Résumé
Charcoal analysis from a short core in a small tropical crater lake (Lake Masoko, SW Tanzania) reveals changes in charred-particle deposition and properties, consistent with high climatic variability for the last 4200 years. This is evidenced by automated image analysis of charcoal size distributions, thermal resistant carbon (soot) and black carbon elemental analysis. Charcoal-particle transport in the lake decreases during dry spells, and increases with stronger runoff regime in the catchment area under humid periods and/or during abrupt low-stands of the lake. However, between 1830 and 1560 cal. yr BP, charcoal-distribution variability and an abrupt increase in particulate carbonaceous particles testify to regional emissions from forest fires and atmospheric transport. This event was followed by a prolonged period of material accumulation but regional climatic drying. After 1560 cal. yr BP, large charcoal fragment accumulation and a rise in the length:width ratio of microcharcoal indicate fires closer to the lake and the opening of the local woodland. These new data give evidence of large-scale fire development just prior to a pollen-inferred vegetation change and late Iron Age activity in the area, which probably coincide with an altered fire regime. In fact, charcoal-size distribution and particulate carbon elemental analysis constrain the sources of fossil carbonaceous particles, and provide an accurate proxy of fire and climatic change in southern tropical Africa.