L’aggradation au Subboréal le long de la marge apennine de la plaine centrale du Pô : aspects géomorphologiques et géoarchéologiques

Abstract

The Apennine foothills at the southern limit of the Po Plain consist of a belt of coalescent, gravelly, alluvial fans, trending to the north, towards the depocenter of the plain. Their aggradation is generally considered to have been controled by climate and linked to the glacial periods of the Middle and Upper Pleistocene. At the transition from the Upper Pleistocene to the Holocene, aggradation of most of the alluvial fans stopped and their surface was subject to weathering, which resulted in deep, rubified Alfisols (Sols bruns fersiallitiques). These developed mainly during the Boreal and Atlantic periods. Beginning in the Sub-Boreal period, fine-textured alluvial deposits buried the distal margin of the alluvial fans and the soils developed on them. These deposits consist of fine-sized sediments of overbank facies, organised in fining upward sets and intercalated by buried Entisols, Inceptisols and Vertisols. The accumulation of alluvial sediments on the alluvial fans, after the prevailing weathering on their top, indicates a main change in the pedo-sedimentary processes and is related to the climatic change that occured between the end of the Postglacial Hypsithermal period and the beginning of the Neoglacial period. The soils on top of the alluvial fans and in later alluvial deposits show intensive human use and occupation from the Neolithic to the Early Bronze Age, and witness a significant change in land exploitation during the Atlantic/Sub-Boreal transition. During the Neolithic (Atlantic period), agricultural practices linked to shifting agriculture had minimal impact on the vegetation mantle and were limited to the plain areas. By contrast, during the Chalcolithic and the Early Bronze Age (Sub-Boreal period), a rather different mode of land use was adopted, as the result of the newly introduced transhumant pastoralism. Deforestation through slash and burn techniques was very intense and widespread, extending far beyond the boundary of the plain, deep inside the mountain range, affecting the Po Plain and the adjoining Apennines. This case study presents an argument for seeing climate change and human activity (deforestation by fire, agriculture and pastoralism) as synergic phenomena that shaped the prehistoric landscape.