AN OPTICAL AGE CHRONOLOGY OF LATE QUATERNARY EXTREME FLUVIAL EVENTS RECORDED IN UGANDAN DAMBO SOILS

There is little geochonological data on sedimentation in dambos—seasonally saturated, channelless valley floors—which serve as important hydrologic, fertility and geochemical reservoirs throughout East and Southern Africa. A number of workers have argued that dambo soils are formed largely through in situ weathering rather than sedimentation, and even where morphological evidence of sedimentation has been presented there are few absolutes dates. Radiocarbon dating is problematic for dambo soils, which range in depth from 1-2 m, due to the potential for contemporary vegetation contamination and rapid oxidation of most organic materials. The equatorial site and semi-arid climate facilitate grain bleaching, while the gentle terrain ensures shallow water columns, low turbidity, and relatively long surface exposures for transported grains prior to deposition and burial. Blue-light stimulation single aliquot regeneration (SAR) techniques were used on quartz grains in the 90 to 250 micron size range. The dominant grain size of each sample site was determined and utilized for Blue-light OSL dating. These Blue-light OSL ages were then compared with IRSL and TL ages from finer grains in the same sample. For this study, we focused on dating sandy strata (indicative of high energy fluvial events) at various positions and depths within a 2nd order dambo in central Uganda. A total of 8 samples were dated, with 6 independent dates at ~ 35, 33, 16, 10.4, 8.5-8.3, and 5.9 ka (cal yr BP). Most of these events can be correlated with rapid global and regional climatic transitions—both arid to humid and visa versa. Evidence from this study suggests that rapid switches in the East African monsoonal climate regime, occurring over decades to centuries, lead to an increase in extreme fluvial events that deposit sand lenses on dambo floors. The optical ages published in this study provide the first detailed chronology of dambo sedimentation, and we anticipate that further work along these lines could provide a wealth of information on the paleohydrology of East Africa.