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Institute
- Institut für Geowissenschaften (62) (remove)
This study examines patterns of climate variability by mapping shoreline changes between 1973 and 2008 for two shallow lakes in Malawi, East Africa. Multi-temporal LANDSAT data covering eight different years within the period of investigation were utilised for lake-area mapping, using image classification techniques. The approach was verified using simple comparisons with local rainfall data and satellite altimetry data. Results indicated that the lake areas varied between the different years investigated and that, although the lakes were affected by the same climate-forcing mechanisms, the individual basin characteristics had a critical effect on their responses. The most likely drivers for the climate-related shoreline variations are the El Nino/Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), causing synchronous changes to the water levels in both lakes, albeit showing differences in climate signal amplification.
The Pleistocene archeological record in East Africa has revealed unusual accumulations of Acheulean handaxes at prehistoric sites. In particular, there has been intensive debate concerning whether the artifact accumulation at the Middle Pleistocene Olorgesailie (Southern Kenya Rift) and Kariandusi (Central Kenya Rift) sites were a result of fluvial reworking or of in situ deposition by hominids. We used a two-step approach to test the hypothesis of fluvial reworking. Firstly, the behavior of handaxes in water currents was investigated in a current flume and the flow threshold required to reorientate the handaxes was determined. The results of these experiments suggested that, in relatively high energy and non-steady flow conditions, handaxes will reorientate themselves perpendicular to the current direction. Secondly, an automated image analysis routine was developed and applied to archeological plans from three Acheulean sites, two at Olorgesailie and one at Kariandusi, in order to determine the orientations of the handaxes. A Rayleigh test was then applied to the orientation data to test for a preferred orientation. The results revealed that the handaxes at the Upper Kariandusi Site and the Olorgesailie Main Site Mid Trench had a preferential orientation, suggesting reworking by a paleocurrent. The handaxes from the Olorgesailie Main Site H/6A, however, appeared to be randomly oriented and in situ deposition by the producers therefore remains a possibility.
TURBO2 - a MATLAB simulation to study the effects of bioturbation on paleoceanographic time series
(2013)
Bioturbation (or benthic mixing) causes significant distortions in marine stable isotope signals and other palaeoceanographic records. Although the influence of bioturbation on these records is well known it has rarely been dealt systematically. The MATLAB program called TURBO2 can be used to simulate the effect of bioturbation on individual sediment particles. It can therefore be used to model the distortion of all physical, chemical, and biological signals in deep-sea sediments, such as Mg/Ca ratios and UK37-based sea-surface temperature (SST) variations. In particular, it can be used to study the distortions in paleoceanographic records that are based on individual sediment particles, such as SST records based on foraminifera assemblages. Furthermore. TURBO2 provides a tool to study the effect of benthic mixing of isotope signals such as C-14, delta O-18, and delta C-13, measured in a stratigraphic carrier such as foraminifera shells.
During the African Humid Period (AHP, 15-5 ka BP) an almost 300 m deep paleo-lake covering 2200 km(2) developed in the Suguta Valley, in the Northern Kenya Rift Data from lacustrine sediments and paleo-shorelines indicate that a large paleo-lake already existed by 13.9 ka BP, and record rapid water level fluctuations of up to 100 m within periods of 100 years or less, and a final lowstand at the end of the AHP (5 ka BP). We used a hydro-balance model to assess the abruptness of these water level fluctuations and identify their causes. We observed that fluctuations within the AHP were caused by abrupt changes in precipitation of 26-40%. Despite the absence of continuous lacustrine data documenting the onset of the AHP in the Suguta Valley, we conclude from the hydro-balance model that only an abrupt onset to the AHP, prior to 14.8 ka BP, could have led to high water levels recorded. The modeling results suggest that the sudden increase in rainfall was the direct consequence of an eastward migration of the Congo Air Boundary (CAB), caused by an enhanced atmospheric pressure gradient between East Africa and southern Asia during a northern hemisphere (NH) summer insolation maximum. In contrast the end of the AHP must have been gradual despite an abrupt change in the source of precipitation when a decreasing pressure gradient between Asia and Africa prevented the CAB from reaching the study area. This abruptness was probably buffered by a contemporaneous change in precession producing an insolation maximum at the equator during September-October. This change would have meant that the only rain source was the Intertropical Convergence Zone (IT CZ), which would have carried a greater amount of moisture during the short rainy season thus slowing the fall in water level over a period of about 1000 years in association with the reduction in insolation. The results of this study provide an indication of the amount of time available for humans in north-eastern Africa to adapt in response to a changing climate, from hunting and gathering to farming and herding.
The late Cenozoic climate of East Africa is punctuated by episodes of short, alternating periods of extreme wetness and aridity, superimposed on a regime of subdued moisture availability exhibiting a long-term drying trend. These periods of extreme climate variability appear to correlate with maxima in the 400-thousand-year (kyr) component of the Earth's eccentricity cycle. Prior to 2.7 Ma the wet phases appear every 400 kyrs, whereas after 2.7 Ma, the wet phases appear every 800 kyrs, with periods of precessional-forced extreme climate variability at 2.7-2.5 Ma, 1.9-1.7 Ma, and 1.1-0.9 Ma before present. The last three major lake phases occur at the times of major global climatic transitions, such as the onset of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker Circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1.0-0.7 Ma). High-latitude forcing is required to compress the Intertropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. These periods of extreme climate variability may have provided a catalyst for evolutionary change and driven key speciation and dispersal events amongst mammals and hominins in East Africa. (C) 2007 Elsevier Ltd. All rights reserved.
Owen et al. [Owen. R.B.. Potts, R., Behrensmeyer, A.K., Ditchfield, P. 2008. Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley. Palaeogeography, Palaeochmatology, Palaeoccology, 269. 17-37], Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation. southern Kenya Rift. Palaeogeography Palaeoclimatology Palaeciecology, 269, 17-37) argued that diatom assemblage variations in the Olorgesailie Formation indicate considerable environmental instability with both wetter and drier periods, contradicting the proposed period of lake stability and wet climatic conditions between ca 11 and 0.9 million years ago as proposed by Trauth et al [Trauth, M.H. Maslin, MA.. Demo. A.. Strecker, M R.. 2005. Late Cenozoic moisture history of East Africa. Science 309. 2051-2053., Trauth, M H. Mashn. M.A., Deino, A., Bergner. A G.N.. Diihnforth, M. Strecker. M.R, 2007 High- and low-latitude forcing of Plio-Pleistocene East African climate and hL.man evolution. journal of Human Evolution 53, 475-486] Contrary to the interpretation of our work by O Nen et al. [Owen. R.B, Potts, R. Behrensmeyer, A.K. Ditchfield. P. 2008], we never said that the proposed periods of large lakes were characterized by stable conditions.
Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa
(2009)
The long-term histories of the neighboring Nakuru-Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modem climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen C-14 and Ar-40/Ar-39 dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.
Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the similar to 300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2 degrees, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.
The development of rise Cenozoic East African Rift System (EARS) profoundly re-shaped the landscape and significantly increased the amplitude of short-term environmental response to climate variation. In particular, the development of amplifier lakes in rift basins after three million years ago significantly contributed to this exceptional sensitivity of East Africa to climate change compared to elsewhere on the African continent. Amplifier lakes are characterized by tectonically-formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area. Such amplifier lakes respond rapidly to moderate, precessional-forced climate shifts, and as they do so apply dramatic environmental pressure to the biosphere. Rift basins, when either extremely dry or lake-filled, form important barriers for migration, mixing and competition of different populations of animals and hominins. Amplifier lakes link long-term, high-amplitude tectonic processes and short-term environmental fluctuations. East Africa may have become the place where early humans evolved as a consequence of this strong link between different time scales. (C) 2010 Elsevier Ltd. All rights reserved.
Plio-pleistocene East African pulsed climate variability and its influence on early human evolution
(2009)
We analyzed published records of terrigenous dust flux from marine sediments off subtropical West Africa, the eastern Mediterranean Sea, and the Arabian Sea, and lake records from East Africa using statistical methods to detect trends, rhythms and events in Plio-Pleistocene African climate. The critical reassessment of the environmental significance of dust flux and lake records removes the apparent inconsistencies between marine vs. terrestrial records of African climate variability. Based on these results, major steps in mammalian and hominin evolution occurred during episodes of a wetter, but highly variable climate largely controlled by orbitally induced insolation changes in the low latitudes.