Late Glacial and Holocene temperature changes at Egelsee, Switzerland, reconstructed using subfossil chironomids

Isabelle Larocque-Tobler 0 1 2 3 Oliver Heiri 0 1 2 3 Michael Wehrli 0 1 2 3 0 M. Wehrli Institut fur Pflanzenwissenschaften, Universitat Bern , Altenbergrain 21, 3013 Bern, Switzerland 1 O. Heiri Palaeoecology, Institute of Environmental Biology, Faculty of Science, Utrecht University , Budapestlaan 4, 3584 CD Utrecht, The Netherlands 2 I. Larocque-Tobler Institut National de Recherche Scientifique (INRS), Centre Eau , Terre et Environnement, 490 De La Couronne, Quebec, QC G1K 9A9, Canada 3 I. Larocque-Tobler (&) Oeschger Centre for Climate Change Research and Institute of Geography, University of Bern , Zahringerstrasse 25, 3012 Bern, Switzerland A temperature reconstruction using chironomids was attempted at Egelsee, Switzerland, a site where pollen and macrofossil records showed a correspondence between vegetation and climatic changes inferred by other proxies in Europe. The general pattern of temperature changes inferred from chironomids during the Late Glacial [i.e. cold temperatures between ca. 16,500 and 14,800 cal BP, close to present-day temperature between 15,000 and 13,000 cal BP and colder temperatures during the Younger Dryas (YD)], and the major temperature changes of the Holocene (i.e. the Younger Dryas-Holocene transition and the Late Holocene cooling trend) at Egelsee, were mirrored in other European climate reconstructions using various proxies. However, the amplitude of temperature changes during the YD was smaller than reconstructed by other proxies at various sites, and the 8,200 years BP event was not apparent. These differences between records were probably due to the dominance of Corynocera ambigua, with percentages reaching 60% in parts of the Egelsee sequence. This taxon was not present in any of the 103 lakes used for the transfer function and its absence may have yielded less accurate inferences. Its presence in samples only associated with cold inferences at Egelsee suggests that this taxon is a cold indicator. However, it was also found in warm Danish lakes and the factors that determine the presence of C. ambigua remain unexplained. Most samples had a poor fit to temperature and instead, dissolved organic carbon seemed to be a factor influencing the chironomid assemblages during the Holocene. These results illustrate the need to better understand the ecology of chironomids and to disentangle the various factors that affect chironomid communities through time. Ultimately, such information will lead to more accurate temperature reconstructions. - Chironomids (Insecta: Diptera) are widely distributed aquatic insects. While the adult stage is a terrestrial winged insect, the larvae develop in or on the surface of different substrates (sediments, plants, dead wood, other invertebrates) in aquatic, humid or semi-terrestrial environments (e.g. lakes, ponds, rivers, estuaries, bogs, marshes). In lake sediments, the chitinous (C6H12O5N) head capsules of chironomid larvae preserve well and can be identified to genus, even to species level in certain cases, making it possible to characterize past assemblages. Subfossil chironomid assemblages have been used both qualitatively and quantitatively to infer various limnological variables such as trophic status (Lotter et al. 1998), oxygen concentration (Quinlan and Smol 2002), salinity (Heinrichs et al. 2001), lake depth (Korhola et al. 2000), acidification (Schnell and Willasen 1996), chlorophyll a (Brodersen and Lindegaard 1999a) and especially air temperature (e.g. Heiri et al. 2003a, 2007b; Larocque et al. 2001, 2006) and water temperature (e.g. Walker et al. 1997). Chironomids have been successfully used to reconstruct major and minor climatic oscillations during the Late Glacial period in the UK (Bedford et al. 2004), Germany (Hofmann 1983a), France (Heiri and Millet 2005), Italy (Heiri et al. 2007a; Lami et al. 1997; Larocque and Finsinger 2008), the Swiss Alps (Brooks 2000; Hofmann 1983b, 2001; Ilyashuk et al. 2009), Norway (Brooks and Birks 2000a) and The Netherlands (Heiri et al. 2007b). During the Holocene, a decrease of temperature through time in the high to mid northern latitudes was inferred using chironomids, with maximum temperature in the early Holocene in Siberia (Porinchu and Cwynar 2002), western Canada (Palmer et al. 2002), Iceland (Caseldine et al. 2003, 2006), Bulgaria (Stefanova et al. 2003) and Sweden (Larocque and Hall 2004), consistent with decreases observed from analysis of other temperature proxies (e.g. Koc et al. 1993; Bradley 2000). Chironomid analysis provided evidence of phases of cooler European summers during the Holocene in Switzerland, including a decrease of temperature dated to ca. 8,000 calibrated radiocarbon years BP (years BP; Heiri et al. 2003b). An event with similar timing was also recorded by chironomids at many sites in northern Sweden (Rosen et al. 2001) and in Finland (Korhola et al. 2002). Good relationships were found between chironomidinferred temperatures and meteorological data (Larocque and Hall 2003; Larocque et al. 2009) indicating the potential of chironomids for accurate temperature reconstructions. Unfortunately, the method can also provide problematic reconstructions (e.g. Kurek et al. 2004; Velle et al. 2005b) probably due to the influence of factors other than climate affecting the chironomid community (e.g. human disturbance, Heiri and Lotter 2005; pH changes, Velle et al. 2005b; Dalton et al. 2005) or limits of the transfer function method (Heinrichs et al. 2004). Here, we describe a temperature reconstruction using chironomids preserved in the sediment of Egelsee, Switzerland. Egelsee is a raised bog and was first chosen for its location in the montane belt (8001,200 m a.s.l) of Central Switzerland where there is little archeological evidence of human settlement (Aerni 1991). A recent pollen and macrofossil diagram from this site (Wehrli et al. 2007) indicated vegetation changes through the Late Glacial and the Holocene, which might be attributed to climate. To better understand the link between climate and vegetation in this part of Switzerland where few other records are available, a temperature reconstruction using chironomids was undertaken. This temperature reconstruction is compared with existing records in other regions of Switzerland and Europe to evaluate its reliability. In its present state, Egelsee is a raised bog located 1 km northwest of Menzingen (Kanton Zug) in Central Switzerland (47 110N/8 350E; Fig. 1). It lies at an elevation of 770 m a.s.l. and has a surface area of 12,000 m2 (Staubli 1998). The peat thickness is 140 cm, representing the last ca. 800 years. Under this peat lies gyttja-type sediment which indicates that the site was previously a lake. The former lake had a catchment of 90,000 m2 (Staubli 1998). The bog lies at the borderline between the Tertiary Molasse basin of the Swiss Plateau and the Prealps. On top of the Tertiary Molasse lies Quaternary till material deposited during several glacia (...truncated)