Research paper Introduction Tufa is a secondary calcium carbonate deposit that forms primarily from the degassing of calcium carbonate-rich waters (Pentecost, 1996; Baker and Sims, 1998). Here we consider tufa deposits formed in cool fresh water, which are common in the Northern Hemisphere (e.g. Pentecost, 1993, 1995; Pentecost and Zhaozhui, 2001; Žák et al., 2002; Davies and Griffiths, 2005; Meyrick and Karrow, 2007). An increasing number of radiocarbon dates became available for this type of tufa during the 1970s and 1980s. Some of these dates suggest that in large parts of Europe the rates of tufa formation were high in the early and mid Holocene but declined strongly thereafter (Weisrock, 1986; Žák et al., 2002), especially in large tufa complexes (Baker and Sims, 1998). In the considerable debate about the possible causes of this decline (Goudie et al., 1993), some authors attributed it to climate change, while others believed that human impact was responsible. One of the sites where postglacial tufa was dated is the Höllgrotten (engl. Hell Grottoes) caves in the Lorze Valley in north-central Switzerland (Figure 1). This tufa complex and its caves developed below highly productive springs that originate in the western slope of the Lorze Valley. Using the 230 Th/ 234 U ingrowth dating method (Eikenberg et al., 2001), the genesis of the tufa complex was dated to the optimal climatic conditions of the Holocene between 9000 and 5000 cal. yr BP (Wyssling and Eikenberg, 2000; Eikenberg et al., 2001), a period when many tufa complexes formed in Europe and Asia Minor (Pentecost, 1995). A hydrogeological study showed that the raised bog Neugrundmoor (Figure 2), located 4 km southeast and 280 m above the tufa complex, is the aqueous recharge area for the springs above the Höllgrotten (Wyssling and Eikenberg, 2000). Without any palaeo- ecological analysis of Neugrundmoor and without radiocarbon dating, these authors proposed the following hypothesis: ‘During the Effects of climatic change and bog development on Holocene tufa formation in the Lorze Valley (central Switzerland) Michael Wehrli, 1 Edward A.D. Mitchell, 2,3,4 W.O. van der Knaap, 1 Brigitta Ammann 1 and Willy Tinner 1 Abstract A massive tufa complex in the lower Lorze Valley (550 m a.s.l., north-central Switzerland) formed between 9000 and 5000 cal. yr BP. In the plain 4 km southeast of the complex lies Neugrundmoor (830 m a.s.l.), a mire in the hydrologic recharge area of the springs responsible for the tufa formation. Here we address an earlier, untested hypothesis that the transition of Neugrundmoor from fen to raised bog caused the end of the tufa formation, by means of pollen and testate amoebae analyses on a peat section. The chronology of the section is based on ten AMS 14 C-dates of terrestrial plant macrofossils. Our results suggest that until 7100 cal. yr BP the plain had shallow ponds, which gradually developed into fens. Starting 5350 cal. yr BP, large raised bogs developed in the fens in a few centuries. This agrees with the hypothesis that the end of tufa formation coincided with the transition from fen to raised bog. It also coincided with the end of the mid-Holocene temperature optimum, when climate became cooler and wetter. We suggest that the main factor causing the end of tufa formation at 5500–5000 cal. yr BP was declining air temperatures, which on the one hand led to diminished calcium precipitation, on the other to enhanced bog formation. Peat has a low hydraulic conductivity, so we suggest that less water became available for tufa formation because of increased thickness of peat layers, exacerbating the direct effect of cooling on calcium precipitation. Noticeable human influence in the surroundings did presumably not start before c. 3500 cal. yr BP and was then very weak, so this was not responsible for the end of the tufa formation. Extrapolation of our results suggests that climatic cooling alone is sufficient to explain the widespread termination of tufa formation after the mid Holocene. Keywords climate change, pollen, raised bog development, Switzerland, testate amoebae, Tufa formation The Holocene 1–12 © The Author(s) 2010 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959683609351901 http://hol.sagepub.com 1 University of Bern, Switzerland 2 WSL, Swiss Federal Research Institute, Switzerland 3 Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland 4 University of Neuchâtel, Switzerland Received 23 February 2009; revised manuscript accepted 8 September 2009 Corresponding author: W.O. van der Knaap, Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern, Altenbergrain 21, CH–3013 Bern, Switzerland Email: knaap@ips.unibe.ch The Holocene OnlineFirst, published on February 19, 2010 as doi:10.1177/0959683609351901