Varved sediments of Lake Yoa (Ounianga Kebir,Chad) reveal progressive drying of the Sahara during the last 6100 years |
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| Authors: | Pierre Francus Hans von Suchodoletz Michael Dietze Reik V Donner Frédéric Bouchard Ann‐Julie Roy Maureen Fagot Stefan Kröpelin |
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| Institution: | 1. Institut National de la Recherche Scientifique, Centre Eau, Terre et Environnement, , Québec, QC, G1K 9A9 Canada;2. GEOTOP Research Center, , Montréal, QC, H3C 3P8 Canada;3. Institute of Geography, University of Leipzig, , D‐04103 Leipzig, Germany;4. Institute of Geography, Dresden University of Technology, , D‐01069 Dresden, Germany;5. Potsdam Institute for Climate Impact Research, , D‐14473 Potsdam, Germany;6. Département de Géographie, Centre d'étude nordique, , Québec, QC, G1V 0A6 Canada;7. Limnology Unit, Department of Biology, Ghent University, , B‐9000 Gent, Belgium |
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| Abstract: | The sedimentological and geochemical properties of a 7·47 m long laminated sequence from hypersaline Lake Yoa in northern Chad have been investigated, representing a unique, continuous 6100 year long continental record of climate and environmental change in the eastern Central Sahara. These data were used to reconstruct the Mid to Late Holocene history of this currently hyper‐arid region, in order to address the question of whether the Mid Holocene environmental transition from a humid to a dry Sahara was progressive or abrupt. This study involved a suite of analyses, including petrographic and scanning electron microscope examination of thin sections, X‐ray diffraction, X‐radiography, granulometry, loss on ignition and magnetic susceptibility. The potential of micro‐X‐ray fluorescence core scanning was tested at very high resolution. Detailed microscopic investigation revealed the sedimentary processes responsible for the formation of the fine laminations, identified the season during which they were formed, and confirmed their annually rhythmic nature. High‐resolution X‐ray fluorescence core scanning allowed the distinction of each individual lamination over the entire record, opening new perspectives for the study of finely laminated sediment sequences. Geochemical and mineralogical data reveal that, due to decreasing monsoon rainfall combined with continuous and strong evaporation, the hydrologically open and fresh Mid Holocene Lake Yoa slowly evolved into the present‐day hypersaline brine depleted in calcium, which has existed for about the past 1050 years. During the oldest part of the investigated period, Lake Yoa probably contained a permanently stratified lower water column that was nevertheless disrupted relatively frequently by mixing events. Deep‐water anoxia became more stable because of increased salinity‐driven density stratification. In parallel, the sediment grain‐size proxies record a progressive increase of aeolian input in the course of the last 6100 years. Altogether, all geochemical and sedimentological indicators point to a progressive drying of the eastern Central Sahara, strengthening previous conclusions based on palaeoecological indicators. |
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| Keywords: | Aeolian input brine evolution grain size magnetic susceptibility palaeoredox X‐ray fluorescence core scanning |
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