A multi proxy core study of the last 7000 years of climate and alpine land-use impacts on an Austrian mountain lake (Unterer Landschitzsee, Niedere Tauern)

Analyses of diatoms, pollen, magnetic compounds, grain size, and selected elements were applied to a 3.45 in long sediment core, spanning the last 7000 years, from a high mountain lake (Unterer Landschitzsee, 1788 in a.s.l.) in the Central Austrian Alps. The pollen record indicates a series of negative climate oscillations superimposed on a longterm trend towards increased humidity that culminated during the beginning few centuries of the 1st millennium BC. Partially these fluctuations - the main follow roughly a 1000-year periodicity - can be related to known climate oscillations and glacier advances in the Alps. The pH was inferred using an Alpine diatom calibration dataset (ALPS) based on weighted average regression; pH levels fluctuated within one unit. Warm and dry periods between 7000 and 5000 cal. BP caused pH levels to rise close to the present value. Cool and wet periods, with extended snow-cover, are the main reason for pH lowering between 5000 and 1400 cal. BP. The core can be subdivided into a pre-anthropogenic section from 7000 to 3500 cal. BP, and an anthropogenic section from 3500 cal. BP to present. The pollen records indicate that high altitude land-use is closely related to climate. Alpine pasturing, which was initiated during the Bronze Age at about 3500 cal. BP, intensified during the Iron Age, and extended during Medieval times. With the formation of alpine pastures, the C/N ratio distinctly declined. The long-term trend of the iron/manganese ratio indicates an inverse distribution, probably due to erosional features. Short-term fluctuations in median grain size and saturation isothermal remanence mainly have been caused by fluviatile transport and catchment erosion related to climate oscillations and deforestation. Processes related to anthropogenic deforestation and to enhanced nutrient fluxes from extended pastures are thought to be responsible for the modifications to the diatom assemblages, which took place around 1000 cal. BP. (C) 2002 Elsevier Science B.V. All rights reserved.