Cosmogenic Constraints on Sediment Generation and Transport, Namibian Rivers

University Of Vermont & State Agricultural College, Burlington VT

Investigators

Abstract

0207669 Bierman Sediment samples were collected from Namibian Rivers with a vision to answer several fundamental scientific questions and in order to make a significant societal contribution. NSF funding will provide for isotopic analyses of these samples. Overall, the data we collect will be used to test the relationship between nuclide activity (a proxy for basin-scale sediment generation rates) and drainage basin lithology, relief, climate, and area. Such relationships will allow us to predict sediment generation rates where landscape conditions are similar, an important tool for land management. Cosmogenic nuclides provide a window into the past behavior of landscapes. When measured in drainage basin sediments nuclides such as 10Be and 26Al allow estimation of basin-scale sediment generation rates over time scales determined by the speed at which the sampled landscape erodes; in Namibia, nuclide activities integrate erosion rates over ~105 years. Samples collected from headwater drainages and drainages with little sediment storage will allow rigorous comparison between landscape-scale erosion rates over the later Pleistocene using cosmogenic nuclides and rates integrated over the Cenozoic using fission tracks. Such comparisons provide the fundamental data that allow one to explain the behavior of Earth's surface over time and to test models of landscape evolution such as those proposed to explain the retreat of great escarpments over time. Namibian rivers have wet headwaters where most floods are generated; the rivers lose water as they cross the hyper-arid Namib Desert. Such loss means that most flood events are discontinuous with sediment in transport being stranded at varying distances along the 300 to 400 km long river courses. Only the largest and rarest floods move sediment to the ocean. We will compare our nuclide-based findings related to sediment virtual velocity and sediment source with sediment composition and the historical flood frequency/magnitude records of the two rivers that we sampled in detail. Understanding long-term sediment fluxes down rivers is important for responsible management of water resources and river corridors. Dams have been placed on several Namibian Rivers and there are plans in the works for others such as a very controversial project in the Kunene River. Clearly, the life time and thus cost-benefit analysis for such massive hydraulic structures depends critically on knowing long-term sediment fluxes. Cosmogenic analysis of river sediments will provide such data. Land use in parts of Africa is intensive. In some places, desertification has been the result of overgrazing and population expansion. Cosmogenic data will determine background rates of sediment generation. Such rates can be compared to modern rates and thus provide the basis for responsible land use management decisions. .