. Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari Aldo Moro, Bari, Italy;5. Department of Environmental Chemistry, IDAEA-CSIC, Barcelona, 08034, Spain;6. GRC Geociències Marines, Departament de Dinàmica de la Terra i de l’Oceà, Facultat de Ciències de la Terra, Universitat de Barcelona, Barcelona, 08028, Spain;1. School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332, USA;2. Institute for Geophysics, Jackson School of Geosciences, University of Texas, Austin, TX, 78758, USA;3. Gunung Mulu National Park, Sarawak, Malaysia;4. Institute of Biodiversity and Environmental Conservation, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Malaysia;5. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, 91125, USA;1. Department of Earth Sciences, University of Oxford, South Parks Road, OX1 3AN Oxford, United Kingdom;2. Geological Survey of Israel, 30 Malchei Israel St, Jerusalem 95501, Israel;3. Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel;4. School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia;5. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom;6. Department of Geography, Hebrew University, Jerusalem 91905, Israel |
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| Abstract: | Geological and geomorphological work in the endorheic Murzuq Basin (Fezzan) in the northern Sahara Desert, SW Libya, has revealed that the Middle Pleistocene Al Mahrúqah Formation consists of at least four limestone beds at four distinct elevations. The corresponding limestone beds are named (from oldest to youngest) the Antalkhatá Member, the Brak Member, the Bi'r az Zallaf Member, and the Aqar Member. The four members are interpreted as lacustrine deposits, and they are well distinguished by their spatial extent and lithology. Twenty-five radiometric and thermal ionisation mass spectrometric (TIMS) 230Th/U ages ranging from about 490–130 ka have been determined on these lacustrine limestones. According to these radiometric 230Th/U ages, the accumulation of the four limestone members of the Al Mahrúqah Formation followed the orbitally-forced (Milankovitch) eccentricity cycle of 100 ka during at least the last 400 ka. The four members formed in humid episodes of several tens of thousands of years' duration, when large lakes might have existed. The δ13C and δ18O values from samples of the four members of the Al Mahrúqah Formation provide independent genetic evidence on the lacustrine origin of the deposits in the Murzuq Basin. The spatial extent of the limestone deposits, and thus the intensity of the humid episodes, decreases at least from the Brak Member to the Aqar Member from about 52,000 km2 to 1400 km2, respectively. In spite of the limited precision and reliability of the new 230Th/U ages, it seems possible that short humid episodes interrupted the intervening arid episodes. Other long-term palaeohydrological records from northern and southern Africa as well as the Mediterranean region support the palaeoclimatic results from the Murzuq Basin. |
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