A study of present day surficial distribution of bulk sediment mineralogy and magnetic mineralogy in Lake Bosumtwi, using x-ray diffraction and magnetic techniques

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Lake Bosumtwi has been the focus of international paleoclimate research (the Lake Bosumtwi Drilling Project) during which continuous sediment cores were recovered for multiproxy paleoclimate information. To determine the present day depositional environment and sediment provenance, 150 samples of the top 2 - 8 cm of the Bosumtwi sediments were collected at different locations throughout the lake with the Ekman dredge. These samples were analyzed for bulk sediment mineralogy and magnetic mineralogy using X-ray diffraction (XRD) and magnetic techniques. Magnetic measurements carried out include isothermal remanent magnetization (IRM), anhysteric remanent magnetization (ARM) and magnetic susceptibility, while XRD measurements were made on powder packs and oriented clay mineral mounts. The wet and dry bulk densities of samples decrease with increasing water depth whereas the % water content and % organic matter content increase with increasing water depth. The magnetic susceptibilities of the samples increased with increasing water depth, whereas % frequency dependence of susceptibility has a gentle increase with water depth. Back field (hard) isothermal remanent magnetization (HIRM)[i.e. (IRM1100mT + IRM300rnT)/2] values of the samples average 34*10-6 Am2/kg. S-ratio (-1*IRM300mT / IRM1100mT) of the samples averaged 0.93, indicating the presence of low coercivity magnetic minerals (such as maghemite and magnetite). Most parameters show an abrupt change at about 18 m lake water depth. The low Hcr values of <36 mT support the presence of low coercivity minerals. These low coercivity minerals in the surface sediments were deposited in the presently warm and wet interglacial climate with reduced Saharal/Sahel dust influx to Ghana. Oriented clay mineral studies using XRD reveal clay mineralogies of smectile, vermiculite, chlorite, illite, kaolinite micas, and their interstratified derivatives, also suggesting a warm wet climate with increased chemical weathering by hydrolysis. The presence of greigite at the central deepwater part of the lake suggests sulphate reduction under anoxic conditions, which indicate deposition in deep water, which further supports the inference of a wet climate. On Dearing’s plot samples mostly fall in the region of SPM enhanced soils. Quartz, orthoclase, anorthosite, muscovite and illite, which emerged from powder XRD results, point to a probable Biriamian Supergroup provenance for these top sediments, most likely from within the crater.
A thesis submitted in partial fulfilment of the requirement for the award of the degree Master of Science (Geophysics, 2005