Holocene alluviation in the Algarve, southern Portugal: The case for an anthropogenic cause

Finding a causal mechanism to account for observed phases of valley alluviation in the Mediterranean region and southern Europe during the Holocene has been a subject of debate for more than 20 years. Three possible causes have been suggested: regional climatic change, anthropogenic induced erosion and the crossing of intrinsic thresholds within fluvial systems so that deposition, rather than erosion, is favoured. In this paper we report investigations carried out in the Algarve region (southern Portugal), where past research is interpreted as suggesting a climatic cause. Mapping and sedimentological investigations of Holocene sediments within three major valleys in this region are reported, together with the association of these deposits with artifacts, a Roman/Moorish mill and bands of charcoal. Chemical, as well as physical sedimentological interpretation, was carried out on the deposits. We also report the results of a detailed investigation and reconstruction of the land use and settlement history of the area and show that episodes of valley alluviation coincide in time with phases of intensive agriculture and clearance within the catchments of these valleys, whereas episodes of valley erosion coincide with episodes of relative stability. Whilst recognizing that temporal correlations are not necessarily causal, we argue that in this area anthropogenic forcing of phases of Holocene alluviation is the preferred mechanism.