Universität zu Köln

Kassa, Tsige Gebru (2015). Holocene Environmental History of Lake Chamo, South Ethiopia. PhD thesis, Universität zu Köln.

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Abstract

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.

Item Type:	Thesis (PhD thesis)
Translated abstract:	Abstract Language Seen aus dem Ost-Afrikanischen Grabenbruch enthalten reichhaltige Informationen zu Klimaveränderungen in der Vergangenheit. Insbesondere das Seesedimentarchiv des temporär endorëhischen Entwässerungssystems des Chamo Sees, der zu den Äthiopischen Riftseen zählt, bietet die Möglichkeit, kurzzeitige Klimafluktuationen und Umweltveränderungen während des Holozäns zu identifizieren. Bisher gibt es noch keine Untersuchungen, die die Umweltgeschichte des Chamo Sees mithilfe seiner Sedimente klären. Deshalb ist das Ziel dieser Doktorarbeit, die Klima- und Umweltgeschichte des Chamo Sees in großer zeitlicher Auflösung für das Holozän zu rekonstruieren. Insbesondere soll der Frage nachgegangen werden, wie groß der natürliche und anthropogene Einfluss auf die Umweltveränderungen im Holozän war und ob die Hypothese eines abrupten Endes der sogenannten, Afrikanischen Feuchtphase“ (AHP) für diese Region Gültigkeit hat. Zunächst werden die kontinuierlichen und geophysikalischen Sedimentkerndaten des Chamo Sees vorgestellt, die mit der Röntgenfluoreszenzmethode (XRF) und dem Geoteck Multi-Sensor-Core-Logger (MSCL) gemessen wurden. Mithilfe dieser Methoden gelingt es, die paläoklimatologischen Bedingungen für das Holozän zu rekonstruieren. Das Alter der Sedimente von ca. 9000 Jahren wurde mittels der AMS-Radiokohlenstoffanalyse ermittelt und anschließend kalibriert. Das Früh-Holozän zeigt durch die Kombination von relativ niedrigen Helligkeitswerten, einem hohen Silizium/Titanverhältnis und minimalen Ca-Gehalten der Sedimente feuchtere Klimabedingungen an. Deutlich erhöhte Gehalte an Kalzium- und Strontium, die als wichtiges Merkmal in der Übergangsphase zum Mittel-Holozän auftreten, werden als Zunahme des Verdunstungs-/Niederschlagsverhältnisses interpretiert und deuten auf größere Trockenheit in diesem Zeitraum hin. Darüberhinaus zeigen hohe Werte der magnetischen Suszeptibilität, des Kaliums (K), des Titans (Ti), des Silikats (Si) und des Eisen (Fe), die im Zeitraum von 1500 bis 800 Jahren vor heute auftreten, einen größeren Einfluss des Landwirtschaft betreibenden Menschen in der Umgebung des Sees an. Weiterhin werden die Anzahl der Holzkohlepartikel und das Vorhandensein von Benzen Polycarbonsäure (BPCA) genutzt, um das Auftreten von Feuern in der Vergangenheit zu dokumentieren und ihre klimatischen oder anthropogenen Ursachen zu differenzieren. Diese Ergebnisse werden genutzt, um eine Korrelation zwischen dem Langzeittrend der Feueraktivität sowie der Klima- und Vegetationsentwicklung, aber auch des menschlichen Einflusses auf verschiedenen räumlichen und zeitlichen Skalen herzustellen. Danach lässt sich ein größeres Auftreten von Feuerereignissen während des Früh-Holozäns feststellen, das sich durch, Black Carbon“ (BC) von Bäumen und Sträuchern zu erkennen gibt. Im Mittel-Holozän dagegen nimmt die Feuerhäufigkeit ab, es herrschen verbrannte Savannengräser vor, die auf eine reduzierte Biomasse aufgrund größerer Trockenheit hindeuten. Das Klima und die Vegetation sind die wichtigsten Faktoren für das Auftreten von Feuern im Zeitraum vom Früh- zum Mittel-Holozän, während die Zunahme der Feuerintensität in den letzten 2000 Jahren vor heute wohl hauptsächlich dem zunehmenden menschlichen Einfluss zuzuschreiben ist. Schließlich wurde die Analyse von Ostrakoden genutzt, um Hinweise auf klimatische und hydrologische Instabilitätsphasen während des Holozäns für Südäthiopien zu gewinnen. Zur Interpretation der Paläoumweltbedingungen wurden die Ostrakoden taxonomisch analysiert, Ostrakodengemeinschaften identifiziert und stratigraphisch eingeordnet. Während feuchter Klimabedingungen sind Ostrakodengemeinschaften seltener und zeigen eine geringere Diversität. Dagegen treten große Ostrakodenvorkommen und eine höhere Diversität in trockeneren Phasen des Chamo Sees auf. Zusammenfassend kann man sagen, dass die geophysikalischen und geochemischen Befunde sowie die Holzkohle- und die Ostrakodenergebnisse zur Rekonstruktion der Klima- und Umweltgeschichte der Region um den Chamo See sowie Ostafrikas genutzt werden können. Dadurch wird das Verständnis zur Klimavariabilität des Holozäns dieser Region erweitert und nachgewiesen, dass das Ende der AHP allmählich erfolgte. German
Creators:	Creators Email ORCID ORCID Put Code Kassa, Tsige Gebru tgebruka@uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-61937
Date:	17 June 2015
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Mathematics and Science Education > Institute of Geography Education
Subjects:	Natural sciences and mathematics Earth sciences
Uncontrolled Keywords:	Keywords Language African Humid Period, Ethiopian Rift Valley Lakes, Lake Chamo, geochemical proxy, charcoal, black carbon, ostracods, Holocene English
Date of oral exam:	22 January 2015
Referee:	Name Academic Title Schäbitz, Frank Prof. Dr.
Projects:	The Collaborative Research Center 806 “Our way to Europe”
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Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/6193