GP51A-1322
High-coercivity minerals from North African Humid Period soil material deposited in Lake Yoa (Chad)

Friday, 18 December 2015
Poster Hall (Moscone South)
Janna Just1, Stefan Kroepelin2, Volker Wennrich1, Finn A. Viehberg1, Bernd Wagner1, Janet Rethemeyer1, Jens Karls3 and Martin Melles1, (1)University of Cologne, Institute for Geology and Mineralogy, Cologne, Germany, (2)University of Cologne, Forschungsstelle Afrika, Cologne, Germany, (3)University of Cologne, Cologne, Germany
Abstract:
The Holocene is a period of fundamental climatic change in North Africa. Humid conditions during the so-called African Humid Period (AHP) have favored the formation of big lake systems. Only very few of these lakes persist until today. One of them is Lake Yoa (19°03′N/20°31′E) in the Ounianga Basin, Chad, which maintains its water level by ground water inflow. Here we present the magnetic characteristics together with proxies for lacustrine productivity and biota of a sediment core (Co1240) from Lake Yoa, retrieved in 2010 within the framework of the Collaborative Research Centre 806 – Our Way to Europe (Deutsche Forschungsgemeinschaft).

Magnetic properties of AHP sediments show strong indications for reductive diagenesis. An up to ~ 80 m higher lake level is documented by lacustrine deposits in the Ounianga Basin, dating to the early phase of the AHP. The higher lake level and less strong seasonality restricted deep mixing of the lake. Development of anoxic conditions consequently lead to the dissolution of iron oxides. An exception is an interval with high concentration of high-coercivity magnetic minerals, deposited between 7800 - 8120 cal yr BP. This interval post-dates the 8.2 event, which was dry in Northern Africa and probably caused a reduced vegetation cover. We propose that the latter resulted in the destabilization of soils around Lake Yoa. After the re-establishment of humid conditions, these soil materials were eroded and deposited in the lake.

Magnetic minerals appear well preserved in the varved Late Holocene sequence, indicating (sub-) oxic conditions in the lake. This is surprising, because the occurrence of varves is often interpreted as an indicator for anoxic conditions of the lake water. However, the salinity of lake water rose strongly after the AHP. We therefore hypothesize that the conservation of varves and absence of benthic organisms rather relates to the high salinity than to anoxic conditions.

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