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Accueil du site → Doctorat → Allemagne → 2018 → Diatom analysis as a tool for reconstructing limnological features and its application to the Late Quaternary sediments from Lake Palaeo-Makgadikgadi (northern Botswana) and Lake Kushu (northern Japan)

Freie Universität Berlin (2018)

Diatom analysis as a tool for reconstructing limnological features and its application to the Late Quaternary sediments from Lake Palaeo-Makgadikgadi (northern Botswana) and Lake Kushu (northern Japan)

Schmidt, Mareike

Titre : Diatom analysis as a tool for reconstructing limnological features and its application to the Late Quaternary sediments from Lake Palaeo-Makgadikgadi (northern Botswana) and Lake Kushu (northern Japan)

Auteur : Schmidt, Mareike

Université de soutenance : Freie Universität Berlin

Grade : Doctor rerum naturalium (Dr. rer. nat.) 2018

Résumé
Diatoms are unicellular, eukaryotic microalgae that form two valves made of silica. After death, these frustules accumulate at the bottom of lake basins. The persistant diatom valves mirror valuable information about water parameters through the lifetime of the organism. The high diversity and the specific ecological requirements allow diatoms to grow in almost all kind of waters and the assemblages accurately identify the limnology. That makes diatoms excellent bioindicators for palaeolimnological reconstructions. In this doctoral thesis, diatoms are used as a tool to study two different lake systems, in different time periods and different hemispheres. The first object is an ancient mega-lake phase of the Lake Palaeo-Makgadikgadi in the Makgadikgadi-Okavango-Zambezi Basin (MOZB) in northern Botswana. The MOZB is a structural depression consisting of five sub-basins, which feature palaeo-lacustrine structures indicative for past humid conditions that resulted in huge lakes (c. 37,000 km2) in the nowadays semi-arid Middle Kalahari. Environmental and climatic implications and fluctuations of the Late Quaternary highstands are controversial and poorly constrained. In this work, a 30 cm thick lacustrine deposit close to the western palaeo-shoreline of the Makgadikgadi Basin was analyzed to obtain a high-resolution diatom record to infer first palaeolimnological parameters such as salinity, pH and trophic level. 60 diatom samples (0.5 cm steps) and 30 samples (each 1 cm) for oxygen isotope analysis (δ18O) were analyzed for a high-resolution reconstruction of the Late Pleistocene lake phase. Optically stimulated luminescense (OSL) dating suggests that the highstand existed during Marine Isotope Stage (MIS) 5d-b (c. 100 kyr BP). The diatom assemblages and δ18O values indicate a shallow oligohaline lake with alkaline conditions and subsequent changes in the hydrologic balance (evaporation / precipitation) that implies variability between warm-wet and cold-arid climate. For the first time, palaeolimnological features of a Lake Palaeo-Makgadikgadi lake phase have been identified. The existence of such a large lake during MIS 5 provides evidence that at least one short-term anomaly of higher humidity occurred during a period that was long thought to be very dry in southern Africa. These findings contribute to the hypothesis that Late Pleistocene hunter-gatherers (Khoisan) have lived permanently in the arid to semi-arid Middle Kalahari at least since MIS 5. In the second part of the thesis, the focus lies on the coastal Lake Kushu on the small Rebun Island (82 km2) in northern Japan. Rebun Island is a key research area of the interdisciplinary Baikal-Hokkaido Archaeology Project (BHAP) to understand population changes and pathways of Holocene hunter-gatherers (Okhotsk, Jomon) from East Siberia towards the Japanese Archipelago and focusses on human-environmental interactions. 43 archaeological sites show the occupation of Rebun Island since the last 6000 years. Records of past climate variabilities and environmental changes in this region are sparse. The need for a nearby archive resulted in the recovery of the 19.5 m sediment core from Lake Kushu, the only freshwater lake on Rebun Island. The obtained sediments covering the last c. 16,600 years (AMS dated) provide the chance to reconstruct climate and environmental changes since the evolution of Lake Kushu in the Late Glacial and to detect human-environmental interactions, when first Holocene hunter-gatherers occupied the island. It also helps to decode the requirements for incipient habitability of Rebun Island. In a first step, it was tested wether the RK12 sediment core represents a continuous palaeoenvironmental archive suitable for multi-proxy analyses.

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Page publiée le 1er novembre 2019