Environmental change during the Holocene : A comparative multi-proxy study of landscape disturbances in Northern Sweden

Sammanfattning: Varved lake sediments were used to provide information on how a landscape is affected by disturbances of different scales, from global (i.e. climatic) to local (i.e. fires), as well as anthropogenic activities. Geochemical and pollen data, biogenic silica (bSi), lake-water total organic carbon (LWTOC) and chlorophyll a were used as proxies to infer past changes in lake-conditions. The goal was to evaluate the response to scale different disturbances and how it differs among sites. By comparing different lake records, it became possible to isolate the climatic signal from the effect of soil development and vegetation establishment, and differences emerging from different catchment characteristics. Climatic trends were reconstructed based on the pollen and geochemical data. The sediment records were then compared to identify the effect of each disturbance on individual lakes. One of the most prominent event observed was the immigration of spruce at about 3000 BP which considerably affected sedimentation trends. The presence of spruce within the catchment appears to promote the input of fine-grained material to the lakes. The timing and intensification of anthropogenic activities was established and it was possible to differentiate the effects of human disturbance from changes caused by natural processes such as climate or landscape vegetation cover changes. The results show that farming practices started earlier in more southern locations and that this timing is site-dependent. Two phases were identified, corresponding to the start of slash and burn farming and later to the expansion of agricultural practices, with a more profound transformation of the landscape.