Climatic variations in MIS 11 recorded by stable isotopes and trace elements in a French tufa (La Celle, Seine Valley)

Transcription

1 JOURNAL OF QUATERNARY SCIENCE (2012) 27(8) ISSN DOI: /jqs.2567 Climatic variations in MIS 11 recorded by stable isotopes and trace elements in a French tufa (La Celle, Seine Valley) JULIE DABKOWSKI, 1,2 * NICOLE LIMONDIN-LOZOUET, 2 PIERRE ANTOINE, 2 JULIAN ANDREWS, 3 ALINA MARCA-BELL 3 and VINCENT ROBERT 2 1 Département de Préhistoire du Muséum National d Histoire Naturelle (UMR CNRS 7194), Paris, France 2 Laboratoire de Géographie Physique (UMR CNRS 8591), Meudon, France 3 School of Environmental Sciences, University of East Anglia, Norwich, UK Received 17 February 2012; Revised 12 June 2012; Accepted 14 June 2012 ABSTRACT: Marine Isotope Stage (MIS) 11 palaeoclimate has so far been documented in marine and ice sheet isotopic records. However, excepting some lacustrine pollen records, very little is known about palaeoclimatic conditions in continental areas. This study uses geochemical records in calcareous tufa deposits from rivers as a basis for reconstructing temperate palaeoclimatic conditions. Tufa deposits are now proven to record high-quality palaeoclimatic information in recent to Holocene deposits. Work on older interglacial tufas is just starting and in this paper we present the first comprehensive results from a MIS 11 tufa. The tufa comes from the Seine Valley (La Celle, northern France). Geochemical data in the tufa calcite are interpreted to record primarily air temperature (d 18 O) and humidity (d 13 C and Mg/Ca and Sr/Ca). The combined data identify a warm and wet climatic optimum followed by two temperature decreases associated with oscillations in humidity. These marked climatic variations recorded through the La Celle profile are strongly coherent with the palaeoenvironmental reconstructions from malacological data. The abrupt climatic and environmental events recorded could be related to short-term degradation of vegetation cover in Europe, which is itself controlled by global palaeoclimatic events. Copyright # 2012 John Wiley & Sons, Ltd. KEYWORDS: calcareous tufa; stable isotopes; trace elements; MIS 11; palaeoclimate. Introduction Geochemical data from terrestrial carbonates such as lake deposits or speleothems (e.g. Fontes et al., 1996; Xia et al., 1997; Von Grafenstein et al., 1999) are now widely used to reconstruct palaeoclimatic conditions. Tufa deposits that result from calcite precipitation under open-air conditions in streams, rivers and lake shorelines are part of this family of terrestrial carbonates, and their mineralogy (typically >95% CaCO 3 ) makes them highly suitable archives of stable isotope (d 18 O and d 13 C) and trace element (Mg, Ca) information, from which palaeoclimatic conditions can be inferred. Recently, scientific interest in Pleistocene tufas as palaeoclimatic archives has increased (Antoine et al., 2007; Ashton et al., 2008; Domínguez-Villar et al., 2011; Martín-Algarra et al., 2003; Ortiz et al., 2009; Preece et al., 2007) but, to date, most of the geochemical work on tufas has been on recent and Holocene aged material (see reviews by Andrews and Brasier, 2005; Andrews, 2006), with just a small number of studies focused on older material, principally from the last interglacial (Brasier et al., 2010; Dabkowski et al., 2011). In this paper we present the first results from a Marine Isotope Stage (MIS) 11 tufa from the Seine Valley (La Celle, northern France). There is much current palaeoclimatic research focus on MIS 11 because of its similarity, in terms of Milankovitch orbital configuration, to the Holocene and thus its potential application in understanding future climate states of the Earth (Droxler and Farrell, 2000). MIS 11 is defined in marine records as the period between 424 and 362 ka when ice volume was low (Imbrie et al., 1984; Lisiecki and Raymo, 2005). The warmest part of MIS 11 was during the first ka an exceptionally long interglacial with a late thermal optimum around ka when compared to preceding temperate periods (Droxler et al., 2003; EPICA Community Members, *Correspondence: J. Dabkowski, 2 Laboratoire de Géographie Physique, as above. dabkowski@mnhn.fr 2004; Jouzel et al., 2007; McManus et al., 1999; Raynaud et al., 2003). Various palynological records from lakes and peat bogs in central and northwestern Europe (e.g. Le Velay in southern France and Dethlingen in northern Germany) suggest that temperate forest developed in Europe between 415 and 400 ka (Desprat et al., 2007; Koutsodendris et al., 2010), broadly corroborating the marine thermal optimum; however, excepting these lacustrine pollen data, very little else is known about MIS 11 climate in continental areas (Rousseau, 2003). A detailed study of La Celle tufa deposit from northern France is thus an excellent opportunity to contribute new data to the MIS 11 palaeoclimatic record in northwestern Europe. We here discuss stable isotope data, supported by trace element analyses which can be interpreted in terms of relative changes in palaeotemperature and palaeorainfall. These interpretations can be assessed in the context of the wider palaeoenvironmental data from mollusc assemblages. We can then speculate on how palaeoclimatic variations recorded in La Celle tufa might fit within the framework of wider global climatic events already proposed for this time period. Study materials and site La Celle tufa is located in northern France (Fig. 1A), on the righthand valley slope of the Seine River, 2.5 km upstream from its confluence with the Loing River ( N, E). In its lower part the tufa lies on fluvial deposits of a middle terrace of the Seine (units 16 and 15, Fig. 1B), about 15 m above the modern alluvial plain: its upper part directly overlies calcareous Eocene bedrock (unit 17). La Celle tufa formed from springs located upslope and accumulated as a body 8 15 m thick covering an area of 12.5 ha (Tournouër, 1877; Bourdier, 1969). New investigations undertaken at La Celle since 2003 have focused on stratigraphy, palaeontology, dating and archaeology and have concentrated on the western part of an old quarry where tufa appears best preserved. Chronological attribution of the tufa to MIS 11 was previously based on its Copyright ß 2012 John Wiley & Sons, Ltd.

2 CLIMATIC VARIATIONS IN MIS 11 IN A FRENCH TUFA 791 Figure 1. (A) Location map of La Celle tufa. (B) La Celle Long profile : stratigraphy. (C) Site map of La Celle with location of the 2003 surveys (along the old face of the quarry) and 2005 boreholes, demonstrating the maximum westward extent of the tufa, and of the Long profile along the slope. (D) Detailed stratigraphical profile of column G1 sampled for geochemistry from the La Celle Long profile (D ). (E) Details of La Celle sequence at about m along the Long profile. This figure is available in colour online at wileyonlinelibrary.com/journal/jqs. Copyright ß 2012 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 27(8) (2012)

3 792 JOURNAL OF QUATERNARY SCIENCE morphological position within the Seine valley terrace system (Lautridou et al., 1999). This age has since been reinforced by new palaeontological evidence (occurrence of the malacological Lyrodiscus assemblage ) and confirmed by several radiometric dates (U/Th thermal ionization mass spectrometry on calcite and electron spin resonance U/Th on tooth enamel) that suggest an age around 400 ka (Limondin-Lozouet et al., 2006; Bahain et al., 2010). The malacological succession has allowed detailed palaeoenvironmental reconstruction and shows a progressive development of forest biotopes (Limondin-Lozouet et al., 2006). Identification of temperate conditions is strengthened by the occurrence of leaf and fruit prints of Mediterranean taxa (Buxus, Ficus) together with records of several thermophile mammal species including Hippopotamus and Macaca. The latter were found associated with Acheulean lithic artefacts that demonstrate human occupation of the site during the climatic optimum (Limondin-Lozouet et al., 2010). We sampled La Celle tufa in column G1 located 12 m along the northnorthwest edge of the Long section (details in Limondin-Lozouet et al., 2010), where most of the units are present (Fig. 1B). The coarsest layers could not be sampled (unit 9, top of unit 6 and bottom of unit 2). At the bottom of the sequence, the transition between unit 15 (greenish detrital silt) and unit 14 (tufa silt) was not well defined and samples from this part of the column may actually belong to the underlying alluvial deposits. In total, 117 samples were taken every 5 cm through 6.30 m of vertical section (Fig. 1D, D 0 ). Stratigraphy The lithology of the tufa is variably silty to sandy, with indurated layers, coarse-grained levels and fine, greyish, carbonaceous horizons (Fig. 1B D). Column G1 comprises 10 stratigraphic units (Fig. 1D, E). At the base a white, fine, silty unit 14 is directly overlain by homogeneous dense grey silty to sandy tufa of unit 12. This unit is laterally variable and corresponds at this location to a greenish tufa including abundant shell fragments (unit 12b in Limondin-Lozouet et al., 2010). Unit 11 is a light orange-brown sandy tufa which coarsens upward. Unit 9 is composed of light-yellow, sandy-silty tufa with abundant indurated tufa blocks (Fig. 1E; unit 9b in Limondin-Lozouet et al., 2010). Downslope, unit 9 is associated with large fragments of tufa and in situ bioconstructions, included in the light-orange silty tufa of unit 10 (Fig. 1B). Unit 8 is a light-yellow to white, fine, sandy-granular tufa. Overall these facies suggest an in-place tufa deposit formed under slow to moderate stream flow, downstream from the feeding springs (Casanova, 1981). Unit 7 is a fine, sandy, greyish horizon containing burned tufa blocks and flints, and detrital components (mainly quartz and fragments of tufa and mollusc shells). It is associated with a prominent block level at its base, where an erosion surface cuts into the underlying units 8 and 9 (Fig. 1B). This level indicates a local fire event which temporarily destroyed vegetation and initiated movement of bedrock limestone and indurated tufa blocks by colluvial processes (Limondin-Lozouet et al., 2010). Unit 6 also contains a detrital component that records evidence of colluviation up to the top of the unit. Unit 3 is composed of light-brown to orange layers of silty tufa including detrital material reworked from the slope. It contains meandering channel deposits that suggest more dynamic fluvial environments than in units Unit 3 is truncated by another block level, which forms the base of unit 2. This last is similar to unit 7, again with evidence of fire at the top of the slope (Limondin-Lozouet et al., 2010). Finally, unit 1 is a light-yellow, soft tufa with discontinuous bedding including fine to sandy-granular layers that indicate deposition close to low flow springs. This final tufa unit is overlain by the modern soil (unit 0). Overview of the malacological record The mollusc assemblages from La Celle have provided a highquality, nearly continuous record of palaeoenvironmental variations throughout the sequence (Limondin-Lozouet et al., 2006). Malacological data from two profiles (S2 and S5) excavated in 2003 (Figs. 1C and 2; Limondin-Lozouet et al., 2006) can be correlated with column G1 on the basis of stratigraphical similarities. Their molluscan succession reveals four episodes of landscape evolution described by mollusc zones (Fig. 2). The earliest malacofaunas at the base of the tufa are similar to pioneer assemblages that recolonized valley bottoms in northern France at the beginning of the Lateglacial interstadial (Limondin-Lozouet, 2011). They indicate a marshy open ground and correspond to the first stage of malacological communities that develop when climatic conditions improve. The second mollusc zone is characterised by first occurrence and rapid development of shade-loving species, under interglacial conditions. These assemblages reflect establishment of forest biotopes together with wet open-ground habitats. Forest expansion increases upward and maximum development of woodland molluscan communities occurs in unit 7 (Fig. 2). This climatic and environmental optimum is part of the third mollusc zone characterized by highest numbers of forest species. However, the number of shade-loving molluscs declines quickly after unit 7, while hygrophilous gastropods are much less common than in the previous zone. These observations suggest permanence of forest habitats but under drier conditions, which is somewhat unusual as forest biotopes are expected to be stable and should result in diversified and abundant malacological populations. However, this unstable episode may be linked with colluvial activity that undoubtedly disrupted the record. In the last mollusc zone, correlative of unit 1, a strong change affected mollusc communities (Fig. 2). This abrupt transition was probably linked to a pause in sedimentation at the bottom of unit 1. Most of the previous forest species vanish, indicating a decline of wooded environment, while two taxa, representative of humid forest today, appear for the first time. In addition, several gastropods typical of wet and marshy habitats appear again. Comparison with the early Holocene biological records demonstrates strong correlation between forest evolution shown by pollen data and occurrence of specific mollusc species (Limondin-Lozouet, 2011). The very peculiar zone 4 fauna might well correspond to the spread of a wet coniferous forest environment, as emphasized in Pleistocene vegetation cycles established from palynological data (Van der Hammen et al., 1971). Methods Screening for diagenetic alteration Before geochemical analysis the tufa was examined in impregnated thin section for evidence of diagenetic alteration. All units contain microbial micritic fabrics, either clotted or around the clasts. Most of the tufa is attributable to cyanobacteria and microalgae that can be taxonomically determined from crystal morphology (Freytet, 1997). Broutinella sp./plaziatella sp., associated with Koeniguerella sequanensis (Freytet, 1997, 1998) are predominant, but all units also contain Wallnerella fascinans bioconstructions, which are believed to represent an intensive stage of Oocardium crystal diagenesis (Freytet, 1998). There is no evidence of sparry calcite cementation of void space except in unit 3, where crusts of fungal-related needle-fibre calcite (Verrecchia and

4 CLIMATIC VARIATIONS IN MIS 11 IN A FRENCH TUFA 793 Figure 2. Synthetic record of La Celle malacological succession from selected profiles. Numbers labelling stratigraphical levels of profile S5 are similar to those used for the isotopic record (Fig. 1). (A) Curves of number of species and number of shells belonging to molluscs of shaded habitats. These curves show progressive expansion of the forest with identification of an optimum episode within level 7, followed by a strong decline of forest mollusc populations. (B) Percentages of present ecological groups show development of wet biotopes at both the bottom (zone 1) and the top (zone 4) of the sequence. Highest extension of open-ground molluscs is recorded during zone 3 after the climatic optimum.

5 794 JOURNAL OF QUATERNARY SCIENCE Figure 3. Oxygen and carbon stable isotopes from La Celle tufa regarding the stratigraphical profile (see Fig. 1 for keys). Verrecchia, 1994) are present around the pore edges: these do not constitute a significant mass of calcite. The observed diagenetic fabrics are mostly syndepositional, forming under similar environmental and climatic conditions as the tufa precipitates. For these reasons, we believe the La Celle tufa geochemical data largely record palaeoenvironmental signals preserved at the time of tufa formation. Stable Isotopes Approximately 1 g of each sample was sifted to <250 mm and then cleaned of volatile organic matter by low temperature (<808C) oxygen plasma etching for 6 h at 300 W forward power in a Bio-Rad PT 7300 plasma barrel etcher. Stable isotope analyses were performed on CO 2 derived from mg plasma ashed sample reacted with anhydrous H 3 PO 4 at 908C. Isotope ratios were measured on a Europa Sigma Hybrid, with an in-house automatic sampler at the Stable Isotope Laboratory of the University of East Anglia (UK). Replicate analyses of the laboratory standard (n ¼ 75) gave 2s precision of 0.09% for d 18 O and 0.3% for d 13 C. These data are plotted in Fig. 3. Trace Elements About 500 mg sifted tufa was dissolved in 25 ml of 10% acetic acid for 1 h. After filtration, the acid was diluted with ultrapure water up to 100 ml. Sr, Mg and Ca concentrations were measured on these solution on a fast sequential atomic absorption spectrometer (AA240FS, Varian), at the Laboratoire de Géographie Physique, CNRS Meudon (France). Confidence intervals were calculated according to the ISO standard for each element and then converted (with error propagation) to Mg/Ca and Sr/Ca. Ratios that fell outside confidence limits after error propagation were rejected, explaining the data gaps in Fig. 4. Results Complete geochemical data (stable isotopes and trace elements) are provided online (Supporting information, Table S1). Stable Isotopes A summary of the stable isotope and trace element data are given in Table 1 and shown in stratigraphic order in Fig. 3. The d 18 O values range between 5.79 and 4.32% (mean value: %, n ¼ 116; Table 1) whereas d 13 C values range between 12.1 and 7.9% (mean values: %, n ¼ 116; Table 1). These d 18 O and d 13 C means and ranges are consistent with those from late Quaternary temperate tufas from northwestern Europe, where continentality and aridity/ evaporation effects are low (Andrews, 2006). The first four values at the bottom of the sequence were excluded from our interpretations because their exact relationship with the underlying alluvium is not clear (see above). The remaining d 18 O profile (Fig. 3) is clearly separable into three parts that concur with sedimentary unit boundaries. In the lower part (units 12 7), the highest d 18 O values, around 4.8%, are registered (Fig. 3 part A and Table 1). At the boundary between units 7 and 6, a clear decrease to values around 5.0% occurs and values remain at this level throughout part B (Fig. 3) until close to the boundary of units 2 and 1. The lowest values (< 5.2%) are recorded within unit

6 CLIMATIC VARIATIONS IN MIS 11 IN A FRENCH TUFA 795 Figure 4. Carbon stable isotopes and trace elements as Mg/ Ca and Sr/Ca ratios from La Celle tufa regarding the stratigraphical profile (see Fig. 1 for keys). 1 (part C). The clear decrease in d 18 O values are highlighted by the mean values for each part (from 4.77 (n ¼ 48) to 5.03 (n ¼ 37) to % (n ¼ 27) as shown in Table 1). In part A, d 18 O values are quite constant, with a standard deviation (s) of 0.15%. Amplitude of variation is higher in units 7 and 8 where maximal values are recorded (up to 4.32%; Table 1). In part B, d 18 O values are also quite constant (s ¼ 0.12%; Table 1). Part C shows a clear variation in d 18 O with a decrease/ increase/decrease succession upward. The lowest d 18 O values of the whole sequence are registered at the top of unit 1 ( 5.72%; Table 1). The d 13 C profile (Fig. 3) mainly shows stratigraphical relationships similar to those observed for d 18 O, the exception being part A, where three distinct subsections (A1 A3, Fig. 3) are distinguishable. In part A1 (units 14 and 12), d 13 C values show a dramatic upward decrease of about 2% from 8%. In part A2 (units 11 and 8), d 13 C values are around 10%, overall higher than the mean d 13 C value for the whole sequence (Table 1). Part A3 (unit 7) has the lowest values in part A, 11.5%. In part B, d 13 C values are similar to those in part A2, while the lowest values are recorded in part C. Overall d 13 C values are very constant in units 11 and 8 (s ¼ 0.1%) and also in units 7 (s ¼ 0.2%) and unit 1 (s ¼ 0.3%), but much more variable, with clear spikes in units 3 and 2 in particular. Trace Elements Concentrations of Mg and Sr range between 130 and 622 ppm and 34 and 181 ppm respectively (Table 1) and are plotted in stratigraphical order as Mg/Ca and Sr/Ca molar ratios in Fig. 4. In unit 12, Mg/Ca ratios show a trend to lower values, whereas the Sr/Ca values show a contrasting (increase/decrease) succession. However, from unit 11 to the top of the sequence, the trace element profiles exhibit similar patterns of variation. Both ratios are quite constant in units 11 and 8 around the following partial mean values: 20 þ5 / (s ¼ ; n ¼ 22) and (s ¼ ; n ¼ 21) respectively. The ratios decrease abruptly at the top of unit 8. Within unit 7 and the first half of unit 6, Mg/Ca and Sr/Ca partial mean values are 13 þ4 / (n ¼ 15) and (n ¼ 11). Ratios remain quite constant (s ¼ and ) up to the second part of unit 6. From there to Table 1. La Celle stable isotope and trace element data summaries. d 18 O(% V-PDB) d 13 C(% V-PDB) Mg/Ca (10 4 ) Sr/Ca (10 4 ) No. of samples (n) 116 (48/37/27) 116 (36/12/37/27) Range of values 5.79 to to to to 2.7 Mean of values 5.00 ( 4.77/ 5.03/ 5.43) 10.1 ( 9.9/ 11.0/ 9.8/ 11.7) 16 þ4 / Precision Max: þ6 / 3 Max: þ0.2 / 0.2

7 796 JOURNAL OF QUATERNARY SCIENCE the bottom of unit 3 (subunit 3d 0 ), Mg/Ca and Sr/Ca ratios increase steadily to reach maximal values of 26 þ6 / and 2.7 þ0.1 / (Table 1). In the following units 3b and 2, they decrease regularly towards their minimum in unit 1 (partial mean values: 7 þ2 / and (n ¼ 26; Table 1). Within this last stratigraphical level recorded values are very constant for both ratios (s < and s < ). Interpretation Oxygen Stable Isotopes Assuming isotopic equilibrium between calcite and its depositing water, as observed for recent northwestern European tufa deposits (Andrews et al., 1997; Garnett et al., 2004), longterm variations in tufa d 18 O are likely to have been caused mainly by (i) change in the water temperature at the time of calcite precipitation, and/or (ii) variation in the isotopic composition of the water from which tufa calcite precipitates. The first mechanism is described by the Craig s thermodynamic equation, where a 18C increase in water temperature results in a 0.24% change in d 18 O of the precipitating calcite (Craig, 1965). Changes in the isotopic composition of the tufaprecipitating water (mechanism ii above) is caused by changes in the mean annual isotopic composition of the meteoric water that recharges groundwater-fed springs (Clark and Fritz, 1997; Darling, 2004): in Europe, mean rainfall d 18 O is known to be strongly correlated with mean tufa calcite d 18 O (Andrews et al., 1997; Janssen, 2000). Isotopic composition of meteoric water in temperate regions at a given latitude and altitude is controlled mainly by air temperature change and the amount effect (Dansgaard, 1964). However, because of homogenisation of water in aquifers, groundwater isotopic values are usually close to the long-term mean value of rainfall (Darling, 2004); isotopically anomalous rainfall events are thus unlikely to be recorded in well-mixed groundwaters. Average air temperature and stream water temperatures are likely to be linked, and the respective isotopic effects of processes i and ii (above) oppose one another. However, it has been shown that in temperate settings the temperature of calcite formation instream typically dampens the stronger signal from water composition change; Andrews (2006) calculated that Holocene European tufa calcites probably recorded about 40% of the actual change in air temperature. This has been similarly demonstrated for lacustrine carbonate sediments (Eicher and Siegenthaler, 1976; Leng et al., 2006) and seems a reasonable assumption to use in the current study. We thus infer that the d 18 O signature of meteoric water (groundwater) feeding the La Celle springs from which the tufa calcite precipitated was most strongly influenced by air temperature in addition to some influence from rainout and source effects (Dansgaard, 1964; Rozanski et al., 1993). There is no evidence to suggest that the dominant source of air masses in the Seine valley during MIS 11 has been different from those of the present. Indeed the similarity of the La Celle tufa d 18 O values to those from other modern and Holocene northwestern European sites (Andrews, 2006) supports this assumption. If change in La Celle tufa d 18 O was controlled predominantly by air temperature changes, then trends toward higher d 18 O values would correspond to increasing temperatures (Andrews et al., 1994). On this basis, decreasing values in the d 18 O profile of La Celle should indicate cooling, which is most apparent in two steps: at the transition between parts A and B (significant decrease of 0.27% between respective partial mean values) and at the transition between parts B and C ( 0.40%; Table 1 and Fig. 3). The highest temperatures are recorded in part A, in units 8 and 7 (d 18 O maximum; Table 1 and Fig. 3). Carbon Stable Isotopes and Trace Elements Variations in tufa d 13 C principally reflect the relative contributions of carbon from sources that comprise the dissolved inorganic carbon (DIC) of the depositing waters (Janssen, 2000; Andrews, 2006). The main sources are (i) isotopically negative d 13 C derived from soil organic matter (Cerling et al., 1989; Amundson et al., 1998), (ii) dissolution of the aquifer marine limestone by groundwater (Hudson, 1977), in this case Cenozoic carbonate with d 13 C values close to 0 or slightly positive, and (iii) equilibration of the aquifer and spring water with atmospheric CO 2 (Usdowski et al., 1979). While a statistically positive relationship was found between d 13 C and d 18 O for the whole dataset (r ¼ 0.58; n ¼ 116; P-value 0.001), the situation is more complex when the data are analysed in each part (A C, Fig. 3). Data from parts A and B plot in similar isotopic space in Fig. 5. Part B data show no significant relationship between d 18 O and d 13 C, while the data from part A show a weak inverse correlation (r ¼ 0.57; n ¼ 48; P-value 0.001). If our d 18 O interpretation is correct, the warmer climatic phases (higher d 18 O values) appear to have been accompanied by a stronger component of isotopically light, soil-derived carbon. This interpretation is plausible since warmer periods would be related to more active plant growth and higher soil biological activity producing a strong component of isotopically light carbon. Variations of in the d 13 C record in part A may thus in part be interpreted as modifications of soil activity and plant growth caused by temperature variations; this, however, does not explain all the larger d 13 C variations in part B. Data from part C are distinct from the others (Fig. 5) and show a weakly positive relationship (r ¼ 0.42; n ¼ 27; P-value 0.05) between d 18 O and d 13 C. The variation in d 13 C is, however, rather small compared to parts A and B, and it is difficult to attach too much significance to the variation, other than to note that d 13 C does not appear to respond to the clear change in d 18 O in the middle of part C. While changes in isotopically negative soil carbon might be linked to climatic temperatures (see above), increased soil moisture from rainfall would probably have promoted soil Figure 5. Plotted d 18 O vs. d 13 C from La Celle tufa according to their stratigraphical position: parts A, B and C are defined according to both stable isotope profiles in Fig. 3 (see text for details).

8 CLIMATIC VARIATIONS IN MIS 11 IN A FRENCH TUFA 797 biological activity and plant growth also. Rainfall also influences in-aquifer processes, aquifer limestone dissolution being controlled in part by recharge characteristics. More importantly, during dry periods there is likely to be more exchange between groundwater DIC and atmospheric CO 2 in the vadose zone, which will encourage prior calcite precipitation (PCP; Fairchild et al., 2000). Both equilibration with atmospheric CO 2 and PCP lead to isotopically heavier remaining DIC values in the aquifer water which will be transferred to the precipitating tufa (Andrews, 2006). Dry conditions will also result in lowered soil zone activity and a decrease in the contribution of isotopically negative biological d 13 C (Andrews et al., 1993, 1997; Garnett et al., 2004). These in-aquifer processes are also controls on trace element concentrations in groundwater. Mg, Sr and Ca derive from limestone dissolution and various processes in the aquifer control their concentrations: (i) the higher dissolution rate of calcite compared to dolomite allows a possible increase of Mg/ Ca once groundwater is saturated with Ca (Chou et al., 1989; Fairchild et al., 2000); (ii) PCP, by degassing in air pockets, leads to Ca concentration decrease relative to Mg and Sr (Fairchild et al., 2000, 2006); and (iii) selective leaching of Mg and Sr with respect to Ca even from non-magnesian limestones (Fairchild et al., 1994). These processes are favoured by longer residence time and higher occurrence of air pockets during dry periods, leading to the Mg/Ca and Sr/Ca increase (Fairchild et al., 2000; Garnett et al., 2004). Accepting the discussion above, there should be a clear relationship between tufa trace element ratios and d 13 C (Garnett, 2003; Ihlenfeld et al., 2003). This is what we found: a statistically strong positive correlation between Mg/Ca and Sr/Ca molar ratios (r ¼ 0.95; n ¼ 73; P-value 0.001), resulting in quite similar profiles (Fig. 4), and a very strong sympathetic relationship between both of them and d 13 C(r Mg/Ca ¼ 0.92, n ¼ 96 and r Sr/Ca ¼ 0.91, n ¼ 87; P-value in both cases). These relationships suggest strongly that variations in palaeorainfall were a major control on La Celle tufa geochemistry. Moreover, the mollusc record supports the idea that palaeorainfall characteristics strongly influenced the tufa d 13 C and trace element variations. For example, the clear decrease in d 13 C values in part A3 correspond to the climatic optimum phase identified by maximum development of woodland molluscan communities. Similarly, in part C (unit 1), a possible rainfall maximum indicated by low d 13 C values is concordant with the new development of several hygrophilous mollusc species and the appearance of particular gastropods specific to wet woodland (Limondin-Lozouet et al., 2006; Dabkowski et al., 2011). Overall, the d 13 C and trace element profiles suggest increasing rainfall (humidity) at the base of the sequence (units 14 and 12, Fig. 4), followed by relative stability through units The earliest wet phase is recorded in unit 7. After a marked decrease of humidity in the upper part of unit 6, a general trend, clearer in the trace elements than in d 13 C, towards higher rainfall contribution then develops in units 3 and 2. Unit 1 appears to have been the wettest episode of the whole sequence. In overview the geochemical records of the tufa sequence at La Celle, can be interpreted to record mainly variations in temperature (d 18 O) and humidity/aridity (d 13 C and trace elements). A general cooling with time is associated with alternating dry/wet periods. In the lower part (part A), temperatures are relatively higher and humidity increases. The highest temperatures are associated with increased rainfall (units 8 and 7) and correspond to the climatic optimum, identified by the malacological fauna (Limondin-Lozouet et al., 2006; Dabkowski et al., 2011). At the transition between parts A and B (limit units 7 and 6), a cooling in temperatures is accompanied by drier conditions recorded in the whole of unit 6 up to the bottom of unit 3. After this apparently driest episode, increasing humidity is then recorded up to unit 1. At the top of the sequence, part C (unit 1) appears to record the highest rainfall intensity, accompanied by the coolest temperatures. Discussion: Integrating the Molluscan and Geochemical Records The four malacological zones shown in Fig. 2 (see also Limondin-Lozouet et al., 2006) define the likely broad landscape evolution at the site. At the base of the tufa, the pioneer assemblages of mollusc zone 1 indicate improving climatic conditions but supporting geochemical data are not currently available. In the second mollusc zone, assemblages are characterised by species indicating development of closed habitats with wet open ground, under interglacial conditions. The stable isotope and trace element interpretations indicate an increase in temperature and rainfall and appear thus in good agreement with the malacology. Forest expansion increases upward and all proxies identify an optimum phase during unit 7 with the highest temperatures and rainfall and maximum development of woodland molluscan communities (Figs. 2 and 3). Above unit 7, malacological assemblages suggest permanence of forest habitats but under drier conditions; however, the malacology and sedimentology also suggest disturbance. This interpretation is consistent with the geochemical proxies which register a clear decline in temperature and rainfall intensity at the transition between units 7 and 6 (Figs. 3 and 4). This unstable episode, recorded in geochemical part B and malacozone 3, may be linked with colluvial activity that undoubtedly disrupted geochemical and malacological signals. The dramatic change in mollusc communities at the transition with the last zone, correlative to unit 1, corresponds to a clear second step recorded in all the geochemical proxies (Figs. 3 and 4). Zone 4 assemblages reveal an environment of open forest and wet ground which is interpreted as the malacological response to the spread of wet coniferous forest (Fig. 2). The stable isotope and trace element data are again consistent, registering decreasing temperature and the highest intensity of palaeorainfall. Overall, our interpretations of the sedimentology, malacology and geochemistry are thus strongly concordant. Previous geochemical studies of calcareous tufa have mostly been made on Holocene deposits where robust chronology is possible through 14 C dating (Andrews et al., 2000; Garnett et al., 2004). Such chronology is not typically achievable for Middle Pleistocene tufas, which are typically too detritally contaminated to allow high-resolution U-Th dating, while other datable materials (bones, teeth, burnt flints), or clean tufa carbonate suitable for U/Th dating, are located in a few discrete stratigraphical levels. Moreover, the duration of the deposition of old tufas cannot reliably be inferred using estimates of calcite deposition rate as this is known to be very variable (usually around 1 3 mm a 1 but up to 5mma 1 when associated with vegetation; Janssen, 2000; Hoffmann, 2005; Pentecost, 2005), and because hiatuses are frequent and difficult to interpret in terms of time elapsed. At La Celle, the mollusc assemblages give some information to help position the tufa sequence in a likely chronological framework. Pioneer assemblages in the underlying silts and then the first occurrence and rapid development of shadeloving taxa at the bottom of the calcareous sequence are comparable with Last Interstadial and early Holocene assemblages (Limondin-Lozouet, 2011). Part A of La Celle sequence

9 798 JOURNAL OF QUATERNARY SCIENCE is thus inferred to represent devolvement of MIS 11 temperate forest known to start around 415 ka in Europe (Desprat et al., 2007; Koutsodendris et al., 2010). Increasing warm and wet conditions led to a climatic optimum recorded at La Celle both by molluscs and geochemical proxies. This optimum and especially the maximum of temperature in the unit 7 could be related to the MIS 11 thermal optimum which has been recorded around ka in marine and ice cores (EPICA Community Members, 2004; Imbrie et al., 1984; Jouzel et al., 2007; Lisiecki and Raymo, 2005) and is consistent with dates (around 400 ka) available at La Celle. Following the optimum, a general cooling is recorded at La Celle occurring in two main steps. These correspond to two major hiatuses in the stratigraphical succession, leading to important changes in mollusc communities and stepped geochemical profiles that may be linked to episodes of forest regression observed in high-resolution pollen sequences (Kukla, 2003; Müller, 1974). Such events could furthermore be related to abrupt changes in Northern Atlantic circulation as recorded in some marine cores (Bauch et al., 2000; Kandiano and Bauch, 2007; Kukla, 2003; Müller, 1974). Conclusion 1 Combined stable isotope (d 18 O and d 13 C) and trace element (Mg and Sr) data from the tufa of La Celle show a clear record of palaeoclimatic variations. d 18 O is interpreted mainly to record air temperature changes, which reaches a maximum in units 7 and 8. Significant cooling then follows to the top of the sequence. Trace elements and d 13 C are interpreted to be mainly controlled by rainfall (humidity) and record alternations of dry and wet periods. The most humid periods correspond with the highest reconstructed temperatures. 2 A strong statistical relationship between tufa trace elements ratios and d 13 C is observed, resulting in similar shaped profiles. Combining trace element data with d 13 C makes interpretations more secure, in this case the variations probably being controlled by humidity (palaeorainfall). 3 Stable isotope and trace elements data from the MIS 11 La Celle tufa are strongly concordant with palaeoenvironmental reconstructions from sedimentology and mollusc assemblages. Abrupt climatic and environmental events at La Celle may be related to short-term degradation of the vegetation, itself linked to global drivers such as abrupt modification of the North Atlantic circulation. Supporting Information Additional supporting information can be found in the online version of this article: Table S1 La Celle complete geochemical data. Acknowledgements. The first author thanks Christine Hatté and Dominique Genty from the LSCE (CNRS-CEA, Gif-sur-Yvette, France) for their advice. This work is a contribution to the SITEP project (Signature climatique des Interglaciaires dans les Tufs Européens, réponse des Environnements et impact sur le Peuplement paléolithique Programme CNRS-INSU: Eclipse II). We are grateful to the Mairie de Vernou-La-Celle and the Conseil Général de Seine-et- Marne for help in organizing and funding fieldwork. Paul Dennis is thanked for his help and support in the UEA stable isotope laboratory. The authors thank Enrico Capezzuoli and Alexander Brasier for their constructive comments, which improved the quality of this manuscript. Abbreviations. DIC, dissolved inorganic carbon; MIS, Marine Isotope Stage; PCP, prior calcite precipitation. References Amundson R, Stern L, Baisden T, et al The isotopic composition of soil and soil-respired CO2. 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