New radiocarbon dating of the transition from the Middle to the Upper Paleolithic in Kebara Cave, Israel

Journal of Archaeological Science 38 (2011) 2424e2433

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New radiocarbon dating of the transition from the Middle to the Upper Paleolithic in Kebara Cave, Israel N.R. Rebollo a, S. Weiner b, F. Brock c, L. Meignen d, P. Goldberg e, A. Belfer-Cohen f, O. Bar-Yosef g, E. Boaretto a, h, * a

Radiocarbon Dating and Cosmogenic Isotopes Laboratory, Kimmel Center for Archaeological Science, Weizmann Institute of Science, 76100 Rehovot, Israel Kimmel Center for Archaeological Science, Weizmann Institute of Science, 76100 Rehovot, Israel Oxford Radiocarbon Accelerator Unit, Research Laboratory for Archaeology & The History of Art, University of Oxford, Dyson Perrins Building, South Parks Road, Oxford OX1 3QY, UK d Université Nice Sophia Antipolis, Campus Saint-Jean-d’Angély SJA3 e CEPAM e UMR 6130 CNRS, 24, avenue des Diables Bleus, 06357 Nice Cedex 4, France e Department of Archaeology, Boston University, 675 Commonwealth Avenue, Boston, MA 02215-1406, USA f Institute of Archaeology, The Hebrew University of Jerusalem, Mt. Scopus 91905, Israel g Department of Anthropology, Peabody Museum, Harvard University, Cambridge, MA 02138, USA h Department of Land of Israel Studies and Archaeology, Bar Ilan University, 95100 Ramat Gan, Israel b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 February 2011 Received in revised form 15 May 2011 Accepted 16 May 2011

The Middle to Upper Paleolithic transition (MP-UP transition) is considered a major technological and cultural threshold, at the time when modern humans spread “out of Africa”, expanded from the Levant into Europe and possibly into central and northern Asia. The dating of this techno-cultural transition has proved to be extremely difficult because it occurred sometime before 40,000 radiocarbon years before present (14C years BP), which is close to the end of the effective dating range of radiocarbon. Other dating methods such as Thermoluminescence (TL) or Electron Spin Resonance (ESR) are not sufficiently precise to date the recorded archaeological MP-UP transition in the Levant. Here we report a consistent set of stratified radiocarbon ages on freshly excavated charcoal from Kebara Cave, Mt. Carmel (Israel), that span the late Middle Paleolithic (MP) and Early Upper Paleolithic (EUP) This study applied novel strategies to improve sample preparation techniques and data analysis to obtain high-resolution radiocarbon models. From this study it is proposed that the MP-UP transition for this site can be placed immediately after 45,200
700 14C years BP and before 43,600
600 14C years BP or from 49/48 to 47/46 radiocarbon calibrated years before present (years Cal BP). Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Middle to Upper Paleolithic transition Radiocarbon dating Paleolithic Archaeology Levant Human evolution

1. Introduction Kebara Cave in Mt Carmel (Israel) produced one of the beststudied late Middle Paleolithic (MP) and Early Upper Paleolithic (EUP) occurrences (Fig. 1A and B). When the archaeological contexts in Kebara Cave were compared to other sites in the region, it became obvious that the first entity that marked the onset of the Upper Paleolithic was missing. Within the accepted terminology the earliest manifestation is generally labeled as the Initial Upper Paleolithic (IUP) (Bar-Yosef et al., 1992; Bar-Yosef and Meignen, 2007). The missing archaeological prehistoric entity (often referred to as “culture”), is known in southern Levant as the Emiran culture. Similarly to other entities it is identified by its lithic assemblage and * Corresponding author. Radiocarbon Dating and Cosmogenic Isotopes Laboratory, Kimmel Center for Archaeological Science, Weizmann Institute of Science, 76100 Rehovot, Israel. Tel.: þ972 8 934 3213; fax: þ972 8 934 6062. E-mail address: [email protected] (E. Boaretto). 0305-4403/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2011.05.010

thus is characterized by the first systematic production of blades, dominance of Upper Paleolithic (UP) tool types such as endscrapers, and the presence of a special point, bifacially trimmed at its proximal end, called the Emireh point (Garrod, 1954; Marks and Kaufman, 1983; Volkman, 1983). The Emiran was recovered from two sites in the region: Emireh cave (the Galillee, Israel), where it was first defined, and Boker Tachtit levels 1 and 2 (the Negev, Israel) where level 1 was dated to 47,280
9050 (SMU-580) and 46,930
2400 (SMU-259) 14C years BP (Marks, 1983; Gilead, 1991). A similar IUP assemblage was uncovered in Tor Sadaf layer AeB, in Jordan (Fox, 2003). In the northern Levant the IUP assemblages contained chamfered pieces shaped by transversal removals at the tip of the blank. The IUP assemblages with chamfered pieces were found in Ksar’Akil cave layers XXV-XXI, as well as in Antelias and Abu Halka (all three in Lebanon), and as a surface collection in the Negev (Goring-Morris and Rosen, 1989). These assemblages lack radiocarbon dates, and were labeled during the course of lithic analysis as “Ksar’Akil Phase

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Fig. 1. A: Late Mousterian site (MP) sites in the Levant, including Kebara Cave in Israel. B: Initial (IUP indicated with a star) Upper Palaeolithic and Early (EUP indicated with a circle) Ahamarian Upper Palaeoloithic sites.

A” (Azoury, 1986; Copeland, 1975; Ohnuma, 1988; Bergman, 1987). Other IUP assemblages with chamfered flakes/blades recently discovered in the northern Levant bearing clear affinities to the assemblages from Ksar’Akil were reported from Üçagizli cave (Turkey) (Kuhn et al., 1999; Kuhn, 2009). This is the only site that provided radiocarbon dates. The earliest stratigraphic unit is dated by Accelerator Mass Spectroscopy (AMS) readings to the range from 41,400
1100 to 35,500
1200 14C years BP (AA-37625 and AA-52050) prepared using the standard acidebaseeacid (ABA) pretreatment and 39,817
383, 36,915
335 and 33,874
271 14C years BP (AA-68965, AA-68962 and AA-68963) by ABOx pre-treatment of three samples (Kuhn, 2009). The Emiran and its contemporary IUP entities are overlain by the Early Ahmarian, commonly regarded as the earliest full manifestation of an Upper Paleolithic culture and thus considered as Early Upper Paleolithic (abbreviated as EUP). The importance of Kebara cave is indeed the presence of the first UP full-blown blade industry, uncovered in unit IV-III. A similar industry is present also in Yabrud II (Bar-Yosef and Meignen, 2007; Bar-Yosef, 2000; Bachdach, 1982; Bar-Yosef and Belfer-Cohen, 2010; Rust, 1950). No fossil human remains were recovered in the Levantine sites of this period. Not less important is the Middle Paleolithic sequence that underlies the Ahmarian deposits in Kebara Cave (units XII to V), that was culturally identified as Late Mousterian and produced a Neanderthal burial, as well as numerous isolated human bones including those of juveniles and young adults (Bar-Yosef et al., 1992; Arensburg and Belfer-Cohen, 1998; Tillier et al., 2008). The entire Mousterian sequence in the cave has been dated to ca. 64/60,000 to 48,000 years BP using TL and ESR (Valladas et al., 1987; Schwarcz

et al., 1988). Unit VI yielded a TL date (Valladas et al., 1987) of 48,300
3500 years BP but no secure dates were available for unit V, the youngest Mousterian layer. The dates for the overlying Early Ahmarian of unit IV were 42,500
1800 100 14C years BP (Pta-5002) and 42,100
2100 100 14C years BP (Pta-4987) clearly indicating the presence of a cultural gap caused by the missing Emiran entity. It was assumed that the transition from the MP to the UP took place sometime between 46,000/45,000 and 43,000 14 C years BP (Bar-Yosef et al., 1996). In radiocarbon laboratories around the world, charcoal samples are routinely pre-treated using an AcideBaseeAcid (ABA) method (sometimes referred to as AcideAlkalieAcid, or AAA). In recent years, some alternative pretreatment methods for radiocarbon dating of charcoals have been proposed and are under development, e.g. hydrogen pyrolysis (HyPy) (Ascough et al., 2009, 2010), plasma ashing (Bird et al., 2010) and a wet oxidation/stepped combustion method (ABOx-SC) (e.g. Bird et al., 1999; Brock et al., 2010). Among the most tested and developed of these is the ABOx-SC method (often referred to simply as ABOx), which applies a much harsher chemical regime than a standard ABA protocol, followed by a pre-combustion to remove further contaminants. This method has been successfully applied to several different sites, including the one relevant to this study. At some sites ABA and ABOx pre-treatments result in similar dates for a sample, but at others over 25,000 14C years BP such as Devil’s Lair, Australia (Turney et al., 2001), Border Cave, South Africa (Bird et al., 2003), Grotta di Fumane, Italy (Brock and Higham, 2009), and Kostenki 14, Russia (Douka et al., 2010) the ABOx method can yield significantly older dates which make much more sense archaeologically than the younger ABA ones.

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A recent study of Paleolithic charcoal from Europe and the Mediterranean Rim reports four new dates for Kebara Cave (Brock and Higham, 2009) from Units IIIBf and V taken from the excavation in 1990; the dates obtained produced similar results using both the ABA and ABOx pre-treatment methods. Two dates are reported for a sample from Unit III: 41,200
450 14C years BP (OxA-18425) using the ABA method, and 40,350
400 14C years BP (OxA-18424) using ABOx. These dates are within the ranges obtained earlier (Bar-Yosef et al., 1996) where the youngest was 35,600
1100 14C years BP (OxA-1567) and the oldest was an infinite date >43,800 14 C years BP (OxA-3977). For a sample from Unit V the two new dates (Brock and Higham, 2009) are: 47,300
800 14C years BP (OxA-18427; ABA), 46,250
700 14C years BP (OxA-18426; ABOx); a sample from the same Unit and square gave an infinite date of >44,000 14C years BP in the previous study (Bar-Yosef et al., 1996). The renewed excavation at the Kebara Cave in 2006 was aimed at improving the accuracy and precision of the dating of the relevant archaeological layers by obtaining fresh charcoal samples from the same strata as those of the earlier study (Bar-Yosef et al.,1996) and to select and process them for radiocarbon dating using new insights into the stability and structure of fossil charcoal (Cohen-Ofri et al., 2006; Cohen-Ofri et al., 2007; Rebollo et al., 2008). In the present study samples of charcoal are pre-screened to identify the quality of preservation before dating, using both the standard ABA and ABOx methods. In order to further advance the understanding of the applicability of these methods to fossil charcoal, a systematic analytical comparison is made of the quality of the charcoal and the dates produced using the two different methods. 2. Material and methods The samples for this study were taken from the southern section of the central part of the cave, where both UP and MP layers are present. In previous excavations, the section was divided into two main depositional ensembles (Bar-Yosef et al., 1996) labeled A and B correspondingly with different sediment characteristics. These two stratigraphic ensembles are subdivided into seven units according to differences in sediment composition (Laville and Goldberg, 1989; Goldberg and Laville, 1991) and bedding characteristics (Bar-Yosef et al., 1992; Bar-Yosef et al., 1996; Schick and Stekelis, 1977). Extensive classification of lithic assemblages demonstrated that the upper half of this section is composed of UP assemblages (units I to IV) overlying the MP layers. In the southern section that was the subject of the dating projects only three of the Mousterian (i.e. MP) units, V to VII, are exposed (Fig. 2). The relevant units for the study of the transition from the MP to the UP are the hearth areas in unit V, the uppermost Mousterian layer and the lowest UP units III and IV. Unit V contains mainly Mousterian artifacts characterized by intensive use of Levallois technique, presence of well-retouched Mousterian points, and a few UP pieces, presumably intrusive from unit IV that are attributed to the Early Ahmarian. The assemblages of units IV and III resemble those of Ksar’Akil, layers XIX-XV (Ohnuma, 1988). Photographs and drawings of the stratigraphy of the southern section following the two excavation seasons of 1990 and 2006 were compared for achieving an accurate match between the location of the samples and the obtained dates. The section from the 1990 excavation (Figs. 1 and 3 in (Bar-Yosef et al., 1996)) corresponds to mid-squares Q and the one from 2006 corresponds to mid-squares R of the same section, i.e., 0.5 m from the boundary grid line of squares Q (Fig. 2A in the present study). The slope of the sediments of the southern profile in the central area, like most sediment beddings within the cave, is generally tilted uniformly from the entrance toward the rear wall of the cave due to subsidence into an

underlying sinkhole. When photographs of these two sections are scaled and overlapped according to the numeric square and depth subdivision (Fig. 4 in (Bar-Yosef et al., 1996) and Fig. 2A and B in the present study) the slope of sediments are very similar, and there is good correspondence between the units and the hearth sampled for charcoal in both seasons of excavations. These similarities allowed a correlation between the contexts of all the samples. Collection of charcoal samples for this study was conducted during the 2006 excavation season, and was primarily limited to hearth areas which we considered to be well-defined intact contexts. Special care was taken to identify, delimit and avoid burrows. The context and specific coordinates of every collected piece were recorded. Twenty-three samples from the 2006 excavation, composed of single large pieces of charcoal were chosen for this study. Nine samples derive from the UP (4 from unit III, and 5 from unit IV) and 14 from the MP (10 from unit V and 4 from unit VII) strata. One major problem in dating the MP-UP transition with radiocarbon is the difficulty of ensuring the effective removal of contaminants present in very old samples in poor state of preservation and with radiocarbon contents close to the measurable limit (Bird et al., 1999; Brock and Higham, 2009). It is thus preferable to perform pre-screening analyses to check that charcoal is relatively well preserved, and that no residual contaminant remains after the chemical pre-treatment. The need to develop novel sample preparation methods for radiocarbon analysis, through a better understanding of the charcoal structure subject to changes in environment and in-vitro has been recognized in recent publications (Bird et al., 2010). Other recent studies (Rebollo et al., 2008; Ascough et al., 2011) have shown that the chemical stability of fossil charcoal largely depends on the dissociation susceptibility of carboxyl groups in charcoal subject to pH changes in the environment and in-vitro. A direct consequence of this susceptibility is the high weight loss incurred during both the ABA and ABOx treatments, which seem to correlate with the preservation state of the sample. Charcoal samples were initially selected on the basis of the reliability of the context and sample size. When possible single pieces of charcoal were selected and processed. When several pieces had to be pooled in order to obtain the minimal necessary weight (around 100 mg) it was known from field observations that the charcoal fragments were all derived from one depositional unit. Every sample was checked for the presence of extraneous material (e.g. roots, sediments) and when necessary these materials were removed mechanically. Samples were not botanically identified. Since the expected ages were in the order of 35e50 ky, the old wood effect was considered negligible. All samples were first homogenized by light crushing using an agate mortar and pestle, and then sieved through a 250 mm mesh size sieve. The fraction 250 mm was used as the small particle size improves contact with the solution. Batches of 50e200 mg were placed in glass tubes and were subject to the following ABA treatment: (a) Initial acid treatment (3 ml of 1 N HCl solution (pH 1)) for 1 h, followed by rinsing with Nanopure water until reaching pH 6 (this is the measured pH of the Type-1 Grade Nanopure Water Barnsted Int.Ò) and then slow drying in an oven at 80  C overnight; (b) Base treatment: 3 ml of 0.1 N NaOH solution (pH 14) for 1 h, followed by Nanopure water rinsing to pH 6 and drying; then repeated twice more; (c) Final acid treatment: 3 ml of 1 N HCl solution for 1 h in hot water bath at 80  C, water rinsed to pH 6 after cooling down the pre-treated solution to room temperature and drying. Freshly prepared solutions, both acid and alkali, were used throughout the experiment. Drying in an oven at 80  C was added so that the weight loss could be measured before continuing to the next step. The glass tubes for the chemical treatments were previously cleaned in chromic acid,

N.R. Rebollo et al. / Journal of Archaeological Science 38 (2011) 2424e2433

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Fig. 2. A. Photograph of the southern section in Kebara Cave after the excavation in 2006. For a description of the exposed Upper Paleolithic (UP) and Middle Paleolithic (MP) layers, see Section 2. The numbers in circles indicate the location from where the samples where taken for the present study (see coordinates in Table 1). The photograph has been scaled to the schematic below. B. Schematic reproduced from (Bar-Yosef et al., 1996). This schematic corresponds to square Q, while the photograph above corresponds to square R, 0.5 m behind square Q. Note that most features (and especially the hearths) as regards both the slope and the stratigraphy are remarkably similar. The radiocarbon dates from the study in 1996 are subdivided into: finite dates, infinite dates, dates near burrows.

rinsed and dried in an oven at 150  C. Every chemical treatment was performed under continuous rotation (30 rpm) to enhance contact between the sample and the solution. The average weight losses after the ABA pre-treatment registered for the 23 pre-screened samples were: 61.0
9.5% in Unit III, 75.1
14.7% in Units IV/V, and 90.9
13.6% for samples in Unit VII. It was then possible to discriminate among samples that were most likely to be better preserved within every Unit based on their weight losses. Based on this criterion, a total of 11 out of the 23 prescreened and pre-treated samples (Table 1) were chosen for dating following the ABA pre-treatment: three from Unit III, four from Unit IV and four from Unit V. No samples for Unit VII were chosen for dating due to their high weight loss (>95%). All the 23 samples were pre-screened before, during and after the ABA pre-treatment. Fourier Transform Infrared (FTIR) spectroscopy was used for identification of mineral content and detection of the presence of fossil charcoal (Cohen-Ofri et al., 2006; Rebollo et al., 2008), characterized by C]C, COOe and COOH bands. The thermal stability behavior of constituent components throughout the treatment was monitored by Thermo-gravimetric and Differential Thermal Analysis (TGA/DTA). Details of these pre-screening procedures are described in (Rebollo et al., 2008) The FTIR analysis showed the presence of siliceous aggregates in all of

the untreated charcoal samples. Siliceous aggregates are known to be present in the layers of ash surrounding the hearth areas (Albert and Weiner, 2001). At the end of the ABA treatment only the presence of charcoal was detected with FTIR for every sample. The DTA analysis shows that all the samples exhibit their major exothermic reaction between 450 and 500  C, which is characteristic of wood charcoal (Cohen-Ofri et al., 2006; Beall and Eickner, 1970). The residue left after TGA analysis was identified by FTIR as siliceous aggregates. The pre-screened ABA-treated samples were sent to the Oxford Radiocarbon Accelerator Unit (ORAU) for dating. There they underwent an additional acid treatment with 1 M HCl at 80  C for 1 h followed by washing in Milli-QÒ ultrapure water to remove any atmospheric carbon dioxide which may have been absorbed by the samples in the time between the initial ABA treatment and their arrival in Oxford. In parallel to the standard ABA method, 95e250 mg untreated homogenized material from each sample (with the exception of R19aIV_1) was also sent to the ORAU to be pre-treated using the ABOx-SC method, as described by (Brock et al., 2010). Sample R19aIV_1, was only pre-treated with ABA, as it was only dated to establish the boundary between the intrusive UP and a nearby hearth (Bar-Yosef et al., 1996; Rebollo et al., 2008; Alon et al., 2002).

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Table 1 Radiocarbon dates for the present study. Dates for 7 samples from the UP layers (R-1 to R-3 from Unit III and R-4 to R-4 to R-7 from Unit IV) and 4 samples from the uppermost MP layer (R-8 to R-11 from Unit V) in Kebara are shown with associated uncertainties of
1s (68.2% probability). Specific coordinates from the excavation (square, unit and X, Y, Z or upper and lower Z1 and Z2 limits) and assigned laboratory numbers for Weizmann are also listed. OxA-V numbers are for samples pre-treated in Weizmann and OxA-X numbers are for samples after ABOx-SC pre-treatment that were smaller than the corresponding standards run for AMS measurement and had low %C. Two parameters relevant to sample preservation (Weight losses after pre-treatment) and wood charcoal characteristics (carbon% on combustion) are shown for every sample (Section 1). Calibration of these radiocarbon dates into calendar years and sequences built for the two pre-treatment methods used on homogenized portions of every sample are shown in Fig. 4 (Section 3.2 for discussion). Sample Name

Square

1

R16cIIIb_2

R16c

2

R17aIIIb,f

3

Unit

Sample Coordinates

Pre-Treatment

X

Y

Z

Z1

Z2

IIIb

100

10

533

e

e

R17a

IIIbf

e

e

e

511

520

R16cIIIb_1

R16c

IIIb

95

25

522

e

e

4 5

R19aIV_1 R17aIV

R19a R17a

UP Channel IV

e e

e e

e e

548 551

552 562

6

R19aIV_2

R19a

IV

e

e

e

553

555

7

R19aIV_4

R19a

IV

e

e

e

554

559

8

R19aV_2

R19a

V

e

e

e

570

579

9

R15cV

R15c

V

70

16

632

e

e

10

R19 aV_4

R19a

V

e

e

e

573

580

11

R19 cV

R19c

V

e

e

e

587

594

a b c

Weightloss % after Pre-treatment

Lab. No. Weizmann

%C (combustion)

OxA

ABA

74.9

RTO 5679-1

58.0 60.9

ABOx-SC ABA ABOx-SC ABA ABOx-SC ABA ABA ABOx-SC ABA ABOx-SC ABOx-SC

95.4 53.9 98.4 56.4 94.3 68 48 84.7 79.6 95.7 96.5

RTOX 5679-2 RTO 5590 RTOX 5796-2 RTO 5589 RTOX 5589-2 RTO 5591 RTO 5680-1 RTOX 5680-2 RTO 5681-1 RTOX 5681-2 RTOX 5797-2

72.1 57 64.3 59.7 49.3 55.7 69.8 67.7 55.9 29.5 27.7

ABA ABOx-SC ABA ABOx-SC ABA ABOx-SC ABA ABOx-SC ABA ABOx-SC

77 97.4 77.9 94.9 70.3 89.3 77 93.8 88.6 91.0

RTO 5799-1 RTOX 5799-2 RTO 5682-1 RTOX 5682-2 RTO 5798-1 RTOX 5798-2 RTO 5800-1 RTOX 5800-2 RTO 5801-1 RTOX 5801-2

46.7 10.6 53.8 13.3 62.3 51.7 55.6 33.1 56.6 21.8

V-2253-42 V-2253-43a Combinedc 18458 V-2220-42 18791 V-2220-41 X-2222-32 V-2220-43 V-2253-44 18459 V-2253-45 18402 18801b Combinedc V-2269-35 X-2264-29 V-2253-46 X-2252-7 V-2267-43 18792 V-2267-45 18803 V-2267-46 18804

Date (yrs BP)

s

40,500 40,600 40,550 41,050 42,600 42,800 42,850 41,400 34,540 41,650 40,400 43,600 40,300 35,160 37,493 36,110 40,500 45,200 36,300 46,250 44,800 49,600 50,600 51,500 44,300

400 400 283 450 500 650 550 1200 250 450 400 600 550 310 284 330 1200 700 650 650 650 1000 1600 1200 1000

(
)

Note: AMS measurement repeated for this sample as a standard procedure at ORAU. Note: This sample was pre-treated twice, and each fraction subjected to AMS measurement, as a standard quality control procedure at ORAU. Note: For these samples, combined dates are used in Fig. 4.

All the samples were freeze-dried, combusted, graphitized and AMS-dated at the ORAU as described by Brock (Brock et al., 2010). Modern and background charcoal standards were subjected to ORAU’s standard pre-treatment procedures and combusted and dated alongside the Kebara samples. 3. Results 3.1. Sample pre-screening and radiocarbon dating Table 1 lists the 11 samples selected and analyzed for radiocarbon contents. The samples are ordered by their stratigraphic contexts from the youngest to the oldest strata (Fig. 2A). In all cases the uncertainties associated with every date correspond to one standard deviation (s.d.) of the distribution. All the samples measured yielded finite dates, including samples in Unit V for which no finite dates were attained from the previous systematic radiocarbon study of Kebara Cave in 1996 (Bar-Yosef et al., 1996). The average weight loss undergone after ABOx treatment was 93.8
4%, which is not surprising given the rigorous nature of the pre-treatment and is consistent with samples from other sites. Because of the low yields obtained by this method a purity check after the sample pre-treatment is not possible. A second parameter listed in Table 1 for every measurement is % C on combustion. This parameter is sample-characteristic, and therefore indicates whether the source of carbon obtained is likely to be wood charcoal. Charcoal samples are expected to yield carbon percentages between 50 and 70% (Braadbaart and Poole, 2008; Braadbaart et al., 2009); for the ABOx-treated samples 7 out of 11 gave %C below 50% with most of them as low as 10e20% for the

oldest Units. These results show that most of the material left after ABOx was not only charcoal. In some instances, it is likely that some silica wool from the pre-combustion was present in the sample, which would have also influenced the %C content of the sample. In comparison, for the ABA-treated samples for which the purity check was performed, 10 out of 11 samples yield %C within the expected range for charcoal. The radiocarbon dates obtained with the two preparation methods are summarized in Table 1 and Fig. 3. The distribution of the data shows that ABOx-treated samples are often younger than their ABA-treated counterparts (Fig. 3). These

Fig. 3. ABA and ABOx-SC radiocarbon age (range limits in brackets) comparison showing an overall trend of younger dates obtained using the latter pre-treatment. Every black rhombohedron is a chart data point of each pair of dates obtained for a given sample. Labels next to each data point correspond to the sample number in Table 1. OxA-X samples, highlighted as empty rhombohedrons, are smaller than the AMS standards.

N.R. Rebollo et al. / Journal of Archaeological Science 38 (2011) 2424e2433

observations point to the possibility that the extraneous material in the samples, which is not charcoal, may be promoting the introduction of modern contamination. From the FTIR analysis, it is known that siliceous aggregates were present in charcoal samples and surrounding sediments. Silica minerals have large charged surface areas and a tendency to adsorb carbon dioxide and most probably carbon monoxide from the atmosphere at ambient temperatures (Regev et al., 2010). Thus the presence of a minor fraction could cause a small increase in the percent Modern Carbon (pMC), especially during sample preparation, if the sample is exposed to the modern atmosphere. From TGA/DTA analysis we found that the amount of charcoal (fraction burnt between 300 and 600  C) relative to the remaining siliceous aggregates (analyzed residue after TGA/DTA) is higher for ABA than for ABOx-treated samples. Therefore, the siliceous aggregates that are not eliminated during the ABA and ABOx pre-treatments and are present in relative higher concentration in the ABOx fractions, are probably the reason for the younger ages obtained in general with the ABOx. As a final remark we note that samples with low %C are normally not dated. Also, three of the ABOx samples (R16cIIIb_1,R19aIV_4 and R19aV_2) were approximately half the size of the AMS standards alongside which they were dated. This is not normal protocol, but these samples were dated at the time when the ABOx method was still in the early stages of development at the ORAU, hence the samples were issued with OxA-X, rather than OxA, numbers. As the method is now more established at the ORAU it is less likely that samples with such low carbon contents would be issued with OxA or OxA-X numbers now, and the samples would probably be failed prior to dating. The dates reported on samples with %C lower than 50% should not be taken as reliable. They are only shown in order to emphasize the risks incurred when pre-screening procedures such as purity checks are not routinely applied (which may be necessary if samples are small and/or are subject to high weight loss during

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pre-treatment). Based on the data analyses presented in this section, only the samples that meet the pre-screening criteria were used for date calibration in the following section. 3.2. Calibration of dates The chronology for the MP to UP sequence at Kebara is shown in Fig. 4 using a modeled Bayesian sequence generated with OxCal 4.1 (Bronk Ramsey, 2009). Three datasets were compared. Sequence (A) built with finite dates from radiocarbon measurements published by (Bar-Yosef et al., 1996) from Units III and IV. Sequences (B) and (C) are built with radiocarbon dates obtained from the present study for pre-screened (Section 3.1) ABA and ABOx-SC pre-treated samples correspondingly for Units III, IV and V. The probability distribution ranges within 1 s.d. (68.2% Probability) are listed in Table 2. For the dataset with the largest number of reliable dates (sequence B), based on criteria described in Section 3.1, we ran 4 models as follows: (i) for phases III and IV only; (ii) with the one date within the range of the calibration curve for Phase V (R19aV_2 (OxA-V-22253-46)); (iii) with the one date within calibration range in Phase V set as a Terminus Post Quem; and (iv) with all four dates from Phase V included. There was no significant difference between the models including the date for R19aV_2 (OxA-V-22253-46) either within the boundary for Phase V or as a Terminus Post Quem. We also ran the model with no dates in Phase V five times to check the sensitivity of the models e comparison of the modeled ages showed no significant variation between the models. The model with all four dates from Phase V was discarded because three of the dates extended beyond the calibration curve. The model with a Terminus Post Quem is not applicable to the archaeological context as it considers a fixed upper limit for Phase V. Therefore, the simplest model within the calibration range was option (ii) including one date from phase V (Fig. 4).

Table 2 Comparison between age-calibration models of Kebara Dates. Un-modeled and chronological modeled ranges are listed for the finite dates available from (Bar-Yosef et al., 1996) and for the pre-screened samples (Section 3.2) in the present study. Because of the proximity to the end of the calibration curve, the dates are modeled at the 68.2% range. The corresponding probability distributions of these calibrated dates are shown in Fig. 4. Sequence

Unit

Sample Number

Sample Namea

Lab. Number

From

To

From

To

1996 Excavation (Finite dates Fig. 2B)

III

16 in Fig. 2B 14 in Fig. 2B 11 in Fig. 2B

Q17IIIB Q16IIIBf Q18IIIBf

Pta-4267 OxA-1567 OxA-3976

42,180 42,040 48,800

40,140 38,970 45,390

9 8 7 1

Q16/Q17 IVB Q16 IVB Q1616b/Q15d IV R16cIIIb_2

47,880 47,840 48,720 44,670

44,160 44,530 45,630 44,180

42,213 42,110 46,000 46,820 49,210 49,050 49,290 44,690

40,080 38,560 44,060 44,750 46,175 46,020 46,550 44,180

46,170 46,410

45,350 45,460

46,070 46,210 47,020 46,300 45,500 47,690 48,660 49,880 49,530 45,100 46,143 47,220 44,680 47,960 48,780 49,600

45,330 45,440 45,960 46,200 44,770 46,360 46,830 48,400 47,020 44,400 45,370 45,720 44,000 46,250 46,860 48,060

Boundary IV_III IV

2006 Excavation ABA pre-treatment (Samples Fig. 2A)

III

Boundary IV_III IV

2006 Excavation ABOx pre-treatment (Samples Fig. 2A)

a

Start IV End V V Start V III Boundary IV_III IV Start IV End V V

in in in in

Fig. Fig. Fig. Fig.

2B 2B 2B 2A

Unmodeled range (BP) 68.2%

2 in Fig. 2A 3 in Fig. 2A

R17aIIIb_f R16cIIIb_1

Pta-4987 Pta-5002 Pta-5141 OxA-V-2253-42 OxA-V-2253-43 Combined OxA-V-2220-42 OxA-V-2220-41

6 in Fig. 2A 5 in Fig. 2A

R19aIV_2 R17aIV

OxA-V-2253-45 Oxa-V-2253-44

47,310 45,490

45,870 44,780

8 in Fig. 2A

R19aV_2

OxA-V-2267-46

49,430

47,560

1 in Fig. 2A 2 in Fig. 2A

R16cIIIb_2 R17aIIIb,f

OxA-18458 OxA-18791

45,090 46,320

44,410 45,470

5 in Fig. 2A

R17aIV

OxA-18459

44,680

44,000

9 in Fig. 2A

R15cV

OxA-18792

48,910

47,030

Note. Sample names were assigned to Bar-Yosef’s finite dates (Bar-Yosef et al., 1996) according to their stratigraphy, Unit and square.

Modeled range (BP) 68.2%

Fig. 4. Bayesian model from MP-UP transition layers in Kebara produced using OxCal 4.1 (Bronk Ramsey, 2009) Sequences built with three different datasets are shown: (A) Finite dates for Units III and IV from (Bar-Yosef et al., 1996) (Table 2), (B) Dates obtained with ABA pre-treated samples from the present study, (C) Dates of ABOx-SC pre-treated samples from the present study. According to the archaeological context, the model considers a contiguous boundary between Units III and IV (one starts as the other ends) and a sequential boundary between Units IV and V (considers a possible gap between phases). Individual un-modeled radiocarbon distributions are shown in light gray, and modeled distributions are shown in darker gray for every sequence. Because of the proximity of the dates to the end of the calibration curve, the models assume that all ages are less than 50,000 years Cal BP. The samples labeled with a “[P: %]” are outliers and therefore are not considered in the model to build the sequence. For discussion of these results refer to Section 3.2.

N.R. Rebollo et al. / Journal of Archaeological Science 38 (2011) 2424e2433

Overall, the distribution ranges for Sequence A are much wider than for the other two sequences, due to the relatively lower precision of radiocarbon dates from 1996; however, they provide a reference time frame to compare with the newest data. Most dates in Sequence B (ABA treatment, see Table 1) follow a chronological sequence within the stratigraphic order. The dates and stratigraphic order hold when these new dates are intercalated with old radiocarbon dates in Sequence A (Fig. 2B). For Sequence B (ABOx treatment, see Table 1) there are three samples marked as outliers. Sample R19aIV_1 (OxA-V-2220-43) is younger than the dates of Unit III as it would be expected from a sample taken from the intrusive UP Channel; R17aIV (OxA-18459) demonstrated 100% probability of being an outlier upon modeling; R19aIV_4 (OxA-V-2269-35) was excluded from the model as it contained less than 50% carbon upon combustion. For Sequence C only three samples, namely: R16cIIIb_2 (OxA-18458), R17aIIIb,f (OxA-18791), and R15cV (OxA-18792) had %C higher than 50% (a pre-screening parameter in this work); these three samples follow a chronological sequence, and the modeled ranges agree with the other two sequences. In this last sequence R17aIV (OxA-18459) is again statistically an outlier. By comparing these three sequences, the inferred earliest appearance for UP layers (Units III and IV) is between 46/47,000 years Cal BP and the end of the Mousterian layers (Unit V) is around 48/49,000 years Cal BP. It should be noted that calibrating radiocarbon dates of this age could be problematic as models may be constrained by the end of the calibration curve. We have addressed this issue by modeling at 68% range and reporting calibrated dates as “greater than” ages where distributions are truncated because of the end of the curve. The interpretation of these observations within the context of the MP-UP transition is discussed in the following section. 4. Discussion The key to dating the MP-UP transition in Kebara Cave is Unit V. The stratigraphy of the southern section profile (Bar-Yosef et al., 1992; Bar-Yosef and Meignen, 2007; Bar-Yosef et al., 1996), its geology and post depositional processes (Goldberg et al., 2009) are sufficiently well known to allow a clear separation between disturbed sediments and well preserved intact hearth areas from the uppermost Mousterian layer (Unit V) and the lowest UP layer (Unit IV). Unit V dips toward the back of the cave and consists of compact, crudely bedded to laminated gritty dark brown sandy silt with stringers and fragments of combustion features, recognizable in the section profile (Fig. 2). The tilting, bedding, and slumping of layers in this section were possibly associated with an increase in wetter conditions that started at the end of the Mousterian and caused the sinkhole to be reactivated (Goldberg et al., 2007). Another time of increased precipitation is evidenced by an irregular layer rich in organic matter and diatoms in the middle of the UP sequence (Bar-Yosef et al., 1992; Bar-Yosef and Meignen, 2007). Units V and IV were slightly affected by slow moving sheet flow from the entrance that caused some dispersal of charcoal specks but sometimes left intact the structure of the hearths. The most outstanding later intrusion feature is an irregular vertical channel of about 1.5 m deep (called “UP channel” on Fig. 2) that cuts across the MP and UP layers filled with laminated, cross-bedded silts containing numerous UP artifacts (Bar-Yosef et al., 1996). Before the present study, the upper age limit for the MP-UP transition was determined by the oldest date obtained from a sample at the limit between Unit IV and V (Pta-5141 e43,700
1800 14 C years BP), but without finite radiocarbon dates for Unit V. A provisional estimation of this transition was deduced by combining the dates from Unit IV, the lower limits given by infinite dates within Unit V and the upper limit for Unit VI of a TL date of 48,300
3500

2431

years BP (Valladas et al., 1987). However, the time span for the “gap” between Units IV and V could not be known until Unit V was dated. Most recently a finite date for Unit V was obtained (Brock and Higham, 2009) by re-dating a sample from the 1980’s excavation, obtaining 47,300
800 14C years BP. The present study reports for the first time finite dates of freshly excavated samples throughout MP and UP layers. The absence of Emiran material as well as field observations determined that there is a temporal hiatus between MP and UP units. As mentioned above, Unit V contains mostly a Late Mousterian assemblage with less than 10% of small tool types (including a few bladelets) traditionally attributed to UP. This intrusion is possibly due to trampling and vertical penetration through minicracks observed in the fine clay terra rossa, sediments that originated from the entrance of the cave. Conversely, in Unit IV more than 90% of the tools are typical UP artifacts. In addition we note that the few flakes of Mousterian type that were retrieved from this layer could be part of an EUP tools as often observed in Levantine sites (Azoury, 1986; Copeland, 1975; Ohnuma, 1988). We should note that none of the characteristic Mousterian retouched points frequently recovered in Unit V were found in Unit IV. It therefore seems that the small flaky component was not derived from the Mousterian Unit V. In addition the charcoal samples were collected from the hearths and thus, even if there are some mixing of small lithics at the top of Unit V, the chosen samples are depositionally from secure un-mixed contexts. Based on the finite new set of dates of 45,200
700 and 43,600
600 14C years BP (Table 1; Fig. 4), we conclude that the chronological gap in the occupation of the cave, between the radiocarbon date markers of the assemblages of Units V and IV was 1500e2000 years or slightly shorter. This may correspond to the length of time when the Emiran culture existed elsewhere in the southern Levant. In sum, if this conclusion is sustained by further studies of wellstratified sites, the beginning of the UP would be around 45,200 14C years BP. Since a new recommended atmospheric radiocarbon age curve for radiocarbon dates from 0 to 50,000 years Cal BP IntCal09 is available (Reimer et al., 2009) a comparison can be made between the calendar dates from Kebara. An overall comparison of calibrated dates, applying the latest available Bayesian model, shifts the boundary between Units V and IV to a range from 49,000 years Cal BP (end of Unit V) to 47,500 years Cal BP (start of Unit IV) (Fig. 4). This study presents for the first time a finite sequence of radiocarbon ages for the youngest MP context in Kebara Cave, and by so doing sets a lower age limit for the oldest known MP-UP transition in the Levant. The oldest estimates for the MP-UP transition published previously from Boker Tachtit and Kebara cave are all minimum estimates and have much larger errors than the dates presented here. TL and ESR dates are available (Fig. 4), but as their errors are so large, they do not contribute significantly to the dating of the transition with the precision presented here. Earlier estimates (Bar-Yosef et al., 1996) suggested that the appearance of UP technology in the Near East took place not later than 45,000/44,000 14C years BP, and when calibrated would be 49,000 to 47,000 years Cal BP (Reimer et al., 2009) and possibly fit the Üçagizli dates. Our finite Kebara radiocarbon dates demonstrate that this range represents the earliest evidence for a context of UP in Western Asia. Assuming that in the future such dates will be supported by samples from Ksar’Akil, the Levantine IUP and EUP assemblages herald the diffusion of UP technologies into Europe where the dates of the first entry of Modern humans are still debated (e.g. Hoffecker, 2009 and references therein). Although the eastewest dispersal of Modern humans occurred at least through two main tracks (the central European (through the Danube river valley) and the southern track are well known), the archaeological manifestations along the way are still partially misunderstood. In western European prehistory

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N.R. Rebollo et al. / Journal of Archaeological Science 38 (2011) 2424e2433

the earliest Upper Paleolithic culture was the Aurignacian, which is assumed to arrive from the east, although there is no evidence that this entity did indeed originated therein. As the Levantine IUP is currently viewed as the ancestor of the earliest Modern humans in Europe, be it the Bachokirian, the Bohunician and other entities, it became clear that the first colonizers of Europe were makers of different kinds of lithic tool kits and were more closely related to the Levantine IUP industries (e.g., the technology of the Boker Tachtit assemblages which is comparable with that of the Bohunician). Thus the classical Aurignacian cannot be considered as being the first cultural EUP entity. (Bar-Yosef and Meignen, 2007; Mellars, 2006). Indeed, the early dates of the IUP in the Levant mark the onset of Modern human dispersal into Europe including the eastern lands, e.g., the Russian plain (Anikovich et al., 2007; Klein, 2008). Scholars who struggle with the dating of the EUP contexts across Europe are familiar with the massive legacy of radiocarbon dates, many of which could be inaccurate due to contamination. The accuracy and precision of radiocarbon dates should be evaluated before the measurement based on independent parameters such as %C, percent of material remaining after pre-treatment and infrared analysis. The novel pre-screening and sample processing techniques used here open the way to re-dating many more sites. Additional precise dating of the earliest archaeological records across Eurasia will contribute to a better understanding of the rate at which the cultural changes from the MP to the UP occurred in various regions (e.g. Klein, 2008; Derevianko, 2009) 5. Conclusions The southern section of the large excavated area was selected because it exhibits a depositional sequence that spans the MP-UP transition. We determined that: A. The absolute archaeological radiocarbon dates obtained extend into the MP beyond 50,000 14C years BP, testifying to the success of the pre-screening strategy, and the effective removal of contaminants. B. The new dates demonstrate that the time gap between the Mousterian deposits and the UP units in Kebara could have been shorter than earlier estimates. They also indicate the antiquity of the onset of the Early Ahmarian (EUP) in this site, making it the currently earliest occurrence of EUP in the Levant. C. The dates suggest the interval lasted from 49/48 to 47/46 years Cal BP, thus providing the date of the IUP Emiran entity. D. Concerning the two pre-treatment methods employed we note that the chemically harsher ABOx method may not always render the most reliable dates. An older date may not be a sufficient indication of a reliable date. Crosschecking with other parameters, independent of the radiocarbon content, may help to more objectively determine the reliability of a date. It is thus necessary to independently evaluate the applicability of the chemical purification treatment used, and the reliability of the date obtained based on its archaeological context. Acknowledgments We thank F. Berna, D. Dumarché, J. Berg, E. Mintz, C. Bronk Ramsey and T. Higham for their contributions to this project. This research was funded by Israel Science Foundation grant no. 05/1040. Financial support was also obtained from the Kimmel Center for Archaeological Science at the Weizmann Institute of Science, the American School of Prehistoric Research (Peabody Museum, Harvard University), as well as a generous gift from the late Mr. George Schwartzmann. S.W. holds the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.

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