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Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years

(2013) SEDIMENTOLOGY. 60(4). p.911-934
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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.
Keywords
X-ray fluorescence core scanning, MAGNETIC-SUSCEPTIBILITY, palaeoredox, EVOLUTION, ECOSYSTEM, BEHAVIOR, ORIGIN, CLIMATE, GRAIN-SIZE ANALYSIS, HOLOCENE, DUST, LACUSTRINE, magnetic susceptibility, grain size, brine evolution, Aeolian input

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Citation

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MLA
Francus, Pierre, Hans Von Suchodoletz, Michael Dietze, et al. “Varved Sediments of Lake Yoa (Ounianga Kebir, Chad) Reveal Progressive Drying of the Sahara During the Last 6100 Years.” SEDIMENTOLOGY 60.4 (2013): 911–934. Print.
APA
Francus, P., Von Suchodoletz, H., Dietze, M., Donner, R. V., Bouchard, F., Roy, A.-J., Fagot, M., et al. (2013). Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years. SEDIMENTOLOGY, 60(4), 911–934.
Chicago author-date
Francus, Pierre, Hans Von Suchodoletz, Michael Dietze, Reik V Donner, Frédéric Bouchard, Ann-Julie Roy, Maureen Fagot, Dirk Verschuren, and Stefan Kröpelin. 2013. “Varved Sediments of Lake Yoa (Ounianga Kebir, Chad) Reveal Progressive Drying of the Sahara During the Last 6100 Years.” Sedimentology 60 (4): 911–934.
Chicago author-date (all authors)
Francus, Pierre, Hans Von Suchodoletz, Michael Dietze, Reik V Donner, Frédéric Bouchard, Ann-Julie Roy, Maureen Fagot, Dirk Verschuren, and Stefan Kröpelin. 2013. “Varved Sediments of Lake Yoa (Ounianga Kebir, Chad) Reveal Progressive Drying of the Sahara During the Last 6100 Years.” Sedimentology 60 (4): 911–934.
Vancouver
1.
Francus P, Von Suchodoletz H, Dietze M, Donner RV, Bouchard F, Roy A-J, et al. Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years. SEDIMENTOLOGY. 2013;60(4):911–34.
IEEE
[1]
P. Francus et al., “Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years,” SEDIMENTOLOGY, vol. 60, no. 4, pp. 911–934, 2013.
@article{4091827,
  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.},
  author       = {Francus, Pierre and Von Suchodoletz, Hans and Dietze, Michael and Donner, Reik V and Bouchard, Frédéric and Roy, Ann-Julie and Fagot, Maureen and Verschuren, Dirk and Kröpelin, Stefan},
  issn         = {0037-0746},
  journal      = {SEDIMENTOLOGY},
  keywords     = {X-ray fluorescence core scanning,MAGNETIC-SUSCEPTIBILITY,palaeoredox,EVOLUTION,ECOSYSTEM,BEHAVIOR,ORIGIN,CLIMATE,GRAIN-SIZE ANALYSIS,HOLOCENE,DUST,LACUSTRINE,magnetic susceptibility,grain size,brine evolution,Aeolian input},
  language     = {eng},
  number       = {4},
  pages        = {911--934},
  title        = {Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years},
  url          = {http://dx.doi.org/10.1111/j.1365-3091.2012.01370.x},
  volume       = {60},
  year         = {2013},
}

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