ESR and U-series analyses of enamel and dentine fragments of the Banyoles mandible
Introduction
In April 1887, a human mandible was found by the stonemason Llorenç Roura during quarrying activities near the town of Banyoles (Bañolas), Catalonia, Spain. The mandible was passed on to Pere Alsius, the well-known local pharmacist and naturalist, the pioneer of prehistory of Catalonia, who published its first description (Alsius, 1915). The descendants of Pere Alsius are still the owners and custodians of this specimen (and are, indeed, also pharmacists). Figure 1 shows a map of the location, along with a photo of the travertine section and the approximate depth (4 m below the surface) at which the mandible was found (Alsius, 1915). This travertine section was completely removed a long time ago and, at present, it is not possible to correlate this site to any of the accessible travertine formations.
The mandible (Fig. 2) is nearly complete but is cracked along the left corpus and ascending ramus and damaged in the superior parts of the ascending ramus on both sides. It is most likely that the mandible was initially complete and that the missing parts were lost during the extraction of the mandible from the site of discovery. All the fractures appear to be post-mortem (Hernández-Pacheco and Obermaier, 1915, Maroto and Soler, 1993). The crowns or roots of all the teeth are in place, but the dentition shows peculiar spacing and extraordinary occlusal wear, especially marked on the buccal surfaces of the first and second molars. The mandible is certainly non-modern in morphology and large in terms of its corpus thickness and dental size (as far as can be inferred given the extreme wear). Its taxonomic assignment remains controversial. Originally, it was considered as a fairly typical Neandertal (Hernández-Pacheco and Obermaier, 1915, MacCurdy, 1915, Sergi, 1917/8). However, others emphasized its large size or the absence of derived Neandertal features, such as a retromolar space and posterior position of the mental foramina, implying the lack of midfacial projection. These researchers concluded that its morphology was more likely “pre-Neandertal” (Bonarelli, 1916, de Lumley, 1971/2, de Lumley, 1973, de Lumley, 1982, Stringer et al., 1984, Turbón, 1987, Roth, 1989, Roth and Simon, 1993) or, alternatively, demonstrated the continuity of primitive characters with those of later Neandertals (Hublin, 1988). After the travertine matrix adhering to the mandible was dated to 45,000 ± 4000 years using U-series dating (Julià and Bischoff, 1991; see below), suggestions of a pre-Neandertal morphology caused some unease because no other pre-Neandertal sites in Europe have yielded age estimates anywhere near as young. Based on its age, some workers have grouped the Banyoles mandible with Neandertals (e.g., Stringer and Grün, 1991, Sánchez, 1993, Strauss et al., 1993), while, alternatively, it has been suggested that the date of the travertine should be regarded as a minimum age because the mandible could be significantly older than the enclosing matrix due to reworking processes (Lalueza et al., 1993).
The travertine matrix of the mandible was removed in 1956 (Alcobé, 1993) and was first dated by radiocarbon to 17,600 ± 1000 BP (UCLA-930; Berger and Libby, 1966). This date includes a 2400 year correction for the dead carbonate fraction. Berger and Libby (1966) regarded the ages as too young for a Neandertal and suggested that either more modern travertine had contaminated older travertine or that the mandible was reworked from older deposits. Yokoyama et al. (1987) reported U-series dates on one travertine and one bone sample from a supposedly equivalent position in a nearby quarry, yielding closed system U-series ages of 73,000 ± 4000 years for the travertine (by α-spectrometry) and for the bone, a Th/U age of 16,200 ± 3200 years and a Pa/U age of 17,100 ± 900 years (γ-spectrometry). Using the above-mentioned radiocarbon age, Yokoyama et al. (1987) engaged in some open system modeling, which resulted in a best age estimate of years for the travertine. Julià and Bischoff (1991) carried out a systematic U-series dating study on the travertine matrix, as well as samples from quarries in the near vicinity of the original site of the mandible, yielding an isochron age of 45,000 ± 4000 years for the matrix of the mandible, and isochron ages between about 50,000 (near the surface) to 140,000 years (base at about 2.8 m depth below the surface) for the Les Prederes quarry (about 200 m SW of the original location of the mandible) and 45,000 (surface) to 70,000 years (base at 4.7 m) for the Mata quarry (about 700 m W of the mandible location). The exact locality of the Yokoyama travertine sample is not known, but the result is compatible with those from the Les Prederes quarry. Brusi (1993) studied the complex travertine formations of the Banyoles basin and placed the site of the mandible in a travertine overspill called Estunes-Formiga, which contains the Les Prederes quarry. Maroto (1993) published a comprehensive review on all aspects of scientific research on the mandible up to that date including, for example, the notes of Alcobé (written in 1956 and published in 1993) on how the matrix was removed from the mandible.
In order to address the question of whether the mandible could have been reworked from older units, we carried out an ESR analysis on an enamel fragment removed from the right M3 (Fig. 3A). The analysis of the fragment followed procedures similar to those that were used to establish the antiquity of the BC5 mandible from Border Cave (Grün et al., 2003). On the dentine, laser ablation in situ U-series analysis was carried out. A small piece of the original travertine matrix was collected (Fig. 3B) for the estimation of the external dose rate (K, U, Th, and porosity), as well as for additional mass spectrometric U-series analysis. To make sure that the travertine sample was indeed from the encasing matrix, the sample was re-fitted to a cast of the mandible (Fig. 3C).
Section snippets
Methods
The enamel fragment was mounted in a Bruker ER 218PG1 programmable goniometer and measured at each dose step at 10° angle intervals for 400° (the spectra past 360° were used to check for short-term fading effects). ESR measurements were carried out on a Bruker Elexys spectrometer with a 15 kG magnet and a rectangular 4102 ST cavity. The samples were recorded with the measurement parameters routinely applied at the ANU ESR laboratory: accumulation of between 500 (natural sample) and 200 scans
Results
Figure 5A shows the U and Th scans of a cross section of the enamel piece. The measured average concentrations in the enamel were 0.19 ± 0.18 ppm U and 0.015 ± 0.034 ppm Th. Initially, we were unable to obtain a dentine sample for further analysis, so we decided to scan the interior surface of the enamel (Fig. 5B). The surface contains significantly higher U-concentrations than the enamel cross section, fluctuating between about 3 and 10 ppm, and reaching up to 21.8 ± 1.8 ppm in one 100 μm diameter
Discussion
Although it might be surprising that the dentine shows more than a hundredfold higher U-concentration than the matrix, a similarly high U-concentration (43 ppm) was measured on a bone by Yokoyama et al. (1987). This result is most likely due to the high porosity of the travertine, which allows groundwater to circulate and mobilize uranium quite freely. The open-system nature of the travertine is clearly evident in the U-series data (see Fig. 7B). The 232Th-normalized U-series data do not fall on
Conclusions
The travertine matrix of the Banyoles mandible shows signs of open-system behavior, and thus it cannot be ruled out that its true age is older than 50,000 years. If this is the case, the best age estimate for the mandible is given by the combined U-series/ESR result of 66,000 ± 7000 years. If the mandible was reworked from another deposit, its mean age would not exceed 60,000 years and would most likely be only a few thousand years older than its matrix. However, there is no taphonomic evidence
Acknowledgments
We thank the Alsius family for providing access to the mandible and allowing us to carry out our analyses. We thank Àngel Galobart, Institut de Paleontologia “M. Crusafont” of Sabadell, for having the courage to remove the enamel and dentine fragments from the mandible, and Joaquim Soler, Universitat de Girona, for helping us to overcome any language problems in our collaboration. We thank Daniel Turbón, Secció d'Antropologia of Universitat de Barcelona, for providing the travertine matrix
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