Local environmental changes recorded by clay minerals in a karst deposit during MIS 3 (La Chauverie, SW France)

Quaternary International 241 (2011) 26e34

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Local environmental changes recorded by clay minerals in a karst deposit during MIS 3 (La Chauverie, SW France) Abderrazak El Albani a, *, Alain Meunier a, Roberto Macchiarelli b, c, Florian Ploquin a, Jean-François Tournepiche d a

Université de Poitiers, UMR 6269-HydrASA, UFR SFA, Bât. Sciences Naturelles, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France UMR 7194, Département de Préhistoire, Muséum National d’Histoire Naturelle, 1 rue René Panhard, F-75013 Paris, France Département Géosciences, Université de Poitiers, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France d Musée d’Angoulême, 1 rue Friedland, 16000 Angoulême, France b c

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 18 March 2010

The four stratigraphic units of the Late Pleistocene (MIS 3) clay-rich deposit of La Chauverie (Charente, SW France) have been characterized for mineral composition. The lower part of this 75 cm-thick karst deposit preserves a mammal fauna typical of temperate climate, followed by an assemblage pointing to a colder phase. The clay fraction of both horizons mostly results from mineral transformations of the clays contained in the sediments of the Paleogene formation surrounding the cave. Because clay mineral properties and changes intimately depend on the physico-chemical conditions and reaction kinetics prevailing at a given time in the soils, they are highly temperature-related. Notably, changes are rapid under temperate conditions, but slow in tundra-like cold contexts. As shown by the altered inherited mica-illite, as well as by the increase of crystallinity of kaolinite particles, at La Chauverie these transformations were marked in the stratigraphically lower part of the deposit formed under temperate conditions. Conversely, as reflected by the reduction of the kaolinite/smectite ratio characterizing the sediments of the upper horizon, the pedological evolution was limited during the subsequent cold phase. Here, both illite/smectite and kaolinite/smectite mixed layers become smectite-richer than their equivalent in the Paleogene. The parallelism between paleontological evidence and mineral signature in recording a relatively rapid (millennial-scale) shift towards colder conditions suggests that clay mineral assemblages from cave deposits can be used to assess paleoclimatic and paleoenvironmental dynamics at local scale. Nonetheless, future research should test this potential tool in more appropriate, thicker deposits investigated using other independent paleobiological and geochemical indicators. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Global paleoclimatic changes are classically referred to d18O isotope record from ice accumulated in polar zones (Raymo et al., 2006; Chester and Langway, 2008; Sellén et al., 2008; Villa et al., 2008). The impact of the climatic changes on erosionedeposition cycles is also recorded in marine sediments which give an average picture of environmental modifications at continent scale (Shackleton et al., 2004; Kitamura and Kawagoe, 2006; Moss and Kershaw, 2007; Patterson et al., 2007). Lake sediments are now largely used to focus the investigations at more regional scales

* Corresponding author. Université de Poitiers, Geology, UMR 6269-HydrASA, UFR SFA, Bât. Sciences Naturelles, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France. Tel.: þ33 549 45 39 26; fax: þ33 549 45 42 41. E-mail address: [email protected] (A. El Albani). 1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.03.007

(Wang et al., 2001; Bar-Matthews et al., 2003; Fagel et al., 2003; de Beaulieu et al., 2006; Scholz et al., 2007; Bohncke et al., 2008), while phytolith assemblages (Barboni et al., 1999; Bonnefille et al., 2004), d18O and d13C values of paleosol carbonates (Aronson et al., 2008) and (paleo)faunal assemblages (e.g., Bobe and Behrensmeyer, 2004), are widely used as proxies of paleoecosystems at basin scale (reviewed in Behrensmeyer, 2006; Kingston, 2007). However, in spite of the relative abundance of paleobiological, paleoanthropological, and cultural remains from anthropic levels preserved in Pleistocene cave deposits (occupation sites), until now the impact of relatively short-term climatic oscillations at local scale is poorly documented. Cave sediments are considered as primary sedimentological and paleontological archives (Bertran et al., 1997, 2008; Delagnes et al., 1999; Texier, 2001, 2009; Bertran, 2005; Pirson et al., 2006), as well as critical sources for organic matter geochemistry (Panno, 2004; Bertran et al., 2008), speleothem petrography and dating (Quinif

A. El Albani et al. / Quaternary International 241 (2011) 26e34

and Bastin, 1993, 2006; Quinif et al., 1994; Genty, 2002; Genty et al., 2005), authigenic aluminium phosphate minerals (Karkanas et al., 2000), magnetostratigraphy (Hladil et al., 2003). Nonetheless, while clayey sediments are particularly abundant in these contexts, only a few investigations have primarily considered their mineral composition and transformations in a paleoclimatic-paleoenvironmental perspective (Foos et al., 2000; Polyak and Güven, 2000). As clay minerals necessarily come from the surroundings of the caves where they are deposited, their crystal-chemical properties intimately depend on the physico-chemical conditions prevailing at that time in the soils and weathered zones. Accordingly, it is likely that clays can be used as proxies to characterize, at very local scale, the evolution of the environmental conditions triggered by global climatic changes. If clays could really record that evolution, it would be of great relevance for further studies of free-fossil caves. In this perspective, the purpose of this study is to characterize for their mineral composition the clays embedding two Late Pleistocene mammal assemblages from the La Chauverie cave

27

deposit (Charente, SW France), indicating a short-term climatic shift from temperate to cold conditions during MIS 3.

2. La Chauverie: geological and stratigraphic setting The karst cave of La Chauverie is hosted in a Cretaceous (TuronianeCampanian) limestone plateau at mid-slope of the Ronsenac river valley (Fig. 1). Excavations were run in 2003e2005 by the Musée d’Angoulême. The site (CVR) currently consists of a network of metres-wide ducts. The investigated duct represents a long and relatively narrow cavity filled by a 75 cm-thick clay-rich deposit. A single 14C date, obtained at the radiometric laboratory of the University of Oxford [Ref. OXA-10480 (LYON-1383)] from a mammalian bone selected from the so-called lower faunal level (LFL) of the deposit (Fig. 2), indicates 36  2 ka BP. In terms of biochronology, the assemblage from La Chauverie, which includes a temperate faunal level (LFL) quickly followed by a more typically

Fig. 1. Location of La Chauverie cave site, SW France, and simplified geological map of the area.

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A. El Albani et al. / Quaternary International 241 (2011) 26e34

cold mammal assemblage forming the so-called upper faunal level (UFL), is fully consistent with attribution to MIS 3. The deposit, which represents a sedimentary continuum with no evidence of erosion, consists of four units which have been distinctly sampled for mineral analysis (Table 1). From bottom to top, the stratigraphic units are as follows: - unit 1: w10 cm-thick dark brown clayey sediment lying above the Cretaceous limestone bedrock (CVR 1 and CVR 2 samples); - unit 2: w40 cm-thick sandy and silty-claystone sediment (CVR 3 sample); - unit 3: w8 cm-thick lenticular grey claystone sediment (CVR 4 sample); - unit 4: 17 cm-thick (on average) poorly sorted mixture of gravel, sand, silt, and clays, where gravels are oriented or dispersed from place to place in the sediment (CVR 5 sample). For comparison, the series of five samples from the investigated section (CVR 1e5, of 1 kg each) has been integrated by two additional samples (same quantity) derived from the two main geological formations surrounding the cave: the Cretaceous limestone and the Paleogene siliciclastic sandyesilty marlstone. Taphonomic and paleontological investigations at La Chauverie revealed the presence of two stratigraphically distinct faunal assemblages (Fig. 2). The LFL lower faunal level, from unit 1, includes the following mammal taxa suggesting a rather temperate climate: Equus caballus L., Equus hydruntinus REGALIA, Sus scrofa L., Bovinae (mostly consisting of Bos primigenius BOJANUS), Cervus elaphus L., Megaloceros giganteus BLUMENBACH, Capreolus capreolus L., Crocuta crocuta spelaea GOLDFUSS. The UFL upper faunal level (units 2e4) includes E. caballus L., E. hydruntinus REGALIA, Eléphantidae, Bovinae (including Bison sp.), C. elaphus L., Rangifer tarandus L., M. giganteus BLUMENBACH, C. crocuta spelaea GOLDFUSS, Ursus cf. spelaeus ROS. and HEINROTH, Panthera spelaea GOLDFUSS. As a whole, compared to LFL, this later assemblage is consistent with a colder, sub-arctic climate. The currently available record thus indicates that the sediments accumulated during a short time period characterized by a rapid (likely millennial-scale) climatic (environmental) change, which is typical of the highly variable MIS 3 (Dansgaard et al., 1993; Fauquette et al., 1999; d’Errico and Sánchez Goñi, 2003).

Table 1 List of the samples (1 kg each) from La Chauverie cave deposit (CVR) and from the surrounding area used for clay mineral analysis. Sample origin U4 U3 U2 U1

Stratigraphic Stratigraphic Stratigraphic Stratigraphic

Label unit unit unit unit

Paleogene marlstone Cretaceous limestone

CRV 5 CRV 4 CRV 3 CRV 2 CRV 1 Paleogene Cretaceous

3. Methods In the case of the samples from the cave and the Paleogene marlstone (Table 1), the <2 mm fraction was separated using centrifugation after gentle grinding, while the Cretaceous limestone was decarbonated using diluted acetic acid. The insoluble remains were washed with distilled water and air-dried. XRD patterns were performed from oriented preparations and randomly-oriented powders using a Siemens D500 diffractometer (Ni-filtered CuKa radiation). Analytical conditions were 2.5e30 or 3e65 2q scanning angular ranges: 0.025 2q scanning step 5 or 6 s counting time per step, for oriented mounts and randomlyoriented powders, respectively. The diffractograms were numerically recorded by a DACO-MP recorder piloted by a micro-computer using the Diffrac-AT software (SOCABIM). Oriented preparations were run in the air-dried (AD) and ethylene-glycol solvated (EG) states. The diffractograms were decomposed into elementary GaussianeLorentzian bands using the DECOMPXR program according to Lanson (1997). In order to determine the position of the (002) harmonic of I/S or K/S MLMs, the decomposition was performed in the 8e14 2q angular range from EG samples. Each elementary band is characterized by three parameters: position ( 2q CuKa), intensity (counts), and full width at half maximum (FWHM,  2q CuKa). These parameters are integrated in an XRD theoretical pattern calculation for two-component systems using the NEWMOD program (Reynolds, 1985). This procedure allows the identification of dominant categories of mixed-layer minerals (MLMs) and to approximately determine the coherent scattering domain

Fig. 2. Sedimentary profile of the 75 cm-thick clayey deposit of La Chauverie investigated for mineral analysis, consisting of four stratigraphic units (U1eU4; see the text for description). LFL, lower faunal level (temperate horizon); UFL, upper faunal level (cold horizon); CVR 1e5, origin of the samples. The star (*) indicates the provenance of the mammal bone selected for 14C dating.

A. El Albani et al. / Quaternary International 241 (2011) 26e34

size (CSDS) of each clay species (methodological details in Annex). Even if the results obtained are not fully satisfying from a crystallographic point of view (presence of three-component MLMs, for instance), they are sufficiently accurate to comparatively characterize the investigated samples.

29

corresponds to that of the (002) harmonics of randomly ordered I/S and K/S MLMs, respectively. A two-component NEWMOD simulation has been performed to reproduce these positions, where the asymmetrical profile of the experimental XRD pattern is the given angular range. This procedure provided the following results: the composition and CSDS (N: number of layers) of the I/S and K/S MLMs are 55% illitee45% smectite (N ¼ 3e14), and 35% kaolinitee65% smectite (N ¼ 3e8), respectively. The asymmetry is reproduced with 66% K/S þ 34% I/S (Fig. 4a). The XRD pattern of the Cretaceous sample does not exhibit the same broad band in the 8e14 2q CuKa angular range, but rather an asymmetry of the 10 Å peak towards high angles (Fig. 3a). The decomposition shows that it is due to the presence of the (002) diffraction band of the randomly ordered illite/smectite mixed layer at 9.70 Å (Fig. 3b). As shown by the two-component NEWMOD simulation (Fig. 4b), the composition of this I/S is roughly 60% illitee40% smectite (N ¼ 3e14).

4. Results 4.1. Cretaceous and Paleogene formations The insoluble residue of the Cretaceous limestone (less than 5% weight) and Paleogene marlstone are composed of quartz and several phyllosilicate species. In both formations, two clay phases have a high CSDS: mica or well-crystallized illite (FWHM <0.2 2q CuKa; Meunier et al., 2004) and kaolinite (Fig. 3a). The asymmetry of the (001) peaks of illite and kaolinite towards low angles is due to the contribution of a low CSDS particle population to the diffracted intensity. The 10.23 and 7.33e7.27 Å bands correspond to 5e12 or 3e10 layers particle populations for illite and kaolinite, respectively. As shown by the (001) diffraction peak shift from 15.4 (AD) to z17 Å (EG), an expandable phase is present in both samples. According to Inoue et al. (1989), the high saddle/peak ratio of the 17 Å peak militates of randomly interstratified illiteesmectite MLMs (R0 I/S). However, the XRD pattern of the Paleogene sample shows a diffraction band in the 8e14 2q CuKa angular range, with a high FWHM and asymmetrical profile (Fig. 3b). This indicates that the interstratification is more complex. Indeed, two bands are separated by decomposition. Their position at 9.11 and 8.37 Å

4.2. Clay sediments in the karst deposit In the CVR 1 and CVR 2 samples from the stratigraphic unit 1 formed under more temperate climatic conditions (LFL), the nonphyllosilicate mineral phases (XRD patterns of randomly-oriented powders) are calcite and, in a lower amount, quartz (Fig. 5a). For these samples, the clay phases <2 mm fractions significantly differ from the picture provided by the three samples (CVR 3e5) from the upper cold series (Fig. 6a, b).

17.01

a

3.57 4.25

4.99

b

7.16

Paleogene 3.34

7.20

10.04

15.43

Paleogene

7.21

E.G.

9.99 3.57

3.34

4.25

5.02

3.34

10.02 16.99

Cretaceous

3.57

5.01

4.25

7.16

9.99 15.32

8.42

A.D.

Cretaceous

7.16

9.98

3.34

E.G.

15

3.57

4.26

5.00 10

7.23

9.23

7.16

9.99 5

7.37 10.23

A.D. 20

25

30

8

9

10

11

12

13

14

Fig. 3. XRD patterns of clay fraction (<2 mm) obtained for the Paleogene and Cretaceous sediments sampled from the La Chauverie’s surrounding area. a e oriented preparations in the air-dried (AD) and ethylene-glycol solvated (EG) states, b e decomposition of the EG XRD patterns in the 8e14 2q angular range using DECOMPXR (Lanson, 1997).

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A. El Albani et al. / Quaternary International 241 (2011) 26e34

a

Paleogene

temperate C al c i t e

17.29 17.1

Qu art z

a b

44

50

56

Calcit e

Qu art z

Calci t e

C al c i t e

38

62

Calcite

32

C a l ci t e C al c i t e Qu art z

C a l ci t e

C al c i t e

26

Qu ar tz

20

Calcite

14

Qu ar tz

Qu art z C l ay s 8

Calcite Calcite Qu ar tz

2

Calcite

14

Cretaceous

Calcite

12

Calcite

10

cold

Qu ar tz

8

Clay s Qu a r tz

6

Clay s

4

Calcite

C l ay s

8.33

C a l ci t e

8.36

9.10

2

Calcite

16.9

Fig. 4. XRD patterns of the 2.5e15 2q range calculated by NEWMOD software (Reynolds, 1985). a e for the Paleogene, the composition and CSDS (N: number of layers) of the I/S and K/S MLMs are 55% illitee45% smectite (N ¼ 3e14) and 35% kaolinitee65% smectite (N ¼ 3e8), respectively; the peak asymmetry is reproduced with 66% K/S þ 34% I/S. b e the clays from the insoluble fraction of the Cretaceous limestones are composed of I/S (w60% illitee40% smectite; N ¼ 3e14).

4.2.1. Mica-illite In CVR 1e2, the symmetrical 10.0 Å peak is decomposed into a single broad low intensity Lorentzian band (Fig. 6b). Compared to the cold sample series, in which the 10 Å peak is sharp and intense, the mica-illite phase here is typically formed of low CSDS particles. 4.2.2. Kaolinite The peak asymmetry towards low angles is important in both samples (Fig. 6b). As shown by the presence of two bands at 7.16 and 7.31e7.32 Å, it is due to the contribution of two particle populations. These bands correspond to high and low CSDS of >20 layers and 3e9 layers, respectively. 4.2.3. R0 I/S and K/S MLMs The (001) diffraction peak shifts from 15.2 Å (AD) to 17 Å (EG) and, compared to the CVR 3e5 sample series, in CVR 1 and 2 it is more symmetrical (Fig. 6a). The saddle/peak ratio is low, indicating that, in the two samples from the temperate horizon, the smectite amount of the R0 I/S and K/S MLMs is higher than recorded in the subsequent cold units. Also, the decomposition shows that two smectite-rich randomly ordered MLMs co-exist in the samples (Fig. 6b): 1) an R0 I/S MLM, whose (002) harmonic is at 9.02e9.07 Å (w60% smectite; CSDS ¼ 3e8 layers), and 2) a kaolinite-smectite MLM (R0 K/S), whose (002) harmonic is at 8.34e8.36 Å (70% smectite; CSDS ¼ 3e10 layers). The position of the (001) peak of the R0 K/S being at 17.33 Å (Fig. 6a) explains why

2

8

14

32

38

44

50

56

Calcite

26

Qu ar tz

14

Qu a r t z

12

Calcite

10

Qu ar tz

8

Qu ar tz Calcite

6

Qu ar tz

4

Calcite

2

Do lo m ite Qu ar tz

Hy d r o x y lap atite

20

Qu ar tz Clay s

9.45

Do lo m ite Hy d r o x y lap atite

Calcite

17.1

62

°2θ aCu Kα

Fig. 5. Bulk mineral composition of samples from the cave deposit. XRD patterns from randomly-oriented powders enhance the non-phyllosilicate mineral phases. a e temperate period, b e cold period.

the I/S and K/S phases expand up to 17.25 Å following NEWMOD modelisation (Fig. 7b). In the three samples from the stratigraphic units 2e4 formed under colder climatic conditions (UFL), the non-phyllosilicate mineral phases are quartz, calcite, dolomite, and hydroxyapatite (Fig. 5b). In CVR 3e5, the clay phases of the <2 mm fractions have a similar composition, including highly and poorly crystalline species (Fig. 6a). 4.2.4. Mica or high CSDS illite The sharp (001) and (002) peaks at 10.00e9.99 Å and 5.01 Å, respectively, do not shift after ethylene-glycol solvation (CSDS up to 30 layers). The peak asymmetry towards low angles is due to the contribution of a low CSDS (3e10 layers) particle population (10.3 Å band) (Fig. 6b). 4.2.5. Kaolinite The (001) peak at 7 Å is asymmetrical because of the contribution of two particle populations. This is confirmed by the decomposition into two bands at 7.16 Å and 7.30 Å, which correspond to large and low CSDS of >20 layers and 3e9 layers, respectively (Fig. 6b). 4.2.6. R0 I/S and K/S MLMs In CVR 3e5, the (001) diffraction peak shifts from 15.1 Å (AD) to 17 Å (EG). The peak profile exhibits a high saddle/peak ratio

7,26

10

8,16 9,01

10,4

9

11

12

A.D.

14

11

12

13

14

cold

10

7,26

9

8,16 9,01

10,4

cold

10,02

3.57

E.G.

13

7,18

3.34

8

CVR 4 3.34

3.57

10

15

7,26 8,16 12

13

14

9

25

30

2 Cu Ka

13

14

13

14

7.31 7.16

8.34

9.02

8

10

11

12

CVR 1

8

9

10

7.32 7.16

8.36

9.07

10.00

temperate

3.34

3.57 20

11

10.00

3.57

3.34 3.34

3.57

4.26

5. 9

9,01

10,07

3.34 4.26

A.D.

A.D.

5

10

E.G.

E.G.

5.01

5.01

7.16

CVR 1

7.16

9

3.57

8

CVR 2

4.26

5. 9

5.01

5.01

CVR 2

7.16

7.16

8.65

10.00 10.00

10.00 8.67 10.00

15.17

10,4

3.34 3.57

4.26

5.01

7.17

17.34

A.D.

17.33

15.18

CVR 3

5.01

7.16

CVR 3

E.G.

10.01

15.03

9.99

16.98

3.34

8

7,17

A.D.

temperate

4.26

10,02

3.57

4.26

3.34

E.G.

3.57

5.01

4.26

5.01

7.17 7.17

7,18

3.34

b

3.57

5.01

4.26

5.01

7.16 7.16

9.99 10.00

CVR 4

10.00

15.02

31

CVR 5

CVR 5

17.01

15.14

9.98

a

17.01

A. El Albani et al. / Quaternary International 241 (2011) 26e34

11

12

Fig. 6. Mineral composition of the clay fraction (<2 mm) of the cave sediments. a e XRD patterns from oriented preparations in the air-dried (AD) and ethylene-glycol solvated (EG) states, b e decomposition of the EG XRD patterns in the 8e14 2q angular range using DECOMPXR (Lanson, 1997).

(Fig. 6a), which indicates that the expandable phases are mixedlayer minerals (Inoue et al., 1989). The decomposition of the XRD pattern in the 8e14 2q CuKa angular range shows the presence of two bands at 9.01 and 8.16 Å (Fig. 6b). They correspond to the (002) harmonics of two randomly ordered MLMs, whose composition and CSDS are 40% illitee60% smectite (N ¼ 3e8) and 50% kaolinitee50% smectite (N ¼ 3e10), respectively (Fig. 7a). 5. Discussion Following Foos et al. (2000), several sources and dynamics can be envisaged in the formation of clayey deposits, including erosiontransport-deposition of weathered rocks or soils; inheritance of insoluble minerals after limestone dissolution; microbiological alteration of the bedrock or cave deposited materials. At La Chauverie, the clays forming the 75 cm-thick sedimentary deposit are mixed with detrital minerals (silt and sand fractions),

which suggests that they originated from the erosion of the surrounding soils and geological formations. As post-sedimentary conditions are usually rather stable in cave deposits, the rate of neo-formation and/or clay mineral transformation are negligible, and the clay mineral assemblages forming the deposit can be considered as representative of the original sediment. Consequently, the changes occurred in their mineral composition can be assumed as mostly reflecting modifications of the environmental conditions prevailing in the source area. Even if the morphology of the stream(s) converging to La Chauverie original cave may have slightly changed with increasing erosion through time, the clay source area concerned certainly remained limited. The translocation processes of clay particles are different during temperate and cold periods (in Velde and Meunier, 2008). Temperate periods favour the development of thicker soils, in which the clay fraction experiences significant pedogenetic transformations, which are mostly activated by biogenic actions. Water circulates in the soils

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A. El Albani et al. / Quaternary International 241 (2011) 26e34

a

16.9

cold

17 . 5

16.5

9.00

2

4

6

8

10

8.14

12

14

16

2 Cu K

b

temperate

17.25

17.22

16.64

9.01

2

4

6

8

8.31 8.30

10

12

14

16

Fig. 7. XRD patterns of the 2.5e15 2q range calculated by NEWMOD software (Reynolds, 1985). a e randomly ordered MLMs whose composition and CSDS are 40% illitee60% smectite (N ¼ 3e8) and 50% kaolinitee50% smectite (N ¼ 3e10), respectively, b e the simultation shows that two smectite-rich randomly ordered MLMs coexist in each sample: 1) R0 I/S MLM whose (002) harmonic is at 9.02e9.07 Å (w60% smectite; CSDS ¼ 3e8 layers); 2) kaoliniteesmectite MLM (R0 K/S) whose (002) harmonic is around 8.30 Å (70% smectite; CSDS ¼ 3e10 layers).

during almost all the year. Thus, clay translocation continuously impoverishes the upper horizons. Conversely, during cold periods, water fluxes are abrupt and concentrated from the end of spring to summer (snow and ice melting). The soils, generally thin and fragile, are partly eroded and the denuded rocks are incised; part of the clay particles form suspensions stable enough to be transported on short distances and settled in the nearby cavities. The clay mineral assemblages forming cave sediments do not simply record local temperature oscillations, but more complex vegetationesoil interactions, including microbial weathering processes, i.e., their composition reflects “environmental conditions”. To produce measurable effects on clay mineral composition, the environmental conditions must remain stable for at least some hundred years during temperate periods, and for some thousands years during cold phases. In fact, while erosion and deposition occur each year during the cold periods, they are less frequent during the temperate ones. Indeed, stable temperate environmental conditions imply only modest erosion intensity and limited sediment deposition. Conversely, any abrupt climatic oscillation induces substantial modifications of the local environment and

increases erosion of the soft superficial soil horizons, thus favouring sedimentation in surrounding cavities. Accordingly, clayey cave deposits potentially record cycles of climatic-environmental stability-crisis at local scale. Present results show that the mineral composition of the clay assemblages from the stratigraphically lower, temperate sample series of La Chauverie (LFL) significantly differs from that of the surrounding Cretaceous and Paleogene formations. Notably, the high CSDS particle populations are reduced to one species (kaolinite). The fact that the mica-illite phase is entirely formed of low CSDS particles indicates a stronger alteration degree typical of soils formed under temperate, not cold climatic conditions. While the R0 I/S MLMs are practically identical in the lower and the upper (UFL) series (w60% smectite), in CVR 1e2 samples the R0 K/S MLMs are enriched in smectite (70% vs. 50%). Within the lower sample series, the increase of expandable layer amount in MLMs is typical of biologically active soils (Velde and Meunier, 2008). The mineral reaction kinetics is much more rapid under temperate than cold conditions (Velde et al., 2003; Couchoud et al., 2009), where the most reactive part of the soils is represented by the organo-biological A horizon (Norberg and Madsen, 1994; Barré, 2007; Barré et al., 2007). Kaolinite/smectite mixed-layer minerals (K/S) are a major component of the clay fraction of soils and weathered mantles developed on volcanic rocks, either acid (Yerima et al., 1985; Quantin et al., 1988; Takahashi et al., 1993), or basic (Wada et al., 1990; Bühmann and Grubb, 1991; Vingiani et al., 2004). Evidence from a variety of contexts, including the Gange plain (Mohindra and Parkash, 1990), Scotia, and Australia (Wilson and Cradwick, 1972; Churchman et al., 1994), show that they are particularly abundant in soils formed on recent sediment deposits, having been identified as dominant mineral phase of the <0.01 mm fraction of fresh soils (Hubert et al., 2009). In spite of the fact that K/S may form under a wide range of climatic conditions (temperate to tropical) in contrasted rain regimes (from India monsoon areas, to arid savannah), it seems that they preferentially occur in dry grassland landscapes, which are favourable to the development of large herbivore populations. At La Chauverie, a similar scenario is compatible with the evidence from the lower faunal level (LFL) of the deposit. Compared to the record from LFL, the mineral composition of the clay fraction from the upper cold series is similar to that of the Paleogene sediments (mica-illite, kaolinite, R0 I/S and K/S MLMs). The high CSDS phases, such as mica-illite and kaolinite, are practically unchanged. Nonetheless, the R0 I/S MLMs are more smectitic in the deposit than in the unaltered sources. Such a minor difference between the units 2e4 (UFL) and the Paleogene indicates that the mineral transformations were only limited in the weathering zone and that, according to the evidence from tundra-like environments in Northern Canada and Europe (Protz et al., 1984, 1985; Gillot et al., 1999), they more likely occurred under cold climatic conditions. In sum, the clay fraction of both the temperate and cold sample series from La Chauverie are mainly inherited from the soils developed on the Paleogene sediments which covered the plateau at that time, and the degree of mineral transformation experienced by these sediments was higher in the soils developed under temperate, rather than cold climatic conditions. 6. Conclusions Cave sediments typically result from the erosion of soils and weathering profiles developed in the surrounding areas. As clay minerals are extremely reactive in soil organic horizons (e.g., Norberg and Madsen, 1994; Barré et al., 2007), any period of time during which a soil develops modifies their crystal-chemical

A. El Albani et al. / Quaternary International 241 (2011) 26e34

properties, at least in the A horizon (Barré, 2007). With this respect, the reaction kinetics is largely temperature-dependent, i.e., relatively rapid under temperate, slow under cold conditions. Accordingly, the mineral signature of cave clayey deposits can be considered an archive potentially preserving the memory of complex plantesoil interactions at local (site) scale, thus as a proxy for (paleo)climatic-(paleo)environmental conditions. In agreement with the paleontological evidence, the extent of clay mineral changes recorded within the 75 cm-thick sedimentary deposit of La Chauverie cave, SW France, reflects a relatively rapid (millennial-scale) shift from temperate to cold climatic conditions occurred at ca. 36  2 ka BP, a scenario which is coherent with the amplitude and frequency of the climatic oscillations typical of MIS 3 (for a review, see d’Errico and Sánchez Goñi, 2003). In particular, the presence of large amounts of K/S within the stratigraphically lower portion of the deposit, characterized by a temperate fauna (LFL), suggests that, at the time, the landscape was that of a rather dry grassland. As shown by the faunal remains from the upper portion of the deposit (UFL), quite suddenly a climatic shift towards colder climatic conditions shaped the grassland into a tundra-like environment. As a whole, the evidence from La Chauverie corroborates current knowledge about the intimate relationships between local climateenvironment dynamics and clay mineral composition (chemical properties), and also show that the signature of such interactions can be preserved in and extracted from Pleistocene cave deposits. However, because of the limited extent of the investigated sedimentary filling, we still ignore the degree of resolution of such signature and its reliability in recording subtle time-related changes within thicker deposits. In the future, this should be investigated in more appropriate sites and compared to the results from other independent paleobiological and geochemical indicators.

33

Ordered stacking sequence (R ¼ 1): a stacking sequence is forbidden (for instance, smectiteesmectite). Randomly ordered K/S mixed-layer minerals are characterized by a non-rational series of peaks at 17e18 Å, 8.50 to 7.15 Å, etc. The greater the kaolinite content, the more the 2nd- order diffraction band shifts towards 7.15 Å (see Annex Fig. 1).

Acknowledgments We are indebted to B. Lanson for helpful discussion. We are also grateful to the Museum of Angoulême (Charente, France) for facilities and field assistance. Research was supported by the French CNRS-INSU.

Fig. 1 Annex. Calculated XRD patterns of kaolinite/smectite randomly ordered mixedlayer minerals using NEWMOD (Reynolds, 1985). Note that the (001) wide diffraction band at 18 Å progressively disappears with increasing kaolinite %, while the second order shifts from 8 to 7.15 Å.

Annex. X-ray diffraction properties of mixed-layered clay minerals

References

X-ray diffraction is the most adapted tool to identify clay minerals. However, the common interpretation of XRD patterns being based on the peak position, confusion and errors are unavoidable. A rigorous interpretation needs to consider not only the position, but also the peak profile and its intensity. An acceptable mineral composition is obtained when experimental patterns of a given sample in different saturation states (air-dried and ethylene-glycol solvated, at least) are fitted by a unique calculated solution. This procedure is rigorous but time-consuming. A less heavy method providing approached solutions is based on the decomposition of the XRD peaks (Lanson, 1997) and NEWMOD calculation (Reynolds, 1985). The most commonly described mixed-layer minerals (MLMs) are two-component: illite and dioctahedral smectite (I/S), kaolinite and dioctahedral smectite (K/S), chlorite and saponite (C/S). A mixed-layer mineral composed of two components AeB occurring in varying relative proportions, WA and WB, may present different stacking modes which are described by succession probabilities of A and B layers (Brindley and Brown, 1980). Random stacking sequence (Reichweite ¼ 0): the succession probability of A and B layers depends only on the relative proportions WA and WB.

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