Elsevier

Quaternary International

Volumes 593–594, 20 August 2021, Pages 85-94
Quaternary International

New climatic approaches to the analysis of the middle Paleolithic sequences: Combined taxonomic and isotopic charcoal analyses on a Neanderthal settlement, Les Canalettes (Aveyron, France)

https://doi.org/10.1016/j.quaint.2020.11.042Get rights and content

Highlights

  • δ13C variability from individual analysis of Pleistocene charcoal is similar to that of current references frames.

  • δ13C from charcoal record environmental and climatic changes from Paleolithic.

  • Isotopic signal and charcoal taxonomic analysis seem have complementary resolutions.

  • Isotopic signal can record climatic envent

Abstract

Over the last decade, several studies have indicated the potential for the δ13C isotope signal of charcoals to act as a new local paleoclimatic/paleoenvironmental proxy, complementary to taxonomic analyses.

These studies mainly focused on archeological charcoal from the Holocene series, but the potential of the method to be applied Pleistocene sequences is still under debate. Understanding climate-driven variability in stable carbon isotope ratios of modern samples is fundamental for the accurate characterization of past climate information based on the δ13C of charred material. In a previous study, we laid the methodological foundation to apply the method to Pleistocene contexts.

Here, we present the δ13C isotopic signal from Pinus spp. charcoal in a well stratified middle Paleolithic site: “Les Canalettes” (Aveyron, France). We demonstrate that the use of isotopic analyses on ancient charcoal of the genus Pinus from the quaternary period could provide a reliable paleoclimatic marker with high resolution and provide data that is complementary to taxonomic studies. A continuous increase in humidity is observed, with a warming episode identified at the top of the sequence. These results place the occupation of the site in a less rigorous phase at the beginning of MIS 4 and underline the importance of the Causses in the mobility system of several generations of Neanderthals.

Introduction

The Upper Pleistocene is a long period marked by climate and environmental changes in Europe, and it is heavily influenced by the last phase of glaciation. Understanding the way in which ancient societies faced environmental conditions is critical issues for prehistorians. Charcoal analysis is a reliable method to reconstruct the regional changes in forests (see previous proceedings from the 4th, 5th, and 6th International Anthracology Meetings) (Badal et al., 2011; Damblon, 2008; Ludemann and Nelle, 2017). The frequency of taxa and their changes over time are interpreted by analogy with current floristic compositions, as a diachronic history of the structure and ecology of forest stands. Furthermore, anthracology provides complementary information on the wood supply and its management by ancient societies (Olive and Taborin, 1987; Théry-Parisot, 1998, Théry-Parisot, 2001).

However, different limits in anthracology can be identified for Upper Pleistocene sequences. Indeed, these sequences sometime exhibit anthracologic assemblages with low taxonomic diversity, mainly represented by Pinus species as woody refuges in a steppe landscape or as broad forest cover. Moreover, anthracology does not provide information on forest density. Most of the time, the lack of taxonomic diversity does not allow us to specify whether it comes from a monospecific environment or is attributable to human practices (i.e., wood selection). In addition, Pleistocene anthracological assemblages are exposed to significant post-depositional processes, which reduce the number, size, and quality of the charcoal (Chrzazvez et al., 2014; Théry-Parisot et al., 2010). Methodological studies from anthracology suggest a relatively significant number of charcoal remains from relatively long human occupations are required to provide a reliable quantitative signal of past environments (Chabal, 1992, 1997). Thus, low numbers of charcoal fragments in Pleistocene sequences are sometime limited to a qualitative interpretation with a low stratigraphic resolution. To overcome this limitation, complementary approaches have recently been developed.

In recent decades, studies focusing on the δ13C isotopic signals of plants have provided a reliable method to reconstruct past climatic and environmental conditions. During plant absorption of atmospheric CO2, a discrimination (Δ) of carbon isotopes occurs. It is defined, in C3 metabolism, as the result of discrimination against 13C induced by stomatal conductance (a = 4.4‰) and the RuBisCO enzyme during carboxylation (b ≃ 30‰), with an intensity that depends on the ratio of intracellular CO2 to atmospheric CO2 content (ci/ca): Δ ≃ a + (b-a) ci/ca (1) (Farquhar and Lloyd, 1993; Farquhar et al., 1982). Thus, changes in stomatal opening, controlled by environmental and climatic parameters (including atmospheric parameters), constrain the δ13C record in the plant (Farquhar et al., 1989). Correlation between the δ13C wood signal and different stress factors (drought, high temperatures, etc.) can be observed over scales at various resolutions (seasonal wood, tree-rings, tree-ring clusters) (Cernusak and English, 2015; McCarroll and Loader, 2004). In particular, correlations between the wood δ13C signal and water availability parameters were observed in a major study, which corresponds to the first recurrent limiting factor (Battipaglia et al., 2014; Dawson et al., 2002; Farquhar and Richards, 1984; Gessler et al., 2014; Panek and Goldstein, 2001; Warren et al., 2001). In the presence of optimal levels of atmospheric moisture or in the global warming, temperature or Irradiation also dominate the relationship between isotopic and climatic signals (Gagen et al., 2007; McCarroll and Pawellek, 2001; Seftigen et al., 2011).

The analysis of the isotopic signal (δ13C) in charcoal remains from archeological contexts is an additional climatic proxy, which raises new perspectives. Pioneering research have attested to its potential to reconstruct long-term climatic trends, in both the Holocene (Aguilera et al., 2012; Baton et al., 2017; Ferrio et al., 2006; Fiorentino et al., 2012a; Masi et al., 2013; Voltas et al., 2008) and the Pleistocene archeological context (Audiard et al., 2019; Hall et al., 2008; Vernet et al., 1996). However, the resolution and application of isotope analyses in Pleistocene sequences is an important issue. Indeed, certain issues are common for all isotopic studies on charcoal. For example, the effect of charring on isotopic signal evolution is now well documented. The measurement of the carbon content (%C) as an indicator of the carbonization state allow for isotopic signal corrections (Audiard et al., 2018; Ferrio et al., 2006; Resco et al., 2011; Steinbeiss et al., 2006; Voltas et al., 2008).

In contrast, major interpretative and methodological challenges must be investigated, according to specificity of the context and chronology or different anthracological assemblage features. Thus, the method needs to be adapted for each archeological site. In particular, the type of charcoal deposit (spread, hearth), the sampling method (manual, or sifting), the intensity of the taphonomic processes and fragmentation (size, mass and number of charcoal samples; reliability of taxonomic identification) can restrict the potential of the isotopic study (Audiard et al., 2019). In addition, the taphonomic processes and the nature of the sediment can lead to exogenous carbon contamination and necessitate pretreatment, which can decrease the mass of charcoal (Vaiglova et al., 2014). Moreover, interpretation of the δ13C signal presents several challenges, particularly for the Pleistocene period, for which we lack understanding of the signal response over longer periods and under different environmental conditions from those of the present day. Indeed, we are only able to analyze a mean signal from isolated charcoals. Thus, the resulting values have automatically integrated the intra-individual variability, the intra-species variability (inter-individual variability) and the inter-species variability (Francey, 1981; Leavitt, 2002, 2007, 2010; Leavitt and Lara, 1994; Leavitt and Long, 1986; Ramesh et al., 1985; Robertson et al., 1997; Tans and Mook, 1980). Consequently, the use of δ13C from charcoal of Pleistocene sequences raises questions about (i) the nature of the isotopic signal obtained and (ii) the possibility to use it as a climatic and environmental proxy of low frequency changes, despite significant natural variability and a limited amount of charcoal for analyses. Lastly, since the variability of δ13C is taxon-dependent, it is crucial to analyze the same taxon for the entire sequence and to base the interpretations on current studies on the same taxon.

In addition to environmental and methodological issues, the relationship between the isotopic signal of charcoal and the wood collected environment raises questions about the impact of the diversification of collection areas on the isotopic signal. The integration of the anthracological data and the isotopic signal could then allow to discuss the Neanderthal behaviours in relation to the wood harvesting and environmental evolutions.

The methodological bases of the application of δ13C to Pleistocene can be found in (Audiard et al., 2018) and a first application to the study of the Middle Paleolithic sequence of La Combette (Vaucluse, France) has shown the potential of δ13C as a paleoclimatic proxy (Audiard et al., 2019). In the continuation of this work, we present here, the first application of the method to charcoal, with individual analyses from a major Middle Paleolithic settlement: Les Canalettes rock shelter (Aveyron, France). The results of previous paleoenvironmental studies from les Canalettes (fauna, microfauna, geoarcheology and anthracology) lead to divergent interpretations of the evolution of climatic conditions over the sequence (Brugal, 1993; Kervazo et al., 1995; Marquet, 1993, 1995; Théry-Parisot, 1998; Vernet, 1993). Our aim was to enrich the sequence of new data increasing the resolution in order to document the paleoenvironmental context and to discuss human/environment interactions at the local scale. The variability and the climatic resolution of the isotopic signal will be discussed with respect to previous, complementary paleoenvironmental results. In addition, methodologic issues will be addressed regarding previous isotopic works in paleolithic sequences with charcoal grouping analyses (Audiard et al., 2019).

Les Canalettes site is a rock shelter (680 m in altitude) located in a wide alveolus (extended Polje) of the Larzac Causse Plateau (700–900 m) in the Aveyron region (Fig. 1). Oriented south-eastward in a wider karstic depression (Canaule) the shelter is separated by a tectonic escarpment of the Vallée du Durzon (550 m in altitude, tributaries of the Dourbie) (Ambert, 1993).The Les Canalettes rock shelter holds a strategic position in a natural passage between the Causse plateau and the valley, which is particularly favorable for raw material and environmental resources (Lebegue, 2012; Meignen, 1993b).

The site presents a 2-m-thick stratigraphic sequence where numerous lithic remains assigned to the Mousterian have been found along the deposit (Fig. 2). The sedimentary dynamics of the shelter are fairly homogeneous over the entire thickness of the stratigraphy, with a sedimentological composition of loose dolomitic sands more or less rich in heterometric gravel. The sequence has been divided into 4 stratigraphic units, mainly on changes in color of the sandy matrix and a decrease of coarser fraction in the middle of the stratigraphy. However, US 1 (black silty-sandy sediment, with coarser fraction mainly composed of 15–25 cm blocks) corresponds to a humic level of variable depth. The humic development is responsible of the uneven limit with US 2 (25–60 cm thick, sandy, yellow sediment with coarser fraction from small gravel to 25–30 cm blocks). The US 1 and 2 are impacted by runoff and percolation, principally close the wall and at the shelter limit. US 1 is represented by heterogeneous archeological material (Mousterian, Neolithic, Middle Age, Historic Period), and US 2 is mostly represented by Mousterian material with a few exceptions against the wall. In contrast, US 3 (30–60 cm thick, sandy, yellow-orange sediment with coarser fraction from gravel to block) and US 4 (25–60 cm thick, sandy, brown sediment with coarser fraction from little gravel to block) are well preserved with horizontal limits observed during excavation (US 2/3 and US 3/4) and rich Mousterian material (Ambert, 1993; Kervazo et al., 1995; Meignen, 1993a).

The geomorphologic dynamic and the lithic material indicate a relatively short and continuous deposit phase (Ambert, 1993; Meignen, 1993a). The single dating study was realized by thermoluminescence on the upper part of US 2 and indicates occupation during the MIS5a and MIS4 transition at 73500 ± 6000 years BP (Valladas et al., 1987).

The well-preserved archeological material and the multidisciplinary approaches enabled reconstruction of the behavior of the Neanderthals present in Les Canalettes, in a landscape long considered austere. It seems to be a long-term and recurrent occupation camp, undoubtedly seasonal (during the good season, late spring to early fall), and partly linked with easy access to abundant herbivore fauna from the plateau and forest of the alveolus (extended polje) as well as its proximity to the raw material (Brugal, 1993; Lebegue, 2012; Meignen, 1993b; Meignen et al., 2001). Moreover, Les Canalettes is also known as the site with the oldest use of the lignite as fuel (Théry-Parisot and Meignen, 2000; Théry et al., 1995, 1996), found in the same relative proportions in US 4 to 2. This material, accessible in the valley, was used by Neanderthal either for a specific use (long fire without flames), (ii) or for its ease of use (harvesting, storage and transport easier than healthy wood) (Théry-Parisot and Meignen, 2000).

The multidisciplinary program at Les Canalettes has provided numerous and complementary paleoenvironmental results. But, the palynologic tests were negative and the homogeneity of the deposit only allows to define deposit conditions in temperate to cold conditions (Ambert, 1993). Similarly, the low number of identified faunal remains prevented the authors from obtaining reliable information on environmental evolution (Brugal, 1993). However, the recurrence of horse and deer throughout the sequence indicates a relatively open and temperate to cold environment. Moreover, a slight increase in hare, mammoth and marmot (one to a few remains), between US 3 and 2, allowed the author to propose a climatic degradation.

The high number of charcoal remains identified (1811) provided reliable information on the paleoenvironment (Théry-Parisot, 1998; Théry-Parisot and Meignen, 2000; Théry et al., 1996; Vernet, 1993). The anthracological diagram (Fig. 3) presents a consistent trend supporting progressive transformation of the environment under the effect of climate change (Théry-Parisot, 1998; Vernet, 1993) with a diversification of taxa, from 8 taxa in US 4 to 17 taxa in US 2. US 4 is characterized by the presence of mountain taxa (Pinus, Betula and Juniperus), which largely dominate the assemblage, with a few occurrences of mesophilic (Quercus f.c., Corylus) together with xeric heliophilic taxa (Buxus, Rosaceae). US 3 is still largely dominated by pine, but mesohygrophilic taxa are diversified (Ulmus, Fraxinus). US 2 shows a significant increase in mesophilic elements (notably Quercus), and is enriched by new taxa (Fagus, Acer) and a marked decrease in mountain taxa.

Thus, the charcoal assemblages of US 4 and 3 are quite similar, whereas US 2 shows a marked change in the composition of the anthracological spectrum. This could suggest a change in wood harvesting behavior at US 2, but this assumption seems weak as the ecological dynamics are very consistent with changing climatic conditions. At the base of the sequence (US 4), charcoal remains suggest the existence of an open environment of pine groves that had developed under rather cold and dry conditions. Gradually, a very slight improvement in the climate is reflected in the localized development of mesophilic elements, while the frequency of mountain taxa begins to decrease. US 3 then appears to be a transition level. US 2 shows a marked change in climatic conditions and a more forest-like environment with a mixed series of pine and oak in which pine dominates but the mesophilic elements remain well represented, suggesting humid temperate conditions that were slightly cooler than current conditions.

The anthracological assemblage thus reflects a consistent dynamic in the evolution of taxa frequencies, arguing for environmental closure related to less severe conditions and higher humidity at the top of the sequence.

Microfaunal studies have also yielded a sufficient number of identifications in each of the lower three US (145–492) to provide reliable paleoenvironmental information. The data correlated with the anthracological study and support the hypothesis of a relatively temperate period with a mosaic landscape around the site (temperate forests, watering place) dominated by an open landscape (not very arid steppes with wet grasslands). Given their higher resolution, the microfaunal results show a slight progressive reduction in dryland species through the stratigraphy (Marquet, 1993, 1995).

All paleoclimatic studies (Brugal, 1993; Marquet, 1993, 1995; Théry-Parisot, 1998; Vernet, 1993) suggest a predominantly open environment with colder temperatures than at present, connected to more forested and/or a humid environment (e.g. Canalettes alveolus, valley) which certainly would have fostered human settlement. However, the authors were divided on the possible climate change between the layers. While sedimentary and large fauna studies provide data in favor of climatic degradation (US 4 to US 2), microfauna and anthracological studies propose a more humid climate, which would favor the increase in a forest environment. The stratigraphic homogeneity and the low number of fauna identifications are put forward as a possible explanation for this opposition in climate signals.

Section snippets

Materials

The isotopic study focused on charcoal identified as Pinus type sylvetsris/nigra (including mugo/cembra). Indeed, pines not only composed the majority of the charcoal remains of Paleolithic sites but are also one of the most represented species in the isotopic studies of wood. Nevertheless, Pinus species cannot be anatomically discriminated. However, several studies on isotopic variability between species of the same genus reveal that it is possible to combine their signals (same response) (

Results

The raw results are synthesized below by artificial stratigraphic sub-unit (décapage, Dec) and US in Table 1, and are then compared with the charcoal taxonomic and microfaunal data in Fig. 4.

Firstly, we observed relative low variability by décapage. The isotopic standard deviation between charcoal from the same décapage does not exceed 1‰ and is mainly lower than 0.5‰. Moreover, except for Dec 8 in the US 2 (3.3‰), no décapage shows isotopic deviation above 2.5‰, and more than 2/3 of the δ13C

Isotopic signal variability

One of the most important issues that arises from this study is related to the climatic/environment significance of the mean isotopic signal from isolated charcoal analysis in the Pleistocene context. Nevertheless, the constant carbon content of about 60% that was found for all samples reveals the same degree of carbonization and allowed us to avoid a “carbonization correction” of the values in δ13C (Audiard et al., 2018; Ferrio et al., 2006). However, this observation had already been made in

Conclusion

This is the first isotopic study (δ13C) of isolated charcoal (individual values) in a Pleistocene sequence in Europe. The use of this method on the charcoal remains of Les Canalettes shelter has thus been able to show:

  • -

    Relatively low variability of the isotopic signal by décapage (artificial sub-stratigraphic unit) compared to the evolution of the mean δ13C signal along the sequence.

  • -

    A correlation between the general isotope signal data and other paleoenvironmental data (anthracology,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was co-funded by the Région-Sud (doctoral grant) and by the VEOLIA company (“Les poubelles de la Préhistoire" project, PI Théry-Parisot). We thank the ONF for their partnership. We would also like to thank VEOLIA for essential financial support for our analysis. Diagrams were drawn with software developed by M. Macías from Universitat Politècnica de València.

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