Brahim Damnati

Equatorial extension of the younger Dryas event: rock magnetic evidence from Lake Magadi (Kenya)

Abstract Magnetic parameters measurements (magnetic susceptibility χ and Isothermal Remanent Magnetizations (IRMs) and sedimentological analyses have been performed on Late Pleistocene/Holocene laminated deposits from Lakes Natron and Magadi (Kenya). Accelerator Mass Spectrometry (AMS) 14 C dating on the Total Organic Matter (TOM) locates a continuous laminated sequence in the 12-10 ka B.P. time interval. The results show the occurrence of a two-stepped lacustrine optimum. A first humid period, resulting in a high-stand above a 635 m altitude geographical barrier, reached its maximum between 12 and 11 ka B.P. Between 11 and 10.7 ka B.P., the lake level dropped below the 635 m altitude barrier, which resulted in the separation of Lake Natron and Lake Magadi. A second maximal high stand period occurred from ca. 10 ka B.P., at the beginning of Holocene times. The 11-10 ka arid event recorded in Lake Magadi confirms previous studies which suggested an equatorial extension of the Younger Dryas event. Magnetic parameters and microfacies analyses suggest that this event was produced by a progressive and general weakening of monsoonal rainfall in East Africa.

Solar and ENSO signatures in laminated deposits from lake Magadi (Kenya) during the Pleistocene/Holocene transition

Abstract Sedimentological and geochemical analysis of two cores (8.7 and 9.6 m) taken from lake Magadi (Kenya, 1°50′S, 36°15′E) revealed a laminated sequence, particularly between a depth of 10 and 280 cm. This unit documents a wet and high lake level and was dated by accelerator mass spectroscopy 14C dates on total organic matter, at between 12,000 and 10,000 years B.P. Investigation of these laminated deposits shows a pair of alternating microfacies. The first lamina is dark and rich in lacustine organic matter; the second is light and rich in detritus, carbonates and magadiite. Assuming continuous regular sedimentation, counting of laminae controlled by 14C dates gives an average cyclicity of 4.30 yrs for the deposition. Spectral analysis, applied to the thickness variation of laminae, demonstrated other periods of around 6.45 years, 8.23 years, 8.79 years, 9.44 years, 13.82 years, 18.94 years, 24.18 years and 30 years. This multiannual climatic variation could be related to the sunspot cycle and/or its double (11 and/or 22 years), and the ENSO cycles. These periodicities also reflect the interdependence of sedimentation and cyclic astronomical phenomena that, in part, regulate the climate.

Sedimentology and geochemistry of lacustrine sequences of the upper Pleistocene and holocene in intertropical area (Lake Magadi and Green crater lake): paleoclimatic implications

Sedimentological and geochemical analyses have been carried out on lacustrine deposits of East Africa, at Lake Magadi (2°S, 36°E, Kenya) and at Green Crater Lake (0°S, 36°E, Kenya), to determine the parameters controlling climatic and environmental dynamics during late Pleistocene and Holocene. These sedimentary sequences were collected with a stationary piston corer. At Lake Magadi (Fig. 1), sedimentary and geochemical control show three phases of lake level variation which corresponds to climatic change occurring during the last 40 thousand years. These phases were defined by three lithostratigraphic units. Laminated deposits of Lake Magadi were formed during a wet period. Analysis of these laminae define two microfacies: a dark lamina, characterised by lacustrine organic matter and a light lamina enriched in detritus, carbonates (CaCO3) and magadiite (NaSi7O13(OH)3, 3H2O). The formation and preservation of each couplet was favoured by climatic contrast, lake stratification and various origin of the sediments (autochthon and allochthon) in the drainage basin. Therefore a relative chronology can be derived from laminae counting and the duration of deposition of each couplet. Spectral analysis applied on variation of the laminae thickness, shows the existence of three main periods, 4–7 years, 8–14 years and 18–30 years, respectively (Fig. 2). These cyclicites of the lacustrine environment precise former determinations established on more recent lacustrine sequences from East Africa. They are related to the global climatic cycle (quasi-biannual oscillations, El Nino Southern Oscillations and the sun spot cycles). At Green Crater Lake, the study of the sedimentary sequence was completed by physico-chemical analysis of the waters and interface sediments which demonstrate the carbonate, sodium, bicarbonate composition and the thermal and chemical stratification of the modern lake. The sedimentary sequence is characterized by volcanic deposits overlain by physico-chemical analysis of the lake waters and interface sediments which demonstrate the carbonate, sodium, bicarbonate composition and the thermal and chemical stratification of the modern lake. The sedimentary sequence is characterized by volcanic deposits overlain by silt and clays deposited before 7400 years B.P., followed by loweing of the lake level at 3000 years B. P. Results from lake Magadi document the occurrence of a wet period starting at about 12,000 years B. P. The methodology applied on modern Green Crater lake provides base of interpretative models for other Holocene sequence lacustrine systems of intertropical zones.