Soil erosion history and past human land use in the North Pare Mountains : a geoarchaeological study of slope deposits in NE Tanzania

摘要

Landscapes are the legacy of past environmental processes. The understanding of current environmental problems such as land degradation is strongly enhanced when trajectories of pastlandscape development are taken into account. In the Pare Mountains of north-eastern Tanzania widespread exposure of subsoil and saprolite indicates large-scale land degradation, which was advanced in the mid-19th century when the first European travellers reported widespread deforestation. The present study explores the timing, causes and consequences of past soil erosion to assess whether the spread of agriculture, large-scale iron working, or agricultural intensification during the 19th-century caravan trade were the main drivers of present day land degradation. Geoarchaeological investigations drawing on a multi-proxy approach including pedological investigations of slope deposits and palaeoecological analysis of swamp sediments, suggest that enhanced soil erosion and corresponding accumulation of slope deposits started about 2000 years ago, roughly contemporaneous with the arrival of new subsistence strategies like agriculture and the spread of iron working. Three distinct periods of soil erosion characterised by an increasing intensity of land use have been distinguished by macroscopic soil features and analytical measurements: Slow topsoil erosion from about 300 BC on, accelerated runoff-based erosion of subsoils since the 15th century and ongoing land degradation under intensive agricultural land use since the 19th century. Progressive land clearance and continuous soil erosion depleted topsoil and later subsoil resources progressively, but resulted in rapid changes of environmental processes when internal thresholds were crossed. Topsoil exhaustion in the 15th century caused a shift from slow aggregate-based to accelerated runoff-based erosion, whereas localised colluviation is identified as having dammed the Lomwe swamp in the 6th century. This research highlights the importance of cumulative impacts of prolonged human land use, whether forest clearing or cultivation, for landscape development. Rather than abrupt climate change, the impact of slow but continuous anthropogenic degradation processes is critical when assessing longterm stability of environment systems or the sustainability of land use practices. The investigation of past soil erosion based on its corresponding terrestrial archives produces detailed, site-specific reconstructions of past environments and their dominant processes and allows conclusions about human land use practices and settlement history. This is particularly important where the archaeological record is restricted due to anthropogenic erosion of past land surfaces.