East African Rift Valley Lakes hold a rich source of information for palaeoclimate change. Specifically, the sediment archives of Lake Chamo, one of the Ethiopian Rift Valley Lakes, reveal short-term climatic fluctuations and environmental instability during the Holocene, since it is located in a temporary endorhëic system. Currently there are no substantial studies yet that investigate palaeoenvironmental history of Lake Chamo. The objective of this thesis is to reconstruct the Holocene climatic and environmental history of Lake Chamo at high temporal resolutions. The specific aim of the project is to estimate climate-driven and anthropogenic environmental change during the Holocene, and to test a hypothesis that there were rapid climate fluctuations during the termination phase of the “African Humid Period” (AHP).
Initially, the first continuous and high-resolution geochemical and geophysical core data from the sediments of Lake Chamo using X-Ray Fluorescence (XRF) core scanner and Geotek Multi-Sensor-Core-Logger (MSCL) respectively, is presented. Using these techniques palaeoclimatic conditions of the region during the Holocene are reconstructed. Additionally, the core chronology is established using Accelerator Mass Spectrometry (AMS) radiocarbon analysis, the results of which show that the core dates back to 9000 cal yr BP. Early-Holocene can be seen to be characterised wetter climatic conditions as recorded from the relative lower lightness values, high Silicon to Titanium ratio (Si/Ti), and minimum calcium concentration in the sediment. Pronounced peak in calcium and Strontium content, which are the main features of the early-mid Holocene transition period, are ascribed to a high evaporation to precipitation ratio, implying the aridity of the region in this time frame. In addition, the peak values of magnetic susceptibility (MS), Potassium (K), Titanium (Ti), Silicon (Si), and Iron (Fe) during 1500–800 cal yr BP are found to be associated to a change in intensity of anthropogenic land use in the area surrounding the lake.
Subsequently, the charcoal counting and detection of benzene polycarboxylic acids (BPCA) methods are implemented to estimate paleofire occurrence in relation to climatic and anthropogenic impact of Lake Chamo. These results are used to correlate the long term trends in fire occurrence in relation to climate, vegetation and human activities at different spatial and temporal scales. Fire occurrence was found to be higher during the early Holocene, typically identified through black carbon (BC) from woody or shrub vegetation sources. The occurrence of fire was found to be lower during the mid-Holocene, due to the presence of predominantly grass savannah which usually results in reduced biomass burning in response to drier condition. Climate and vegetation were found to be the main factors for fire occurrence during the early and mid Holocene, whereas the increased fire intensity since 2000 cal yr BP record is potentially attributed to anthropogenic forcing.
Finally, ostracod analyses are used to gain evidence about climatic and hydrological instability of southern Ethiopia during the Holocene. The ostracod study focuses on the taxonomy, stratigraphy and the use of ostracod assemblages to interpret the palaeoenvironments established during Holocene period. Ostracod assemblages were found to be infrequent and of limited diversity in the sediment profile, implying a period of wetter conditions. The highest abundance and more diverse ostracod assemblage were found to be associated with periods of drier climatic condition in the Lake Chamo records.
To summarize, the geophysical, geochemical, charcoal, and ostracod data analysis, alongside core chronology results of this thesis have been used to reconstruct Holocene climatic and environmental history of Lake Chamo region, and of the East Africa region as a whole. The main results of this study provide significant input for the understanding of climate variability in the Holocene, as well as identifying termination of the African human period was gradual in Lake Chamo region.
Gebru Kassa, T. (2015): Holocene Environmental History of Lake Chamo, South Ethiopia.
|Authors||Gebru Kassa, Tsige|
|Title||Holocene Environmental History of Lake Chamo, South Ethiopia|