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Optical dating of pottery, burnt stones, and sediments from selected Quebec archaeological sites1 M. Lamothe
Abstract: Recent developments in luminescence technologies applied to sediment dating is used to better constrain the age of archaeological events. Suitable geoarchaeological material includes sediments and fired objects, such as pottery and burnt stones. The assessment of archaeological ages illustrated here are based on single aliquot regeneration (SAR), with both infrared and blue stimulation on the same fine-grained aliquot being detected. These new approaches in optical dating were tested on polymineralic extracts of an Archaic burnt stone, a Woodland ceramic, and a soil containing Plano artefacts. The results demonstrate that whether or not these sites had yielded datable radiocarbon material, luminescence would have provided a reliable chronological framework given that the appropriate procedures to correct anomalous fading are incorporated. Résumé : Les récents développements dans les technologies de la luminescence appliquées à la datation de sédiments sont utilisés afin de mieux délimiter l’âge d’événements archéologiques. Le matériel géoarchéologique adéquat comprend des sédiments et des objets cuits tels que de la poterie et des roches chauffées. L’évaluation des âges archéologiques présentés ici est basée sur la méthode de l’aliquote unique en régénération (SAR, « single aliquot regeneration ») avec double stimulation bleue et infrarouge sur la même aliquote à grains fins. Ces nouvelles approches en datation par stimulation optique ont été mises à l’épreuve sur des extraits polyminéraux d’une pierre chauffée de la période Archaïque, une céramique du Sylvicole et un sol contenant des artefacts de la culture Plano. Les résultats démontrent que, même pour des sites sans matériel radiocarbone datable, la luminescence peut fournir un cadre chronologique fiable à condition que des procédures appropriées pour corriger le fading anormal soient appliquées. [Traduit par la Rédaction]
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Introduction The first successful application of luminescence to the dating of archaeological material was reported 40 years ago (Aitken et al. 1964), but it took several more years and considerable research and development for this method to achieve the status of a reliable dating tool (Aitken 1998). Luminescence dating is a chronological alternative to radiocarbon dating for sites at which organic material is not available. Furthermore, it provides opportunities for dating events beyond the 40 ka limit of 14C dating. However, the technological advances acquired over the last decade are of such importance that the method may yield ages that can be as accurate as radiocarbon dating but more closely related with the archaeological event investigated, such as the timing of firing of ceramics. A spectacular example of the importance of accurate absolute age chronology in archaeology is the application of thermoluminescence to burnt flint in the Levant and its role in the recent reinterpretation of the history of
modern Man (Aitken and Valladas 1992). Optical dating of sediments additionally is becoming a major tool in the investigation of the origin and development of H. sapiens sp. (Roberts 1997). Luminescence research has benefited greatly from the pioneering work of Huntley et al. (1985) in the development of optically stimulated luminescence (OSL) techniques designed specifically for dating sediments. Some inherent difficulties in applying luminescence dating to sedimentary environments drove the objectives of recent research towards the analysis of single aliquots and single grains (Duller 1991; Lamothe et al. 1994; Murray and Roberts 1997). These new approaches developed for sediment dating are now being used to better constrain the age of archaeological events (e.g., Henshilwood et al. 2002). Fired objects like pottery, burnt stones, and baked sediments are obvious candidates since datable material commonly contains luminescent minerals, including quartz and feldspar. The purpose of this paper is to describe and demonstrate the potential of OSL
Received 16 July 2003. Accepted 7 April 2004. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 9 June 2004. Paper handled by Associate Editor J.R. Desloges. M. Lamothe. Département des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, C.P. 8888, Succ. Centre-Ville, Montréal, QC H3C 3P8, Canada (e-mail:
[email protected]). 1
This article is one of a selection of papers published in this Special Issue on Applications of earth science techniques to archaeological problems.
Can. J. Earth Sci. 41: 659–667 (2004)
doi: 10.1139/E04-032
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technological advances through three case studies. Materials used are geoarcheological objects that had been previously investigated with thermoluminescence in the Montréal Lux laboratory. They include an Archaic burnt stone, a Woodland ceramic, and a Plano artefacts-bearing soil.
Can. J. Earth Sci. Vol. 41, 2004 Fig. 1. Location of the three archaeological sites in Quebec investigated in this study. Site EiBg43 is a 2.5–3.0 ka fire place near Blanc Sablon; site CgEq14 is near Cap Tourmente and contains 1.5–2.0 ka ceramics; site DcEd1, located a few kilometres east of Rimouski, is a soil in which are found 8.0 ka Eastern Plano artefacts.
Description of the samples The three different types of studied material are from archaeological sites located around the Saint-Lawrence estuary (Fig. 1). The first sample is a burnt Cambrian arkosic sandstone from an Archaic hearth found in Blanc Sablon (EiBg43; Pintal 1989) that was radiocarbon dated at 2700– 3200 calendar years BP using charcoal. Several burnt stones from this fireplace structure were dated using thermoluminescence (TL) by Bertrand (1991). She used TL from quartz and feldspar minerals of different grain sizes extracted from the burnt stones. The ages that were obtained span the expected time period, except for some feldspar-bearing subsamples that returned young ages in the order of 2300 years. The second sample presented herein is from pottery that was collected from a Middle Woodland site (CgEq14) near Cap Tourmente, a few kilometres north of Québec City. This site has been extensively studied by Bossé (1992), and thousands of ceramic fragments have been found, described, and dated on stylistic grounds. The CgEq14 site was excavated at the surface of a marine terrace dated at ca. 2300 years BP. Thermoluminescence dating has been carried out by Pelletier (1996) who reported, for this particular site alone, 10 ceramic TL dates that ranged from 1300 to 2000 years. These age estimates were consistent with an extended Woodland occupation. The age range could also be explained by the occurrence, to different degrees in each ceramic sample, of anomalous fading from constituent feldspars. The third sample is a soil near Rimouski, in which points and blades of distinctive Plano heritage have been identified (DcEd1, Chapdelaine 1994). The oldest charcoal found at the site was radiocarbon dated at 8.15 ka BP and was from the Plano artefacts-bearing eluviated Ae horizon of a podzolic soil. A series of preliminary additive dose infrared stimulated luminescence (IRSL) ages have been obtained, which ranged from 12 400 years for the Bf horizon, to < 5000 years for the uppermost A horizon (Lamothe 1994). In each of these archaeological cases, the ages obtained from the feldspar extracts were younger than the quartz ages. This problem of age underestimation is the result of anomalous fading of feldspar luminescence, for which appropriate solutions have only recently emerged. The correction for fading developed by Huntley and Lamothe (2001) is particularly suitable for young geological material and this correction is applied herein.
Luminescence dating: some recent innovations Luminescence results from complex physical processes wherein light is emitted by minerals as they are externally stimulated (Aitken 1985, 1998). Light is emitted as electrons trapped during geological or archaeological time, gain sufficient energy from an external stimulus following which
they may undergo radiative recombination. The presence of electrons in the dating trap results from the interaction of natural radiation with the geological material. If the stimulus is thermal, then thermoluminescence is emitted as an indirect measure of the radiation dose acquired since the last firing event or since crystallization of the mineral. Optical stimulation produces optically stimulated luminescence (IRSL for infrared stimulation) that is proportional to the time elapsed since the dating trap was last emptied by light or heating. If one can evaluate how much luminescence is induced per unit of absorbed radiation, and if the natural radiation intensity is constant throughout the time period under consideration, then the sample can be dated using Age (years) = Paleodose (Gy) / Annual Dose (Gy / year) The paleodose (P) is the dose that has been received by the mineral in the environment. The equivalent dose (De) is the laboratory dose required to induce the observed natural luminescence intensity. The postulate herein is that the latter corresponds to the former. One typical example of P ≠ De is for feldspars that exhibit anomalous fading (Visocekas 1985; Spooner 1993). The electrons at the source of the luminescence signal are expected to remain in the dated trap for periods of time exceeding the time scale investigated. For feldspar, however, luminescence resulting from laboratory irradiation is observed to decrease as a function of storage time. The consequence is that the laboratory dose response for feldspar is apparently higher than the dose response in the natural © 2004 NRC Canada
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environment and, accordingly, the age deduced from the equivalent dose is underestimated. Single aliquot dating The classical approach for equivalent dose assessment is through the analysis of a growth curve constructed from several aliquots of the same sample, some in their natural state and some artificially dosed (Fig. 2). Using several aliquots, one equivalent dose value is computed, and hence, this procedure is known as the multiple aliquot method. In their seminal paper, Huntley et al. (1985) had suggested that several OSL measurements could be extracted from a single aliquot without significantly depopulating the dating trap. Duller (1991) was the first to develop a single aliquot method with some success. Later, to detect and isolate luminescence arising from only well bleached grains in a sediment sample, Lamothe et al. (1994) assessed the equivalent dose in feldspar, on a grain to grain basis. The age of the dated event is selected by analysing an “age distribution” diagram. Later, Murray and Roberts (1997) extended the method to single grains of quartz and this research prompted the development of an innovative dating technique known as “single aliquot regeneration” or SAR (Murray and Wintle 2000; Murray and Olley 2002; Murray and Wintle 2003). The SAR method allows for an evaluation of the distribution of doses among aliquots, and hence can be used to produce age distributions, a significant advantage over the single age derived from multiple aliquot growth curves. Another advantage of SAR is of particular interest when dealing with archaeological material, as the method requires only a very small sample volume. Some experimental caveats are described and dealt with below. The following experimental steps are carried out using the SAR protocol, as it was originally described by Murray and Wintle (2000). Newly developed experimental conditions, some specific to feldspar, are indicated. (1) The aliquot is first preheated to remove the contribution of thermally unstable luminescence; for quartz, preheats of 220 to 260 °C for 10 s are commonly used; for feldspar, Lamothe et al. (2001) recommended 250 °C for 60 s, following the original preheat experimental conditions of Lamothe and Auclair (1999). One may vary the preheat temperature, and hence assess the thermal dependence of De. (2) The aliquot is then optically stimulated. For quartz, the preferred stimulation wavelengths are in the visible part of the spectrum, using green (ca. 525 nm; e.g., Galloway 1996) or blue (ca. 470 nm; e.g., Bøtter-Jensen et al. 2000) diodes. Both wavelengths can stimulate luminescence in feldspar. But for this mineral, it is appropriate to use near-visible infrared photons for stimulation, as there is a well known resonance within the dating trap, near 880 nm. The emission for feldspar is selectively in the blue–violet range, whereas for quartz, ultraviolet (UV) light is measured. The resulting luminescence intensity is referred to as Li. (3) As the same aliquot is used repeatedly in the SAR routine, there are significant luminescence sensitivity changes that are occurring at each dose-preheat-optical stimulation step. These sensitivity changes are corrected by monitoring
661 Fig. 2. Thermoluminescence (TL) glow curves and determination of the paleodose for polymineralic fine grains extracted from ceramics, using the additive dose method (from Pelletier 1996). The emission is through blue filter combinations. The delay between irradiation and glowing is 6 months. De, equivalent dose.
the luminescence intensity following a standard radiation dose (called a test dose). As a consequence the aliquot is irradiated by a standard dose in the third step, commonly a fraction of the presumed paleodose (ca. 25%– 50%). (4) The aliquot is preheated again, but to minimize sensitivity changes, the preheat is kept at a minimum, i.e., enough to remove phosphorescence. For quartz, the recommended preheat is at 160 °C/0 s or 220 °C/10 s (other combinations have been proposed as it may be sample dependent; Bailey 2000). In the case of feldspar, Lamothe et al. © 2004 NRC Canada
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(2001) suggested using the same preheat for both dose and test dose. (5) The sample is optically stimulated again as in (2), and the resulting luminescence intensity is Ti. (6) The sample is given a known radiation dose, and the procedure is repeated from step 1 to 5. A growth curve is then constructed with several dose-points (i = 1 to n; Fig. 3). The last irradiation step is a repeat of the first one and a recycling ratio, is devised. This ratio is equal to [(Ln/Tn)/(L1/T1)], and it is mostly verified to be within 10% of unity (Stokes et al. 2000). The dose-response curve (Li/Ti versus laboratory dose, including a “zero dose”) is plotted, and the natural luminescence intensity (LN/TN) is interpolated from it to determine the equivalent dose (Fig. 3). In Fig. 3, the 3 Gy dose point is repeated, and the Li/Ti from the two measurements is so similar that it is not distinguishable on the graph. The recycling ratio is an indication that the SAR protocol does accurately monitor the changes in luminescence sensitivity. However, it had been demonstrated by Wallinga et al. (2000) that the luminescence resulting from the natural radiation dose in feldspar may not be correctly monitored by the following test dose, as the former had been acquired before any thermal treatment. Lamothe et al. (2001) have tested different experimental conditions to find the most reliable preheat procedure. The suggestion has been made that the appropriate procedure is the one in which both the dose and the test dose are similarly preheated before measurement. An example of the malign effect on recycling ratios due to the use of inappropriate preheat treatment is shown on Fig. 4 for a Blanc Sablon sample. An interesting implication of using similar preheat treatments for both stimulation domains (IR and blue) is that one can monitor luminescence from two distinctive emissions, in the blue–violet emission band (ca. 400–450 nm) emission and in the UV band (ca. 300–350 nm). The former emission is commonly observed in feldspar IRSL and the latter in quartz OSL. The concept of using the same aliquot for assessing the dose in quartz and feldspar has been first suggested and developed by Banerjee et al. (2001) and is known as the “post-infrared blue” measurement, termed here PIB. This approach had been initiated with the objective to isolate the quartz luminescence signal in a fine-grained polymineral aliquot. The idea herein is that the IR stimulation should deplete the so-called fast component of the OSL from the constituent feldspar grains, hence it is termed “an infrared wash.” However, in the case of an aliquot that contains a significant amount of feldspar, there is a possibility that, even after an IR wash, there could be a contribution from feldspar that could deplete rapidly and contribute to the “fast component” (Fig. 5). This “feldspar residual fast component” may be due to transfer of electrons from the IR trap to the blue-sensitive trap, while it is IR stimulated, or from recuperation or because of an increase in energy from the 1.4 eV of the IR photons to 2.5 eV for the blue emission. The fast component residual from feldspar, if present, will be adding its contribution to the blue stimulated luminescence of quartz and this PIB could, therefore, include a fading component. The reliability of using IR and post-IR blue UV emissions from single fine grains aliquots is tested below, using a similar
Can. J. Earth Sci. Vol. 41, 2004 Fig. 3. Single aliquot regeneration dose-response curve for sample EiBg43. The measurement protocol is explained in the text. The stimulation used is from infrared diodes and the emission spectrum is detected in the blue. The luminescence intensities used in the calculation of Li/Ti are the first 2 s of stimulation after subtraction of the last 40 s. IRSL, infrared stimulated luminescence.
preheat treatment for both dose and test dose and applying a fading correction to both emissions. Correction for anomalous fading As observed by several workers in the past, luminescence that results from a laboratory radiation decays in most feldspar minerals without any stimulation. The general observation is that the decrease in luminescence intensity is linearly related to the logarithm of time elapsed since the end of the irradiation. Visocekas (1985) was the first to suggest that quantum mechanical tunnelling was the culprit for such leakage. To date accurately any feldspar sample, methods of corrections for this anomalous decay have been recently introduced in the luminescence literature by Lamothe and Auclair (1999) and Huntley and Lamothe (2001). This last correction has been shown to be reliable when the feldspar natural luminescence intensity lies in the linear part of the dose-response curve. Hence, it is appropriate for young samples (≤20 ka), and is the one used in this study. The correction is based on an iterative equation: T = 1 − κln − 1 T t c
Tf
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Fig. 4. Recycling of the luminescence signal from a test dose of 9.88 Gy (80 s beta irradiation), normalized by a test dose of 2.47 Gy (20 s beta irradiation) for 11–63 µm-size grains extracted from a laboratory heated Cambrian arkose (BSRM) similar to EiBg43. (a) Li/Ti for post-infrared blue (PIB) ultraviolet emission with a preheat of 220 °C/10 s and a cut heat of 160 °C (solid circle) compared with the ratios obtained if both dose and test dose are preheated at 220 °C for 10 s (open circle). (b) Similar experiment for IRSL blue emission, except that preheat for dose is 250 °C/60 s.
Fig. 5. (a) Shine down curve for quartz (Qz), under blue stimulation with detection in ultraviolet window. (b) Shine down curve for K-feldspar (KF) under similar conditions, after it has been exposed to infrared (IR) photons for 100 s (PIB, post-infrared blue). (c) Synthetic sum (SUM) of the two shines, as it would appear for a fine-grained aliquot not separated into specific mineralogical species. OSL, optically stimulated luminescence.
Application of the SAR technique to geoarchaeological objects in Quebec where κ=
g 100 * ln(10)
The measured fading rate is g, tc is a fixed time constant, Tf is the age before correction, and T is the true age. Auclair et al. (2003) described how SAR could be used to assess the anomalous fading rate. An example is provided on Fig. 6 for sample RS5 from the Rimouski site.
Systematic measurements of the IRSL and the PIB of polymineral fine-grained extracts from a burnt stone, a pottery fragment, and a soil component at three Quebec archaeological sites are described. The objective here is to assess the reliability of multiple age determination using the luminescence of both the infrared (blue and UV emission) and the blue stimulated luminescence (UV emission only) on two populations of aliquot for the same sample. In this “double SAR” protocol, a first set of aliquots is measured using the blue emission (with a filter combination of BG39 + Corning 7-59) upon IR illumination. The other set of aliquots is stimulated © 2004 NRC Canada
664 Fig. 6. Logarithmic decay of luminescence measured over time for one aliquot of sample RS5. The delay is the time elapsed between the end of the irradiation and the luminescence measurement and includes about half of the irradiation time. The slope of the decay is used to calculate the fading rate parameter g.
Can. J. Earth Sci. Vol. 41, 2004 Fig. 7. Luminescence ages for every single aliquot measured, shown on a radial graph (Galbraith 1990). The experimental conditions are explained in the text and the mean ages are found in Table 1. The shaded area is the 14C age. In (a) EiBg (Blanc Sablon) and (c) DcEd (Rimouski), the luminescence and radiocarbon age should be similar; in the case of (b) CgEq (Cap Tourmente), the radiocarbon age dates the emergence of the terrace on which the archaeological objects were found, hence the luminescence age should be younger than 14C.
twice, and both measurements are detected in the UV emission band (BG39 + U-340). Therefore, three results are shown for this set: the IR stimulated luminescence, the PIB stimulated luminescence, and the sum of the two luminescence intensities (SUM = IR + PIB). The addition of the two luminescence outputs was decided after it was realized that, in the three cases investigated, the sensitivity of “true quartz” was very low and the contribution of feldspar to the PIB is significant. A clear proof of this is that the PIB luminescence exhibits anomalous fading, contrary to expectations, if this emission would be purely from quartz. Addition of the luminescence intensities generates a synthetic SUM aliquot shown in Fig. 5c. For the four equivalent dose results, a SAR growth curve and an anomalous fading correction are computed. In Table 1, the SAR ages obtained from the IR, the PIB, and the luminescence SUM of the fine-grained aliquots are presented, and the results are displayed on radial plots (cf. Galbraith 1990) in Fig. 7. Blanc Sablon In the original experiment of Bertrand (1991) in Blanc Sablon, four rocks were split into different grain sizes and mineralogical separation had been carried out on the sand-sized fraction of each stone. One of those rocks, EiBg43.3b, had been dated at 2.78 ± 0.20 ka using quartz TL, 2.52 ± 0.18 ka using feldspar TL, and 2.79 ± 0.21 ka using polymineralic fine grains TL. The associated 14C charcoal ages range from 2.69 ± 0.06 to 2.87 ± 0.06 ka BP, all uncalibrated, which translates into an average calibrated age of 3.00 ± 0.30 ka. The thermoluminescence ages obtained in the original study were relatively close to the expected age, even though the feldspar ages are somewhat younger. The TL dates were measured after a relatively long time delay after irradiation (ca. 6 months), as it was commonly the case in luminescence laboratories a decade ago. This partly explains the close © 2004 NRC Canada
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Table 1. Luminescence age estimates from fine grains SAR. Optical ages (ka) c
LN (counts 0–2 s)
Equivalent dose (Gy)
Fading rate, gd (%/decade)
Corrected for fading
Sample
Methoda
Detectionb
Blanc Sablon EiBg43 43.3b
IRSL IRSL PIB SUM
Blue UV UV UV
4448±340 511±62 1424±44 1154±249
6.41±0.31 5.81±0.59 7.80±1.71 6.90±1.18
8.0±0.9 8.9±1.6 3.7±2.5 7.0±1.2
1.75±0.14 1.57±0.20 2.12±0.44 1.87±0.29
3.57±0.68 3.52±1.05 2.54±0.45 3.33±0.79
Cap Tourmente CgEq14 CG4
IRSL IRSL PIB SUM
Blue UV UV UV
3452±310 430±28 1127±80 1557±105
5.86±0.17 4.91±0.65 5.43±0.39 5.27±0.27
9.6±1.0 9.7±0.8 8.4±2.0 9.0±0.9
0.87±0.04 0.73±0.10 0.81±0.07 0.79±0.05
2.09±0.50 1.86±0.43 1.60±0.64 1.79±0.36
Rimouski DcEd1 RS5
IRSL IRSL PIB SUM
Blue UV UV UV
20754±710 4270±1095 10782±2363 16349±1881
16.62±0.30 12.60±1.12 14.46±0.40 14.02±0.49
6.5±1.2 10.7±1.7 2.2±1.1 6.8±0.8
5.22±0.48 3.94±0.32 4.54±0.36 4.40±0.33
9.11±1.53 15.19±6.78 4.85±0.56 7.87±1.18
Uncorrected
a
IRSL, Infrared stimulated luminescence; PIB, post-infrared blue stimulated luminescence; SUM, sum of the luminescence intensities. Filter combinations for detection are BG39 + Corning 7–59 (Blue) and BG39 + U340 (UV, ultraviolet). c Average net counts for natural signal following late light substraction. d Values are normalized to a tc = 48 h. e Dose rate values in Gy/ka are 3.68 ± 0.26 for EiBg43, 6.73 ± 0.44 for CG4, and 3.20 ± 0.28 for RS5. b
correspondence of the quartz and feldspar ages. Table 1 and Fig. 7a show the optical dates obtained on the fine-grained aliquots. Following the fading correction of Huntley and Lamothe (2001), the IRSL and SUM ages are similar, but significantly higher than the PIB result. Overall, the radial plot on Fig. 7a is a good indication that the optical dates obtained from multiple measurements are consistent among themselves. Most of these results are found inside the 2σ uncertainty around the mean of the radiocarbon age. The best luminescence age estimate for the Blanc Sablon hearth using the 24 fine grains aliquots is 3.47 ± 0.12 ka (central value model, Galbraith et al. 1999; standard error on the average age). Cap Tourmente At Cap Tourmente, only fine-grained material had been TL-dated by Pelletier (1996; Fig. 2) as the local ceramics are poor in sand-sized temper. The large range of TL dates, from 1300 to 2000 years BP, had been taken as a possible indication that this site might have been occupied over an extended time period, periodically or permanently. A second alternative is that some pottery fragments might have been reworked from older sites. A third more likely alternative is that the spread in TL dates reflects a range of feldspar mineral abundance, and hence of variable anomalous fading rates among the different shards. In spite of the low optically stimulated luminescence intensities (see LN in Table 1) and poor reproducibility of the optical measurements, the optical ages obtained from each experimental set up is similar, with an average age of 1.88 ± 0.15 ka (11 aliquots, Fig. 7b). This number is coherent with the geological context, as the archaeological site sits on a marine terrace dated at 2.45 ± 0.30 ka (from an uncalibrated charcoal radiocarbon age of 2.36 ± 0.12 ka BP). This optical age agrees as well with the expected archaeological age, based on ceramic styles of the period (Bossé 1992).
Rimouski This critical site in the Quebec archaeological record (Chapdelaine 1994) yielded originally four datable sediment samples, in which the sand-sized fraction had been further separated into feldspar and quartz aliquots. The multiple aliquot IRSL age for the sediment sample RS5, the one most likely related to the archaeological finds, is 8.35 ka (with a large uncertainty of ± 1.29 ka; Lamothe 1994). In this optical dating study, there is a significant luminescence sensitivity in the fine-grained extracts, even for the post-infrared blue stimulated luminescence, which is normally expected to arise from quartz. Considering that the quartz coarse grains had no measurable OSL sensitivity (unless heated to 500 °C), it is concluded that the fast component in the PIB emission is therefore from feldspar. Fading is poorly reproducible in these aliquots and at a much smaller rate (g = 2.2 ± 1.1%; Table 1) than for the IR stimulated luminescence (g = 10.7 ± 1.7%). Contrary to the other two samples, the PIB is herein much too young compared with the expected age of 9.15 ± 0.35 ka. Nevertheless, the luminescence age of the finegrained extracts using both the blue emission of the IRSL and the SUM result from the UV emissions are in close agreement with the presumed burial age of the Plano artefacts, as the average optical age is 8.73 ± 0.37 ka (18 aliquots, Fig. 7c).
Conclusion The three examples selected represent different geoarchaeological contexts. These Quebec case studies provide clear evidence that anomalous fading rates need to be measured for every fine grain sample investigated, as an infrared “wash” may not be sufficient to eradicate the contribution of feldspar to PIB luminescence. “Double SAR” age estimates from fine-grained aliquots have originally been suggested by Banerjee et al. (2001) and tested by Roberts and Wintle © 2004 NRC Canada
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(2001) and Stokes et al. (2003). Even though the PIB age for sample RS5 would be considered unreliable, overall for the three cases investigated here the PIB result is systematically underestimating the expected age. This is similar to the underestimation found by Roberts and Wintle (2001) in their study on Chinese loess. This double SAR procedure has strong potential in luminescence dating, as it allows the practitioners to measure the natural signal twice on one aliquot, thus representing a definite improvement over more traditional experimental procedures. There is however an urgent need to develop a more precise methodology to assess fading rates, as the one introduced by Auclair et al. (2003) generates a high level of uncertainties for samples of low luminescence sensitivity. The SAR optical ages obtained from the fine-grained extracts using both blue and UV emissions are in good agreement with the expected archaeological ages for each site. Should those sites not have yielded any datable radiocarbon material, optical dating would have provided a reliable chronological framework given that the appropriate procedures had been carried out. The extent to which luminescence methods are considered and exploited by North American geoarcheologists should perhaps be reconsidered in the light of the most recent developments in the field.
Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC grant no. RGPIN 37375). The author would like to thank S. Huot and M. Auclair for data processing and editing, M. Laithier for the figures, and S. Stokes, from the Geography Dept., Oxford University, for reviewing the final manuscript. Olav Lian and an anonymous reviewer greatly improved the original version of the paper. The author wishes to thank Chantal Pelletier, Karen Bertrand, Jean-Yves Pintal, and Claude Chapdelaine for letting him use their samples.
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