Mineral magnetic analyses of sediment cores recording recent soil erosion history in central Tanzania

Sediment cores, covering the period from ca. 1835 to 1988 AD, were retrieve d from Lake Haubi, located in a severely eroded area in the Kondoa District , central Tanzania. The results of mineral magnetic analyses undertaken on the sediment cores reflect two distinctly different depositional environmen ts. Before ca. 1902 AD the basin formed a seasonally inundated swamp, which subsequently turned into a lake. The swamp sediment is black, uniform, and extremely clay-Rich. It contains antiferromagnetic minerals (e.g. haematit e) but lacks ferrimagnetic minerals (e.g. magnetite) due to post-deposition al dissolution. The lake sediment is also very clay-rich but laminated. Her e ferrimagnetic minerals (magnetite) dominate the magnetic assemblage. The soil erosion history of the catchment has been reconstructed using results based on the mineral magnetic analyses and on the sedimentation rates obtai ned from Pb-210 datings, whereby variations in magnetic concentrations and ratios, attributed to variations in sediment influx, are assumed to reflect soil erosion within the catchment. The results from the magnetic analyses are in general agreement with the sedimentation rates. High sediment accumu lation occulted around the turn of the century, and increased generally sin ce 1935, with particularly high rates between ca. 1945 and 1950, and from c a. 1955 to the present. The reconstructed soil erosion history has been com pared to both historical records of anthropogenic activity in the Kondoa Di strict and to rainfall data. From this comparison we infer that effects on soil erosion from variations in rainfall are subordinate to those induced b y man. (C) 1999 Elsevier Science B.V. All rights reserved.