Holocene and Lateglacial summer temperature reconstruction in the Swiss Alps based on fossil assemblages of aquatic organisms: a review

The taxonomic composition of chironomid, cladoceran and diatom assemblages in small lakes in the Alpine region shows a strong relationship with summer temperature. Since fossils of all three organism groups preserve well and remain identifiable in lake sediments, summer temperature transfer-functions can be developed based on the modern distribution of these organisms and applied to fossil records to reconstruct past summer temperature variability. We provide a summary of the chironomid-, cladoceran- and diatom-based transfer functions available from the Swiss Alps and discuss the potential problem of co-variation between summer temperature and lake nutrient conditions for transfer-function development. Whereas the diatom-based summer temperature transfer function remains to be evaluated in down-core reconstructions, the cladoceran- and chironomid-based transfer functions have been used successfully to produce summer temperature records on Lateglacial and Holocene time scales that are in good agreement with other temperature reconstructions in the Alps. Major problems that can be encountered when using fossil assemblages of aquatic organisms for temperature reconstruction in the Alpine region are biases in the inferred temperatures associated with human impact on lakes and parameters other than temperature affecting the fossil assemblages. A multi-proxy approach to palaeoenvironmental reconstruction is recommended to keep a close control on past catchment and within-lake processes during the time interval of interest.