Stable isotope composition of Late Glacial land snail shells from Grotta del Romito (Southern Italy): Palaeoclimatic implications

Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 550 – 560 www.elsevier.com/locate/palaeo

Stable isotope composition of Late Glacial land snail shells from Grotta del Romito (Southern Italy): Palaeoclimatic implications Andre Carlo Colonese a , Giovanni Zanchetta b,⁎, Anthony E. Fallick c , Fabio Martini d , Giuseppe Manganelli e , Domenico Lo Vetro d a

d

Dipartimento di Archeologia e Storia delle Arti, Sezione di Preistoria, University of Siena, via delle Cerchia, 5, Italy b Dipartimento di Scienze della Terra, University of Pisa, via S. Maria, 53, Italy c Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK Dipartimento di Scienze dell'Antichità “G. Pasquali”, University of Firenze, Piazza Brunelleschi 4, 50121 Firenze, Italy e Dipartimento di Scienze Ambientali, Università di Siena, via P.A. Mattioli 4, 53100 Siena, Italy Received 3 November 2006; received in revised form 29 June 2007; accepted 4 July 2007

Abstract Stable isotope composition of living and fossil land snail shells was determined at Grotta del Romito (Southern Italy) with the aim to reconstruct environmental and climatic variation in the area during Late Upper Palaeolithic. The investigated succession comprised 15 different excavated layers spanning between ca 13,000 and 14,500 yr cal BP. The oxygen isotope composition of snail shells indicates a marked decrease at the layer D8 suggesting a climatic deterioration consistent with the GI 1d climatic event (Older Dryas). This climate deterioration may have been related to a substantial decrease of mean annual temperature with associated changes in the regional atmospheric circulation. However, the environmental conditions at the time of shell's growth in the other intervals sampled suggest condition comparable to the present day. The carbon isotope composition of fossil snail shells is in agreement with other records, which indicate a general increase of the δ13C values of organic matter during Pleniglacial to Late Glacial caused by substantially lower atmospheric CO2 concentration at that time. © 2007 Elsevier B.V. All rights reserved. Keywords: Land snail shells; Stable isotopes; Late Glacial; Palaeoclimate; Southern Italy

1. Introduction The 13C/12C and 18O/16O ratios of land snail shells are governed by a wide number of physiological and environmental factors which make it difficult to obtain a simple quantitative reconstruction of the past climate parameters by fossil shells (e.g. Lécolle, 1985; Goodfriend et al., 1989; Goodfriend and Ellis, 2002; Balakrishnan and Yapp, 2004; Zanchetta et al., 2005). Specifically, the ⁎ Corresponding author. E-mail address: [email protected] (G. Zanchetta). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.07.005

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O/16O ratio is related to the isotopic composition of environmental water ingested by snails (e.g. water vapour, dew, local meteoric precipitations, e.g. Goodfriend et al., 1989; Lécolle, 1985; Zanchetta et al., 2005), an isotopic effect linked to the exchange of fluid between the external environment (through the body of the snails) and internal fluid (Balakrishnan and Yapp, 2004), relative humidity (Yapp, 1979; Balakrishnan and Yapp, 2004) and the temperature of carbonate precipitation. Different direct correlations between oxygen isotope composition of meteoric water and oxygen isotope composition of land snail shells have been found within living populations

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(Lécolle, 1985; Goodfriend and Ellis, 2002; Zanchetta et al., 2005), but in very arid lands this correlation is often not apparent (Goodfriend et al., 1989). Probably the most complete model to interpret oxygen isotope composition of land snail shells is that proposed by Balakrishnan and Yapp (2004). However, quantitative prediction from this model involves several assumptions which complicate its applicability to past samples (Balakrishnan et al., 2005a). An additional issue is related to the large oxygen isotope variability usually measured both on living and fossil populations (Goodfriend et al., 1989; Leone et al., 2000; Goodfriend and Ellis, 2002; Balakrishnan et al., 2005a,b). If, for fossil remains, this variability can partially be explained by accepting that fossils assemblages accumulated during a certain interval of time (and so shells can actually record variability in the climatic parameters during time), for living populations this spread is probably due to very localised variability of local environmental conditions (e.g. temperature, moisture) during the life of the snails. Furthermore, different species collected in the same locality can have different 18O/16O ratios (e.g. Lécolle, 1985; Balakrishnan et al., 2005b) but the wide isotopic variability found in modern populations can hide small vital offsets (Zanchetta et al., 2005). The carbon isotope composition has mainly been related to the isotopic composition of the ingested food (e.g. Stott, 2002; Goodfriend and Ellis, 2002; Metref et al., 2003; Balakrishnan and Yapp, 2004). This depends on the fact that land snails incorporate more CO2 through respiration than CO2 from the surrounding environment (McConnaughey et al., 1997). However, ingestion of carbonates (Goodfriend and Hood, 1983; Yates et al., 2002), exchange with atmospheric or soil CO2, and metabolic rate (Balakrishnan and Yapp, 2004) may complicate the picture. Scrutiny of the data available in literature suggests that the carbon isotope composition of snail shells is more species-dependent than for 18O/16O (e.g. Yapp, 1979; Lécolle, 1983, 1984; Yates et al., 2002; Zanchetta et al., 2004, 2005). Moreover, the land snails adjust the time of shell growth during the most favourable condition avoiding both too high and too low temperature, and in function on the local humidity (Balakrishnan and Yapp, 2004), so that the isotopic composition of their shells is not strictly governed by the “average” local environmental conditions and other factors like seasonality can have an important role (e.g. Leng et al., 1998). In conclusion one could argue that the use of isotopic shell composition to infer past climatic change can be partially hidden by the large isotopic variability we can find on past assemblages or through the effect of smoothing of the climatic signal regulated by physiological requirement. Despite these

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limitations, land snail shells are commonly preserved in continental deposit of many regions and their isotopic study can represent an additional important source of information for the understanding of the past environments (e.g. Goodfriend and Ellis, 2000; Leone et al., 2000). Moreover, most species of land snails present in the Quaternary fossil record are extant, so it is possible to establish the relations between isotopic composition of fossil and living samples (Goodfriend and Ellis, 2000). In particular, land snail shells are common fossil remains of archaeological excavations and, the study of their isotopic composition can aid palaeoenvironmental reconstructions at time of human occupation (Abell and Plug, 2000; Goodfriend and Ellis, 2000; Balakrishnan et al., 2005a). The aim of this paper is the palaeoenvironmental reconstruction, through stable isotope record obtained from land snail shells, of the Late Glacial deposit preserved at the Grotta del Romito (Southern Italy, Fig. 1) archaeological excavation. This archaeological excavation supplies a high resolution, well dated, stratigraphic continental succession between ca 13,000 and 14,500 yr cal BP, and represents the opportunity to test if the isotopic composition of land snail shells is a suitable tool to reconstruct rapid climatic fluctuations. 2. Geological and climate setting The Grotta del Romito (39° 54′N, 15° 55′E) is a small horizontal cave open in the western side of Monte Ciagola limestone hill at 275 m a.s.l., about 25 km from the Tyrrhenian coast (Fig. 1). This region is characterised by mountains rising to more than 2000 m, the Apennine ridge, which descend steeply toward the Mediterranean Sea. The study area is dominated by winds blowing from west supplying air masses that are partially dammed by the Apennine ridge resulting in an abundant annual precipitation (from 904 mm to 1502 mm according to local elevation and slope aspect). Mean monthly temperatures at the site vary from a mean minimum of ∼8.5 ± 4 °C, during January to February, to a maximum of ∼27±3.1 °C in August (Fig. 2). The mean annual oxygen isotope composition of meteoric water at the site is ca −6‰ (Longinelli and Selmo, 2003; Leone and Mussi, 2004). The deposits of Grotta del Romito contain one of most important sequences of the Upper Palaeolithic of the Italian peninsula (from Gravettian to Late Epigravettian), recording a detailed prehistoric occupation of ca 13,000 yr in the immediate entrance of the cave (e.g. Graziosi, 1962, 1971; Martini et al., 2003; Cattani et al., 2004; 2004a,b). Since the first seasons of excavation Grotta del Romito had revealed a detailed prehistoric settlement, several important manifestations of prehistoric art and an intense

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Fig. 1. Location map of the studied area.

funerary use, thus assuming an important position in the frame of the Late Upper Palaeolithic of the Mediterranean basin. Fig. 3a shows the general stratigraphy of the archaeological excavation within the cave, while Fig. 3b shows the location of the excavated area. Area A concerns the old excavations performed at the beginning of 1960, whereas areas B and B′ are the recent excavations that started in 2000. 3. Materials and methods This study investigated 15 archaeological layers relative to the Late Epigravettian succession (ca 13,000 and 14,500 yr cal BP; Table 1) coming only from excavated area B (Fig. 3b) to avoid any problem of stratigraphic correlations. 14C ages were obtained on charcoals and are in stratigraphic order, indicating no reworking from different layers (Table 1). Land snail shell remains are relatively abundant, but to reduce the possibility of analysing reworked specimens and to control the preservation state, complete well preserved shells were selected. Two herbivorous species were chosen for stable isotope analyses: Discus rotundatus (Müller, 1774) (Punctoidea, Patulidae), and Helix cf. ligata (Müller, 1774) (Helicoidea, Helicidae). D. rotundatus is a

semelparous species (Cameron, 1982) living in habitats characterised by sufficient moisture such as damp sheltered places, in woods, maquis, under stones, and fallen trunks in damp herbaceous vegetations (Giusti et al., 1995; Kerney and Cameron, 1999). H. cf. ligata is a poorly known species endemic to Central–Southern Italy. According to Settepassi and Verdel (1965) its optimum seems to occur between 400 and 1000 m a.s.l. preferring woody areas and avoiding long periods of drought. H. cf. ligata is probably an iteroparous species as other large species of subgenus Helicinae. For each archaeological layer 2 to 6 shells were analysed for the appropriate species. Living land snail shells of D. rotundatus, and H. cf. ligata were also collected at the cave entrance during 2004–2005. Random samples of archaeological and modern shells were analysed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to look for evidence of recrystallization. The results indicate that the internal aragonite structures were preserved. D. rotundatus occurs more abundantly from layer C to D8, whereas in lower levels it is less abundant and fragmented. By contrast H. cf. ligata has well preserved and complete specimens only below layer D8. Complete shells were cleaned in an ultrasonic bath, rinsed several times in deionised water, powdered and ashed in a low temperature oxygen plasma to remove organic

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Fig. 2. Monthly precipitation and temperature for the year 2004–2005 in the area of Grotta del Romito.

contaminants (Zanchetta et al., 2005). The samples were analysed at SUERC (East Kilbride, Scotland) with the AP2003 mass spectrometer equipped with a separate acid injector system, after reaction with 105% H3PO4 under He atmosphere at 70 °C. Isotopic results (Tables 2 and 3) are reported using the conventional δ‰-notation. Mean analytical reproducibility (± 1σ) was ± 0.08‰ and ±0.15‰ for carbon and oxygen, respectively. The δ18O and δ13C are reported relative to V-PDB, whereas the δ18O of waters are quoted to V-SMOW. 4. Results and discussion Table 2 shows stable isotope results of living specimens, including mean values and standard deviation,

while Table 3 shows the same information for the fossil shells. Shells of living Discus rotundatus and Helix cf. ligata have essentially comparable δ18O values indicating that vital offset between these species is negligible or, if present is hidden by the variability found in natural population (Table 2, Fig. 4). The range of δ18O values for D. rotundatus (∼0.9‰) and H. cf. ligata (∼1.8‰) are in the variability found for other living populations studied in western Mediterranean (Lécolle, 1985; Zanchetta et al., 2005). The smaller δ18O variability of D. rotundatus compared to H. cf. ligata is probably due to the more stable microenvironment condition where D. rotundatus lives (D. rotundatus has been collected under stones in the cave entrance and H. cf. ligata mainly in ravine between stones).

Fig. 3. a) Stratigraphy of the archaeological excavation. b) Location of the excavation within the cave. A: old excavated area; B–B′: recent excavation areas.

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Table 1 C dates available for the Grotta del Romito archaeological excavation. Calibration was performed using the program Calib Rev. 5.0.1 (Reimer et al., 2004)

14

14

C C1 C2 C3 C4 D D1 D2 D3 D4 D5 D8 D9 D11 D12

11.060 ± 100 13.170–12.890

–

Beta-160295

11.090 ± 70 11.380 ± 70 11.250 ± 70 11.580 ± 70 11.660 ± 70

– – – – –

Beta-160296 Beta-160297 Beta-160298 Beta-160299 Beta-160300

C yr BP

14

δ13CV-PDB‰ Lab. code

Layer

C cal yr BP (±2σ)

13.190–12.890 13.500–13.140 13.460–13.010 13.240–13.190 13.606–13.419

12.060 ± 90 13.970–13.830 12.170 ± 60 13.857–14.171

– −26.8

Beta-160302 CeDaD-LTL234A

12.334 ± 75 14.013–14.748 12.404 ± 80 14.107–14.876

−24.7 −25.6

CeDaD-LtL238A CeDaD-LtL235A

However, Helix cf. ligata has, on average, lower δ13C values compared to Discus rotundatus probably related to the different physiology and diet, whereas δ13C variability (∼2.5‰ and ∼3‰, for Helix and Discus respectively) is comparable. Other studies have already shown that Helix spp. have the lowest δ13C values of shells when different species are compared (Lécolle, 1983, 1984; Yates et al., 2002). This may be due to the efficiency of the oxygen transport system of medium–large land snail, such as those of the Helicids, compared to small land snail, which regulates the amount of CO2 adsorbed from air (McConnaughey et al., 1997). This is in agreement with the Stott's (2002) results which show that 13C/12C of Helix aspersa shell (Cornu aspersum according to recent literature) is little influenced by diffusion of atmospheric CO2 into the hemolymph. However, it is also possible that D. rotundatus includes in its diet more carbonate than Table 2 Stable isotope data of living shells collected at the entrance of the cave δ18OV-PDB‰

Mean

Discus rotundatus 0.13 0.54 − 0.08 − 0.35 0.06 Helix cf. ligata − 0.39 0.88 0.10 − 0.93 0.87 0.10

SD

δ13CV-PDB‰

0.38

− 10.85 −9.78 − 12.78 − 12.64

0.79

− 12.03 − 13.29 − 13.24 − 14.43 − 12.98

Mean

− 11.51

− 13.20

SD

1.45

0.90

Table 3 Stable isotope data of the archaeological excavation. Mean and standard deviation for each layer are also reported Layers δ18OV-PDB‰ Discus rotundatus C − 0.20 − 0.29 − 0.53 − 0.19 C1 − 0.27 − 0.49 − 0.09 − 0.24 C2 0.46 0.28 0.53 1.00 C3 0.70 0.35 0.70 0.57 C4 0.75 0.39 1.12 D − 0.20 − 0.30 0.13 − 0.12 0.50 − 0.48 D1 0.08 − 0.15 − 0.53 0.46 D2 − 0.76 − 0.52 − 0.19 D3 − 0.48 0.54 0.54 − 0.73 D4 − 0.16 0.70 0.49 − 0.29 D5 − 0.07 0.21 − 0.66 D8 − 3.43 − 2.15 − 4.16 Helix cf. ligata D9 − 0.18 − 0.50 − 1.39 D11 − 0.22 0.44 D12 − 0.28 − 0.20

Mean

SD δ13CV-PDB‰

− 0.30

0.16

− 0.27

0.17

0.57

0.31

0.58

0.17

0.75

0.37

− 0.08

0.35

− 0.04

0.41

− 0.49

0.29

− 0.03

0.67

0.19

0.48

− 0.17

0.45

− 3.25

1.02

− 0.69

0.62

0.11

0.47

− 9.34 − 10.72 − 10.52 − 10.72 − 10.14 − 8.83 − 8.50 − 9.22 − 8.20 − 12.11 − 10.56 − 9.26 − 9.89 − 10.82 − 10.59 − 10.03 − 8.30 − 7.51 − 9.10 − 10.43 − 11.80 − 11.40 − 9.21 − 10.08 − 9.61 − 10.48 − 9.59 − 8.41 − 9.60 − 10.42 − 10.75 − 10.09 − 10.62 − 9.82 − 9.58 − 11.39 − 10.93 − 10.44 − 10.70 − 10.50 − 9.05 − 10.12 − 11.81 −10.33 − 9.96 − 11.96 − 8.96 − 9.05 − 9.61 − 9.55 − 9.00 − 8.66 − 8.67

Mean

SD

−10.33 0.66

−9.17 0.71

−10.03 1.69

−10.33 0.44 −8.30 0.80

−10.42 1.01

−9.52 0.85 −10.42 0.33

−10.35 0.82

−10.64 0.22 −10.33 1.39 −10.75 1.06

−9.20 0.35 −9.27 0.39

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Fig. 4. δ18O–δ13C diagram of the whole set of isotopic data from Grotta del Romito.

H. cf. ligata (e.g. Yates et al., 2002). According to the experiments Stott (2002) and Metref et al. (2003) performed on H. aspersa raised on controlled food, the δ13C of the living shells of Romito is compatible with a diet mainly composed of C3 plant material. A minor effect can be due to annual/seasonal variability of 13C/12C ratio due to different sources of food, soil CO2 production or onset of reproductive activity (e.g. Magaritz and Heller, 1983; Leng et al., 1998; Baldini et al., 2007). For each layer fossil populations show δ13C and δ18O ranges of values within that found for living counterpart, supporting the view of the absence of reworked material in the set of shells analysed. The only exception is the δ13C values for layer C2 (range: ∼3.9‰) but the δ18O has a range within of that of the living population (∼0.7‰). As for living populations the δ13C shows larger ranges compared to δ18O. The larger δ18O range of values was found at layer D8 (∼2‰). Zanchetta et al. (2005) have suggested that the increasing δ18O spread of values found in living populations may indicate an increase of environmental stress. Comparison of the δ18O and δ13C of the living shells with those of the fossil populations (Figs. 4 and 5, Tables 2 and 3) shows some interesting differences. The Discus rotundatus and Helix cf. ligata δ18O values are roughly similar in both fossil and living populations. Only the three samples coming from layer D8 appear strongly 18O-depleted compared to the whole set of data. The SEM analyses of a specimen from this layer rule

out a possible diagenetic effect. The δ13C values of H. cf. ligata fossil populations are basically 13C-enriched, whereas for the δ13C values of D. rotundatus a partial overlap is apparent (Fig. 3). According to Goodfriend and Ellis (2000) snail populations which lived after the industrial revolution (ca after the AD 1860) may have been affected by the progressive 13 C-depletion of atmospheric CO2 due to the burning of fossil fuel (Friedli et al., 1986) superimposed on the natural δ13C variability usually found in living populations. The δ13C reduction of the atmospheric CO2 is today of ca 1.5‰ with respect to the 19th century. If this reduction is completely propagated, via ingested food, to the shell of living land snails it may account for most of the observed δ13C variation between fossil and living populations of D. rotundatus but only a part of the difference found in the H. cf. ligata. It is important to note that δ13C values of plants depend on which part of the plant is considered, but more important they are species-dependent and vary according to many environmental parameters, including the isotopic composition of atmospheric CO2, atmospheric CO2 concentration and water stress (e.g. White et al., 1994). Indeed, the decrease of the δ13C of the atmospheric CO2 in the last century is not clearly observed in the δ13C records obtained by tree-rings (Friedli et al., 1986). Therefore, this may not be the ultimate reason for the observed δ13C values in the fossils of Romito. Rather, it is interesting to note that many δ13C records of “continental organic

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matter” (e.g. bone collagen, leaf, organic lake sediment) show a significant decrease in δ13C between Late Glacial and Holocene transition, with δ13C values substantially higher during the Last Glacial and Late Glacial (Stevens and Hedges, 2004 and references therein) despite of a negligible δ13C decrease of the atmospheric CO2 (ca 0.3‰, Leuenberger et al., 1992). The higher δ13C values of many proxies during Late Glacial have been mainly interpreted as due to lower CO2 concentration (Krishanamurthy and Epstein, 1990; Stevens and Hedges, 2004). Therefore, the higher δ13C values of snail shells during Late Glacial at Romito compared to living populations are in agreement with this very general tendency of the organic matter of plants and herbivorous tissues and bones

to have higher δ13C values during the Last and Late Glacial compared to Holocene. However, the difference in δ13C between living and fossil snails, especially evident for H. cf. ligata, can be due to other factors. For instance, this difference could be easily produced by a diet of the late Pleistocene snail populations containing basically C3 vegetation with different proportions of different plants (e.g. shrubs and tree ratio and/or evergreen and deciduous species) and/or with mixed C3 and C4 diet. It is also known that environmental stress like lower meteoric precipitation and humidity can produce increases in plant δ13C values (Stuiver and Broziunas, 1987). Fig. 5 shows the detailed trend of the stable isotope composition of fossil specimens of Discus rotundatus and

Fig. 5. Grotta del Romito δ18O and δ13C records. For comparison the isotopic values of living specimens are also reported.

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Helix cf. ligata. As already observed the most consistent variation is recorded by δ18O in layer D8 (13,857 to 14,171 yr cal BP) with a strong 18O-depletion. Apart from this layer the trend does not show significant differences compared to δ18O values of living population. The D8 layer δ18O excursion is in very good agreement with the GI 1d climatic event (or Older Dryas, Björck et al., 1998; Fig. 6) in the GISP 2 ice core record (Stuiver et al., 1995). In Fig. 6 the ages of layers C1, D2 to D4 and D9 were linearly extrapolated from ages of the bracketing layers. Although, this is not strictly correct because it assumes a continuous sedimentation within the cave succession, it does not introduce particular bias concerning layer D8 age being directly dated. As discussed in the Introduction a general model for interpreting the oxygen isotope composition of snail shells in term of past climatic changes is not yet available. For the Italian peninsula, living populations of land snail shells show a good relationship between shell δ18O and local δ18O of meteoric precipitation (δ18Os and δ18Op respectively) given by the equation (Zanchetta et al., 2005): d18 Op ¼ 0:65 d18 Os  5:44

ð1Þ

According to this equation layer D8 may have recorded a 18O-depletion of ca 2‰ in the local mean precipitation compared with the average. If the calculated variation of the δ18Op values was mainly due to changes in average temperature the layer D8 may have recorded a decrease in temperature of ca 5 to 10 °C taking into account that today the relation between the mean δ18 Op and surface temperature in the Mediterranean ranges from 0.2 to 0.4‰/°C (Hauser et al., 1980; Bard et al., 2002; Argiriou and Lykoudis, 2006). Obviously, these estimates rely on the assumption that the δ18Op/°C ratio was valid also for pre-Holocene periods, an assumption supported by climate simulations on very different climatic modes performed by Bard et al., 2002 for the western Mediterranean basin. Despite this, a so drastic decrease in mean annual temperature, as suggested by δ18O values of shells, is probably unrealistic and the low δ18Op values could have been associated both to a change in temperature and atmospheric circulation. A substantial decrease in δ18Op could have been produced by the socalled “amount effect” (Dansgaard, 1964), in which a decrease in δ18Op is associated to an increase in meteoric precipitation. This mechanism has been invoked as one of the most important in driving many δ18O proxy records in the Mediterranean basin (e.g. Bar-Matthews et al., 2000; Bard et al., 2002; Drysdale et al., 2004; Zanchetta et al., 2007). However, pollen (e.g. Sadori and Narcisi, 2001; Follieri et al., 1989) and δ18O records on speleothems (e.g.

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Bar-Matthews et al., 2000; Drysdale et al., 2004, 2007) and lake carbonates (Baroni et al., 2006; Zanchetta et al., 1999, 2007) have shown that in the Mediterranean basin an increase in precipitation is related mainly to warm condition whereas cold phases are usually associated to dry conditions on land. Therefore, the striking correlation of the event recorded in the layer D8 with GISP 2 and the fact that precipitation at basin scale seems to increase mainly during warming phases, suggest that the δ18O of shell at layer D8 was mainly influenced by a substantial decrease in the δ18O of the precipitation due to decrease in temperature but not associated to the “amount effect”. It is worthy of mention that the other layers show isotopic composition essentially similar to the living shells, which suggest conditions at time of shell growth similar to the present day. This may also support the interpretation that δ13C values of fossil shells are lower compared to living shells mainly because of lower CO2 concentration rather than different climate conditions. The δ13C record of Discus rotundatus does not show marked variation in layer D8 and also general trends are not apparent (Fig. 4). This probably may indicate that the inferred climatic changes at layer D8 did not produce significant isotopic impact on the snail diet and it is also in agreement with the fact that a consistent part of the δ18Op variation inferred by shells is not probably related to temperature alone. The δ13C values of Helix cf. ligata (layers D9, D11 and D12) appear particularly high if compared to living Helix and, once normalized to Discus rotundatus adding 1.7‰ as mean differences observed in the living population (Table 2), these are the highest δ13C values of the record. The high δ13C values of Helix at layers D9, D11 and D12 can be tentatively explained by the lower atmospheric CO2 concentration during the early Bølling (Monnin et al., 2001). 5. Conclusion Despite the origin and environmental significance of the isotopic composition of land snail shells being particularly complex, this study demonstrates that stable isotope composition (in particular 18O/16O ratio) of land snail shells can retain a signal of short term climatic changes. Owing to the large isotopic variability of living and fossil populations only stratigraphic successions sampled with adequate resolution and detail will usually deliver significant results; this can often be achieved at archaeological excavations. However, archaeological layers can suffer different processes of reworking and stratigraphic disturbances produced by human occupation.

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Fig. 6. Comparison between the δ18O records of Grotta del Romito and GISP 2. For Grotta del Romito the δ18O values are the mean values for each layer.

The data discussed indicate that this was not the case for Grotta del Romito. The isotopic measurements on two species of land snail not previously analysed show that there is no substantial offset in the oxygen isotope composition between these two species, but carbon isotopes appear to be speciesdependent. This is in agreement with previous findings on other species (e.g. Zanchetta et al., 2005). The oxygen isotope composition of land snail shells shows a good agreement with the general trend observed in the GISP 2 δ18O ice core record. In particular, the δ18O record indicates a clear brief climatic deterioration chronologically consistent with the GI 1d climatic event. The carbon isotope composition of living land snail shells appears 13C-depleted compared to fossil populations (in particular for fossil Helix cf. aspersa). The higher δ13C values of fossil snail shells are in agreement with other records suggesting a general increase of δ 13 C of

“continental organic matter” during Last to Late Glacial as effect of substantial lower CO2 concentration at that time. However, other causes can have produced the 13 C-enriched values of fossil populations like mixed C3/C4 plant or different proportions of C3 species in snails diet. Acknowledgements The authors wish to thank T. Donnelly, J. Dougans, and A. Tait for their support in stable isotopes analyses. This work is partially funded by the University of Pisa (ex 60% G. Zanchetta). SUERC is funded by a consortium of Scottish universities and NERC. This paper is part of the Ph.D. thesis of A.C. in “Preistoria - Ambienti e Culture” at the University of Siena, Sezione di Preistoria, XVIII ciclo. We also thank two referees for the comments and suggestions which greatly improved the quality of the paper.

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