Chert procurement for inferring Neanderthal mobility in central western Iberia

doi:10.21203/rs.3.rs-7684583/v1

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Chert procurement for inferring Neanderthal mobility in central western Iberia

https://doi.org/10.21203/rs.3.rs-7684583/v1

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Chert was one of the preferred lithic raw materials used across Prehistory whatever the continent. To acquire it, people moved across large distances and developed networks with other territories and with the other groups therein. Some of the reasons for such effort are the ability of chert to produce extremely sharp cutting edges, the great predictability of its knapping, retouch and shaping processes, and the endurance of the edges.

Because stone-tools are the most available and reliable sources of information on prehistoric human behavior, and because chert sources have specific macroscopic and geochemical features, this raw material is one of the most used to infer the mobility and economical patterns of prehistoric populations.

Neanderthals inhabited western Iberia from ca. 200 ka to 40 ka, and most of the Mousterian sites have chert in their assemblages. In this study we analyze the chert assemblages from the Mousterian layers of Gruta Nova da Columbeira, Praia Rei Cortiço and Mira Nascente through macroscopic, PIXE and pXRF analysis. Results suggest as possible provenance regions the nearby coastal bluffs and stream deposits draining across the Jurassic bedrock toward the Óbidos valley. Little is still known about the chert sources from Portugal and how they fit into the archaeological sites. With Gruta Nova da Columbeira, Praia Rei Cortiço and Mira Nascente being reference sites to the Portuguese Mousterian, this study allows us to deepen the study of human population dynamics and the relationship with regional resources and landscape in the Late Pleistocene.

Western Iberia

Neanderthals

Mousterian

Lithic assemblages

Chert

Mobility was a key feature of Pleistocene human adaptation and can be particularly difficult to understand in the archaeological record, but lithics provide one of the most reliable datasets to access it.

The investigation of raw material sourcing is done by comparing the characteristics of the raw materials in which stone-tools were made with samples taken from natural sources. This can be done through several methods with different resolution (high, medium, low), damage (destructive, non-destructive), sample size, cost, and type of output data (visual, numerical) (Abrunhosa et al. 2020). With the ongoing technological development, there is an increased use of non-destructive and high-resolution methods. In large lithic assemblages (in the order of several hundred or several thousand artefacts) the analytical methods tend to be limited to a few samples. Therefore, macroscopic analysis is usually the first step to identify groups and define subgroups (Aubry et al. 2005; Moreau et al. 2016).

In the last decade, studies on raw material sourcing, processing and distribution have been used in identification of sourcing areas (Clarkson and Bellas, 2014), and natural availability of types and subtypes of raw material (Knutsson et al. 2016). They have also led to inferences about behaviour (de Lombera-Hermida and Rodríguez-Rellán 2016), economy (Doronicheva et al. 2017), culture and mobility patterns (Aubry et al. 2012; Perreault et al. 2013; Pereira et al. 2017b; Ichinose et al. 2022). This work has also informed discussions of cognition and in archaic human groups (Braun et al. 2008, 2009), behavioural complexity (Brown et al. 2009; Nash et al. 2013; Wilkins et al. 2017) and the emergence of social complexity (Cassen et al. 2012; Petrequin et al. 2012; Pétrequin et al. 2012; Pétrequin and Pétrequin 2016).

Lithic raw material use patterns can elucidate Neanderthal selection preferences (Pereira et al. 2017a; Abrunhosa et al. 2019; de Lombera-Hermida et al. 2020), transport and curation (Baumler 1996; Burke 2006), and overall exploitation territory even if the exact position of some sources are not yet recognized (Matias 2016; Doronicheva et al. 2017; Gómez de Soler et al. 2019, 2020; Abrunhosa et al. 2020; Eixea et al. 2022). The curation of rare and exotic raw materials typically reflects the presence of long-distance networks of exchange among modern humans (Mcbrearty and Brooks 2000; Bar-Yosef 2002), but Neanderthals also displayed this behaviour (Zilhão et al. 2010, 2020; Cortés-Sánchez et al. 2011; Hoffmann et al. 2018a, b; Nabais and Zilhão 2019). Despite evidence that Iberian Neanderthals had relatively wide territories and complex networks, the available lithic raw material data appears to show short to medium range acquisition of chert in most cases, often following stream valleys (Sunyer 2016; Abrunhosa et al. 2020; de Lombera-Hermida et al. 2020; Rios-Garaizar and Eixea 2021; Eixea et al. 2022). Still, studies about raw material sourcing tend to highlight the primary rather than secondary sources, meaning that such distance would have been the hypothetical maximum range of acquisition and not necessarily the maximum range of their territories.

The study of lithic raw material sourcing by Neanderthals in Portuguese Estremadura is only beginning (Matias 2012, 2016; Pereira et al. 2015), but the available inventories show abundant chert at sites located in chert-rich territories, and its dramatic decline at sites with different geology, even in regions adjacent to chert-rich areas (Pereira et al. 2012). This suggests that, overall, Neanderthals may have had relatively small exploitation territories.

1.1. The geomorphological background of the sites

Portuguese Estremadura is a coastal region located in central western Iberia (Fig. 1) belonging to Iberia’s Western Mesocenozoic Edge, particularly the Lusitanian Basin. This sedimentary basin was formed during the opening of the North Atlantic and has different lithologies (Kullberg 2000; Kullberg et al. 2006) (Table 1).

Table 1
Phases, ages and sediments of the Lusitanian basin.
Phase	Age	Sedimentary sequence
Pre-rift	Late Triassic and the Early Jurassic	Sandstones, siltstones, conglomerates, halite, gypsum, and lacustrine limestones and shales
Syn-rift	Middle Jurassic and Early Cretaceous	Marine limestones, sandstones, siltstones, and mudstones organic-rich shales, reef and carbonate platform deposits, and some volcaniclastic layers
Post-rift	Late Cretaceous and the Cenozoic	Marine limestones and marls, sandstones and shales, flysch-type deposits, and alluvial and deltaic sands and gravels

The modern landscape is mostly based on uplifted Lower to Upper Jurassic limestone and dolomite units building the ca. 800 km² Maciço Calcário Estremenho (MCE), the limestone massif of Estremadura (Martins 1949, Manuppella et al. 1985) that spread to the submerged nearby continental platform (Ribeiro et al. 1979; Almeida et al. 2000; Ferreira 2000). The MCE is marked by bounding tectonic faults that enclose wide karst depressions and valleys, dividing the region into distinct plateaux (Martins 1949; Reis et al. 2023). In the major river valleys (e.g. Tejo, Lis, and Mondego) the Miocene to Quaternary deposits and terraces, and also Upper Cretaceous beds, have abundant silicic gravels mostly composed of quartzite and quartz (Cunha 2019).

In the karstic areas, these silicic deposits are less common, but within the Middle Jurassic (Bajocian and Bathonian) and Upper Jurassic limestone beds chert lenses and nodules are well known (Matias 2012; Jordão 2023). The incised drainage network exposes at several locations bedrock layers displaying chert nodules that can be found detached in the foot slopes, in terraces and, ultimately, in the shoreface (Matias 2012; Aubry et al. 2012; Pereira et al. 2016; Jordão 2023).

There are nine main chert sourcing areas: Leiria, Serra d’Aire e Candeeiros, Lisbon, Torres Vedras and Cesaredas have several primary chert outcrops. Secondary chert deposits are found as fluvial gravels in the valleys of the rivers Zêzere, Maior and Alenquer. At the coast, the Nazaré outcrop has several chert layers that feed the coastal gravels (Fig. 2).

Due to its position, the region was always deeply influenced by sea level oscillations, causing the Pleistocene coastline to be, for most of the time, further west (Daveau 1980; Quézel 1985; Boski et al. 2000), and most probably exposing extensive gravel deposits. Presently, the coastal limestone is usually covered by marine and continental siliciclastic sediments, including gravels, of the same ages as the river terraces (Gouveia et al. submitted). At significant locations of the study areas outcrop regionally developed Upper Cretaceous gravel layers rich in quartzite and other hard silicic rocks with a provenance from the Iberian Massif, devoid of sedimentary limestone related cherts. Sometimes those deposits raise up to dozens of meters as steep cliffs as result of the regional diapirs and erosion. Above them the overall landscape is covered by Pleistocene and Holocene dune fields (Daveau 1980; André et al. 2009; Cunha 2019; Haws et al. 2020).

1.2. The sites

Gruta Nova da Columbeira (39°17'53.42"N | 9°12'2.60"W) (Figs. 1 and 2 - #1, Figs. 4 and 5) is a cave with seven Mousterian layers, rich in fauna and lithics, located in a canyon that runs across the Cesaredas Plateau (Raposo and Cardoso 1997, 1998; Cardoso et al. 2002; Fernández-Laso et al. 2015; Carvalho et al. 2018; Figueiredo et al. 2018). At its mouth, the canyon meets the Óbidos valley, a depressed area close to the seashore and to where converge several streams coming from the western façade of the Serra de Aire e Candeeiros.

Mira Nascente (39°42'21.56"N | 9° 2'58.74"W) (Figs. 1 and 2 - #3, Fig. 3 top) is a single layer, open-air site with hundreds of lithics and several refittings, located at the middle of an eroding coastal cliff (Benedetti et al. 2009; Haws et al. 2010, 2020). At this site, 30 m of late Quaternary sand and pebbles overlie the limestone bedrock which outcrops on the beach. Chert nodules such as those found at the site are not visible in the immediate area, but it may have been available in the shore face or in bedrock outcrops that have since been covered or eroded.

Praia Rei Cortiço (39°25'22.38"N | 9°14'35.69"W) (Figs. 1 and 2 - #2, Fig. 3 bottom) is a single layer, open-air site, with hundreds of lithics and several refittings, located at the foot of an eroding coastal cliff (Benedetti et al. 2009; Haws et al. 2010, 2020) The cliff exposes Late Pleistocene and Holocene valley fill deposits incised into the Cretaceous conglomerate bedrock, which is exposed adjacent to the site and contains abundant quartz, quartzite, and chert pebbles. The presence of this chert near the site results from the combination of cliff erosion and transport by colluvial and fluvial processes.

A multi-method analysis composed of macroscopic and mesoscopic characterization and elemental analysis by pXRF and PIXE was carried out to determine the variability of the raw materials from Gruta Nova da Columbeira, Mira Nascente and Praia do Rei Cortiço, the probable origin of the chert, and to compare the results between these sites. The geological samples for comparison were from the reference collection LusoLit (Pereira et al. 2016) (red dots at Fig. 2).

Macroscopic observations were made using a 30x magnification loupe, focusing on diaphaneity, fracture, color variation, porosity, surface texture, homogeneity, and the presence of microfossils following established descriptive criteria (Dunham 1962; Embry and Klovan 1971; Hurlbut and Klein 1977; James 1984; Vernon 2004; Abrunhosa et al. 2020). Raw materials were grouped and subgrouped based on shared macroscopic characteristics. Color, including its variability and homogeneity, was described using up to three Munsell rock color chart codes (Munsell Color 2009) per specimen: two for internal color variations and a third for the cortex, if distinguishable (Ferguson 2014) (Table 2).

Table 2
Results from the macroscopic analysis of the Gruta Nova da Columbeira samples.
	Sample Name	Porosity		Transparency	Luster	Singular Components	Colour Variability	Munsell 1	Munsell 2	Munsell 3 (Cortex)
Mudstone	CH1	Compact		Semi-Opaque	Dull / Waxy	Chalcedony	Variable	10YR 6/2	5YR 8/1	-
	CH6	Semi-Porous		Semi-Opaque	Waxy	Geodes	Homogeneous	N9	10YR 7/4	-
	CH9	Compact		Translucent	Dull	-	Variable	5Y 8/1	5YR 4/1	5YR 5/6
	CH11	Semi-Porous		Semi-Opaque	Dull / Waxy	Liesegang Rings, Iron Oxides (5%), Bioclasts (Gastropods)	Homogeneous	5Y 8/1	10YR 8/6	-
	CH12	Compact		Semi-Opaque	Dull	-	Variable	5Y 8/1	5B 7/9	-
	CH13	Compact		Semi-Opaque	Waxy	Liesegang Rings, Iron Oxides (1%)	Homogeneous	10YR 8/2	10YR 6/6	-
	CH15	Compact		Translucent	Waxy / Dull	-	Homogeneous	5YR 5/2	-	10YR 8/2
	CH17	Compact		Opaque	Waxy	-	Homogeneous	N5	N4	-
	CH19	Compact		Semi-Opaque	Waxy	Iron Oxides (1%)	Homogeneous	5YR 2/1	5YR 5/2	-
	CH20	Compact		Opaque	Waxy	-	Homogeneous	N3	-	10YR 6/2
Mudstone to Wackestone	CH3	Compact		Semi-Opaque	Waxy	Bioclasts	Variable	5R 4/6	10R 6/2	-
	CH10	Compact		Semi-Opaque	Dull / Waxy	Liesegang Rings, Iron Oxides (30%), Bioclasts	Variable	5RP 8/2	10YR 8/6	-
	CH16	Compact		Semi-Opaque	Waxy	-	Homogeneous	5YR 6/4	-	5Y 5/2
	CH18	Compact		Opaque	Dull	Bioclasts	Variable	5R 4/2	10R 7/4	-
	CH21	Compact		Semi-Opaque	Waxy	Iron Oxides (20%)	Variable	5R 3/4	10R 2/2	-
	CH23	Compact		Translucent	Dull	Bioclasts	Homogeneous	10YR 6/2	-	-
Wackestone	CH2	Compact		Semi-Opaque	Vitreous / Waxy	Geodes	Variable	10R 5/4	5YR 2/1	-
	CH4	Compact		Semi-Opaque	Dull	Bioclasts	Variable	5YR 8/1	5YR 6/1	-
	CH5	Compact		Semi-Opaque	Dull / Waxy	Bioclasts, Chalcedony	Variable	10YR 68/2	10YR 8/6	-
Packstone	CH7	Semi-Porous		Semi-Opaque	Dull	Bioclasts	Variable	10YR 8/2	10R 2/2	-
	CH14	Compact		Semi-Opaque	Dull / Waxy	Geodes, Iron Oxides (1%)	Variable	5Y 5/2	10YR 8/6	10YR 8/2
	CH22	Compact		Semi-Opaque	Dull	-	Variable	10R 4/2	5R 8/2	-
Boundstone	CH8	Semi-Porous		Opaque	Waxy	Geodes	Variable	N9	5YR 5/6	-
Boundstone	CH24	Compact		Semi-Opaque	Dull	-	Variable	5YR 5/2	10YR 8/2	-
Summary of Chert Type Classification.
Chert Type	Texture		Porosity			Main Features	Examples
Mudstone-Dominated Cherts	Compact		Low			Liesegang rings, iron oxides, bioclasts	CH1, CH9, CH10, CH11, CH12, CH13, CH15, CH17, CH19, CH20
Wacke-Dominated Cherts	Compact		Low			Bioclasts, geodes, chalcedony	CH2, CH3, CH4, CH5, CH16, CH18, CH23
Packstone Cherts	Semi-porous to compact		Moderate			Bioclasts, iron oxides, geodes	CH7, CH14, CH21, CH22
Boundstone-Associated Cherts	Compact		Low to moderate			Geodes, silicified boundstone	CH8, CH24

Geochemical elemental analysis was performed in 2017 solely on the Gruta Nova da Columbeira assemblage. A portable Bruker™ S1 Titan® energy dispersive X-ray fluorescence spectrometer, equipped with a rhodium X-ray tube, FAST® SDD detector, 5 mm collimator, S1RemoteCtrl filter, and S1Sync software, was used. The spectrometer was operated in a closed desktop configuration. Elemental analysis, covering elements from Magnesium (¹²Mg) to Uranium (⁹²U), was conducted on cleaned, flat, homogeneous surfaces. Each analysis consisted of a 240-second measurement, split equally between heavy and light elements (120 seconds each). Multiple analyses were performed per specimen. The default "Application Geochem General Method Dual Mining" calibration was used, consistent with previous successful applications (Carvalho and Pereira 2017; Pereira et al. 2017a, 2021; Paixão et al. 2019). PIXE analyses were carried out on all three assemblages at the IST laboratory in 2014, following their established protocol (Corregidor 2011). Profiting from the external beam setup, the samples could be examined in ambient air, rendering sample size irrelevant. Each specimen was exposed to a 2 MeV and 500 pA proton beam for 15 minutes, targeting an area of 0.9 × 0.9 mm². The resulting X-rays were collected using a 30 mm² SDD detector, and the spectra were processed and quantitatively analysed using the GUPIXWIN software (Campbell 2010). With the chosen experimental conditions, only elements heavier than Si can be detected and quantified. All detected elements were considered to be present in its most common oxide form. While most specimens received a single measurement, some refitted nodules and specimens with multiple-colored zones underwent multiple measurements to verify the consistency of their elemental composition and proportions. In total, 46 specimens yielded 60 readings (Supplementary information Tables 1 and 2).

With both pXRF and PIXE, several of the common elements are measured as their oxides readings (Supplementary information Tables 1 and 2).

Praia Rei Cortiço, Gruta Nova da Columbeira and Mira Nascente contain quartzite, quartz, chert, and other raw materials in trace amounts. However, Mira Nascente is dominated by chert, Praia Rei Cortiço by quartzite, and at all layers of Gruta Nova da Columbeira the amounts of the three main raw material change but are always balanced (Table 3).

Table 3
Raw material distribution from layers 4 to 9 of Columbeira Cave, Mira Nascente and Praia Rei Cortiço.
Context	Flint	Quartz	Quartzite	Others	Reference
Columbeira Cave Layer 4	37,4	35,5	27,1	0	Raposo et al, 2002
Columbeira Cave Layer 5	36,5	36,5	26,9	0,1	Raposo et al, 2002
Columbeira Cave Layer 6	30,8	43,8	25,4	0	Raposo et al, 2002
Columbeira Cave Layer 6a	34,1	36,2	29,7	0	Raposo et al, 2002
Columbeira Cave Layer 7	30,4	37,7	31,8	0,1	Raposo et al, 2002
Columbeira Cave Layer 8	26,9	39	34	0,1	Raposo et al, 2002
Columbeira Cave Layer 9	30,6	47,9	21,5	0	Raposo et al, 2002
Mira Nascente	86,6	12	1,4	0	Haws et al. 2020
Praia Rei Cortiço	3,1	29,5	67,0	0,4	Haws et al. 2020

Quartzite and quartz have some internal variability, appear always as rounded pebbles with very good and excellent knapping qualities. These raw materials are highly abundant in multiple and vast Cenozoic deposits widespread across the Lusitanian Basin region. They arrive by fluvial transport from the Iberian Massif Zone to the east, or by remobilization from the nearby outcropping Cretaceous conglomerates (Praia Rei Cortiço). They were most probably collected from local gravels, and for this reason they do not give relevant information about human mobility.

Chert, in contrast, is only present at some specific locations with different geochemical compositions. This allows a relatively good understanding of their distribution across the landscape as primary and secondary sources and, consequently, to infer human mobility and territoriality. However, it cannot be ruled out the possibility of other primary and secondary sources have been used but that are presently undersea, buried, eroded or unknown.

3.1. Macroscopic analysis

The macroscopic analysis of chert from Gruta Nova da Columbeira, following the methodology described above, identified five primary groups (Table 2).

The macroscopic characteristics of these chert assemblage shows diversity in texture, mineralogy, and color, suggesting different depositional and diagenetic conditions. Most samples are fine-grained, ranging from mud-supported (Mudstone, Wackestone) to grain-supported (Packstone, Boundstone).

Mud-supported cherts indicate low-energy sedimentation of the host limestone layers, while grain-supported types point to higher-energy environments or biogenic frameworks. Cherts are generally compact, indicating low primary porosity, or high diagenetic silicification though some semi-porous varieties (e.g., CH6, CH7, CH8, CH11) may show secondary porosity from diagenesis. Iron oxides, Liesegang rings, and geodes indicate post-depositional alteration, including fluid-rock interactions and oxidation, while chalcedony and bioclasts contribute to variability in luster.

Mud-supported cherts, such as CH1, CH6, and CH9, are compact to semi-porous with predominantly semi-opaque to translucent transparency. These samples exhibit dull to waxy luster, with some showing notable chalcedony content (e.g., CH1). CH6 is distinguished by the presence of geodes, suggesting a diagenetic overprint involving silica-filled void spaces. CH9, on the other hand, stands out for its high translucency, possibly indicating a high degree of silica purity and a lack of detrital impurities. The Munsell color values range from light grayish brown (10YR 6/2) to darker gray tones (5YR 4/1), suggesting generally low iron content with subtle variation, and potential organic matter influences.

Wacke-supported cherts (e.g., CH2, CH4, and CH5) are compact with low porosity, exhibiting semi-opaque transparency and a mix of dull, vitreous, and waxy lusters. CH2 is notable for the presence of geodes, which suggest cavity-filling silica precipitation during late-stage diagenesis. CH4 and CH5 contain bioclasts, indicating biological input and potential silica replacement of carbonate components. CH5 further distinguishes itself with the presence of chalcedony, reinforcing the interpretation of a mixed biogenic and chemically precipitated silica origin. Color variability within this group is significant, with hues ranging from reddish brown (10R 5/4) to light gray (5YR 8/1), likely reflecting iron variations and diagenetic oxidation processes.

A subset of samples, including CH3, CH10, and CH21, exhibit characteristics indicative of mixed mud-wacke compositions. These cherts maintain compact textures and semi-opaque transparency but display notable diagenetic features such as Liesegang rings (CH10, CH21) and iron concentrations. The Liesegang banding, particularly in CH10, suggests rhythmic silica precipitation influenced by fluctuating redox conditions. Iron-rich samples, such as CH21, exhibit darker hues (5R 3/4, 10R 2/2), consistent with ferruginous silica cementation.

Pack-supported cherts (CH7, CH14, and CH22) exhibit semi-porous to compact textures, with dull to waxy luster and moderate variability in bioclastic content. CH7, for instance, contains bioclasts and shows significant color heterogeneity (10YR 8/2 to 10R 2/2), potentially indicative of depositional variations in oxygenation levels. CH14 contains minor iron, which contribute to subtle reddish-brown tones (5Y 5/2, 10YR 8/6). These cherts may represent higher-energy depositional settings, where silica replacement of pre-existing carbonate frameworks was influenced by more dynamic diagenetic conditions.

Boundstone-associated cherts (e.g., CH8, CH24) exhibit a more massive texture, typically with opaque to semi-opaque transparency and waxy or dull luster. These samples occasionally contain geodes (CH8), suggesting synsedimentary or early diagenetic silicification of carbonate-rich sediments. Their color profiles, ranging from white (N9) to reddish-brown (5YR 5/6), indicate a combination of primary mineral composition and later diagenetic alteration.

Overall, the chert assemblage reflects a spectrum of silica diagenetic processes, from primary biogenic sources to secondary chemical precipitation and silica replacement of pre-existing carbonate substrates. Variability in iron content, Liesegang banding, and porosity characteristics further suggest differential diagenetic histories, likely controlled by localized geochemical conditions and fluid-rock interactions.

It was possible to detect secondary patinas reflected mostly on the lustre of the assemblage. The great majority of the pieces analysed present a waxy to dull lustre. Thermal alterations caused thermal domes, cracking and a waxy sheen. Likewise, these elements may have alterations in their colouring and fracture pattern which partly limit the correct correlation of these pieces with macroscopic groups. When thermic alterations were evident, these elements were excluded from the raw material identification analysis. However, their presence is important to note because it confirms the occurrence of fires inside the cave, as suggested by the ash detected during the excavation (Cardoso et al. 2002).

3.2. pXRF results

The pXRF analysis (Supplementary information Table 1 and Fig. 6a) was only performed in the assemblage from Gruta Nova da Columbeira and a wider number of samples from LusoLit. Results show the predominance of silica often with values close to 100% but shifting in some cases down to 81,5%. Phosphate, sulfur, and chlorine are present in all samples. Calcium is present in all archaeological samples from Gruta Nova da Columbeira and in several geological samples from primary outcrops and gravels nearby them, reflecting the environmental inputs from the cave and their original association with limestone or dolostone.

Nickel is present in very few specimens, and it only appears in the samples from further north in the eastern façade of the Serra d’Aire and Candeeiros (Arneiro da Milhariça and Espinheiro). Au appears in several archaeological and geological samples, the latter from the eastern drainage of Serra d’Aire and Candeeiros. Other samples to the west of this mountain are associated with quartzite, quartz, and chert cobbles in the gravels of Leiria and Nazaré, but never in primary position. Tungsten, barium, and silver appear in geological samples from this same region on either side of the Serra d’Aire and Candeeiros. Some of the geological samples from this area may have moved North-South due to the river dynamics, but they seem to be relatively consistent in their position. The samples with less silica are those that tend to have higher magnesium. Aluminum is present in most but not all specimens, which is a common feature of several sources in western Iberia.

The cluster analysis of single readings and mean values of multiple readings show several branches linking the Gruta Nova da Columbeira specimens with the Cesaredas at west, the Nazaré outcrop and Leiria at north, some sources from the top the Serra d’Aire and Candeeiros, and with gravels from the western parts of Rio Maior and Alenquer.

3.3. PIXE results

Silica is dominant but its frequency shifts between 99,9% and 61,4%. Phosphorus, sulfur, chlorine, potassium, calcium, titanium and manganese are the most common elements while cobalt, nickel, copper, zinc, bromine and strontium are present in lesser amounts. Bromine (usually associated with evaporitic deposits) and strontium are only present in archaeological specimens from Praia de Rei Cortiço and Gruta Nova da Columbeira, which might indicate a local chemical signal that is not from the western face of Cesaredas from where two of the several geological samples already taken were analyzed. Nickel is only present in Nazaré and cobalt is only present in Nazaré and Montes (12 km inland from Nazaré), both areas more than 30 km from the cave.

The cluster analysis shows two major branches. One includes archaeological specimens from Gruta Nova da Columbeira and a single specimen from Praia Rei Cortiço that is chemically and macroscopically distinct from the others recovered from this site, and another with only archaeological specimens from Praia Rei Cortiço and Mira Nascente (Supplementary information Table 2 and Fig. 6b).

4.1. The lithic assemblages

The lithic assemblage of Gruta Nova da Columbeira has three main features: 1) a wide diversity of raw materials with balanced amounts of quartz, quartzite and chert, along with other rocks that appear in very little quantities; 2) little cortex that, when present, is waterworn, and; 3) high curation rates (Raposo and Cardoso 1997, 1998; Cardoso et al. 2002). These features are considered as markers of distance decay by themselves (Renfrew 1969, 1977; Wilson 2007; Clarkson and Bellas 2014).

Interestingly, this pattern of low waterworn cortex (that does not demand a necessary cortex removal) also occurs with quartz and quartzite, which are abundant and ubiquous across the landscape. The absence of the configuration phase and low amounts of cortex in the cores (particularly the pre-determinate ones) and on the blanks across the stratigraphy indicate that they were shaped and produced elsewhere and brought as so into the cave.

This is the opposite of what happens at Mira Nascente and Praia Rei Cortiço, where cortex is present, along with some Levallois flakes where configuration products refit into cores (Benedetti et al. 2009; Haws et al. 2020).

4.2. The analyses

Macroscopic analysis has strong limitations because the characteristics are qualitative and, even if the evaluation parameters are defined in advance, their evaluation is partly subjective to the observer (Agam and Wilson 2019). However, it is necessary on large assemblages such as that of Gruta Nova da Columbeira, where a great variability of raw materials can be noted by naked eye to create big groups for further analysis.

The macroscopic groups do not correspond directly to different probable sources because a source can have internal variability. Sometimes, this is even visible within a single artefact, allowing to create affinity subgroups. When there is great variability, it is common to create sub-groups that inform on the reasons behind the selection, while also helping to understand how much variability can be discerned from a given source. Nevertheless, macroscopic features can be created by post-depositional (mechanical and chemical weathering) conditions. Petrographic analysis of thin sections would be necessary to best determine the environment of origin of the defined chert types, but this destructive analysis has not been authorized at this stage of the work. However, results from macroscopic analysis, combined with non-destructive geochemical analyses, may justify future destructive analyses to refine the data.

Geochemical analysis also brings important advantages after some caution being considered. Chemical weathering caused by water flow in the cave can react with the minerals and form new minerals and soluble salts, particularly if the water is acidic. This reaction results in a patina on the artefact surface but also alters the chemical surface analysis results. It is for this reason that the use of combined methods is recommended.

Although PIXE can be considered a non-destructive and used as a non-invasive technique, small and almost imperceptible alterations to the surface could sometimes be perceived. In these type of samples this is just the result of the formation of “colour centres” likely manifesting as localized darkening most probably on limestone rich areas. Elemental concentrations are then not altered and sensitivity and reliability of results maintained. The two low-power lasers employed in the setup serve solely to define the analysis point, determined by the intersection of their beams. Given their minimal energy output, they pose no risk of damaging the sample. The major downfalls are the high costs of both the equipment and analysis. On the other hand, pXRF is practical and more affordable, but there are many machines available providing very different resolutions and degrees of reliability.

In this analysis, the geochemical readings also indicate some chemical coating or input caused by post-depositional action, with Praia Rei Cortiço and Mira Nascente having an open-air, coastal signal, while Gruta Nova da Columbeira an inland cave signal. This is evident with the high frequency of calcium in the specimens from the latter, compared with low frequency at the formers. The relatively low calcium values do not seem to obscure the relations between the sites and the chert sources of the region.

4.3. The areas of acquisition

It is difficult to find clear geographic, chronological and cultural trends and boundaries for the Iberian Mousterian based solely on the lithic technology and typology (de la Torre et al. 2013). Variation can be quite visible, however, in the use of raw materials. Dozens of sites converge towards a regional trend where artefacts are predominantly made according to the quantity and quality of the raw materials locally available. If chert was good and abundant, it was always the preferential raw material. If other suitable raw materials were more readily available, they were used instead. When several materials were accessible, the sites reflected that diversity as well (Romagnoli et al. 2022).

It is assumed that the range of acquisition of lithic raw materials was broadly the same as the other resources (Sunyer 2016). In this sense, Site Catchment Analysis (SCA) allows development of models for economic areas around the sites (Vita-Finzi et al. 1970; Vita-Finzi 1978), with concentric lines of 5, 20, 30, or > 30 km radius corresponding to short, medium, large, and very large ranges (Geneste 1989). Despite being theoretically sound, SCA is not perfect (Roper 1979), because such concentric lines exclude important geomorphological elements such as topography and hydrography. Therefore, additional perspectives are necessary.

In the case of the study area, even considering the problems associated with the preservation of the material and the limitations of each one of the methods of analysis, the multi-proxy geochemical analyses give consistent results about the potential raw material sourcing of the western-most Neanderthals, and allow us to move forward with inferring about Neanderthal territory.

The PIXE results show that the coastal sites of Praia Rei Cortiço and Mira Nascente seem to be related with the coastal sources of Nazaré, Montes and Cesaredas, at the western façade of the Serra d’Aires and Candeeiros, but not with the inland region of Rio Maior or the others that feed a considerable part of the Gruta Nova da Columbeira.

Since nickel is only present in Nazaré and cobalt is only present in Nazaré and Montes (12 km inland from Nazaré), these trace elements are common and unique to some geological strata of this coastal region. The presence of these elements in some samples from Gruta Nova da Columbeira (> 30 km distant) suggests that this region was part of the territory exploited or that the chert was naturally transported closer to this site. The cluster between Gruta Nova da Columbeira and geological samples from Leiria (more than 60 km to the north) can be explained by the same two factors: human or natural transport. However, if nodules from Nazaré and Leiria were transported closer to Gruta Nova da Columbeira by fluvial and coastal processes, they would be expected to also appear in the Praia Rei Cortiço assemblage which is closer to these sources. This would seem to strengthen the argument for human transportation of these materials to the site.

Unexpectedly, Mira Nascente, although closer to Leiria, does not cluster with the known sources from here, but has similarities with Praia Rei Cortiço. In turn, Gruta Nova da Columbeira shows a diversified and dispersed range of acquisitions that includes the Nazaré outcrop, Leiria Cesaredas, and the western Rio Maior region.

The pXRF analysis corroborates this argument. The chert assemblage from Gruta Nova da Columbeira clusters with different areas located at different distances and directions. One cluster is within the immediate vicinities of the cave. Although only a small number of geological specimens were found here, all in secondary positions, it is not surprising that some acquisition of chert was done just a few meters from the cave. A second area of acquisition is towards the west across the primary sources of the Cesaredas and the coastal gravels, in a range of 15 km. Another targeting area was the primary outcrops from Nazaré, 45 km to the north. Continuing towards the north, the samples also cluster with specimens from both primary and secondary sources from Leiria, in this case, 65 km from the cave. To the north and east are the cluster of sources from Serra d’Aire e Candeeiros. Another cluster is towards the southeast at the western parts of Alenquer and Rio Maior, ca. 25 and 30 km, respectively.

The presence of chert from Nazaré and Leiria in Praia Rei Cortiço and Gruta Nova da Columbeira can be explained by the movement of Neanderthals to those areas, or by the flush of nodules into the sea and then brought southwards by the sea currents.

In between the Gruta Nova da Columbeira and Leiria, the top and eastern façade of the Serra d’Aire and Candeeiros are cut by streams that run across several chert outcrops draining chert nodules close to the coast, namely Vale de Óbidos and to the wide Lagoa da Pederneira (literally “gunflint lagoon”) south of Nazaré. Today, these drainages reach from the coast to 9 km inland (Gonçalves 2007). It is, therefore, highly plausible that fluvial gravels transported to the Lagoa da Pederneira area would have been one of the most prominent chert sources in western Iberia.

The sourcing of chert from the eastern sources seems to have occurred in the western-most parts of the Alenquer and Rio Maior areas. This pattern requires human mobility, as the streams from the eastern façade of the Serra d’Aire and Montejunto drain towards southwest, to the Tejo, and not towards the Atlantic. It is also worth noting that the best passage between the western and eastern façades of Serra d’Aire e Candeeiros is exactly here, between these two mountains.

4.4. The possible Neanderthal territories at western Iberia

The Mousterian sites of Gruta Nova da Columbeira, Mira Nascente and Praia Rei Cortiço, although separated by tens of thousands of years in time, seem to have shared the same territory. This was limited to the east by the mountains of Montejunto, Serra d’Aire, and Candeeiros, south by the Cesaredas Plateau, north by the basin of the river Lis (Leiria area), with incursions through the corridor between these mountains reaching their southeastern façades (Fig. 7).

This territory is adjacent to that exploited by the Neanderthals occupying Oliveira cave (Matias 2012; Linscott et al. 2023).

These coastal Neanderthals seem to have exploited exposed coastal gravels across the shallow continental platform. During the MIS5 and the MIS5/4 transition (Paine et al. 2024), a fall of the sea level between 20 and 60 m would represent a retreat of the coastline up to ca. 18 km from its present position (Alveirinho Dias et al. 1997; Benedetti et al. 2009) (online source: https://portal.emodnet-bathymetry.eu/). This distance between these gravels and the sites may explain why some archaeological cherts do not have correspondence with the reference collection. It may also help explain why chert was probably transported but not shellfish, as is seen at some steep bedrock cliff sites (e.g. Bajondillo and Figueira Brava) where the fall of the sea level would not represent as much retreat (e.g. Bajondillo and Figueira Brava) (Simón Vallejo and Cortés Sánchez 2007; Cortés-Sánchez et al. 2011; Zilhão et al. 2020).

Little is still known about Neanderthal raw material sourcing patterns and territory in western-most Iberia. Our study, still exploratory, places the exploitation territory of the Neanderthals living at Gruta Nova da Columbeira, Mira Nascente and Praia Rei Cortiço within a region well-defined between the shore (wherever it was during each occupation) and the western-most mountains of Portuguese Estremadura, with limited incursions through the corridor linking to the Tejo basin.

The acquisition of raw materials was most probably made broadly in the same gravels associated with the coastal bluffs and stream deposits draining toward Óbidos valley, Lagoa da Pederneira and the mouth of the river Lis. This suggests a territory that could have been of ca. 2000 km², which would fit approximately a circular area with a 25 km radius if the concentric areas of Site Catchment Analysis were considered. Theoretic circular areas always need to be used with caution (particularly in rolling hilling landscape cut by rich fluvial networks and with uneven distribution of resources such as that of western-most Iberia), but that does not mean that the size of the area per se should not be taken into consideration.

Still, some landmarks such as major rivers and steep mountain façades may be essential to define the territory of exploitation of each Neanderthal site. That would have been constrained by the contours of the coast, the steep slopes of the mountain façades making Neanderthals follow along the geomorphology of more depressive areas and valleys. It must be highlighted, however, that such territory is to be seen as the main one when using the site and not as the only territory. It cannot be completely ruled out that the groups who occupied Gruta Nova da Columbeira, Mira Nascente and Praia Rei Cortiço may have also occupied other adjacent areas. However, that needs to be corroborated by comparing the raw materials present at each site and the chert sources.

Future research on the assemblages from sites already excavated, new sites, complementary petrographic and geochemical analyses, and further geoarchaeological surveys to enlarge the comparison samples will allow understanding in greater detail the patterns of Neanderthal procurement and exploitation strategies, as well as mobility patterns in the landscape.

Author Contribution

TP: Conceptualisation, funding acquisition, methodology, formal analysis, investigation, writing, reviewing and editing. AA: Methodology, formal analysis, investigation, writing, reviewing and editing. EP, MC, JH and MB: Investigation, writing, reviewing and editing. EA and LA: Methodology, formal analysis.

Acknowledgement

T.P. was funded by Fundação para a Ciência e a Tecnologia between 2011 and 2013 (SFRH/BPD/73598/2010) and between 2014 and 2018 (IF/01075/2013). PIXE analysis was founded by the Archaeological Institute of America’s Archaeology of Portugal Fellowship grants in 2013, awarded to TP. A.A. research is supported by Research Group Support of the Catalonia Government (2023 SGR 01237/2023PFR-URV-01237), Grant PID2022-138590NB-C41 funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU, and the Archaeology of Portugal Fellowship from the Archaeological Institute of America (EUA). The survey and excavations at Mira Nascente and Praia Rei Cortiço were funded by grants from the U.S. National Science Foundation to J.H. (BCS-0455145, BCS-0612923, BCS-1118155) and M.B. (BCS-1118183).

Data Availability

Data is provided within the manuscript or supplementary information files.

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Chert procurement for inferring Neanderthal mobility in central western Iberia

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Version 1

Abstract

Figures

1. Introduction

1.1. The geomorphological background of the sites

1.2. The sites

2. Methods and Materials

3. Results

3.1. Macroscopic analysis

3.2. pXRF results

3.3. PIXE results

4. Discussion

4.1. The lithic assemblages

4.2. The analyses

4.3. The areas of acquisition

4.4. The possible Neanderthal territories at western Iberia

5. Conclusion

Declarations

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1