Climatic interpretation of terrestrial malacofaunas of the last interglacial in southeastern France

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1 ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Climatic interpretation of terrestrial malacofaunas of the last interglacial in southeastern France Denis-Didier Rousseau a,b,ł, Jean-Jacques Puisségur 1 a Paléoenvironnements et Palynologie, UMR CNRS 5554, Institut des Sciences de l Evolution, Université Montpellier II, place E. Bataillon, case 61, Montpellier cedex 5, France b Lamont-Doherty Earth Observatory of Columbia University, Palisades NY 10964, USA Received 25 September 1997; accepted 24 November 1998 Abstract The sequence of the Ruisseau de l Amourette, in the French Alps, covering the isotope stages 5e, 5d, 5c, and part of 5a and 6, was studied for terrestrial mollusks. Correspondence analysis of the mollusk assemblages permits reconstruction of temperature and moisture trends using knowledge of the modern ecology of the recognized species. The classic environmental succession of the last Interglacial Early Glacial (Würmian) is recognized. The Eemian (Sub-stage 5e) suggests warm and moist conditions. It is followed by the cold Melisey I (Sub-stage 5d) showing moisture variation, from dry in the lower part to moist in the upper part. The mollusk equivalent of St. Germain I and II also indicates warm and moist conditions. The Melisey II stadial (Sub-stage 5b) is missing but may be reflected by the deposition of barren gravel layers. All these environmental oscillations agree with the fluctuations described in the literature. However, Stage 5e indicates a particular trend as the mollusks recorded two well individual cool events which seem to correspond to variations identified in various west-european continental records. However, as no particular cold indicator species were identified, these events cannot be related to strong climatic fluctuations as emphasized in the early studies of the Greenland GRIP ice-core or in North Atlantic and western European records Elsevier Science B.V. All rights reserved. Keywords: Eemian; terrestrial mollusks; paleoclimates; Alps; France 1. Introduction Previous analyses of mollusks in Holocene sections (Lozek, 1972, 1985; Piechocki, 1977; Keen, Ł Corresponding author. Fax: C ; denis@dstu.univ-montp2.fr 1 Before the completion of this paper, Dr. Jean-Jacques Puisségur died. All the material described in this paper is deposited at Montpellier, but is accessible upon request to DDR The coded data are archived in Boulder on line at ftp:==.ngdc.gov=paleo=contributions_by_author= rousseau1998= 1981; Alexandrowicz et al., 1984; Meijer, 1984; Alexandrowicz, 1985; Neck, 1985, 1989; Goodfriend, 1988, 1991; Nyilas and Sümegi, 1989; Limondin and Rousseau, 1991; Rousseau et al., 1993, 1994, 1998; Sommé et al., 1994; Limondin, 1995) yielded paleoclimatic information that could be related to other paleoclimatic records. Older interglacial materials are difficult to find mainly because soil pedogenesis generally dissolves the shells. However, in some particular cases, individuals can be preserved, and provide enough material for paleocological interpretations. This is the case in slope deposits /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S (99)

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3 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) as described by Lozek (1964) and Puisségur (1976), or in carbonate deposits such as tufa (Kerney et al., 1980; Alexandrowicz, 1985; Preece et al., 1986; Rousseau et al., 1992). Records that can be studied for the last interglacial are rare as the time resolution is not generally as good as for the Holocene. The aim of this paper is to present results of the statistical analysis of terrestrial malacofaunas, yielded by a well developed Quaternary sequence in the French Alps. This record was not affected by erosion due to the glacier advance. The results will be interpreted in relation to the recent published data from both continental and marine records. 2. Material and methods The Ruisseau de l Amourette section is located in the Trièves basin (Fig. 1), a large depression bordered by the Vercors Mountain to the west, the Oisans Mountain to the east, the Devoluy Mountain to the south and opens north to the Drac valley. This area was not affected by Würmian glaciers (Montjuvent, 1978, 1980). However, an ice dam blocked the valley behind which a lake formed and laminated clays were deposited. 131 samples from a 46 m deep section composed of clays and silts were analyzed for pollen and mollusks (Gremmen et al., 1984). All mollusk species were counted, and broken shells were included following the method developed by Lozek (1964). For each mollusk assemblage, from one stratigraphical layer, the number of species was counted and the Shannon index H 0 computed. The Shannon index describes the diversity of a biological community using the formula: nx H 0 D p k log 2 p k where p k 1 is the frequency of one species in one determined assemblage. Such an index allows characterization of the structure of a given assemblage and permits us to detect any changes affecting the assemblages. Additionally, the analysis of the species diversity allows us to define assemblages relevant for environmental interpretations. Each count was also coded for multivariate analysis by transforming the values into abundance classes on a logarithmic scale, following the method described by Rousseau (1987). Such a coding retains information on representative variations, while suppressing the differences between the well represented and the poorly represented species. Correspondence analysis (Benzecri and Benzecri, 1980) applied to the mollusk data (see Rousseau, 1991; Rousseau et al., 1993) allows simultaneous study of mollusk assemblages (represented by rows) and mollusk species (represented by columns). This allows statistical interpretation of species behaviour in terms of ecological niches. Consequently, results from rows and columns can be plotted in a single diagram, facilitating environmental interpretation. This method has already been successfully applied to Quaternary mollusk assemblages (Rousseau, 1986, 1987, 1991; Rousseau and Puisségur, 1989, 1990; Limondin and Rousseau, 1991; Rousseau et al., 1993; Rousseau and Kukla, 1994). The environmental interpretation of the Ruisseau de l Amourette mollusk assemblages is based on information on the current ecological tolerance of individual living species (thermal and moisture requirements, and vegetation association) (Kerney and Cameron, 1979; Kerney et al., 1983). Indeed, all the recognized fossil species have modern counterparts exhibiting the same ecological characteristics (Rousseau, 1989). Both mollusk and pollen studies show two warm intervals separated by a cold interval (Gremmen et al., 1984) (Fig. 2). This interpretation was accepted later by de Beaulieu et al. (1992) when taking into account the succession of the reconstructed vegeta- Fig. 1. Ruisseau de l Amourette sequence. (a) Map showing the location of the Ruisseau de l Amourette sequence. (b) Detail of the geological area of the Ruisseau de l Amourette sequence: 1 D substratum; 2 D Les Serres gravels; 3 D L Amourette formation; 4 D glacio-lacustrine clays (Würmian); 5 D lower terraces gravels (Würmian); 6 D valley bottom infilling (Holocene); 7 D Würmian front moraines; 8 D area of ice-dammed lakes; Dev D Devoluy Mountain (modified from de Beaulieu et al., 1992). (c) The Ruisseau de l Amourette sequence. Numbering of the samples, and of the corresponding sections studied. Description of the lithology: 1 D calcareous gravels and pebbles; 2 D clay with macro-remains of plants and mollusks; 3 D sandy loam; 4 D shaly lignites with occurrence of lacustrine chalk and wood; 5 D dark shale (after Gremmen et al., 1984 modified).

4 324 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999)

5 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) tion. Although they support the earlier chronology of Eemian to St. Germain I defined for the Ruisseau de l Amourette sequence, de Beaulieu et al. (1992) state that the pollen diagram is atypical leading to a vegetation history peculiar to the southern Alps. This assumption is based on the high percentages of Pinus (pine) during the early temperate phases of the Interglacial and by the significant role of Fagus (beech) associated with Carpinus (hornbeam) and Abies (fir) during the late temperate phase of the Interglacial (de Beaulieu et al., 1992). Preliminary results of oxygen isotope studies of mollusk shells from the Ruisseau de l Amourette support this interpretation (Ch. Hannss, oral commun.). Eight mollusk zones were recognized (Figs. 3 and 4) according to the species composition (Gremmen et al., 1984). The first three are interpreted as corresponding to the previous glacial period of a Riss age. The fourth is a warm interval correlated with the Eemian interglacial. This agrees with the pollen record (Fig. 2). A relatively cold phase (zones E and F) overlies the Eemian, and in turn is followed by another warm interval, zone G, assumed to be the equivalent of St Germain I. This also is in agreement with pollen data (Fig. 2). Gravel layers interrupt the sequence. They are overlain by sediments indicating another warm interval, zone H, showing a truncated upper part by early Würm deposits. 3. Results Mollusk assemblages from the Ruisseau de l Amourette sequence reveal relatively high values of the Shannon index that seem to be in agreement with the earlier biostratigraphical interpretation for interglacial assemblages (Rousseau et al., 1993). The values vary between 0.5 and 4.5, indicating that several environmental variations occurred in the fossil sequence. An earlier survey of modern mollusk assemblages (Rousseau et al., 1993) indicates that mollusk assemblages from forest environments show the highest Fig. 2. Simplified pollen diagram of the Ruisseau de l Amourette sequence (modified from Gremmen et al., 1984). Plot of the main arboreal taxa against depth. The non-arboreal taxa are not represented here. diversity values. The high diversity is related to numerous ecological opportunities present in a forest environment. On the contrary, open environments, and especially those in northern Europe, or at high elevations offer fewer niches, resulting in a lower species diversity. In this case, the index values are low, between 1.5 and 2.5. However, low values are also characteristic of assemblages that are affected by problems in sediment deposition (Rousseau et al., 1993). In fluvial deposits mollusk shells are generally broken or dominated by aquatic taxa. Several assemblages indicate a low diversity, as well as a low number of individuals, suggesting that deposition conditions disturbed the distribution of the species among the assemblages (Fig. 4). This implies that such assemblages cannot be considered for further ecological interpretations. On the other hand, assemblages that show a low number of individuals coupled with diversity values related to assemblages in natural conditions will be used for ecological analyses. Following this procedure, the majority of samples show a diversity index varying between 2.8 and 4.3. There are, however, three assemblages which show low values (0.63, 1.6 and 1.8) (Fig. 4). Such a distribution is in agreement with the occurrence of environments ranging from cool (values tending to 2) to warm climate conditions (values tending to 4.3). Assemblages allocated to warm periods in the earlier biostratigraphic study present the highest diversity values, although the lowest value of the whole sequence is recorded in mollusk zone G. One can notice also that warm intervals show assemblages with diversity values fluctuating between 1.8 and 4. Cold assemblages also vary between 1.5 and 4 (Fig. 4). The correspondence analysis of the set of selected assemblages permits interpretation of these variations (Figs. 5 and 6). The first two axes explain 21.4% of the total variance, based on 98 samples containing 46 taxa recognized. If each taxon had a contribution equivalent to the general variability, the theoretical value would be 1=46 D Each taxon indicating higher values than this threshold is used for the interpretation of the results. The first axis discriminates Abida secale, Clausilia parvula, Pupilla muscorum, Vallonia costata, Punctum pygmaeum, Vertigo pygmaea, Vallonia pulchella,

6 Fig. 3. Simplified mollusk diagram of the Ruisseau de l Amourette against depth. The abundance scale used is a logarithmic one applied for the multivariate analysis. The mollusk zones are from Puisségur in Gremmen et al. (1984). 326 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999)

7 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Fig. 4. Variation of the diversity index H 0 for the terrestrial assemblages plotted against depth. Mollusk zones and pollen biostratigraphy are from Gremmen et al. (1984). In temperate forest conditions, the values are generally varying above 3 (Rousseau et al., 1993). Trichia hispida, and Cochlicopa lubrica on the positive side (Table 1). These species mainly indicate open environment conditions. Species with high contributions on the negative side of the first axis correspond to Helicodonta obvoluta, Cochlostoma septemspirale, Discus rotundatus, Azeca goodalli, Cepaea sp., Aegopinella nitidula, Acicula polita, Aegopinella pura, Ena montana, andacanthinula aculeata (Table 1). This second set groups together species that require a forest cover, or at least shading from trees or bushes. Such discrimination is characteristic in the multivariate analysis of assemblages of land snails. However, in the particular case of the Ruisseau de l Amourette sequence, the open environment species do not include numerous so-called cold species, those that now live in the Arctic tundra, or at high elevations above the tree-line. The second axis discriminates on the positive side Vertigo angustior, Vallonia enniensis, Succinea putris, Vallonia pulchella, Clausilia pumila, Vertigo antivertigo, Carychium minimum, Cochlicopa lubrica, Euconulus fulvus, Zonitoides nitidus, Acicula polita, Vertigo genesii, Vitrina sp.(table1).thesedifferent species indicate moist conditions. Vertigo genesii that represents moist environments, also indicates cold conditions when associated with Columella columella or Pupilla alpicola. The negative side of the second axis groups Abida secale, Discus rotundatus, Cochlostoma septemspirale, Helicodonta obvoluta, Cepaea sp., Ena montana, Aegopinella nitidula, Aegopinella pura, Azeca goodalli, Achantinula aculeata. (Table 1). The highest contribution is due to Abida secale. All these taxa characterize dry environments. In the study of Quaternary snail assemblages

8 328 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Fig. 5. Correspondence analysis of the Ruisseau de l Amourette malacofaunas. Plot of the species on the first factor plane (axes 1 2). The arrows indicate how the species explain the variability of the general data set. Black arrows indicate the highest contributions while white arrows show high values. Determination of temperature and moisture gradients by 40º rotation of the factors. The codes of the species correspond to those indicated in Table 1. from northern France, Puisségur (1976) indicated that particular mollusk assemblages occurred during transitional times, i.e. Late Glacial, that were mainly characterized by substantial numbers of individuals of Abida secale. The distribution of the different species on the first factor plane (Fig. 5) shows that the forest or open-forest species are plotted together in the third quadrant (1 2 ). The other taxa indicating open environmental conditions are plotted following a fan shape with moist species on one hand opposed to dry ones on the other hand. The distribution of the mollusk assemblages in the correspondence analysis (Table 2) resembles that of the species (Fig. 6). However, it presents an L-shape, characteristic of the Guttman effect indicating a relationship between the two axes. Such a distribution has already been described for other mollusk sequences in which either extreme temperature or moisture conditions were lacking. Indeed, when all the boundary conditions are represented among the mollusk assemblages of a given fossil sequence, the first axis opposes cold (i.e. arctic or steppe environments) and temperate (i.e. forests) conditions, while the second axis opposes xeric (i.e. dry grasslands) and damp (i.e. swamp) conditions (Rousseau, 1987). Thus, the lack of one boundary condition leads to the interpretation of the multivariate analysis in terms of climatic gradients, both temperature and moisture after a rotation of the first two axes. We took the forest pole as a reference point for the rotation by 40º because it is best expressed in the analysis (Fig. 6). Species characteristic of forest environment are also well grouped on the third quadrant (1 2 ). Subsequently, we calculated the coordinates of the assemblages (Table 2) and by taking into account both their stratigraphic position and diversity index value, we extracted two relationships, one representing temperature changes and the other the moisture trend. The temperature label

9 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Fig. 6. Correspondence analysis of the Ruisseau de l Amourette malacofaunas. Plot of the mollusk assemblages on the first factor plane (axes 1 2). Determination of temperature and moisture gradients by 40º rotation of the factors. is used better as an indication of climatic changes as the mollusk trend in this case has been shown to compare well to the marine δ 18 O record (Rousseau and Puisségur, 1990). The interpreted temperature values show marked variations from temperate to cold conditions (Fig. 7). The lower part of the sequence (between 46 and 38 m; mollusk zones A, B and C) represents cold conditions, which would agree with the assumption of its late Riss age. The Eemian interval (between 38 and 28 m; mollusk zone D) shows overall temperate conditions which are interrupted by two cool episodes named C1 and C2 (Fig. 7). These cool intervals are sufficiently marked to be considered in the discussion. They were named C for cool and numbered 1 and 2 according to their occurrence in the chronology of the sequence. They apparently are not related to any other record because of the lack of reliable absolute dates. The following interval (between 28 and 14 m; mollusk zones E and F) indicates cold conditions with minor fluctuations. Towards the top a warming trend appears. This is in agreement with the interpretation of the pollen samples of this interval as an equivalent of Melisey I according to the pollen stratigraphy, and corresponds to isotope Substage 5d. The interval from 14 to 5 m depth, mollusk zone G, shows marked variations, ranging from cold to temperate, similar to the Eemian mollusk zone D. The temperate conditions are preceded and followed by cold and cool peaks. This mollusk zone was interpreted as corresponding to the equivalent of St Germain I in the pollen Grande Pile stratigraphy or marine isotope Sub-stage 5c. The uppermost mollusk zone H, between 4 and the top of the sequence, once again shows particularly temperate conditions similar in magnitude to those indicated during the mollusk zones D (Eemian) and G (St Germain I). In the earlier biostratigraphical study, Puisségur (Gremmen et al., 1984) interpreted this zone as being the mollusk equivalent of the Grande Pile St Germain II. The gravels separating zones G and H would correspond in this case to marine Sub-stage 5b (or to

10 330 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Table 1 Correspondence analysis of the malacological samples from the Ruisseau de l Amourette Species Code Factor 1 Factor 2 species Acicula polita s Carychium minimum s Carychium tridentatum s005 Succinea oblonga s Succinea putris s Azeca goodalli s Cochlicopa lubrica s Vertigo antivertigo s Vertigo substriata s Vertigo pygmaea s Vertigo moulinsiana s025 Vertigo genesii s Vertigo angustior s Orcula doliolum s030 Abida secale s Pupilla muscorum s Vallonia costata s Vallonia pulchella s Vallonia enniensis s Acanthinula aculeata s Ena montana s Ena obscura s046 Punctum pygmaeum s Discus rotundatus s Vitrina sp. s Vitrea subrimata s Vitrea contracta s Nesovitrea hammonis s058 Aegopinella pura s Aegopinella nitidula s Oxychilus sp. s067 Zonitoides nitidus s Euconulus fulvus s Clausilia parvula s Clausilia bidentata s080 Clausilia pimila s Perforatella bidentata s Trichia hispida s Euomphalia strigella s Helicodonta obvoluta s Arianta arbustorum s101 Helicigona lapicida s Capaea sp. s Cochlostoma septemspirale s Limax sp. s136 Monacha sp. s Percentage of variance Significant contributions (higher than the theoretical threshold D 1=46) of species to the explanation of the variability of the data set according to the first two factors. Positive and negative signs indicate the location on the axes. Codes of the species used in the factor plane diagram. the Melisey II interval in the Grande Pile chronology). The moisture variations in biozones A, B and C relate to dry conditions with minor fluctuations (Fig. 7). On the contrary, mollusk zone D, of Eemian age, shows a moister environment. However, one can note that the two cool episodes recognized in the temperature curve are different. The first one, C1, is drier than the second one, C2, which presents the moistest values of the sequence (Fig. 7). The following cold interval, mollusk zones E and F, suggests the driest conditions of the sequence with the occurrence of mollusks characteristic of steppe conditions. This is in agreement with the general interpretation of marine Sub-stage 5d. The second part of this interval sees a return to intermediate (from a statistical point of view, i.e., centre of the axis) conditions. The temperate intervals corresponding to mollusk zones G and H are mainly corresponding to moist conditions presenting some minor fluctuations. 4. Discussion The results of the multivariate analysis support the biostratigraphic interpretation of the mollusk assemblages. Biozones A, B, and C correspond to the upper part of the penultimate glaciation. Mollusk zone D is the mollusk equivalent of the Eemian according to pollen data, or marine isotope Sub-stage 5e. Mollusk zones E and F are indicative of cold conditions and are interpreted as the mollusk equivalent of pollen zone Melisey I, or marine isotope Substage 5d. Mollusk zone G is the mollusk equivalent of the St Germain I or Sub-stage 5c. Finding a mollusk sequence which relates to the pollen stratigraphy during marine isotope Stage 5 is of great importance as no similar record has been found in western Europe. However, mollusk data render additional information of interest. First, the Eemian interval, which was recognized also by the pollen content (Gremmen et al., 1984), shows a particular and complex trend with the occurrence of two cool episodes with a different climatic signature. This is also expressed in the pollen diagram through the percentages of arboreal taxa. C1 is expressed by a decrease in Alnus and Quercus and the appearance of Abies whereas C2 shows a strong

11 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Fig. 7. Time series of mollusk assemblages from the Ruisseau de l Amourette sequence on the temperature and moisture gradients against depth. The values correspond to the variability of the general data set along the gradients. C1 and C2 indicate the occurrence of the two recognized cool Eemian events. decrease in Alnus, Quercus, Corylus, Carpinus by high values of Abies (Fig. 2). The first one, C1, at the base of the interval is cool and dry. The second peak, C2, is cool and moist. The recognition of these two cold episodes is of particular importance as similar events were described from the GRIP Greenland icecore as cold events, and named 5e4 and 5e2 (GRIP members, 1993). New analyses of the CH4 record of the GRIP ice-core indicate that these events are related to older ice inserted into the Eemian interval, implying that these cold spells are invalid (Chappellaz et al., 1997). The discussion nevertheless remains open as North Atlantic DSDP 609 (Broecker et al., 1990), NA87-25 (Cortijo et al., 1994), Norwegian Sea V27-60 (Cortijo et al., 1994) also indicate one or two cooling events during Sub-stage 5e in the North Atlantic. Lauritzen (1995) also described an unstable isotope record from Stage 5e from Norwegian speleothems. He recognized two sharp variations in the isotopic signal that he interpreted as related to the GRIP Eemian events. In northern Denmark, Seidenkrantz and Knudsen (1997) indicate the evidence of two major environmental and hydrological changes during the Eemian from both benthic foraminifera and stable isotopes. Southwards, Thouveny et al. (1994) also indicate the occurrence of two environmental variations during the Eemian in the French Massif Central. They are both recorded in the magnetic susceptibility and in the pollen signals and are interpreted as related to the GRIP record. Finally, when calibrating the pollen record of La Grande Pile sequence with both the beetles and the organic matter studied in parallel, Guiot et al. (1993) indicated the occurrence of a cold spell in the early Eemian temperature estimates by including Taxus in the considered taxa for transfer function. Moreover, the deciduous percentages during the Eemian at La Grande Pile indicate a first cold spell in the lower part due to the development of the Taxus forest. This is in agreement with the last temperature reconstruction by Guiot et al. (1993). On top of the Grande Pile Eemian, the deciduous trees drastically and quickly recede (Woillard, 1979) during a second cold and rapid event described by Woillard and known in the literature as the Woillard event (Kukla et al., 1997). It is not the purpose of this paper to discuss the reliability of the Greenland ice-core and the other marine and western European continental records which indicate cold events during the Eemian. However, if these events did in fact occur, it is truly

12 332 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Table 2 Correspondence analysis of the malacological samples from the Ruisseau de l Amourette Sample Depth (m) CTR1 CTR2 F1 F2 rf1 rf2 M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M R R R R R R R

13 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) Table 2 (continued) Sample Depth (m) CTR1 CTR2 F1 F2 rf1 rf2 R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R Significant contributions (higher than the theoretical threshold D 1=98) of the terrestrial assemblages to the explanation of the variability of the data set according to the first two factors (CTR1, CTR2). Positive and negative signs indicate the location on the axes. Coordinates of the assemblages on the first two factors (F1, F2), and on the two climatic gradients (rf1, rf2) after a rotation of 40º of the axes.

14 334 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) amazing that they would seem to have a reduced distribution limited to the North Atlantic and western Europe. Recent investigations from the Nordic Seas (Norwegian, Iceland and Greenland seas) by Fronval and Jansen (1997) support such interpretation indicating that the sea-surface temperature of the Norwegian Iceland Sea was less stable during the Eemian interglacial indicating marked shifts. Concerning the Ruisseau de l Amourette record, the lack of species indicating cold environments, i.e. Columella columella, Vertigo genesii, Pupilla alpicola, which occur presently at high elevations in the Alps (Kerney and Cameron, 1979), leads us to better interpret the cool events recognized in the Alps as regional variations. Such interpretation is reinforced by the lack of any physical dates and would agree with the Southern Alps significance of the pollen diagram as noticed by de Beaulieu et al. (1992). Recent reconstructions from mollusk assemblages in a British late glacial-holocene sequence also indicated a significant cooling event although the typical cold indicator species did not occur (Rousseau et al., 1998). In that case, as in the Amourette sequence, the general composition of the assemblage and the ratios between the different species counts contribute to such reconstructions. Then the placing of these particular events within the chronology of the interglacial needs to be improved. The present results allow only hypothetical correlations with similar events reported elsewhere as it was proposed for those characterized at the Lac du Bouchet (Thouveny et al., 1994). The bipartition of the Melisey I interval is interesting because it resembles the succession described from the GRIP Greenland δ 18 O record (GRIP members, 1993). In that case, however, mollusk zone E would last three times longer than zone F. The fact that the lower part of mollusk zone E is colder than the upper part concurs with both marine and ice-core isotope studies. Not surprisingly, Vertigo genesii, an Arctic indicator species (Kerney et al., 1983), only occurs during that peculiar interval. Mollusk zone G shows a simpler pattern of a temperate episode bracketed by cold and cool intervals. This also agrees with the classical interpretation of marine isotope Sub-stage 5c. Finally the marked warming at the end of mollusk zone H is in agreement with the climatic trend expressed at the base of marine isotope Stage 5a. 5. Conclusions The multivariate analysis of the mollusk record from the Ruisseau de l Amourette sequence indicates several major oscillations, in agreement with the pollen record, that do not correspond to local changes. However, during isotope Sub-stage 5e, two cool events are characterized. The moisture analysis shows that these two cool Eemian events were different, with the first one dry and the second moister. These events could be interpreted as the equivalent of cold events recognized in other continental sequences corresponding to different locations: in the GRIP ice-core and in western European records. The lack of any cold indicator species during these particular events suggests that may reflect local environmental fluctuations. Despite the lack of any absolute time scale, the Ruisseau de l Amourette section nevertheless shows a climatic sequence which is in agreement with the general interpretation of marine isotope Stage 5. Similar events have been recorded in both high and middle latitudes in Europe, from different environmental contexts and biological and non-biological proxy data. This suggests that these new results from mollusk assemblages are mainly responses to changes in the macro-climate over Europe during the past 130,000 years and subsequently further investigations are needed in this region. Acknowledgements We would like to thank V. Lozek, R. Preece and an anonymous reviewer for comments and criticisms about this paper, and Sonia and Greg Hamilton for improving the English. This work was supported by the CEC-EPOCH programme and an NSF-CNRS fellowship (DDR). This is ISEM (Institut des Sciences de l Evolution de Montpellier) contribution Appendix A Coded values used for the correspondence analysis. Code of the assemblages and depth (in metres) m1: Acicula polita; m24: Discus rotundatus; m2: Carychium minimum; m25: Vitrina sp.;

15 D.-D. Rousseau, J.-J. Puisségur / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) m3: C. tridentatum; m26: Vitrea subrimata; m4: Succinea oblonga; m27: Vitrina contracta; m5: S. putris; m28: Nesovitrea hammonis; m6: Azeca goodalli; m29: Aegopinella pura; m7: Cochlicopa lubrica; m30: A. nitidula; m8: Vertigo antivertigo; m31: Oxychilus sp.; m9: V. substriata; m32: Zonitoides nitidus; m10: V. pygmaea; m33: Euconulus fulvus; m11: V. moulinsiana; m34: Clausilia parvula; m12: V. genesii; m35: Cl. bidentata; m13: V. angustior; m36: Cl pumila; m14: Orcula doliolum; m37: Perforatella bidentata; m15: Abida secale; m38: Trichia hispida; m16: Pupilla muscorum; m39: Euomphalia strigella; m17: Vallonia costata; m40: Helicodonta obvoluta; m18: V. pulchella; m41: Arianta arbustorum; m19: V. enniensis; m42: Helicigona lapicida; m20: Acanthinula aculeata; m43: Cepaea sp.; m21: Ena montana; m44: Cochlostoma septemspirale; m22: E. obscura; m45: Limax sp.; m23: Punctum pygmaeum; m46: Monacha sp. 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