Late Holocene environments at Lake Mitsinjo,northwestern Madagascar

Two sediment cores from Lake Mitsinjo in northwestern Madagascar were analysed for evidence of past environmental changes. The unconformities in a 503 cm core from the centre of the lake and a 306 cm core from the shore were correlated to identify a hiatus in sedimentation that occurred just before ca. 1000 BP. The 3500-year stratigraphic record was analysed for fossil pollen, charcoal, algae, organic matter, and carbonate content. Pollen stratigraphy indicates the following vegetational changes: (1) a mosaic of dry forest and grassland from ca. 3500 BP until the time the lake dried up; (2) a savanna from ca. 1000-500 BP; and (3) a highly disturbed grassland with fire-adapted trees and ruderal herbs in the last 500 years. Pollen spectra and lithologic correlation indicate that prior to the hiatus the lake experienced a highstand, and after the hiatus lake levels were lower. The lake-level changes probably reflect the local effective moisture. Pollen evidence indicates a possible human disturbance of the surrounding landscape beginning ca. 1000 BP, with a major escalation in disturbance in the last 500 years. These events coincide with archaeologically derived evidence for herding and later cultivation in the region. Algal evidence suggests cultural eutrophication in the lake began about a century ago. It would appear that both natural desiccation and anthropogenic modification of the landscape overlap temporally with some of the much-debated megafaunal extinctions that have occurred on the island in recent millennia.