JOURNAL OF QUATERNARY SCIENCE (2004) 19(5) 423–430 Copyright ß 2004 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.850 Rapid Communication A major widespread climatic change around 5300 cal. yr BP at the time of the Alpine Iceman MICHEL MAGNY1* and JEAN NICOLAS HAAS2 1 Laboratoire de Chrono-Ecologie, Faculté des Sciences et Techniques, 16 route de Gray, 25 030 Besançon, France 2 University of Innsbruck, Department of Botany, Division of Palynology, Systematics and Geobotany, Sternwartestra e 15, 6020 Innsbruck, Austria Magny, M. and Haas, J. N. 2004. A major widespread climatic change around 5300 cal. yr BP at the time of the Alpine Iceman. J. Quaternary Sci., Vol. 19 pp. 423–430. ISSN 0267-8179. Received 30 September 2003; Revised 20 March 2004; Accepted 24 March 2004 ABSTRACT: Palaeoenvironmental and archaeological data from Arbon Bleiche, Lake Constance (Switzerland) give evidence of a rapid rise in lake-level dated by tree-ring and radiocarbon to 5320 cal. yr BP. This rise event was the latest in a series of three successive episodes of higher lake-level between 5550 and 5300 cal. yr BP coinciding with glacier advance and tree-limit decline in the Alps. This west-central European climate change may have favoured the quick burial and the preservation of the Alpine Iceman recently found in the Tyrolean Alps. It has possible equivalents in many records from various regions in both hemispheres dating to 5600–5000 cal. yr BP and corresponds to global cooling and contrasting patterns of hydrological changes. This major mid-Holocene climate event marks the Hypsithermal/Neoglaciation transition possibly resulting from a combination of different factors including orbital forcing, changes in ocean circulation and variations in solar activity. Copyright ß 2004 John Wiley & Sons, Ltd. KEYWORDS: mid-Holocene; climate change; interhemispheric linkages; solar activity; orbital forcing; ocean circulation. Introduction From several points of view, the Alpine Iceman found in September 1991 in the Tyrolean Alps is the most exciting European archaeological discovery in recent years (Höpfel et al., 1992). Radiocarbon dated to 5320–5050 cal. yr BP (Kutschera and Müller, 2003), he appears to be the oldest well preserved prehistoric body ever discovered in Europe, even older than the Egyptian royal mummies. It has been hypothesised that the striking preservation of this Neolithic man and his equipment resulted from a quick burial below snow and ice cover owing to rapid climate cooling (Baroni and Orombelli, 1996). This paper presents palaeoenvironmental and archaeological evidence of an abrupt climate change at 5550–5300 cal. yr BP at Arbon Bleiche, Lake Constance (Switzerland), i.e. ca. 110 km west of the Tyrolean Alps. Cooler and wetter conditions associated with this climatic oscillation may actually have favoured the rapid burial of the Alpine Iceman. Moreover, climatic changes possibly correlative to this event can be observed in * Correspondence to: Michel Magny, Laboratoire de Chrono-Ecologie, Faculté des Sciences et Techniques, 16 route de Gray, 25 030 Besançon, France. E-mail: michel.magny@univ-fcomte.fr many palaeoclimatic records from both hemispheres. This suggests a more global significance of the climate oscillation recorded at Arbon Bleiche that was responsible for the preservation of the Alpine Iceman, and allows increasing understanding of the mechanisms and underlying forcing factors of this mid-Holocene climatic reversal. The Arbon Bleiche record in west-central Europe The site of Arbon Bleiche (47.30 N, 9.25 E, 397 m a.s.l.) is located on the southeastern shore of Lake Constance (Fig. 1). This lake has the second largest surface area of all European peri-Alpine lakes. The main inflow is the Alpine River Rhine draining an area of more than 6100 km2 in the Alps. Archaeological investigations recently revealed remains of a Neolithic lake-shore village established in the former shallow bay of Arbon Bleiche, which was dendrochronologically dated to 5334–5320 cal. yr BP (Leuzinger, 2000). Figure 1 presents the sediment sequence observed in the investigation area. Palaeoenvironmental studies have focused on the reconstruction of lake-level and vegetation cover before, during and just after the neolithic settlement. The results have been described 424 JOURNAL OF QUATERNARY SCIENCE Figure 1 Upper panel: Geographical location of the site of Arbon Bleiche, Lake Constance (Switzerland). Lower panel: the sediment sequence of Arbon Bleiche extensively elsewhere (Haas and Magny, in press). The present paper deals only with data obtained from units 8 to 5 at the base of the sediment sequence (Fig. 1), which documents the period around 5600–5200 cal. yr BP. Unit 8 is composed of carbonate lake marl and divided into two subunits, 8a and 8c, by a thin sand layer (subunit 8b) where the presence of beach pebbles marks an erosion surface (ES1) responsible for a sediment hiatus in agreement with the radiocarbon dates. The transition between units 8 and 7 coincides with a second erosion surface (ES2) marked by a sharp change in sediment composition, the presence of beach pebbles and the pollen stratigraphy (Table 1). Unit 7 is a sand layer 1 to Copyright ß 2004 John Wiley & Sons, Ltd. 60 cm thick. It is missing in the eastern part of the site as a result of erosion: erosion surface (ES3). Unit 6 is a 5 to 40 cm thick archaeological layer mainly composed of anthropogenic organic material. Unit 5 is a 10 to 30 cm thick sand layer containing remains resulting from the destruction of the Neolithic houses. The contact between units 5 and 4 is marked by a pedogenic horizon. The chronology is based on (i) AMS radiocarbon dates from terrestrial plant macrofossils and (ii) tree-ring dates from wooden piles used by prehistoric people for dwelling construction. The lake-level fluctuations were reconstructed using a specific sedimentological method (Magny, 1992, 1998) in addition to J. Quaternary Sci., Vol. 19(5) 423–430 (2004) GLOBAL CLIMATE CHANGE AT THE TIME OF THE ALPINE ICEMAN 425 Table 1 Changes in lake-level and vegetation cover reconstructed from the sediment sequence of Arbon Bleiche (Switzerland). Note that the death of the Alpine Iceman is radiocarbon-dated to 5320–5050 cal. yr BP (Kutschera and Müller, 2003), i.e. close to the sudden rise in lake-level in Arbon Bleiche at 5320 cal. yr BP Soil/erosion surface Pedogenic horizon Erosion surface 3 Sediment units Lithology Ages Lake level 4 Sand 5 Sand 6 Anthropogenic 5334–5320 cal. yr BP (Zürich Laboratory) organic deposits Sand ca. 5440 cal. yr BP (interpolated age) 7 High Low 4700  40 14C yr BP, i.e. 5583–5317 cal. yr High BP (VERA 2486) Erosion surface 2 8a Carbonate lake-marl Erosion surface 1 8b 8c Sand Carbonate lake-marl 6595  35 14C yr BP, i.e. 7560–7432 cal. yr BP (VERA 1703) Copyright ß 2004 John Wiley & Sons, Ltd. Subboreal (rising values of Fagus, Abies, Picea and Ulmus decline) Low High Low 4835  40 14C yr BP, i.e. 5602–5492 cal. yr High BP (VERA 1701) changes in sediment texture and geometry of the sediment layers (Digerfeldt, 1988). Erosion surfaces 1 to 3 correspond to three phases of low lake-level, each followed by a phase of higher lake-level conditions responsible for the deposition of units 8a, 7 and 5 (Table 1). Morphotypes of carbonate concretions and shells of aquatic molluscs found in units 8a, 7 and 5 support this interpretation (Haas and Magny, in press). Thus, the lowering marked by ES3 favoured the installation of the neolithic village on the southeastern shore of Lake Constance, tree-ring dated to 5334–5320 cal. yr BP. Immediately afterwards, dated to 4700  40 yr BP (i.e. 5583–5317 cal. yr BP), the deposition of unit 5 corresponds to a rapid rise in lake-level, which caused the village to be abandoned. This rise was the latest in a series of three successive episodes of higher lakelevel, the first (unit 8a) occurring at 4835  40 yr BP (i.e. 5602–5492 cal. yr BP) and the second (unit 7) at ca. 5440 cal. yr BP (interpolated age). Regarding the history of the vegetation cover, the transition from the Younger Atlantic to the Subboreal pollen zones occurred after the deposition of unit 8a (Table 1). The changes in climatic conditions at 5600–5300 cal. yr BP, as recorded at Arbon Bleiche by vegetation and lake-level data, appear to be fully consistent with (i) a regional phase of higher lake-level dated to 5650–5200 cal. yr BP by radiocarbon and dendrochronology (Magny, 2004), and (ii) cooler conditions in the Alps indicated by glacier advance at ca. 5450–5000 cal. yr BP (Patzelt, 1977), tree-limit decline at ca. 5600–5200 cal. yr BP and changes in vegetation cover at 5600–5000 cal. yr BP (Schmidt et al., 2002). The development of cooler and wetter climate conditions at that time in the Alps may explain the quick burial and preservation of the Alpine Iceman as hypothesised by Baroni and Orombelli (1996). More than 40 AMS radiocarbon dates (Kutschera and Müller, 2003) indicate that the death of the Alpine Iceman occurred at 5320– 5050 cal. yr BP, i.e. close to the abrupt climate variation recorded by a sudden rise in lake-level in Arbon Bleiche at 5320 cal. yr BP. Furthermore, the 5600–5000 cal. yr BP climate reversal recognised in west-central Europe coincides with maximum values in the atmospheric 14C content marked by three successive peaks at 5600, 5450 and 5300 cal. yr BP, i.e. close to the ages of the three successive episodes of higher lake-level reconstructed at Arbon Bleiche. This supports the hypothesis of an abrupt climate change forced by varying solar activity (Haas and Magny, in press). Pollen zones Younger Atlantic (Quercetum mixtum dominance, Ulmus > 5%) Low High A major widespread climate reversal at 5600–5000 cal. yr BP Mid-Holocene climatic reversal has been recorded by several authors and Fig. 2 presents a map of possibly correlative events in various regions in both hemispheres (based on selected data listed on Table 2). Bearing in mind the difficulties of assessing the exact temporal relationships owing to differences in dating accuracy and sample temporal resolution, it can be observed that most of the events fall in the 5600– 5000 cal. yr BP time window and provide evidence for a generally prevailing climate cooling at that time, as shown by changes in vegetation cover (Heikkikä and Seppä, 2003), glacier advance (Benedict, 1973; Patzelt, 1977; Wenzens, 1999), decline in tree limit in mountains (Bortenschlager, 1977; Rochefort et al., 1994), increasing permafrost and retreating timberline at high latitudes (Payette et al., 2002; Väliranta et al., 2003), cooler sea-surface temperatures (Calvo et al., 2002; Lamy et al., 2002), or ice-sheet isotope records at the poles and in the tropics (Dahl-Jensen et al., 1998; Masson et al., 2000; Thompson et al., 2002). For example, the cooling has been assessed at 1–1.5 C for mean summer temperature in the European Alps (Bortenschlager, 1977; Haas et al., 1998), 0.75 C for mean annual temperature in Finland (Heikkikä and Seppä, 2003), 1–2 C for mean annual temperature in southern Africa (Jerardino, 1995), more than 1 C for seasurface temperature (SST) in the Nordic Seas (Calvo et al., 2002; Risebrobakken et al., 2003), 0.6 C for SST off Chile (Lamy et al., 2002), and 1.5 C in Greenland (Dahl-Jenssen et al., 1998). In addition, as suggested by Fig. 2, the 5600– 5000 cal. yr BP period coincided with drier conditions in central Asia (Morrill et al., 2003), in the northern part of Africa (Damnati, 2000; Chalié and Gasse, 2002; deMenocal et al., 2000), in eastern North America (Kirby et al., 2002), in Central America (Street-Perrott and Perrott, 1990), in the southern Mediterranean region (Carrion, 2002; Bar-Matthews et al., 1999; Arz et al., 2003) and at high latitudes (Nesje et al., 2001; Andreev et al., 2003). Figure 2 also suggests that, at the same time, wetter conditions prevailed over intermediate latitudes between ca. 40 and 60 latitudes in west-central Europe (Starkel, 1991; Langdon et al., 2003; Blaauw et al., 2004; Magny, 2004) and South America (Steig, 1999; Lamy et al., 2002; Noon et al., 2003). J. Quaternary Sci., Vol. 19(5) 423–430 (2004) 426 JOURNAL OF QUATERNARY SCIENCE Figure 2 World-wide climatic change at 5600–5000 cal. yr BP according to the multiproxy records of study sites mentioned in Table 2 Such contrasting patterns of hydrological changes, in addition to a global temperature cooling, appear to be consistent with changes in the atmospheric methane content, which shows an inverse trend to increase from 5200 cal. yr BP in relation with the higher-/lower-latitude CH4 emission ratio (Blunier et al., 1995). Moreover, these hydrological changes reflect a weakening of the African and Asian monsoon linked to cooler SST and a weaker thermal contrast between landmass and adjacent oceans (Damnati, 2000; Morrill et al., 2003). They also suggest (i) a reinforcement and a migration of westerlies toward lower latitudes in response to a stronger thermal gradient between high and low latitudes (Magny et al., 2001; Mullins and Halfman, 2001; Lamy et al., 2002), and (ii) the establishment of modern El Niño periodicity in relation to the onset of a steeper zonal SST gradient over the tropical Pacific Ocean (Rodbell et al., 1999). Among potential causes of the climate reversal at 5600– 5000 cal. yr BP, orbital forcing, changes in ocean circulation and variations in solar activity appear to be possible candidates. As indicated above, the 5600–5000 cal. yr BP event is placed just at the abrupt inversion in the interglacial trend of atmospheric methane values (Blunier et al., 1995) and various other records show an abrupt climate oscillation at 5600– 5000 cal. yr BP close to the beginning of a long-term climate reversal toward cooling (e.g. Calvo et al., 2002; Dahl-Jensen et al., 1998; Hodell et al., 2001). This corresponds to the rapid Copyright ß 2004 John Wiley & Sons, Ltd. mid-Holocene transition from the Hypsithermal to the Neoglaciation (Steig, 1999). Despite apparent contradiction between progressive insolation changes and abruptness of events in many records at 5600–5000 cal. yr BP, model experiments have shown the ability of gradual insolation forcing to produce abrupt climatic oscillations owing to the non-linear sensitivity of the climate system when crossing threshold values (deMenocal et al., 2000; Hodell et al., 2001; Kirby et al., 2002). However, some records also show a more progressive mid-Holocene climatic transition in response to orbitally driven changes in summer insolation. Thus, the ENSO frequency increased progressively as early as ca. 7000 cal. yr BP before the modern periodicity was established at ca. 5000 cal. yr BP (Rodbell et al., 1999). Similarly, lake-level records from the Tibetan Plateau provide examples showing that the climate drying associated with the weakening of the Asian monsoon and the decrease in summer insolation began as early as 6900–6300 cal. yr BP (Gasse et al., 1991), although a synthesis of multiple regional records also suggests an abrupt change at ca. 5000 cal. yr BP (Morrill et al., 2003). The Asian monsoon and ENSO also have been considered as interacting systems affected by changes in insolation (Morrill et al., 2003), and Schmittner et al. (2000) have pointed to the possible significance of the amplitude of ENSO events for the strength of the North Atlantic thermohaline circulation. J. Quaternary Sci., Vol. 19(5) 423–430 (2004) GLOBAL CLIMATE CHANGE AT THE TIME OF THE ALPINE ICEMAN 427 Table 2 Non-exhaustive list for records of world-wide climatic change at 5600–5000 cal. yr BP. The site numbers are those indicated on Fig. 2. Note that site numbers may refer to several study sites Site number Site 1 1 1 2 2 2 3 4 5 6 7 8 9 9 10 10 11 12 13 13 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 27 27 28 28 29 30 31 32 33 34 35 36 37 38 38 39 40 41 42 43 44 Arbon-Bleiche, Lake Constance (Switzerland) Swiss Plateau and Alps Jura and Pre-Alps (France), Swiss Plateau Nieder Tauern (Austria) Rotmoos (Austria) Alps (Austria) The Netherlands Poland Pentland Hills (Scotland) Jostedalsbreen region (Norway) Finland East-European Russian Arctic Nordic Seas Norwegian Sea Feni Drift (North Atlantic Ocean) Subpolar North Atlantic Ocean Gardar Drift (North Atlantic Ocean) Lake Siles (Spain) GRIP core (Greenland) GRIP core (Greenland) GISP2 Treeline (Canada) Western North America (USA) Elk Lake (USA) Shepherd Lake (Canada) Owasko Lake (USA) Fayetteville Green Lake (USA) Colorado Front Range (USA) Lakes Chiconahuapan and Chalco, Mexico Sargasso Sea (North Atlantic Ocean) Ecuador Huascaran ice sheet Peruvian coast Southern Chile shelf Southern Andes (Argentina) Tierra del Fuego (Argentina) Southern Patagonia South Georgia (Southern Ocean) Signy Island, maritime Antarctica South Atlantic sector Taylor Dome Antarctica ice sheet Kerguelen New Zealand Cango Caves (South Africa) Kilimanjaro ice sheet Lake Abiyata (Ethiopia) Off Cap Blanc (Mauritania) Lakes of Northern Hemisphere of Africa Southern Sahara Soreq Cave (Israël) Arabian Sea Sumxi Co (Tibet Plateau) Asian summer monsoon area Lakes of Mongolia Taymyr Peninsula (Arctic Russia) Climatic signal Age (cal. yr BP) References Wetter 5550–5300 Haas and Magny, in press Cooler and wetter Wetter Cooler and wetter Cooler Cooler Wetter Wetter Wetter Cooler and drier Cooler Cooler Cooler Cooler Change in ocean circulation IRD Change in ocean circulation Drier CH4 increase Cooler Higher Polar Circulation Index Cooler Cooler Cooler and wetter Wetter Drier Drier Cooler Drier 5100 Haas et al., 1998 5650–5200 Magny, 2004 5200 Schmidt et al., 2002 5600–5200 Bortenschlager, 1977 5450–5000 Patzelt, 1977 5650, 5550, 5400 Blaauw et al., 2004 ca. 5500 Starkel, 1991 5300 Langdon et al., 2003 ca. 5000 Nesje et al., 2001 5200 Heikkikä and Seppä, 2003 ca. 5600 Väliranta et al., 2003 5200 Risebrobakken et al., 2003 5500 Calvo et al., 2002 5100 Oppo et al., 2003 5400 Bond et al., 2001 5300 Bianchi and McCave, 1999 5200 Carrion, 2002 5200 Blunier et al., 1995 5300 Dahl-Jensen et al., 1998 5000 Mayewski et al., 1997 ca. 5800/5500 Lamb, 1977; Payette et al., 2002 ca. 5700 Rochefort et al., 1994 5400–4800 Bradbury et al., 1993 5550–5300 Haas and McAndrews, 2000 5300 Mullins and Halfman, 2001 5200 Kirby et al., 2002 ca. 5200 Benedict, 1973 ca. 5700–5100 Street-Perrott and Perrott, 1990 Cooler Modern El Niño periodicity Cooler Cooler, modern ENSO onset Cooler, lower salinity Cooler Cooler and wetter Wetter Cooler Cooler and wetter Cooler, sea-ice advance, IRD Cooler Cooler Cooler Cooler Cooler Cooler Drier Drier Drier 5000 5000 5200 ca. 5800 5500 5050–4800 5700 5000 5200 5300–5000 5500–5200 5500 5000 ca. 5700 5300–5000 5300–4850 5200 After 5400 5500 5700–5500 Keigwin, 1996 Rodbell et al., 1999 Thompson et al., 1995 Sandweiss et al., 2001 Lamy et al., 2002 Wenzens, 1999 Heusser, 1998 Steig, 1999 Rosqvist and Schuber, 2003 Noon et al., 2003 Hodell et al., 2001 Steig et al., 1998 Masson et al., 2000 Porter, 2000; Young and Schofield, 1973 Porter, 2000 Jerardino, 1995 Thompson et al., 2002 Chalié and Gasse, 2002 deMenocal et al., 2000 Damnati, 2000 Drier Drier Drier Drier Colder and/or drier Drier Colder and drier 5700–5500 5200 5500 ca. 5450–5000 ca. 5000–4500 ca. 5200 5200 Vernet and Faure, 2000 Bar-Matthews et al., 1999; Arz et al., 2003 Sirocko et al., 1993 Gasse et al., 1991 Morrill et al., 2003 Fowell et al., 2003 Andreev et al., 2003 IRD, ice-rafted debris. Changes in ocean circulation are regarded as other good candidates for provoking rapid climate variations (Broecker, 1992). Several records listed in Table 2 suggest a weakening of the thermohaline circulation (Keigwin, 1996; Bianchi and McCave, 1999; Oppo et al., 2003), a latitudinal shift of the Copyright ß 2004 John Wiley & Sons, Ltd. Antarctic Circumpolar Current (Lamy et al., 2002), or possible expansions of polar waters into the Humboldt and Benguela Currents (Jerardino, 1995) during the 5600–5000 cal. yr BP period. This may result from an intrinsic ocean instability or bipolar seesaw (Maslin et al., 2001). However, in the case of the J. Quaternary Sci., Vol. 19(5) 423–430 (2004) 428 JOURNAL OF QUATERNARY SCIENCE 5600–5000 cal. yr BP event, such a bipolar seesaw appears to be ruled out as suggested by a concomitant cooling of both hemispheres (Fig. 2). Variations in solar activity also appear to have been responsible for successive climate oscillations punctuating the whole Holocene (Denton and Karlén, 1973; Magny, 1993a; van Geel et al., 1996; Bond et al., 2001; Blaauw et al., 2004). They may account for the abruptness, the relative short duration (e.g. Bar-Matthews et al., 1999; Thompson et al., 2002) and the global extension of the 5600–5000 cal. yr BP event (Fig. 2), which coincided with a period of weaker solar activity as indicated by a 5600–5200 cal. yr BP maximum in the atmospheric 14C content (Stuiver et al., 1998). Model experiments developed by Goosse et al. (2002) have shown how variations in solar irradiance may trigger a reduction in the thermohaline circulation quite similar in magnitude, direction and spatial pattern of climate anomalies to those resulting from, for instance, freshwater outbursts from proglacial lakes during the last deglaciation. Furthermore, timberline retreat in North America (Payette et al., 2002; Nichols, 1967, cited by Lamb, 1977), increasing permafrost in the east European Russian Arctic (Väliranta et al., 2003), extension of sea-ice and/or IRD events in North and South Atlantic sectors (Bond et al., 2001; Hodell et al., 2001), higher values of Polar Circulation Index above Greenland (Mayewski et al., 1997), and displacement of westerlies to lower latitudes (Magny, 1993b; Magny et al., 2001; Lamy et al., 2002) suggest an extension of the polar cell in both hemispheres, fully in agreement with the scenarios proposed by van Geel and Renssen (1998) to explain a possible sun–climate link. In summary, far from a straightforward understanding, the 5600–5000 cal. yr BP event offers an example characteristic of the complexity of Holocene climate oscillations which may have resulted from a combination of multiple, non-exclusive factors. It still seems difficult to distinguish between high- or low-latitude climate control in the mechanisms involved in this event (Hodell et al., 2001; Lamy et al., 2002; Leuschner and Sirocko, 2003). Problems with sampling resolution and dating accuracy complicate the debate, in addition to (i) the abruptness of this event, which requires records with high temporal resolution, and (ii) the specificity and the sensitivity of available markers that give either a direct or indirect picture of climate change. Moreover, the Arbon Bleiche record clearly shows that, in certain regions, the mid-Holocene climate reversal appears to be characterised by intermediate warm spells (Fig. 1 and Table 1) within the distinct succession of strong cooling episodes (maxima at ca. 5550, 5450 and 5300 cal. yr BP), probably in relation to a solar forcing. Finally, although the onset of Neoglaciation favoured snow accumulation and glacier advance in the Tyrolean Alps and, as a result, the excellent preservation of the Alpine Iceman, the changes in palaeoenvironmental conditions induced by this mid-Holocene climate reversal may have led, via a more or less complex causal network, to substantial perturbations within human societies, as suggested by changes in cultural development in central Europe (Arbogast et al., 1996; Berglund, 2003; Magny, 2004), strong variations in human settlement patterns (Vernet and Faure, 2000) and by a rapid development of hierarchical societies in the overpopulated Nile valley and Mesopotamia (Sirocko et al., 1993) as well as in South America (Sandweiss et al., 2001). Acknowledgements Financial support for this study was provided by the Archaeological Department of the Canton Thurgau (Switzerland) and by the French Centre National de la Recherche Scientifique (CNRS) (Programme ECLIPSE). 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