The response of sedimentation in three varved lacustrine sequences to air temperature, precipitation and human impact

Human activity and climatic forcing have influenced sedimentation in three of Finland's deepest lakes during the last centuries. High-resolution sediment sequences of Lake Paijanne, Lake Paajarvi and Lake Pyhajarvi represent records of the last 440 years, 839 years and 633 years, respectively. The accumulation rates of dry matter, organic carbon and biogenic silica refer to changes in human activity in the catchments. However, they also reveal the importance of climatic forcing on lake sediment deposition. A significant correlation was found between instrumentally measured records of temperature (163 years) and precipitation (148 years), and varve thickness. Warm winter months indicating a short ice-cover period have the strongest control on varve thickness. This shows that wind-driven resuspension of littoral material is the forcing mechanism of climate on lake sediments. The long-term human-induced erosion pulses observed may even have magnified the climatic signals in some cases. Nevertheless, increased anthropogenic field erosion in the catchment, and the associated leaching of bioavailable nutrients, hampers the observation of climate signals, especially during the last 50 years, in the lakes studied.