Soil erosion history in central Tanzania based on OSL dating of colluvial and alluvial hillslope deposits

The Irangi Hills in Kondoa District, central Tanzania, are severely degraded by sheet, rill and gully erosion. Using recently developed optically stimulated luminescence (OSL) dating techniques, and a detailed study of the hillslope stratigraphy and soils, we have determined the sequence of events that gave rise to this highly degraded landscape. Two major colluvial deposits have been identified on the slopes. The oldest colluvium gave OSL deposition dates of 14,700 +/- 1600, 14,100 +/- 1500 and 11,400 +/- 1300 years ago. These dates coincide with the climatic change from dry to wet conditions, which took place during the Late Pleistocene. It is possible that the erosion and deposition of the old colluvium occurred as a response to this change. This phase of erosion and deposition was followed by a long phase of stability and pedogenetic alteration of the old colluvium. A phase of accelerated soil erosion began not later than 900 years ago, as indicated by a dated alluvial fan, the presence of which indicates that some deeper gullies already existed on upper pediment slopes at that time. The second, more recent colluvial deposit gave OSL dates of 460 +/- 40, 590 +/- 70 and 660 +/- 50 years. A major period of gully formation and incision, with subsequent fan development, occurred sometime between 600 and 300 years ago. The recent phase of erosion ( < 1000 years), which is still continuing, is probably a result of the introduction and/or intensification of agriculture, livestock husbandry and iron smelting practices in the Irangi Hills. (C) 2000 Elsevier Science B.V. All rights reserved.