Stratigraphy, biostratigraphy and lithic industry

The depositional sequence can be divided into four major stratigraphic units (I, II, III, IV) and 12 sedimentary levels (Ia to Ie, IIa and IIb, IIIa to IIIc, IVa and IVb) [7], with 10 archaeological (1, 2, 3, 4a, 4b, 5a, 5b, 6, 7 and 8) and 3 hominin fossil-bearing (6, 5b, 5a) levels [2,5,8] (see table in [9]).

The lowermost unit I includes five levels (Ia - Ie) composed of bedded-sandy-clay with angular gravels [7,9]. This unit, containing mainly carnivore and reindeer remains (archaeological levels 8, 7), including small sized Canis lupus, Crocuta crocuta spelaea, Ursus thibetanus, Vulpes vulpes, Panthera (Leo) spelaea, Ursus deningeri, Ursus arctos, appears to have been deposited under a generally cold climate [1,4] (Figure 2). Unit II, divided into three archaeological levels (6, 5b and 5a) with a preponderance of Cervus elaphus, Dama clactoniana, Capreolus sussenbornensis and Sus scrofa fossils, is composed of silty deposits with eroded gravels, large fallen blocks and speleothem formations [7], corresponding to a humid and temperate climate (MIS9, [1]) (Figure 2). Further up, unit III is composed of three sedimentary levels (IIIa to IIIc) of clayey sand with angular gravels and blocks, with abundant Bovidae fossils, corresponding to a cool and humid climate. Three archaeological horizons (4b, 4a, 3) can be identified (Figure 2). The uppermost unit IV, including two archaeological levels (1 and 2), is composed of clayey deposits with some gravels [7]. This unit marks the last human occupation of the site, and contains predominantly Equus steinheimensis remains, corresponding to a cooler climate and an open landscape [1] (Figure 2).

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Figure 2. Archaeological levels, stratigraphic levels of Orgnac 3, dominant faunal composition, paleoclimate and biostratigraphy.

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Evidence of the emergence of Middle Palaeolithic-type behavior can be observed throughout the depositional sequence with the development of long and complex flaking reduction sequences. In the lower levels (7 to 5a), debitage is mainly represented by centripetal cores. In the middle levels (4b and 4a), the first evidence of Levallois cores can be observed. In the top levels (3 to 1), Levallois cores on flakes are dominant. Two groups of levels may be distinguished by observing the flake-tool kit; levels 8-3 (with a broader diversity of flake-tools) and levels 2-1 (containing a majority of scrapers with thinner retouch). The shaping reduction sequences are limited throughout the whole sequence (bifaces and pebble tools). In levels 2 and 1, the frequency of bifaces is very low (less than 1%), and these are mainly bifacial tools with few removals. Various criteria related to technical behavior and subsistence strategy patterns indicate gradual changes over time towards Middle Palaeolithic-type behavior from the bottom to the top of the sequence.

Previous chronological studies

The first dating of Orgnac 3 was carried out in 1985 [10]. Four speleothem samples from archaeological levels 7 and 6, and between levels 6 and 5b were dated with the alpha spectrometric U/Th method (Figure 3). Based on the results obtained, the author proposed that the mean age of four age results ${339}_{-42}^{+76}$ ka, should be taken as the best age estimate for the speleothem formations. One of the four calcite samples was also analyzed by the electron spin resonance (ESR) method, yielding an age of 309 ± 34 ka [11]. At about the same time, Debard and Pastre [8] described and analyzed fallout volcanic ashes in the upper archaeological level 2, which is composed of lightly brown silty sand [7]. The volcanic ashes there are yellowish inclusions several tens of centimeters in diameter (an example of such an inclusion is given in Figure 4A, marked as ORG-C1). The authors [8] extracted well- preserved angular green pyroxenes, which are characteristic of one of the last eruptions of the Puy de Sancy volcano (Mont-Dore, Massif Central, France) [12]. They proposed an age of about 300 ka for the upper level 2. With the fission track (FT) technique, Khatib [7] analyzed 22 zircons from volcanic ashes from the same level, obtaining an age of 298 ± 55 ka (Figure 3). Masaoudi [13] presented the results of U/Th and ESR dating of bones and teeth and ESR dating of calcite and quartz samples from different levels (Figure 3). The measured dates are overdispersed, and do not conform well to the stratigraphy. Recently ⁴⁰,Ar/³⁹Ar dating was carried out on 16 sets of sanidine grains [9]. Four of them yield ages too old to be acceptable because of contamination by inherited K feldspar grains, while the remaining 12 ages are between 276 and 326 ka with a weighted mean of 308.2 ± 6.8 ka. Roger et al. [14] considered that the tephra layers at the Praclaux and du Bouchet maars (French Massif Central) and at Orgnac 3 all came from the eruption of the Sancy volcano centre. Based on the weighted mean of ⁴⁰Ar/³⁹Ar dates on a series of sanidines from the Praclaux and Bouchet lakes, they assigned an age of 275 ± 5 ka to the Sancy eruption. Recently, Nomade et al. [12] recalculated the age according to ACR-2 at 1.193 Ma and obtained an age of 279 ± 5 ka. Roger et al. [14] also determined a step-heating plateau age of 300 ± 2 ka from sanidine grain populations (Figure 3). For their experiments, the neutron fluence (J) was monitored with a biotite Bern B4B with an age of 17.25 Ma and a sanidine Draz with an age of 24.99 Ma [14], recently recalibrated to 25.42 Ma [15].

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Figure 3. Summary of previously obtained ages and ages obtained in this work.

https://doi.org/10.1371/journal.pone.0082394.g003

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Figure 4. Position of level 2 and of levels 7 and 6 at Orgnac 3.

(A) of volcanic inclusion ORG-C1 in the upper unit level 2 and (B) of calcite samples number PL2-1 to PL2-5 (levels 7 and 6).

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U/Th dating

Isotopic measurements of the fourteen speleothem samples are presented in Table 1 and in Figures 4, 5. The uranium content ranges from 66 ppb to 148 ppb, and thorium from 250 to 32,495 ppt. Samples are mostly free from detrital contamination as indicated by ²³⁰Th/²³²Th activity ratios higher than 20. Only three samples PL1-1, PL1-2 and PL1-2a have low [²³⁰Th/²³²Th] activity ratios (10.0, 10.9 and 17.6 respectively) indicating contamination by detrital materials (Table 1, Figure 5). Note that PL1-2 and PL1-2a are taken from exactly the same position. But compared with PL1-2a (with a [²³⁰Th/²³²Th] activity ratio of 17.6), the more contaminated PL1-2 (10.9) gives a significantly younger age result (218 ka << 275 ka, Table 1). For this, we tend to consider that the samples with a low [²³⁰Th/²³²Th] activity ratio may have undergone metamorphism leading to underestimated age results. Therefore these three (ICP-MS) U/Th ages were excluded (Table 1, Figure 5). The precision of ICP-MS isotopic ratio measurements is much better in comparison with the previous alpha measurements, as shown in Figure 6. For the base of the upper part of the first speleothem PL1, U/Th dates range from 265 ± 4 ka (PL1-3) and 295 ± 8 ka (PL1-4a) (levels 5b-6) (Figure 5. The top of the lower part of this speleothem yielded a date of 312 ± 15 ka (PL1-6) (Figure 5). For the second stalagmite PL2 (Figure 4), U/Th dates range from 281 ± 8 ka (PL2-4a) to 298 ± 17 ka (PL2-3), except for the date of 267 ± 6 ka for sample PL2-4. This youngest date is significantly different from the date, 281 ± 8 ka, of the coeval sample PL2-4a and the mean date of 289 ± 16 ka for the other 5 dates of this speleothem. This abnormally young date is probably biased by post-depositional diagenesis and is thus excluded from this study. U/Th ages indicate that both speleothems from levels 7-6-5b range from 265 ka (marine isotope stage 8, MIS 8) to 312 ka (MIS 9).

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Figure 5. Position of speleothem samples number PL1-1 to PL1-6 (levels 6 and 5b).

https://doi.org/10.1371/journal.pone.0082394.g005

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Figure 6. U/Th diagram obtained from Isoplot program [21] with quotation of previous alpha spectrometer data at 2σ level.

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⁴⁰Ar/³⁹Ar dating

Fifty-seven sanidine grain populations were analyzed with the total-fusion method (Figure 7). In order to obtain precise data, 50-150 sanidine grains were analyzed for each measurement ⁴⁰Ar/³⁹Ar ages with counting errors at 2σ are shown in Tables 2, 3, 4. Ten samples with determined ages older than 550 ka are most likely contaminated by inherited K feldspar grains. These samples, representing 17.5% of the results, were not taken into account (Tables 2, 3, 4). The abnormally old age results may be explained by the presence of old minerals, such as plagioclases or sanidines, extracted from the base rock during the Sancy volcano eruption.

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Figure 7. Sanidine minerals at Orgnac 3.

(A) About 150 grains (200-300 μm) for a total fusion analysis. (B) Orgnac sanidine (SEM). The minerals are sharp-edged and unweathered.

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Note that if 10 out of 57 samples are heavily contaminated, unless only one “inherited K feldspar” grain is enough to cause an abnormally old result, we should consider the possibility that the rest may be more or less contaminated.

Forty-six dates, ranging from 283 to 344 ka, are associated with low atmospheric contamination (featuring over 79.7% of radiogenic argon, % ⁴⁰Ar*). Another sample, K346-18 suffers from high atmospheric contamination (> 23.7% i.e. % ⁴⁰Ar*<76.3, Tables 2, 3, 4), yielding a young age (277.8 ka). Forty of the 47 dates yield an age distribution displayed by a probability density distribution (using Isoplot software, Figure 8). The dominant mode of the distribution is centered at 302.9 ± 2.9 ka (2σ, n=40/47, MSDW=1.2, P=0.16) (Figure 8). This age distribution is better than previous results [9]. In consequence, the weighted mean age of level 2 at Orgnac 3 is 302.9 ± 2.9 ka, corresponding to the transition from MIS 9 to MIS 8.

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Figure 8. ⁴⁰Ar/³⁹Ar ages distribution of Orgnac samples.

The ⁴⁰Ar/³⁹Ar Orgnac ages are quoted at 2σ level with an age distribution (n=40/47) reported by a probability density distribution using Isoplot [21] (7 open circles are rejected). The weighted mean of 302.9 ± 2.5 ka (2σ, n=40/47, MSDW=1.2, P=0.16).

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The measurement of ³⁶Ar in ⁴⁰Ar/³⁹Ar dating allows for the plotting of the inverse isochron graph. The results are a series of data points ranging from pure atmospheric argon to pure radiogenic argon. A regression line through these data points forms an inverse isochron and the point at which the isochron intercepts with the x-axis yields the ³⁹Ar/⁴⁰Ar* of the samples and therefore the age. In Figure 9, a regression line was plotted using ³⁶Ar/⁴⁰Ar and ³⁹Ar/⁴⁰Ar ratios (n=43/57), excluding 14 samples. Ten contaminated samples, K346-20 and K346-11 with high ³⁹Ar/³⁶Ar ratios, K346-21 with a negative ³⁶Ar/⁴⁰Ar ratio and K346-51 were eliminated in order to attain the atmospheric ratio (298.56; [25]) (Figure 9). In these conditions, the atmospheric ratio obtained (⁴⁰Ar/³⁶Ar)₀ is 299.0 ± 41.8 (2σ) and the intercept inverse isochron age is 302.9 ± 5.9 ka (2σ) (MSWD=2.57). This is in agreement with the weighted mean of 302.9 ± 2.9 ka (2σ) (Figure 8).

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Figure 9. ³⁶Ar/⁴⁰Ar vs ³⁹Ar/⁴⁰Ar inverse isochron diagram

for all the data (n=57) with ellipse error at 2σ excluding 14 samples (blue squares): 1) *samples too old (quoted with * in Table 2) (n=10); 2) K346-20, K346-11 with a too high ³⁹Ar/³⁶Ar ratio and K346-21 with a negative ³⁶Ar/⁴⁰Ar ratio because ³⁶Ar was underestimated; 3) K346-51 was eliminated otherwise the MSWD is 3.2 and the atmospheric ratio is not reached.

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U/Th and ⁴⁰Ar/³⁹Ar dating comparison

The ⁴⁰Ar/³⁹Ar weighted mean age of 302.9 ± 2.9 ka and the inverse isochron age of 302.9 ± 5.9 ka are older than the upper limit of the measured U/Th age interval 265 to 312 ka (Figure 10). The ⁴⁰Ar/³⁹Ar apparent ages determined in the higher level 2, should be younger than the U/Th apparent ages obtained in the median levels. This small difference in age (about 30 ka, which is not a major difference but is nonetheless statistically significant), could be explained by a slight contamination of sanidine grains or by a minor excess of ⁴⁰Ar. Another hypothesis which may explain the age difference between the two methods may be that the volcanic minerals were transported to the site tens of thousand years after the Sancy eruption. In order to improve our ⁴⁰Ar/³⁹Ar dating results, we have tentatively used the step-heating method to highlight a possible ⁴⁰Ar excess. In spite of the presence of excess ⁴⁰Ar incorporated in minerals during crystallization [26], Renne et al. [27] have demonstrated in other studies with this method that a sanidine sample less than 2000 years old can be dated with 5% precision. However, for Orgnac 3, this step-heating method, using a VG 3600 mass spectrometer with a Daly detector, required a significant amount of material (up to 1,000 grains, 200 μm) which considerably increased the proportion of inherited K feldspar grains and thus increased the probability of an unreliable apparent age.

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Figure 10. Stratigraphic positions of speleothem samples and volcanic minerals at Orgnac 3 with corresponding U/Th and ⁴⁰Ar/³⁹Ar dates.

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In conclusion, taking into account our total fusion multigrain analyses, U/Th dates and errors, it seems reasonable to conclude that the Orgnac infilling is contemporaneous with MIS 9 and 8 (Figure 11). The U-series date of 265 ka may mark a minimum age for the level 5b, when the Levallois flaking technique began to appear at the site (Figures 2, 5 and 10). This date is concordant with the biostratigraphical pattern which attributes levels 2 and 1 to MIS8 [1].

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Figure 11. MIS and Orgnac infilling after U/Th and ⁴⁰Ar/³⁹Ar dating.

Stacked δ¹⁸O record of benthic foraminifera from [28] after [29] with modifications. The shaded vertical envelope (± 2σ) shows the occurrence of the Orgnac infilling after U/Th and ⁴⁰Ar/³⁹Ar dating, close to the transition between MIS 8 and MIS 9.

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Thus, according to our U/Th ages, preliminary ⁴⁰Ar/³⁹Ar and the comparison between the two dating methods, Orgnac 3 is one of the oldest sites with the systematic use of Levallois knapping. As evidenced in [5,1], this temporal framework indicates the emergence of new technological behavior in southern France and Europe during MIS 8. Standardized core technology such as Levallois knapping can be observed in a few well-dated European sites close to the limit between MIS 9 and MIS 8, such as La Micoque (L2/3) (France), Gran Dolina (TD11/10), Bolomor (Spain) and la Baume Bonne (France) attributed to MIS 8 [1].

U/Th dating

At Orgnac 3, two speleothems of well-crystallized calcite were collected from levels 7, 6 and 5b (Figure 4 and Figure 5). The first speleothem, in levels 6 and 5b, is composed of two pieces of flowstones, each about 10 cm thick (Figure 5). The samples PL1-1 and PL1-2, PL1-2a were taken from the upper part and PL1-3, PL1-4, PL1-4a and PL1-5 from the lower part of the upper piece. One sample PL1-6 was taken from the upper part of the lower piece (Figure 5). Six more samples, PL2-1, PL2-2, PL2-3, PL2-4, PL2-4a and PL2-5 were taken from the top, the middle and the bottom, respectively, of a 46 cm-long stalagmite from levels 7 and 6 (Figure 4).

The selected bulk subsamples were physically cleaned with ultrasonic methods [30]. U/Th chemistry was conducted in a class-10,000 metal-free clean room with class-100 benches at the High-precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University [30,31]. U-Th isotopic compositions and concentrations were determined on a sector-field inductively coupled plasma mass spectrometer (SF-ICP-MS), Thermo Fisher ELEMENT II [17] or a multi-collector ICP-MS (MC-ICPMS), Thermo Fisher NEPTUNE, with a dry introduction system, Cetac ARIDUS [16]. Uncertainties in all ICP-MS U/Th isotopic data were calculated at 2σ level and include corrections for procedure blanks, multiplier dark noise, abundance sensitivity, mass discrimination, and the occurrence of isotopes of interest in spike solution. Age was off-line calculated [17] with decay constants of 9.1705 × 10^-6 yr ^-1 for ²³⁰Th and 2.8221 × 10^-6 yr ^-1 for ²³⁴U [19], and 1.55125 × 10^-10 yr ^-1 for ²³⁸U [20].

⁴⁰Ar/³⁹Ar dating

Volcanic sediment samples were collected from level 2, about 1m below the top of the depositional sequence (Figure 4). As sanidine is a proven chronometer [32], the largest possible and well-preserved sanidine grains (200-300 µm) were extracted using standard heavy liquid methods and then hand picked under a binocular microscope (Figure 7). The obtained sanidine grains are angular and quite well preserved. Their chemical composition was estimated using scanning electron microscopy (Figure 7) with Energy Dispersive X-ray Spectroscopy (EDS) in order to check the homogeneous presence of potassium (Ecole des Mines, Sophia Antipolis, Valbonne, France). The samples were irradiated for 40 minutes with Cd shielding in 5C position at McMaster University Reactor (Hamilton, Canada) and stored for one month at the Geoazur Laboratory (Nice, France). The sanidine grains were subsequently loaded onto a copper plate by sets of about 50-150 grains per hole for multigrain aliquot analyses (Figure 7). Gas was extracted with an infrared continuous laser and purified in stainless and glass extraction line using two Al-Zr getters and a N₂ cold trap. System blanks were run for every two or three analyzed samples. The mass spectrometer is a VG 3600 with a Daly detector. Mass discrimination was monitored by regularly analyzing one air pipette volume. The ultimate accuracy of the ⁴⁰Ar/³⁹Ar method depends on well-dated homogeneous standards [15,23]. J values were calculated using an age of 1.194 Ma [22] for ACS and the total decay constant of [33]. Recent revisions of decay and monitor constants suggest values about 0.6% ([15]; 1.201 Ma for ACS) and 1.0% ([34]; 1.206 Ma for ACS) older than those used here [35]. The implied difference in age is negligible for our samples (about 3 ka), therefore we use the conventional value of 1.194 Ma for ACS for all the ages.