AMS-~(14)C chronology of 40.0 cal ka BP continuous deposits from a crater lake (Lake Massoko, Tanzania) Modern water balance and environmental implications

摘要

Lake Massoko, Tanzania (8°20'S, 33°45'E; 870 m a.s.l.) is a freshwater maar lake belonging to the Rungwe volcanic area, 15 km nothwest of the Livingstone Mountains and Lake Malawi. The rainfall regime is driven by the yearly oscillation of the Intertropical Convergence Zone, which determines a wet summer and a dry and windy winter. The presence of Lake Malawi to the south, and high ranges to the north result in a higher annual precipitation than that observed at a regional scale. Air masses overloaded with humidity bypassing Lake Malawi are submitted to ascending currents, producing rainfall up to 2400 mm yr~(-1) in the Lake Massoko area. Immportant exchanges between the lake and groundwater reservoirs have been demonstrated from hydrological and isotopic balances of Lake Massoko on a regional scale. These exchanges are superimposed on groundwater circulation. The hydrological setting of Lake Masoko is also influenced by the tectonics of the region dominated by the northwest/southeast tilt of tectonic blocks. The sedimentary sequences of Lake Massoko cored in 1995 and 1996 under the EEC-project RUKWA [RUKWA Project-Final Report, 1997] likely register the palaeoenvironmental changes on the catchment. The sediment record is nearly continuous due to the near-constant high water level of the lake. In contrast with many other lacustrine sequences, in which reduced deposition rates, discrepancies and/or gaps characterise arid periods such as the Last Glacial Maximum and the Younger Dryas event, all the AMS-~(14)C dtes on authigenic organic samples fit with the stratigraphy, and the palaoenviromental record of Lake Massoko exhibits a constant mean sedimentation rate of 0.682 mm yr~(-1) for the last ca. 40 cal ka BP.