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.
Francus_etal_2013_Varved_Sediments_Lake_Yoa_Qunianga_kebir_Chad_Drying_Sahara.pdf Accessed 248 times | Last updated 22.09.2014
httphdl.handle.net1854LU-4091827 Accessed 269 times | Last updated 22.09.2014
Francus P., Suchodoletz H. v., Dietze M., Donner R., Bouchard, F., Roy A.-J., Fagot M., Verschuren D., Kröpelin S. (2013): Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years. In: Sedimentology 60: 911-934
|Authors||Pierre Francus and Hans von Suchodoletz and Michael Dietze and Reik V. Donner and Frédéric Bouchard and Ann-Julie Roy and Maureen Fagot and Dirk Verschuren and Stefan Kröpelin|
|Title||Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years|
|Citation||Francus P., Suchodoletz H. v., Dietze M., Donner R., Bouchard, F., Roy A.-J., Fagot M., Verschuren D., Kröpelin S. (2013): Varved sediments of Lake Yoa (Ounianga Kebir, Chad) reveal progressive drying of the Sahara during the last 6100 years. In: Sedimentology 60: 911-934|
|Pages||911 - 934|