A systematic assessment of late Quaternary hydrological change from valley fill complexes in Namibia

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Southern African climate is influenced by both tropical and temperate rainfall systems and the interplay of these into the future will strongly determine climatic changes. Our understanding of how the hydrological dynamics of southwestern Africa changed in the past, however, is not well constrained. Marine sediment records highlight significant environmental changes over southwestern Africa since the last glacial maximum, yet terrestrial records are limited, and the spatial detail of changes throughout the landscape is not recorded by offshore studies. This study will produce records of fluvial activity from two ephemeral fluvial valleys: the Huab in Northern Namibia, and one in Southern Namibia. This aims to understand individually, how local river valleys have responded to past climatic change individually, and when taken together what we can infer about past regional hydrological dynamics. To produce a record of fluvial activity, sediment deposited in valley fill regions will be dated using OSL. A number of debates persist in explaining how the climate changed over this region in the past. The focus on a southern Namibian river valley will be particularly interesting in understanding the shifting boundary between winter and summer rainfall zones. Relating to the interplay between tropical and temperate rainfall systems, the location of such boundary in the past is currently relatively unknown. The Northern Namibian fluvial record aims to address questions relating to southern Africa's response to the African Humid Period. Whilst well constrained in Northern Africa, the response of Southern Africa appears to be spatially complex and not well understood. Alongside these broad scale questions of regional hydrological dynamics, the study of both northern and southern Namibian river valleys aims to have a comparative aspect, where variations in timing of fluvial activity between the two valleys will be used to investigate within-system changes. Optically stimulated luminescence (OSL) dating will be employed to obtain a timeline of sediment deposition. OSL dating investigates the accumulation of radiation within quartz grains. When subjected to sunlight, grains are bleached and the radiation signal zeroed, but once buried a signal accumulates within the grain. A quantification of radiation within the quartz grain can be converted to an estimation of the length of time the grain has been buried for. A fluvial event which deposits sediment in these valley fill regions can therefore be dated in time. In order to understand the fluvial history from such OSL records, an understanding of the sedimentary architecture of the valley fills will need to be obtained. This will potentially involve the use of ground penetrating radar (GPR), and a comparison to existing palaeoarchives as a guide to understanding gaps and clusters in ages. OSL dating has been applied to other Namibian river valleys, but with varying conclusions based on interpretations of the valley fill sediments. Some identify sediment fills as slackwater deposits from flood events and others as river end deposits from periods of reduced river flow. A component of this study will investigate in detail the origin of such sediments. We will gain a better understanding of the composition of the sediment and how transport and bleaching influences OSL ages. This has the aim of presenting a more coherent picture of Namibian fluvial activity, with the correct interpretation of such valley fill sediments. This study will therefore have two facets: to investigate how the OSL signal relates to fluvial activity, and to produce a record of fluvial activity in the two river valleys from the OSL record. This palaeoenvironmental reconstruction aims to provide an understanding of shifts in regional hydrological dynamics, which can add to the body of knowledge vital for model evaluation in this time period.