A palaeoecological study of recent environmental change in the drainage basin of the Lac d'Annecy (France)
Higgitt, Sandra Rosemary
PublisherUniversity of Liverpool
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A palaeoecological study of sediments from the Lac d'Annecy has been undertaken in order to assess the impact of man on soil vegetation systems in the lake basin. The geology of the drainage basin is dominated by weakly magnetic calcareous Jurassic and Cretaceous deposits, but natural soil forming processes and artificial burning have produced assemblages of strongly magnetic minerals within topsoil's of the catchment area. These can be characterized by mineral magnetic parameters which are preserved as the eroded soil fractions become incorporated within the lake sediments. Man's impact on the landscape appears to have been most dramatically felt from c. 1100 AD; fossil pollen assemblages give evidence for widespread forest clearance, associated with arable and pastoral farming activities, which are thought to reflect the earliest intensive agricultural development of the higher slopes within the catchment area. At the same time there is a marked change in the nature of the sedimentary matrix. An increase in the concentration of major cations and magnetic minerals indicates a regime of more intensive soil erosion and a change in the magnetic mineral assemblage itself indicates a shift in the relative importance of different catchment sources to the total allochthonous material flux. Reconstruction of environmental change during more recent centuries has been aided by reference to primary and secondary documentary sources of evidence relating to past patterns of land-use. The mixed farming system of the eighteenth century, characterized by a regime of relatively intensive arable cultivation, was not particularly well-suited to the natural environment. A decrease in the concentration of major cations and magnetic minerals, together with a decline in the total sediment accumulation rate from the midnineteenth century onwards, is thought to reflect a fall in the rate of loss of material from catchment surfaces. It has been suggested this was related to a shift in focus of the rural economy from the semi-arable, semi-pastoral subsistence agricultural system to one which concentrated increasingly on the breeding of livestock for the local dairying industry.