Holocene environmental changes and human impact in the northern Swiss Jura as reflected by data from the Del,mont valley

The construction of the A16-Transjurane motorway revealed evidence of Holocene sediment sequences in the Del,mont valley (Canton of Jura, Switzerland). Certain processes begin during the Younger Dryas. Pine forests dominate this cold period, which was unfavourable for pedogenesis; they remain throughout the first half of the Holocene. The meandering river system then becomes stable for more than four millennia. The first signs of human impact on the vegetal cover begin to appear around 3,500 cal BC (Middle Neolithic). An increase in hydric activity occurs between 3,600 and 2,500 cal BC. However, the earliest evidence of the cultivation of cereals dates only to about 2,000 cal BC (Early Bronze Age). Extensive forest clearing and the emergence of cultivated plants occur after 1,400 cal BC. On the cleared slopes the soil erodes and at their foot colluvium deposits accumulate. The densification of the settlement from the Late Bronze Age (1,350-800 cal BC) until the beginning of the Iron Age (800-650 cal BC) contributes to alluvial destabilization. A palaeosol has been identified in all of the Holocene deposits in the valley. The deforestation intensifies between 400 and 100 cal BC while hydric activity decreases. The first centuries of our era record very limited pedo-sedimentary phenomena. However, human presence becomes less marked after 350 cal AD. The slopes are stabilized and the soil develops. From 550 cal AD, an important increase in hydric activity takes place, a probable consequence of a wet fluctuation in the climate. Contemporary forest clearing causes deep gullies. After 750 cal AD, drier conditions set in. This period of stability, marked by occasional rises in the water level, continues until 1,250-1,300 cal AD (Late Middle Ages). Then superficial flows resume and entrenchment of the main waterways occurs, combined consequences of the Little Ice Age and the upsurge in human activity.