Introduction
The first solid data of “palaeospeleology”, that is, the presence of ancient humans in deep caves, were related to Neanderthals. They were discovered in Bruniquel Cave (France) and dated 176 ka BP. They comprised six circular anthropic structures, made up of approximately 400 fractured speleothems, with more than 18 fire traces inside, 336 m. from the entrance (Jaubert et al., 2016a). In the Upper Palaeolithic, combustion residues related to lighting proliferated in the inner recesses of caves, especially associated with graphic activity. However, archaeological research traditionally focused only on Palaeolithic Art. Analysis of the Internal Archaeological Context (i.e., including the traces of lighting) began in the second half of the 20th century (Clottes, 1993; Clottes & Simonnet, 1990). This subject is now receiving increasing attention and is beginning to be addressed in a holistic and interdisciplinary way in order to obtain global knowledge of the palaeolithic anthropization of caves (Arias & Ontañón, 2020; Garate, 2017; Jaubert et al., 2016b; Medina et al., 2018; Pastoors et al., 2017).
The main references on palaeolithic lighting were published during the 1960s–1980s (Allain, 1965; Beaune, 1987; Delluc & Delluc, 1979). These studies focused mainly on a single type of lighting resource: mobile lamps ignited with animal fat. Remarkably, this light-tool is the least frequent in the archaeological record and the one with the least light capacity. Furthermore, these early studies were conducted at a time when physico-chemical analyses were only incipient within Archaeology. Now, there is an ideal research context to advance on this subject, using portable and non-invasive tools (Carrasco et al., 2016; Donais & Vandenabeele, 2021; Hernanz, 2015; Ziemann & Madariaga, 2021).
Charcoals, related to the use of torches and firelight, are the most recurrent archaeological remains of light in the Internal Archaeological Context. Little attention has been paid to them, however, except in a few caves with exceptional conservation conditions (Théry-Parisot & Thiébault, 2005; Théry-Parisot et al., 2018). In my Ph.D., I studied mainly these lighting residues and the results showed their potential for the integral understanding of palaeolithic underground anthropization, including in non-intact caves (Medina, 2019). In this article, I present the postdoctoral project The “Archeology of Light”: a multiproxy, interdisciplinary and experimental approach to Paleolithic subterranean activities directed by M.A. Medina-Alcaide and supervised by C. Ferrier at the PACEA-UMR 5199 Laboratory (Université de Bordeaux). This project was funded by the Fyssen Foundation with an allocation grant (call 2021). At present, the project is still ongoing in the framework of a postdoctoral contract MSCA-PF (2022–2024). The following sections present the objectives, methodology and results obtained during the first step of project.
Methods
In the framework of the recently published book “Light in Archaeology” (Papadopoulos & Moyes, 2021), the main aim of this project was to improve our knowledge of the early subterranean behaviour of Humanity, through a pioneering and interdisciplinary methodology applied on seldom studied remains: the residues of Palaeolithic light, mainly charcoals and soot from fireplaces (Figure 1a: fireplace from Atxurra with remains of charcoals, ash and rubefacted clay – Garate et al., 2023), torches (Figure 1b: scattered charcoals linked to the use of wooden torches from Alkerdi 2 cave) and lamps (Figure 1c: fixed “lamp” of Nerja cave with soot deposit -green- and micro-charcoal remains -red). This research focuses on outstanding remains of palaeolithic lighting from six European caves1, one of which has been declared a World Heritage Site by UNESCO (Altxerri cave) (Figure 2). These remains were carefully selected in order to: (1) analyze and compare the different lighting systems, (2) extend the analysis to combustion remains of various kinds (beyond charcoals), (3) expand the archaeological record studied at the international level and (4) extend the period studied, whilst incorporating the knowledge of lighting technology possessed by different humanoid species (Neanderthal and H. sapiens). Specifically, the inner anthropization of the selected sites integrates different phases of the Upper Palaeolithic (35–12 ka. BP), including a period of the Middle Palaeolithic (176 ka. BP). This is conducive to a good diachronic understanding of the topic.
Figure 1. Diverse residues of Palaeolithic light systems analyzed in this project.
Figure 2. Site location, chronology, types of lighting and fuel.
The objectives of the project are as follows: 1. Define and analyze the lighting systems used by palaeolithic hunter-gatherers in each cave, by multiproxy and multianalytical analysis (prioritizing on-site and non-destructive analysis). 2. Quantify the physical parameters and elemental characteristics, including their residual aspects, through experimental replication and monitoring. 3. Synthesize the data and evaluate the role of firelight in palaeolithic hunter-gatherer societies, including its significance for the anthropization of caves and the performance of the first symbolic activities in them.
At the methodological level, the A-LIGHT project comprises six Work Packages:
1. Work in caves and sampling: within the framework of the research projects in each cave, a systematic surface survey of each cave was carried out, as well as small probes in particular areas to assess the remains of anthropic activity underground beyond the surface level (Medina-Alcaide et al., 2018). Dino-lite® portable microscopes (models AM4815ZT 20–200x, AM4013MZT4 400–470x, AM4515T8 700–900x were used to conduct a preliminary characterization of the vestiges in caves, and Disto Leica (model X2 modified) for the topographical geolocalization on the 3D or the 2D topography of the caves (Intxaurbe et al., 2020; Trimmis, 2018). The intrinsic and extrinsic characteristics of each combustion residue were recorded for the study. Sampling was carried out as aseptically as possible so as not to contaminate the sample for dating purposes, and was done either manually for the remains on the surface or using a flotation system for the remains from probes.
2. Laboratory analyses: On the charcoals, holistic anthracological analysis was implemented, covering, if possible, a taxonomic approach (to characterize the woody species of origin), a taphonomic approach (to identify the physiological and phenological state of the wood) and a dendro-anthracological approach (to determine the thickness of the wood). Raman analysis was carried out on some of the charcoals to determine the maximum combustion temperature, following Deldicque et al. (2016; 2023). Lastly, 14C-AMS dating of the charcoals was used to determine the date of human use of each cave.
The project also includes the use of various laboratory tools for the determination and study of lighting remains: binocular loupe, light microscope, SEM-EDX and TEM-EDX. The latter two tools include the Energy Dispersive X-ray spectroscopy technique which enables the analytical profiles and distribution maps to be obtained using the dispersed energy. This helps to characterize the nature of the remains that do not present an identifiable structure. Raman and TEM-EDX spectroscopy proved extremely useful to characterize the prehistoric soot remains. This project also included collaboration with other specialists and laboratories, for example, for the fuliginochronological analysis (S. Vandevelde, Université du Québec), for Raman analysis (D. Deldicque, École Normale Supérieure), for GC-AMS analysis and determination of the fatty residues (F. Lafont, University of Cordoba) or for micromorphology analysis, to identify the ash and rubefaction clay, and various phases of visits inside the caves (C. Ferrier, Université de Bordeaux and M. Arriolabengoa, University of the Basque Country).
3. Ethnographical review of firelight: This project includes an exhaustive review of the ethnographic literature on firelight in circumpolar societies (Evenks, Inuits), frequently compared in the literature with Palaeolithic groups for their lifestyles, and on the last human groups to live in caves (the Tao't Bato tribe). Altogether, this will allow the acquisition of techniques and knowledge (assembly system, operation ...) on traditional fire lighting systems that will be fundamental for the development of the experimental activity. This information is key to exploring the social and cultural meaning of firelight beyond practical activity. For example, there is a curious cultural relationship between the qulliq (fat lamp) and Inuit women (Billson & Mancini, 2007).
4. Reproduction and monitorization of Paleolithic light: The experimental replication in a subterranean context under controlled conditions (monitoring and repeatability) of each type of lighting system recorded enabled the physical parameters Palaeolithic light to be quantified. These data enrich our knowledge of Palaeolithic activities developed in the darkness of caves through the application of GIS studies (palaeo-speleological behaviours, visibility of art, techniques used, etc.), and lead to a more realistic and attractive dissemination of the underground heritage in digital environments and physical replicas. These experimental activities were carried out in a natural cave, without archaeological remains, located in Lekeitio (Bizkaia, Spain). Thanks to the Before-Art project team (dir. Diego Garate, University of Cantabria), that manages and has conditioned this cave, the cave has the necessary equipment to carry out the experimentation and light monitoring. The main tools used for the monitoring of the fires were a luminous flux temperature meter with built-in luxmeter, sensors to record temperature, CO2 and humidity, thermocouples, a reflectometer and a spectrophotometer. Fire residues were also recorded from the experimental activity and then mapped for comparison with archaeological records using GIS and to obtain reference samples. During these experimental activities, we collaborated closely with the experimental archeologist Jean Leblanc (Université de Toulouse).
5. Analyses, integration of data and synthesis: The R statistical package for statistical computation and graphics was used to explore the data set, as well as a heatmap tool in QGIS® to create a Kernel Density Map through vector point layers with the location of the lighting traces (archaeological and experimental residues), to observe the different distribution patterns and their relationship with other underground elements (structures, rock art, etc.) and with different human movements inside the cave. Bayesian analysis (©Oxcal 4.4.) was used for data management of the dating results. At the end of the project, the results will be interpreted in light of the available literature on palaeolithic underground activities. Lastly, the role of firelight in palaeolithic underground behaviour will be assessed based on the results obtained throughout the different methodological phases, with emphasis on the importance of light in the performance of the first symbolic activities in the caves and on its economic and socio-cultural implications beyond purely utilitarian aspects.
Results and discussion
In the following, we will present some results obtained in the framework of the postdoctoral project "Archaeology of the Light" during the year 2021–2023.
Wood for light. Beyond a functional aspect?
Inside Nerja cave (Málaga, Spain), we located remains of three different lighting systems (torches, fires and fixed lamps). The anthracological study of 336 charcoals from the woody fuel used in these prehistoric lighting resources shows a preference for the use of Pinus sylvestris-nigra (51.79%) (Figure 3). Other taxa were also identified2: Pinus cf. tp. pinea-pinaster (2.38%), Pinus sp. (3.87%), conifer (12.80%), Leguminosae (4.16%), Prunus sp. (0.30%), cf. Ulex sp. (0.30%), angiosperm (2.98%), indeterminable charcoals (7.44%) and pith remains (0.30%). In summary, we characterized 4 different types of taxa: Pinus tp. sylvestris-nigra (scots pine and black pine), Pinus cf. tp. pinea-pinaster (maritime pine and stone pine), Prunus sp. (blackthorn) and Ulex sp. (gorse). Other charcoals were defined with a lower degree of identification; they could correspond to the taxa mentioned above, or to others not identified at the taxonomic level. This was mainly due to the low consistency of most of the charcoals analyzed, which disintegrated when trying to prepare an optimal section for examination, and to the poor state of preservation of some of the remains.
Figure 3. SEM photograph of Pinus tp. sylvestris-nigra charcoal from a paleolithic lamp.
Pinus tp. sylvestris-nigra is the most common anthracological type found in the interior of caves and is linked to artificial lighting in the Upper Palaeolithic, specifically in the pre-Magdalenian period (Medina-Alcaide et al., 2021). In some caves, it has even been documented almost exclusively: in the Chauvet cave, for instance (Théry-Parisot & Thiébault, 2005; Théry-Parisot et al., 2018), only one fragment of Rhamnus cf. (buckthorn family) was identified compared to 292 fragments of Pinus sylvestris. Environmental availability in the surroundings of the cave, the suitability of this wood for lighting the cave due to its resin content and other cultural aspects have been proposed as the reasons for this preferential choice (Théry-Parisot & Thiébault, 2005; Théry-Parisot et al., 2018).
In Nerja cave all the identified woods used to light fires were also determined in the habitat shelters in the access areas of the cave (Badal, 1996), as well as fitting in with other palaeoenvironmental studies in the south of the Iberian Peninsula, such as the palaeopollen sequences from Bajondillo cave (Torremolinos, Málaga) (López-Saez et al., 2007), from Gorham cave (Gibraltar) (Carrión et al., 2008), from the Padul peat bog (Granada) (Carrión, 2012; Pons & Reille, 1988), etc. In other words, the woody species used for illumination were available in the cave environment and do not seem to come from more distant areas. Specifically, all the radiocarbon dates we have carried out on Pinus tp. sylvestris-nigra charcoals have a Palaeolithic chronology (>15000 years ago), and in no case a Holocene chronology. These palaeoenvironmental data are in line with the anthracological study carried out in the stratigraphic sequence of the Vestibule (external room of the cave) (Badal, 1996), where, although this wood is present throughout the stratigraphic sequence from the Gravettian to the Epipalaeolithic, from the Upper Magdalenian onwards it loses its predominant character and is present in low percentages. Also, thanks to our results, we can suggest that the choice of Pinus tp. sylvestris-nigra wood for lighting-related activities seems to be a cross-cultural aspect, shared by palaeogroups over a long period of time (30,000 years). In particular, the choice of this type of wood for lighting has been recorded from the Aurignacian to the Late Magdalenian in Nerja cave. Likewise, this aspect, reiterative over time, has been indicated for the preferential use of Pinus sylvestris in the Chauvet cave, although only for two different periods of occupation, the Aurignacian and the Gravettian (Théry-Parisot et al., 2018).
Our data also agree with the socio-economic deduction proposed in the previous anthracological study of Nerja cave (Badal, 1996) regarding the avoidance of stone pine wood as fuel, in view of the scarce fragments of this carbonised wood recognised throughout the external stratigraphic sequence and the abundance of pine cone bracts and pine nut shells from this tree. This led Badal and colleagues to consider the exploitation of this tree for food rather than fuel (Badal, 1996; Badal et al., 2014). Our results confirm this management of plant resources by Palaeolithic groups and the prior planning of wood used as fuel depending on the activity. Specifically, the fuel used for lighting is mainly Pinus tp. sylvestris-nigra, with Pinus tp. pinea-pinaster being very sparsely represented (2.38%).
The manual collection of the remains could also be one of the causes of the overestimation of some taxa over others (Chabal, 1997). In fact, when we applied a more exhaustive collection of the carbonised remains (by flotation), for example in the case of the Ledge of Horses in the Atxurra cave (Garate et al., 2023), a greater number of taxa were found. Their variability is not high, however, and the representativeness of some taxa over others continues to be eloquent in favour of one or, at most, two determinations. In this sense, ethnographic studies show that monofunctional fires, i.e. those linked to a specific activity (such as lighting in our case), have a higher degree of fuel selection (Henry et al., 2018).
Concerning the taphonomic alterations of the analysed charcoals, we observed different anomalies linked to the combustion process, such as vitrification (54.14%) and shrinkage cracks (32.41%). Vitrification is related to several factors, some of which may fit with the particularities of the context of origin of our charcoals, for example: a. the sudden interruption of combustion as a trigger for vitrification (Carrión, 2005) could be connected, in our context, to the gradual detachment of the charcoal from the torches (during the pyrolysis stage); b. the burning of small branches (Marguerie & Hunot, 2007) could correspond to the diameter of wood suitable for transport to deep contexts of the caves, as well as its suitability for the manufacture of torches; c. the burning of dense wood with high resin content (Py-Saragaglia & Ancel, 2006; Scheel-Ybert, 1998) could be linked with the taxonomic identification of most of our charcoal samples (pine and conifer wood); d. the aerobic degradation of the wood prior to combustion (Henry, 2011) could be linked to the use of dead wood derived from natural pruning for lighting (Théry-Parisot & Thiébault, 2005; Théry-Parisot et al., 2018). Concerning the last point, our identification of several samples with a strongly altered structure (44.14%) and with low consistency (hyperfragmentation) could also indicate a preferential use of dead wood, which is easier to collect and ignite, for artificial lighting (Asouti & Austin, 2005; Scheel-Ybert, 2001; Théry-Parisot, 2001). In addition, this preference would imply a strong seasonal component for the development of subterranean activity, outside the snow season, or planning of the activity by taking into account the necessary drying time of the wood (at least two years) (Théry-Parisot et al., 2018). Only dried wood guarantees a homogeneous and persistent combustion of flammable gases. However, in our experimental activities, we obtained good results with a slightly shorter dehydration time in a dry and sheltered environment.
Also related to this seasonal aspect, we must highlight the vegetative buds of Pinus sylvestris located in the internal context of the Nerja cave and related to Gravettian charcoals (27000 years ago) linked to the use of wooden torches (Figure 4) (Medina et al., 2015). This element is unique in similar contexts. These slightly resinous buds develop at the ends of branches during autumn and winter (Ruiz de la Torre, 2006). If the wood had been harvested when green, we could infer some seasonal data about the time of year when the visit may have taken place; however, the data obtained to date suggest the harvesting of dead or dry wood, so these shoots may have been part of the fuel collected after natural pruning. However, the discovery of these remains indicates that the fixed fires or torches could include other plant elements apart from woody fuel, such as these resin-rich shoots.
Figure 4. SEM photograph of Pinus sylvestris buds.
Finally, the charcoals examined show brownish stains (22.07%). Based on experimental observations, we attribute these stains to the partial and incomplete burning of the wood. This likely occurred when charcoal fell from the torches before complete combustion; some remains could also be due to partial combustion of a fixed fire.
In addition, traces of reaction wood (Figure 3) were identified (31.72%), together with some knots (1.38%). These alterations suggest that the firewood used must have come from a set of thin branches. The spatial context of the samples (internal context of caves, often of complex transit) and the suitability of this type of wood for the manufacture of torches, conditioned the use of thin wood. In addition, previous studies proposed this choice based on the size of the preserved wood remains in some caves, for example, woody imprints found in the clay from the floor of the Chauvet cave (Clottes, 2001), or wood branches preserved under the concretion in the Aldène cave (Galant et al., 2007).
At this point, we should point out that we tried to carry out a dendroanthracological analysis of the charcoals, although the absence of bark in all the samples (presence of the last ring), the small size and low consistency of the samples severely limited the results in this respect. Of the remains analysed in Nerja cave, only 9 fragments were found to have moderate ring curvature, 21 had strongly curved log rings and the vast majority (259) were classified as having indeterminate ring curvature. This was determined by observing the degree of curvature of the rings in the cross-section of the charcoal (Marguerie & Hunot, 2007). The high frequency of charcoals with indeterminate curvature considerably limits the representativeness of the results obtained. However, most of the determinable curvatures we observed present pronounced curvatures linked to a position of the charcoal fragment close to the pith or related to small diameter twigs; to a lesser extent they are consistent with moderate curvatures related to larger branches in relation to the previous ones or coals further away from the central axis; on no occasion did we observe weak ring curvatures linked to the combustion of large trunks.
The presence of hyphae was also recorded (32.76%), preferably post-combustion (white). This anomaly is linked to the endokarst microbial activity and to the high humidity. As far as possible, we preferred not to send any charcoal with this type of alteration to be dated, as it has been related to the possible rejuvenation of some radiocarbon dates obtained in similar contexts (for example, in the Tito Bustillo cave -Fortea, 2007).
In summary, beyond the species, the state or the size of the wood chosen as fuel, various ethnobotanical studies also show that historical and cultural conditioning factors influenced the selection of wood for firewood, even when other species were abundant in the vegetation (Henry et al., 2009; Henry et al., 2018; Lavrillier, 2007; Zapata & Peña-Chocarro, 2003). At the same time, socio-economic factors have been pointed out in the choice of fuel, such as the previously cited example of Nerja Cave (Badal, 1996; Badal et al., 2014). While the cultural and symbolic conditioning factors are difficult to ascertain from the archaeological record, they must nevertheless be taken into account, particularly in the archaeological context examined, which is often far removed from basic subsistence activities and subject to behaviours of a symbolic or ritual nature.
Prehistoric cave life through soot and charcoal trails
Through the C14-AMS dating of 53 of these charcoals, we constructed a high-precision Bayesian model to identify the minimum number of prehistoric visits to the interior of Nerja cave (Medina-Alcaide et al., 2023). The different phases were defined according to the presence of archaeological materials (and not only through the radiocarbon dating results) of each of the proposed phases in the stratigraphic deposits of the entrance rooms of the cave, or, failing that, to the presence of this type of cultural material in the regional context. For this purpose, using OxCal4.4, we sequenced these suggested Phases, limited by Boundaries determining the temporal distributions associated with the changes of phase. We also used the Outlier Charcoal model (Bronk Ramsey, 2009) to properly down-weight outlier samples, taking into account the long life of the charcoal samples. This allowed us first to confirm the statistical plausibility of the model, then to determine the start date (beginning of the boundary) and the end date (end of the boundary) for the different phases of human presence within the cave, their duration and the transition period between phases (Bronk Ramsey, 1995; Bronk Ramsey, 2001; Bronk Ramsey & Lee, 2013; Morrell, 2019).
The Bayesian model suggests at least 12 distinct phases of visits to the interior of Nerja cave between 41,218 and 3299 cal BP, with an agreement index (Aoverall) of 98. These phases of visits to the interior of the cave correspond to the specific chronocultural periods for the prehistoric regional context and with transitional periods: Early Aurignacian (phase 1), Recent Aurignacian (phase 2), Gravettian (phase 3), Lower Solutrean (phase 4), Middle Solutrean (phase 5), Upper Solutrean (phase 6), transition between the Upper Solutrean and Lower Magdalenian, and Lower Magdalenian (phase 7), Middle Magdalenian (phase 8), Upper Magdalenian (phase 9), Early Neolithic (phase 10), Recent Neolithic (phase 11) and Copper Age (phase 12) (Figure 5).
Figure 5. Phases of visits inside of Nerja cave identified by Bayesian model and micro-layers of soot.
There are 12 possible transition intervals between the different phases. One of them, separating phases 9–10 (between the Upper Magdalenian and the Early Neolithic), has a notable chronological amplitude (> 6000 years). There are however 8 transition periods that could correspond to 0 years (almost-continuous phases, taking into account the minimum values), in particular, those between phases 1–2 (both could be included in the Aurignacian, the former probably belonging to the ancient phase and the latter to the evolved stage), 4–5 (between the Lower Solutrean and the Middle Solutrean), 5–6 (between the Solutrean and the Middle Solutrean), 6–7 (around the Upper Solutrean and the transition between the Upper Solutrean and Lower Magdalenian), 7–8 (between the Lower Magdalenian and Middle Magdalenian), 8–9 (between the Middle Magdalenian and Upper Magdalenian), 10–11 (between the Early Neolithic and Recent Neolithic), and 11–12 (between the Recent Neolithic and Chalcolithic).
Through the identification, microcounting and dating of micro-layers of soot (fulinochronological analysis - Vandevelde et al., 2017; Vandevelde et al., 2018; Vandevelde et al., 2020; Vandevelde, 2021-) present inside a small stalagmite, we specified the minimum number of visits for the last three phases identified in the Bayesian analysis. This multianalytic approach was developed in collaboration with S. Vandevelde, E. Pons-Branchu and H. Valladas (Laboratoire des Sciences du Climat et de L’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay). The chronology of the soot microlayers found in the stalagmite was determined by the C14-AMS dating of CaCO3 layers deposited before and after them, following the methodology previously used in other studies on Nerja cave (Pons-Branchu et al., 2022; Sanchidrián et al., 2017; Valladas et al., 2017). The soot deposits at the base of the stalagmite were deposited between 7562 and 6736 cal. BP; the soot microlayers from the upper level of the stalagmite were deposited between 6836 and 2998 cal. years BP assuming 0% of DCP for the calibration of the dating.
A minimum of 64 occupations were identified between 7000 and 3000 years (a Minimum Number of Occupations—MNO that can be increased to 82 if we also count uncertain soot films and microcharcoal alignments; this second case will be included in brackets from now on). If we relate these data to the phases determined by Bayesian analysis, we can state that at least 58 (71) different occupations occurred in phases 10 and 11 (Early Neolithic and Recent Neolithic) with no apparent hiatus between these two phases (a result that is consistent with the Bayesian model from charcoal), and at least 6 (8) visits in phase 12 (Copper Age). The Figure 5 (modified from Medina-Alcaide, Medina-Alcaide et al., 2023) shows different phases of visits to the interior of Nerja cave identified by a Bayesian model from charcoal (in red) and image of the different micro-layers of soot of the stalagmite, which increases the minimum number of occupations to at least 64 for the last 3 Bayesian phases. The succession of soot films in the carbonates is represented as barcode diagrams. Bars represent soot films and dashed lines represent probable soot films. The long vertical grey line next to the barcode represents speleothem total thickness.
The identification of the soot remains inside the stalagmite as microlayers was characterised by TEM-EDX (following Pawlyta & Hercman, 2016) and Raman analysis (following Sadezky et al., 2005 and Deldicque et al., 2016). TEM–EDX observations revealed spherical carbon particles of soot aggregates (Figure 6). In Nerja Cave, similar particles were observed inside a Palaeolithic fixed lamp in the upper galleries (Medina-Alcaide et al., 2019), but soot residues were also located in another speleothem fragment inside the cave (Del Rosal, 2016; Pons-Branchu et al., 2022). Microscopic observation also revealed clay deposits together with microcharcoal and soot levels, suggesting that clay was not brought in by percolation at different times of the stalagmite formation but that the human visits to the cave contributed to the suspension of clays that re-deposited in the stalagmite together with the particulates from wood combustion (soot and microcharcoal).
Figure 6. TEM-EDX photograph of nanometric spherical particles of prehistoric fossilized soot.
Reliving paleolithic light for a better understanding of paleolithic cave art
The experimental replication of Palaeolithic light was monitored in an endokarst environment in order to resemble the humidity, temperature and moisture conditions of the Palaeolithic context as closely as possible. The physical parameters obtained for the three Palaeolithic lighting systems are given in Figure 7–Figure 8. These light resources were constructed on the basis of an exhaustive compilation of the existing literature on the subject, which also included archaeological data obtained as part of our doctoral thesis and postdoctoral study.
Figure 7. Average values of the parameters measured in each experiment with paleolithic torches, lamps and fireplaces.
Figure 8. Photograph of the experimental recreation of paleolithic lighting with torches.
The functioning of five wood torches was evaluated in a cave context (experiments 1–5 (Figure 7). They were all made from branches of dry juniper wood 1.2 cm thick that were joined together. This type of structure was chosen because it fits with the archaeological data and the form of the few prehistoric torches that have been preserved. For example, it matches the thickness of the remains of branches that have been found in the caves of Aldène and Réseau Clastres and with the morphology of a Hallstatt torch (Medina-Alcaide et al., 2021). Birch bark was included among the juniper branches as a kind of tinder to start combustion. Similarly, the wood was broken up to facilitate combustion (through easier oxygenation) and impregnation with non-ligneous fuel. Thus, pine resin, animal fat, or a combination were added in some cases to assess the function of the torches with those fuel types.
For fireplaces, the wood fuel was thin branches of juniper and oak wood in a dry state, arranged in a tepee-shaped structure. Birch bark was used to start the fire. This was placed inside the combustion structure. The fireplace was 23 cm in diameter and 7 cm high (measured before lighting), had no boundary structure, and was lit on a clay substrate. This experiment was carried out at a distance of 80 cm from the closest wall and 1.60 m from the ceiling in order to better assess the reflection of the light.
The lamps used in the experiments were replicas of the lamp from La Mouthe Cave (Dordogne, France) (Berthelot, 1901). This lamp was made of sandstone, 17 cm long and 12 cm wide, and had a concavity with ≈150 cm3 capacity. Bovid marrow was used as the main fuel, with three woody wicks composed of dried and crushed juniper wood. They were arranged in a tepee shape in the center of the active part of the piece. Pine resin was added in one experiment (number 7) to assess its lighting benefits.
Thanks to the physical parameters of Palaeolithic light thus deduced, we developed pioneer methodologies in GIS environments for the quantitative determination of the degree of visibility, gauging and difficulty of access to Palaeolithic Art (Intxaurbe et al., 2021; Intxaurbe et al., 2022). The Figure 9 show the 3D restitution of the rock art combined with the 3D model of the cave, showing that the panel is located over a cornice, and (B) the viewsheds of the Graphic Unites in Sector J using ArcMap in ArcGIS. The most visible figures are in a high panel. The influence of the archaeological context (in these cases the presence of hearths) is also important to make inferences (Intxaurbe et al., 2020: Figure 6). Besides, we took advantage of the potential of GIS (ArcGIS) to design a cutting-edge methodology to quantify the visibility and capacity in sectors with Palaeolithic Art inside caves. This allowed us to estimate the maximum potential audience for Palaeolithic Art, and the optimal location for visibility, by creating a PYTHON script that was integrated into the software, and provided a comparison with quantitative data between different caves. This innovative methodology was successfully applied in three renowned caves in the Basque Country: Santimamiñe, Altxerri and Atxurra. It showed that, in some cases, there may have been prior planning to improve the visibility of some figures. In all cases, the groups of figures are located in deep and hidden parts of the caves, normally in sectors with a limited capacity to hold people, which would be consistent with a restricted communication system (Intxaurbe et al., 2021).
Figure 9. Viewshed analysis of Sector J of Atxurra cave.
For example, in the paradigmatic case of the shelf in sector J of Atxurra, the location of the hearths at the foot of the rock art ensemble (dated to the same period as the artworks themselves on the adjacent wall) seems to have been determined to provide visibility, as the halo of firelight would have illuminated almost the entire wall with paleolithic art in this zone (Garate et al., 2020a; Garate et al., 2020b; Medina-Alcaide et al., 2021), thus increasing the visibility indices of the figures on the right side of the wall. This is even more prominent for the two main horses of the composition as they feature a number of techniques such as engraving and scraping, and maybe painting.
Secondly, the physical parameters of Palaeolithic light obtained through our archeological and experimental studies were used to propose a scoring methodology to quantify the accessibility of Palaeolithic Art inside the caves. The proposed methodology provides information on the difficulty of access, the estimated time of arrival, the length of the optimal route, etc. (Intxaurbe et al., 2022). We developed a PYTHON script that integrates light and velocity data (experimental tests with professional cavers) for the spatial analysis of the 3D models of the caves from GIS. This spatial study of the paleolithic art in the caves makes it possible to compare the accessibility of the sites with quantitative (and not only qualitative) data, which is essential to investigate which graphical elements are more inaccessible or hidden and why.
This methodology was applied in Atxurra Cave to evaluate the accessibility of all the rock art zone. As a preliminary conclusion, the results obtained seem to show that the artists' choice of panel was directly affected by the difficulty of transit. The Upper Magdalenian groups at Atxurra chose panels that were difficult to access, despite the fact that the options offered by the cave are much wider and less risky. However, the reason for this is unknown: a desire to "hide" this art? evidence of rites of passage? (Arias, 2009; Owens & Hayden, 1997). In some cases, it seems that the locations in high areas could have been related to more functional reasons, such as giving visual prominence to certain figures, as in Sector J. This, as mentioned above, is accentuated by the location of the fires lit in this area of the cave.
Logically, the estimated time to access each sector increases for areas that include constant vertical zones (passable by climbing or traversing). The experience, physical strength or knowledge of the topography of the cave would help to optimise the time needed to access the deep areas.
Conclusion
The first results obtained within the project entitled the A-Light (funded by the Fyssen Foundation between 2021, and MSCActions PF 2022–2024) confirm the potential of the interdisciplinary study of combustion and lighting remains in caves for the knowledge of prehistoric subterranean activities, including rock art. The exhaustive anthracological analysis of charcoals from fires, including taxonomic, taphonomic and dendroanthracological approaches, suggests that there was a selection of fuel for cave lighting probably linked to its environmental availability, its lighting capabilities and for other economic and cultural reasons. Thanks to a pioneering multiproxy and interdisciplinary approach, the C14-AMS dating, the Bayesian analysis, and the interdisciplinary study of the soot deposits, we have been able to define the useful life of the cave (in the case of Nerja cave we have extended the origin of human occupation of this famous cave by 10000 years) and the minimum number of prehistoric visits to the cave interior. Experimental replication and monitoring of the physical parameters of Palaeolithic light has allowed us to better understand the functioning of these lighting systems, as well as to develop innovative methodologies for the evaluation of the visibility and accessibility of Palaeolithic art inside the caves. In short, these data demonstrate that the Archaeology of the Light is here to stay and that it is an essential approach for a holistic understanding of Palaeolithic caves.
Ethics and consent
Ethical approval and written informed consent were not required.
