The Vila Velha do Ródão and Arneiro depressions are located in the furthest upstream reach of the Lower Tejo River, about 20 km from the Spanish border (this is Portuguese Reach I as defined by reference [1
]) (Figure 1
). From the sensitive location of the study area close to the North Atlantic, we can assume strong interaction between marine and terrestrial processes and environmental conditions. This highlights the relevance of this area for research on past climate and environmental change. Previous research in the study area has mainly focused on characterizing the geological setting, geomorphic genesis (e.g., of the terrace staircase), active tectonics, archaeological background and landscape evolution in general (e.g., see review in reference [2
]). Climatic and environmental changes in the Lower Tejo Basin during the last glacial cycle and especially during Marine Isotope Stage (MIS) 3, remain poorly documented.
The Middle Palaeolithic of Iberia has received considerable attention in recent times in connection with the extinction of the Neanderthals [3
], but is also relevant in connection with the early establishment of the Mousterian, the diversity and demise of early hominins and the widespread distribution of the first Neanderthals [16
Since the 1970s, 37 archaeological contexts in the study area have been recorded, ranging from Acheulean to Mesolithic; amongst these, 17 are small surface assemblages of uncharacteristic chopper-like cores and flakes, broadly assigned to the Palaeolithic, and eight were obtained from excavation [22
] (Foz do Enxarrique, Vilas Ruivas, Pegos do Tejo-2, Azinhal, Tapada do Montinho, Cobrinhos, Monte da Revelada and Alto da Revelada) (Figure 1
In this contribution, we focus on the sedimentary record from the lowest terrace (T6) of the Tejo at Vila Velha de Rodão, which represents an important terrestrial archive of relevance to the possible relation between palaeoenvironmental conditions and early human occupation dynamics in the region. Herein, we reconstruct and discuss the climate and environmental conditions during the last glacial cycle, through integration of evidence from optically stimulated luminescence (OSL) dating, grain-size distribution, rock magnetic properties (low field magnetic susceptibility, frequency-dependent magnetic susceptibility and isothermal remanent magnetization curves), sediment mineralogy, phytolites and palynology, as well as reviewing published paleontological and archaeological data.
2. Geological and Geomorphological Setting
The Tejo is one of the largest systems of Western Europe and flows E–W across almost the whole of Iberia; it is an ancient river (c. 3.7 Ma) with an important sedimentary record [23
]. In the uppermost Portuguese reach of the Lower Tejo, the river flows through two quartzite ridges by way of the Ródão gorge (named “Portas de Ródão”), which separates the Ródão (upstream) and Arneiro (downstream) depressions.
The oldest bedrock comprises the Neoproterozoic and lower Cambrian schists and metagreywackes of the Beiras Group and the Ordovician Armorican Quartzite Formation. The latter is dominated by resistant ridges that topographically dominate (by c. 150 m) the extensive adjacent planation surface developed on phylites and metagreywackes. The Cenozoic is represented by the Cabeço do Infante Formation, the Silveirinha dos Figos Formation and the Murracha Group. The first two are dominated by soft sandstones and gravels, while the Murracha Group consists of gravels interbedded with fine sediments [23
In Lower Tejo Reach I, below a culminant sedimentary unit (the Falagueira Formation, at c. +260 m—above the river bed) corresponding to the ancestral Tejo River before drainage network entrenchment, the Pleistocene to Holocene record is summarized as follows [1
] (Figure 1
): (i) T1, with the surface at +111 m, without artefacts; (ii) T2, at +83 m, without artefacts; (iii) T3, at +61 m, without artefacts; (iv) T4, at +34 m, with Acheulean in the basal and middle levels and Mousterian in the uppermost levels; (v) T5, at +18 m, with Mousterian industries through the entire fluvial sequence; (vi) T6, at +10 m, with Mousterian industries at the lower deposits; (vii) Carregueira Formation (aeolian sands) (32 to 12 ka), with Upper Palaeolithic to Epi-Palaeolithic industries; (viii) Alluvial plain and a cover unit of aeolian sands (Holocene), with Mesolithic and more recent industries. Immediately upstream of the Ródão gorge, the modern river bed is at c. 72 m above sea level (a.s.l.). In this area, geomorphological evidence for late Cenozoic tectonics arises from interpretation of valley asymetry and drainage patterns, fault scarps, tectonic lineaments, fracture-controlled valleys, and vertical displacement of planation surfaces and terraces [26
]. The sedimentary controls for the formation of the Lower Tejo terrace staircase (mainly glacio-eustasy and differential uplift) are different from those affecting the Middle and Upper Tejo, because of separation (between the Middle and Lower Tejo) by a lengthy knick zone through hard basement [27
Previous dating of the Pleistocene litostratigraphic divisions was undertaken using Uranium-series, Thermoluminescence (TL), Optically stimulated luminescence on quartz (Quartz-OSL) and infra-red stimulated luminescence (IRSL) on K-feldspar. Quartz-OSL was used to date: the Carregueira Formation (32 to 12 ka; 3 ka) [2
]; the alluvial deposits at the Azinhal archaeological site, linked to T6 (61 ± 2 ka: GLL code 050302); the topmost deposits of T4 at the Pegos do Tejo-2 archaeological site (minimum age of 135 ± 21 ka: GLL code 050301) [29
]. In Reach I of the Tejo, IRSL dating provided a first temporal framework for the Lower Tejo terraces [26
], but at that stage of research T5 and T6 were not yet separated as distinct terraces. Recently, T4 in Lower Tejo Reach IV was dated to c. 340 to 155 ka, using post infra-red stimulated luminescence (pIRIR) [28
Geomorphological, stratigraphical, sedimentological and chronological data were obtained using standard methodology (e.g., [30
]): (1) geomorphological study, complemented by local detailed investigations and the production of a detailed map using Geographic Information System (GIS), (2) field descriptions of the sedimentary units, (3) sedimentological characterization of the deposits and (4) luminescence dating.
3.1. Geomorphological Mapping
Geomorphological mapping was undertaken in three stages: (1) field mapping onto topographical (1/25,000) and geological (1/50,000) base maps, (2) analysis of 1/25,000 aerial photographs and of a digital elevation model (DEM) based upon a 1/25,000 and 1/10,000 topographic databases and (3) field ground truthing.
3.2. Field Work
The T6 deposits at Foz do Enxarrique were studied in detail in order to improve our understanding of the local stratigraphy and sedimentology. Exposures of T5 and T4 in the study area were also revisited. Fieldwork included stratigraphic logging and sedimentological characterization of the sedimentary deposits in order to obtain data on the depositional facies, including sediment colour, texture, maximum particle size, clast lithology, fossil content, bedding and depositional architecture.
At the studied stratigraphic section of T6 at Foz do Enxarrique, continuous sediment sampling was undertaken manually, every 1 cm, to a depth of 5.00 m. Samples were labelled as follows: e.g., for “T6FE0.21”, T6 identifies the terrace code, FE the site and 0.21 the sample depth. We also collected a present-day sediment sample from the Foz do Enxarrique stream bed (FE-modern). Each sampled horizon (1 cm) was characterised according to its colour (using the Munsell system), texture and the relative abundance of carbonate concretions. Phytolith analyses were undertaken on carbonate concretions from five levels within the sequence (T6FE0.72, T6FE2.00, T6FE2.46–2.48, T6FE3.48 and T6FE5.00). An additional seven samples (spanning greater depth) were collected for clay mineralogy and palynological studies (T6FE0.70–0.74, T6FE1.13–1.17, T6FE1.98–2.02, T6FE2.88–2.92, T6FE3.46–3.50, T6FE4.10–4.14 and T6FE4.49–4.52). Palynological study also included samples collected from the T6 archaeological level with fossil bones: codes T6FE/15 0.25–0.30, T6FE/15 0.35–0.40 and T6FE/15 0.45–0.50 (here, the depth refers to the top of the archaeological level). The T6 upper unit at Foz do Enxarrique was sampled for optically stimulated luminescence (OSL) dating: sample 062201 collected at 0.89–0.93 m depth (below the terrace surface); sample 052202 collected at a 5.20–5.30 m depth; sample 052201 collected at a 5.40–5.50 m depth (top of the archaeological and fossiliferous layer).
At the Foz do Enxarrique site, a 1 m thick exposure reveals the lower part of T5. Two samples (T5FE0.15–0.23 and T5FE0.92–1.00) were collected here for clay mineralogy and palynological studies. At this site, two further samples for OSL dating (052204 and 052247) were collected at depths of 1.50 and 2.00 m below the T5 surface. T5 was also sampled for OSL dating at the Vilas Ruivas site (052207, 052231 and 052253).
From T4, samples for OSL dating were obtained from the Vilas Ruivas, Rodense Bolaria (Vila Velha de Ródão) and Pegos do Tejo-2 (Arneiro) sites (Figure 1
3.3. Optically Stimulated Luminescence Dating
From Reach I of the Lower Tejo, samples for OSL dating were previously collected from the sedimentary successions of the T4, T5 and T6 terraces and dated by IRSL, with a correction for the anomalous fading effect [26
], because the quartz-OSL signal was found to be in saturation. However, it was later documented that the fading correction used was inappropriate, leading to age underestimation. In order to improve the chronology of the terrace sequences and their associated lithic industries, we now use the pIRIR protocol. K-feldspar grains from the samples dated in 2008 (IRSL) were measured (Equivalent doses) in 2013 by pIRIR (the most up-to-date protocol); these new results are now presented. The storage of K-feldspar grains does not affect the luminescence properties or the resulting ages. In summary, several samples were selected for OSL dating (pIRIR protocol): three from T6 at Foz do Enxarrique (upper unit), five from T5 (collected at Foz do Enxarrique and Vilas Ruivas; Figure 1
) and four from T4 (collected at Pegos do Tejo-2, Vilas Ruivas and Rodense Bolaria/V.V.Ródão; Figure 1
OSL is an absolute dating technique that measures the time elapsed since sedimentary quartz or feldspar grains were last exposed to daylight [31
]. Exposure to daylight during sediment transport removes the latent luminescence signal from those minerals. After burial, the luminescence signal (trapped charge) starts to accumulate in the mineral grains due to ionising radiation. The annual dose of a sediment sample is related to the decay of 238
Th and 40
K present in the sediment itself, to cosmic ray bombardment and to the water content of the sediment. In the laboratory, the equivalent dose (De
, assumed to be the dose absorbed since the last exposure to light, i.e., the burial dose, expressed in Grays —Gy) is determined by comparing the natural luminescence signal resulting from charge trapped during burial with that trapped during a laboratory irradiation. In this study, the radionuclide concentrations were measured by high-resolution gamma spectrometry [32
]. These concentrations were then converted to environmental dose rates using the specified conversion factors [33
]. For the calculation of the dose rate of sand-sized K-feldspar grains, an internal K content of 12.5 ± 0.5% was assumed [34
]. Dividing the De
by the environmental dose rate (in Gy/ka) gives the luminescence age of the sediment.
Sample preparation for luminescence analyses was carried out in darkroom conditions, at the Department of Earth Sciences of the University of Coimbra. Samples were wet-sieved to separate the 180–250 μm grain-size fraction, followed by HCl (10%) and H2O2 (10%) treatments to remove carbonates and organic matter, respectively. The K-feldspar-rich fraction was floated off using a heavy liquid solution of sodium polytungstate (ρ = 2.58 g/cm3). The K-feldspar fraction was treated with 10% HF for 40 min to remove the outer alpha-irradiated layer and to clean the grains. After etching, the fraction was treated with HCl (10%) to dissolve any remaining fluorides.
At the Nordic Laboratory for Luminescence Dating (NLL), OSL were conducted using automated luminescence Risø TL/OSL-20 readers (Roskilde, Denmark), each containing a calibrated beta source. Small (2 mm) aliquots of K-feldspar were mounted in stainless steel cups. The K-feldspar equivalent doses (De
) were measured with a pIRIR SAR protocol using a blue filter combination [35
]. Preheating was at 320 °C for 60 s and the cut-heat 310 °C for 60 s. After preheating the aliquots were IR bleached at 50 °C for 200 s (IR50
signal) and subsequently stimulated again with IR at 290 °C for 200 s (pIRIR290
signal). It has been shown [36
] that the post-IR IRSL signal measured at 290 °C can give accurate results without the need to correct for signal instability. For all IR50
calculations, the initial 2 s of the luminescence decay curve less a background derived from the last 50 s was used.
3.4. Grain-Size Measurements
Grain-size analyses of uncemented sediment samples was carried out using a Beckman Coulter LS230 laser granulometer (Brea, CA, USA), with a measurement range of 0.04 to 2000 μm and a relative error less than 2%. Visual inspection of grain-size distribution curves allowed the identification and interpretation of unimodal or multimodal subpopulations. The T6FE sediment samples of the 5.00–3.20 m depth interval were analysed at a 5 cm spacing; the 3.18–2.60 m and 2.50–0.30 m depth intervals were analysed at a 1 cm spacing in order to provide a better distinction between fluvial and aeolian deposition.
3.5. Mineral Composition
Analyses of sediment composition were based on binocular microscope observation and X-ray diffraction (Department of Earth Sciences—University of Coimbra), as well as Scanning Electron Microscopy (SEM) and Energy Dispersive Spectometry (EDS) of selected carbonate concretions (UNESP Laboratory, at the University of Rio Claro—Brazil). A Philips PW 3710 X-ray diffractometer (Virginia, USA), with a Cu tube, at 40 kV and 20 nA was used for mineralogical identification within carbonate concretions and for clay mineralogy. The mineralogical composition of the <2 μm fraction was obtained in oriented samples before and after ethylene glycol treatment and heating up to 550 °C. The percentages of the clay minerals in each sample were determined through the peak areas of the mineral present, with the use of specific correction parameters.
3.6. Rock Magnetism
For magnetic susceptibility measurement, samples were dried at 40 °C and transferred into plastic bags for subsequent analysis. Rock magnetic properties were measured in the Instituto Dom Luis, University of Lisboa and in the Department of Earth Sciences of the University of Coimbra, and consisted of low field mass specific magnetic susceptibility (χ in m3/kg), frequency-dependent magnetic susceptibility (Kfd in %) and isothermal remnant magnetization (IRM). Magnetic susceptibility measures the ability of a material to be magnetized and includes contributions (in proportion to their abundance) from all diamagnetic (calcite), paramagnetic (clays), and ferromagnetic (magnetite) minerals present in the sediment. Low-field magnetic susceptibility was measured with a MFK1 (AGICO Inc, Brno, Czech Republic) apparatus operating with magnetic field intensity of 200 A/m and frequency of 978 Hz. Data were reported as mass-normalized values (m3/kg). Frequency-dependent magnetic susceptibility is an indicator of the presence of superparamagnetic particles (SP), generally produced during pedogenic processes. Low (0.47 kHz) and high (4.7 kHZ) frequency-dependent magnetic susceptibility was measured with a Bartington Instruments magnetic susceptibility meter coupled to a MS2B sensor and reported in percentage as follows: Kfd (%) = 100*(Klf-Khf)/Klf. After cleaning by alternating field demagnetization up to 100 mT, samples were subsequently submitted to stepwise isothermal remanent magnetization (IRM) acquisition with an impulse magnetizer (model IM-10-30). We applied maximum fields of 1.2T following approximately 30 steps. Remanence was measured with a JR-6A (AGICO Inc, Brno, Czech Republic) magnetometer.
Data were analysed using a cumulative log-Gaussian (CLG) function with software developed for the purpose [37
]. The S-ratio was calculated with the formula −IRM−0.3T
For phytolith analyses, samples with a volume of 1 cm3
were placed in an Erlenmeyer flask and dissolved in 20 mL of HNO3
solution at 1:4. The material was heated for 3 h at 90 °C on a hot plate. After cooling at ~25 °C, 10 ml of H2
was added, before washing in distilled water, centrifuging (1500 rotations per minute up to neutralization (pH ~ 7.0), and washing with alcohol. For slide preparation, 50 µL of material was extracted by pipette, placed on slides and dried on hot plates. Coverslips were fixed using Entelan®
resin (Hatfield, UK). Phytoliths were analyzed through optical microscopy (×160 and ×640), identified with reference to literature [38
] and named according to the International Code of Phytolith Nomenclature [41
For palynological studies, thirteen sediment samples were selected: two from lower and middle levels within T5; three from the T6 archaeological level and eight samples from the upper division of T6 (silty very fine sands and sandy silts). These samples were subjected to a physical and chemical pollen concentration pre-treatment. The pollen residue was isolated with a standard palynological preparation methodology [42
], with some modifications: omitting acetolysis and sieving, the latter in an attempt to increase pollen concentration. The pollen residue was assembled on thin glass slides to allow its identification and counting. It was embedded in glycerine and sealed with histolaque, to permit movement of the grains for more complete observation of the morphological features of pollen and non-pollen-palynomorphs. Grains were identified (based on references [43
]) and counted using an optical transmitted-light microscope.
3.9. Geochemical Analyses
Geochemical analyses on sediment samples collected from the upper unit of T6 were performed, at the laboratory, with a X-ray fluorescence spectrometer (Niton XL3t Ultra Analyser—Thermo Fisher Scientific; Waltham, MA, USA).
Regarding the environmental interpretation of the T6 record in the study area, the lower boulder–pebble gravel, 0.4 m thick and overlying a strath cut in metamorphic basement, corresponds to the coarse river-bed sedimentation near the margin of the energetic Tejo palaeochannel, probably during the interval c. 60–45 ka (MIS 3). No lithic artefacts or fossils, which could have helped in the interpretation, were found in this bed.
The T6 upper unit, mainly consisting of fine to very fine sands, grading upwards to coarse silt, is attributed overbank sedimentation. The detailed environmental interpretation of the various stratigraphic subunits is discussed in the following paragraphs.
The c. 20 cm lower bed (at 5.60–5.40 m depth), comprising Mousterian artefacts and fossil bones in a matrix of gravelly micaceous fine sands, is interpreted as overbank deposits close to the channel margin that also record hominin activities of hunting and butchery [48
]. During this period, the Tejo channel moved laterally towards the west, preserving this record of human occupation. Thus, this confined place at the confluence of the Enxarrique and Açafal streams was used by animals for drinking and they were easily hunted. As this thin layer, now dated to 44 ± 3 ka, represents the last regional evidence of Mousterian industries and the megafauna, it may correspond to a cold and dry period that negatively impacted the animals and the Neanderthals. It is possible the Neanderthals relied heavily on some specific biotic resource that may have been reduced during cold climatic conditions, so that they faced difficulties in adapting. The remaining Neanderthals could have been induced to move toward the better climate conditions of SW Iberia and were later absorbed into modern human populations. Also relevant to this discussion is the fact that the ages of the stratigraphic levels with the youngest Mousterian industries in westernmost Iberia are progressively younger toward the SSW: the Cantabrian region, c. 48–45 ka [71
]; central Portugal, c. 44–34 ka (e.g., Foz do Enxarrique, c. 44 ka; Almonda, c. 32 ka; Mira Nascente, c. 42–40 ka; Gruta da Figueira Brava, c. 34 ka) [58
]; Murcia, 37 ka [11
]; Gibraltar, between c. 33 and 24 ka [9
The sediments of the 5.40–4.55 m depth interval within T6, consisting of micaceous fine sands with some interbedded thin gravel stringers, are also attributed to fluvial overbank deposition close to the channel margin. No fossils or artefacts were found here or in the upper deposits of T6. The onset of a new sedimentation phase (overbank fine-grained sediments) without artefacts is not necessarily evidence for cultural breakdown. However, in other areas of the Lower Tejo where T6 is preserved, no Mousterian artefacts were found in younger stratigraphic levels.
Regarding the T6 upper unit, the coarse silts from 4.55 cm depth to the surface are attributed to overbank sedimentation, but some characteristics point to the possibility of short-distance transport by wind, namely the lack of lamination and of erosion surfaces, the absence of dispersed coarser grains, the low clay content, a mean grain size in the coarse silt range, the fine skewed distributions and evidence of aeolian abrasion provided by phytolith analysis. The literature shows grain-size-distribution curves of loess deposits to be very similar to these silty T6FE deposits (e.g., [76
]. Possible short-distance aeolian transport of exposed overbank fines could have been promoted by strong winds coming from the west and penetrating through the Ródão gorge (Figure 1
). However, if there had there been significant aeolian transport, the resultant sediments should also cover higher terrace levels, and this is not evidenced by field observation.
Sediment magnetic properties have been widely applied to fluvial sediments and loess and may provide useful information about fluvial activity, climate and environmental changes, as well as pedogenesis [50
]. In fluvial sediments from Beijing, for example, high magnetic susceptibility values generally reflect warm-climate conditions, whereas lower values match colder periods [84
]. In wind-blown sediments and buried soils from southern Siberia [81
], colder high-wind periods that are associated with an absence of soil formation show low values of frequency-dependence of magnetic susceptibility, whereas higher values are observed in episodes with less wind. In the classic loess–palaeosol sequence of the central Chinese Loess Plateau, there is a striking correlation between magnetic susceptibility and grain size [85
], which are good indicators of summer and winter monsoon intensity respectively [86
]. In general, ferromagnetic crystals in soils derive from both primary (detrital) and secondary (enhanced) iron minerals. The latter are most often of stable single-domain size or less and associated with the clay fraction, whereas the former are usually associated with sand and coarse silt-size fractions [49
]. Regarding the studied T6 sediments, no significant correlation (R2
= 0.0261) is observed between sedimentary grain size and magnetic susceptibility, suggesting that the latter is not controlled by mineralogy (ex. paramagnetic clay and phyllosilicates versus diamagnetic quartz) (Figure 5
). In contrast, a striking correlation between SIRM and magnetic susceptibility (R2
= 0.9554 for magnetite; R2
= 0.769 for hematite) indicates that magnetic susceptibility is dominantly controlled by the iron-oxide content, in the form of magnetite and hematite (Figure 10
), probably of a detrital origin. The proportion and contribution of magnetite and hematite in the bulk remanence is mostly similar in all samples, as illustrated by narrow S-ratio values of 0.86–0.92. In particular, the IRM curves of the T6 samples are very similar to those of modern sediments (Figure 9
), suggesting a common source for the entire sedimentary profile during the last millennium. Superparamagnetic particles, generally interpreted as a product of pedogenic processes, are present in all samples and may have been formed in situ or transported from the surrounding soils. The presence of numerous rhizoliths observed in the field suggests that pedogenic magnetic particles may have precipitated in-situ, during soil development. However, the poor correlation (R2
= 0.0145) between mass specific magnetic susceptibility and frequency-dependent magnetic susceptibility indicates that pedogenic processes alone cannot explain the short-term cyclical variations observed in the magnetic susceptibility curve of the T6 profile (Figure 4
). Conversely, a slight but significant correlation (R2
= 0.5023) between the S-ratio, i.e., the relative proportion of magnetite versus hematite, and magnetic susceptibility imply that the short-term cycles may correspond to changes in weathering regime and climate. More exactly, warmer/drier periods would enhance the oxidation of magnetite (or maghemite) and promote precipitation of hematite, and the reverse. Because the magnetic susceptibility of hematite is lower than magnetite, this provides a potential explanation for the cyclical oscillations observed in the magnetic susceptibility curve.
The 0.60–5.40 m depth interval contains c. 30 thin levels of calcium carbonate concretions and rizoliths intercalated in uncemented coarse silt. The characteristics of these levels, which have no evidence of any erosive surface and dip toward the palaeochannel, progressively increasing in thickness, point to a secondary origin for the carbonate concretions. A relatively stable surface and a certain amount of rainfall during the represented periods are indicated. The most probable source of calcium carbonate for these pedogenetic concretions is the dissolution of dolomitic and calcium carbonates that occur at the base of the Cabeço do Infante Formation (Paleogene), which crops out at a short distance from the site. Upstream sources of calcium carbonates are located at least at 200 km away (in the Madrid Cenozoic basin).
The aggradation of T5 (c. 140–70 ka) correlates with the very high sea levels of MIS 5, whereas the following period of river down-cutting (c. 70–60 ka) indicates to have been mainly determined by the low-sea-level conditions during MIS 4 and the aggradation of T6 (c. 60–32 ka) seems to correlate with the higher sea levels and high sediment supply coeval with MIS 3 [2
]. During this interval (c. 140–32 ka) Iberia was influenced by several climatic (e.g., [88
]) and oceanographic changes [93
], registered in the North Atlantic region and in records from Greenland Ice Cores (e.g., [95
The results obtained from palynological study of the T6 and T5 deposits do not allow palaeoenvironmental interpretation. Regarding the interval represented by the T6 terrace, the palynological record of the MD95-2039 ocean core points to an open landscape with steppe vegetation and low values of tree pollen, suggesting a severely cold and dry climate, during 61–59 ka; during the interval 57–31 ka, there are fluctuations in the expansion and contraction of arboreal pollen and Ericaceae related to the alternation of warmer and humid conditions during the interstadials and the cold and dry stadial minima of the last glacial cycle [92
]. In the Puente Pino sequence (Toledo, Spain), for the 42–34 ka interval, declining woodland and the increasing herbaceous pollen taxa are observed, related to adverse climate conditions of cold and dry character [97
]. The palaeoenvironmental data provided by these two adjacent contexts support the possibility of an open landscape in the region of the study area and could explain the several levels with pedogenic calcium carbonate rizoliths and concretions in the T6 upper unit.
The mineralogical composition of the <2 μm fraction of the samples collected from T6 and T5 are similar, consisting of smectite, illite and kaolinite, although T6 has less smectite. Even if a significant part of the clay minerals could have been sourced from erosion of the local Paleogene and Miocene formations, very rich in smectite [23
], this clay mineral association seems compatible with the regional climatic conditions during MIS 5–3 [91
By 32 ka, the climate had changed to cold and dry conditions and aeolian deposition dominated the valley landscape, preserving Upper Palaeolithic industries.
Updated ages from the three Lower Tejo terrace sequences, containing Mousterian industries, were obtained by pIRIR, as follows:
(i) OSL dating of the oldest Mousterian industry, stratigraphically situated in the uppermost T4 deposits, suggests a probable age of c. 200–170 ka for the arrival of the Neanderthals in this region, probably by way of the Tejo River valley from central Iberia;
(ii) T5 dates from c. 140 ka at the base and 70 ka at the top;
(iii) T6 dates from c. 60 ka at the base and c. 35 ka near the top;
(iv) The new date of 44 ± 3 ka for a level located at the base of the T6 upper unit records the last regional occurrence of a Mousterian industry and of the megafauna.
The T6 upper unit (so far without Mousterian or Early Upper Palaeoleolithic industries), consists of fine sands to coarse silts interpreted as overbank sediments. It has a large number of intercalated thin levels of carbonate concretions and rizoliths, suggesting episodic evaporation and development of paleosols in a seasonal dry period, in agreement with the occurrence of phytoliths.