FIFTY YEARS OF LUMINESCENCE DATING*

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1 Archaeometry 50, 2 (2008) doi: /j x Blackwell Oxford, ARCH Archaeometry X XXX Original Fifty A. *Received University G. years Wintle UK Articles Publishing 13 of November luminescence Oxford, Ltd ; accepted dating 29 November 2007 FIFTY YEARS OF LUMINESCENCE DATING* 8 University of Oxford, 2008 A. G. WINTLE Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK Luminescence techniques for dating both heated materials (e.g., pottery and burnt stone) and sediments have gone through three phases in the past 50 years. The first 22 years, from 1957 to 1979, were devoted to thermoluminescence (TL) techniques applied to heated material. In the next six years, from 1979 to 1985, it was found that TL dating could be applied to sediments. However, the TL signals of both quartz and feldspar grains observed for modern sediments were small, but were not zero, thus making them useful only for older sediments. A new luminescence signal that was totally zeroed by exposure to sunlight for a relatively short period of time, a few hours or less, was found; this led to the development of optically stimulated luminescence (OSL) dating techniques in Further developments have continued to the present day. Within the past 22 years, 1999 was also a year with major developments. A reliable procedure for single aliquots of quartz that have a rapidly bleached ( fast ) OSL component was formalized and an instrument that allowed rapid measurement of equivalent doses for single grains was constructed. These developments have led to OSL becoming a major dating tool in Quaternary geology, at least for the past years, and in archaeology, particularly as related to the dispersal of modern humans. KEYWORDS: LUMINESCENCE DATING, TL, OSL, IRSL, HISTORY, HUMAN DISPERSAL INTRODUCTION Thermoluminescence dating was developed by Martin Aitken in the early 1960s, as a means of obtaining the firing age of ceramic materials found at archaeological sites. The quartz and feldspar grains found in pieces of pottery or other types of fired clay acted as dosimeters, i.e., they were able to record the amount of radiation to which they had been exposed. In the case of pottery, this radiation dose was that received by the grains since the time when they had been heated. The heating erased the previous TL signal and with it information on the previous radiation exposure, i.e., that related to the time elapsed since the minerals had been formed. Subsequently, the TL signal builds up with time as the grains are exposed to the almost constant dose rate resulting from the decay of naturally occurring radioactive isotopes, 40 K and 87 Rb, and 238 U, 232 Th and 235 U and their daughter products, together with a usually small contribution from cosmic rays. Thus the basis of a dating method for heated materials was born. Combining the dose in the grains today, found from TL measurements, and the dose rate, obtained from measurements of the concentrations of the radioactive isotopes in the ceramic object and in the sediment surrounding it, the time since firing is obtained. This paper seeks to provide a chronological review, following on from that by Aitken (1989) in an earlier issue of Archaeometry and another by Roberts (1997) in Radiation Measurements. *Received 13 November 2007; accepted 29 November 2007 University of Oxford, 2008

2 Fifty years of luminescence dating 277 THE FIRST 22 YEARS, Apart from the initial announcements of TL dating in Nature (Aitken et al. 1964; see also Aitken et al. 1968) and in Science (Mazess and Zimmerman 1966), the main papers related to TL dating carried out in the 1960s were published in Archaeometry, as this journal was produced in the Research Laboratory for Archaeology and the History of Art, of which Martin Aitken was Deputy Director. Thus, Archaeometry became the most appropriate journal for developments in this new technique for dating archaeological material. The first paper on TL dating to be published in Archaeometry (Tite and Waine 1962) summarized previous work in Switzerland by Houtermans and in the USA by Daniels. Tite and Waine (1962) attributed to these two scientists the possibility of applying thermoluminescence as a method of dating, due to the proportionality relationship between the amount of light emitted and the time since the last heating. Shortly afterwards, Fremlin and Srirath (1964) presented photographs of the TL emitted by slices of pottery that had been irradiated using protons in order to provide sufficient TL output to register on a Polaroid film. These photographs indicated the heterogeneity of the clay used to make a strong ceramic. Fine grains and coarse grains The seminal papers on the TL dating of fine (silt-sized) grains by Zimmerman (1967, 1971a) and of sand-sized (coarse-grained) quartz inclusions by Fleming (1966, 1970) recognized and exploited the different radiation fields to which grains of different sizes were exposed in the natural environment. The terms fine-grained and coarse-grained dating have been firmly entrenched in the literature ever since. Fine grains (taken as 4 11 micrometre (μm) diameter grains with a mixed mineralogy, i.e., containing both quartz and feldspars) receive gamma radiation from the sediment surrounding the piece of pottery, and both beta and alpha radiation from within the piece of pottery. Coarse grains (taken as greater than 90 μm, and usually less than 250 μm, in diameter) receive the same gamma dose as the fine grains, but the amount of beta dose they receive from their immediate surroundings will be reduced by attenuation, and the alphairradiated region can be removed entirely (or nearly so) by etching in hydrofluoric acid. Dose rate considerations The complexity of the natural radiation environment (e.g., the non-uniformity of uranium, thorium and potassium in the sediment surrounding the piece of ceramic), the absorption in water of part of the dose delivered by the radioactive decay and the possibility of the loss of the gaseous element, radon, was recognized early on (e.g., Aitken 1969; Mejdahl 1970). These authors took advantage of the commercially available TL dosimeters that were being developed at that time, materials such as natural CaF 2 (Aitken 1969) and CaSO 4 :Dy (Mejdahl 1970). Burial of these dosimeters in the archaeological section for about one month produced a measurable TL signal generated by the gamma radiation from most sediments. Such an in situ measurement would give a dose rate that takes account of the water content and radon loss related to the sediment, both assumed to be the same as over the whole burial period. At that time, the TL dosimeter CaSO 4 :Mn was also used to measure the radiation from the ceramic itself (Mejdahl 1969). In the 1970s, the major effort in TL dating of ceramic material was focused on improving calculation of the dose rate. In addition, a rigorous method for calculation of all error terms

3 278 A. G. Wintle was developed (Aitken and Alldred 1972; Aitken 1976) and it is still used today. The lower efficiency of alpha particles, compared with beta and gamma radiation, was investigated (Zimmerman 1972). The TL produced when very small diameter grains were irradiated with alpha particles of different energy produced by a Van de Graaff accelerator was measured (Aitken and Bowman 1975). These studies were used to provide better estimates of the effective natural alpha dose rate, derived from alpha particles with different energies, and the effective laboratory dose rates observed for fine grains irradiated by artificial sources. Quicker methods of determining the in situ gamma dose rate, and one that did not risk contaminating the laboratory with highly sensitive artificial dosimeters (such as CaF 2 and CaSO 4 :Dy), using scintillations from a sodium iodide crystal, were explored. Portable gamma detection systems were used, both with energy spectrum measurements and as a scintillometer (Murray et al. 1978); the wish to calibrate these gamma detection systems led to the construction, in the back garden of 6 Keble Road, Oxford, of a set of concrete blocks doped with U, Th and K (Aitken 1985, ). These blocks are still used for calibration of sodium iodide crystals, although they were relocated to the rear of the Dyson Perrins Building, South Parks Road, Oxford when the Research Laboratory for Archaeology and the History of Art moved there in January 2006 (Rhodes and Schwenninger 2007). For determination of the contribution of U and Th from within the ceramic to the alpha and beta dose rates, alpha counting was widely used, following its early use by Tite and Waine (1962). This relatively simple technique (Aitken 1985, 86 94, ) is still used today. During the development of alpha counting, emanation of radon from the crushed ceramic was often observed when scintillation rates were compared, first with the counting cell open to allow gas exchange with the atmosphere, and then sealed. Radon emanation was further explored for pieces of solid ceramic using a gas counting cell, particularly with regard to the effect of water on radon retention (Desai and Aitken 1974). Also, during the 1970s, new information on the energies and rates of the alpha, beta and gamma emissions became available in periodically updated tables of range and stopping power. In 1978, Warren noted that the decay of 87 Rb, an isotope of rubidium that is taken into crystals together with potassium, also contributes to the dose rate. In the late 1970s, Bell (1976, 1977, 1979) revised the values provided by Aitken and Bowman (1975) and provided a consolidated tabulation of the relevant decays. These compilations by Bell were used to provide the tables of annual dose data in Aitken (1985, ). Over the next 20 years, these were replaced with revised nuclear data, first by Nambi and Aitken (1986) and then by Adamiec and Aitken (1998). Gamma spectrometry was also proposed for determination of U, Th and K in ceramics (Meakins et al. 1979). This was never widely adopted because of the expensive instrumentation, the complicated set-up and the need for long counting times for small samples. However, in later studies, the possibility of radioactive disequilibrium occurring in sediments became a concern, with early discussions of the impacts of a time-dependent dose rate by Readhead (1987) and by Murray et al. (1987). Olley et al. (1996) reported disequilibrium for a number of fluvial sediments and also provided conversion factors to obtain dose rates from measured decay rates for different parts of the uranium and thorium decay chains; updated conversion factors are given by Stokes et al. (2003). This greater understanding of the energy released in each transition led to a greater awareness of the need to calculate attenuation factors. The calculation of beta attenuation factors for sand-sized grains of different diameter was carried out by Mejdahl (1979) for the different beta-emitting isotopes. These calculations were performed in order to establish the attenuation

4 Fifty years of luminescence dating 279 factors for the wider range of grain sizes that he encountered in the dating of burnt stones, an important feature of archaeological sites in Denmark, where he had established a TL laboratory. These calculations were also vital for the assessment of the internal dose rates to large feldspar grains that were also to be found in these stones. The contributions to the dose rate from 40 K and 87 Rb in feldspars, particularly potassium (alkali) feldspars that contain up to 14% K, result in higher dose rates being obtained in these K-feldspar grains than in quartz grains of the same diameter. At the same time, quartz grains were being found not to be as simple as originally proposed. In his study, Fleming (1970) reported that his quartz grains had very low radioactivity. However, for a small number of samples, Sutton and Zimmerman (1978) found grains with substantial radioactive contents. They employed fission-track mapping to demonstrate the presence of radioactive inclusions in these grains. Fortunately, such grains appear to be rare. In addition, Bell and Zimmerman (1978) reported that quartz grains etched with concentrated hydrofluoric acid for 40 min did not have their surfaces removed isotropically. Some grains were found to have tunnels into their interior and to have some alpha-irradiated surface area remaining. This was variable from sample to sample and from grain to grain. Using an optical microscope, the etched grains were seen as being either shiny or frosty, and this effect on the grain transparency was shown to affect the calibration of the beta dose rate (Bell and Mejdahl 1981). Some of these effects may still be causing us problems with reproducibility in OSL measurements of single quartz grains. An early application During the 1970s, several exciting applications of TL dating of pottery and fired clay were published in Archaeometry. These included the dating of pieces of fired clay from the excavations at Dolni Vestonice, in the Czech Republic, carried out several decades earlier (Zimmerman and Huxtable 1971). These clay lumps were found in the same layer as animal and human figurines, also made of fired clay. The recently developed fine-grain method (Zimmerman 1971a) was employed and an average age of 33 ± 3 thousand years (ka) was obtained using measurements on three lumps of fired clay, giving the oldest TL ages obtained to date. It was also possible to compare these ages directly with a radiocarbon age of ± 170 bc obtained on organic material collected at the same location during the original excavation in the late 1920s. It seems that this material was probably contaminated in the 30 years before the radiocarbon dating was carried out. This study includes one of the first plateau tests to be published (Fig. 1). For samples of such antiquity, it was necessary to determine that the part of the glow curve being used for dating was thermally stable. Figure 1 (a) shows typical glow curves for polymineral fine grains, with one broad peak in the natural TL and two for the TL obtained after an additional laboratory irradiation. A rapid heating rate was used (up to 20 C s 1 was usually employed in order to increase the rate of TL output). The ratio of the two curves shows complete loss of TL below 200 C during burial, partial loss between 200 and 300 C, and slight loss between 300 and 400 C. This led the authors to select TL above 400 C for the dating signal. Such plateau tests can still be used today to provide information on whether particular traps are sufficiently thermally stable. Use of the TL signals directly provides the information more rapidly than plots of equivalent dose as a function of preheat temperature, as is usually used in OSL dating. However, for sediments, simple application of the TL measurements (as in Fig. 1) is not applicable unless the unbleachable part of the TL signal is identified and removed.

5 280 A. G. Wintle Figure 1 (a) TL glow curves; that is, signals measured as a function of temperature when heating at 20 C s 1. Signals have been obtained for fine-grain discs from fired clay with an age of ~30 ka, one disc giving the natural TL and the other disc having been given an additional dose of 100 Gy. Also shown is the blackbody radiation measured for one disc after the TL had been measured. (b) The ratio of the two signals in (a), showing that a plateau is reached for signals above 300 C. Below this temperature, some of the trapped electrons giving rise to the TL signal have been lost during the last 30 ka (redrawn from Zimmerman and Huxtable 1971). Authenticity testing By the late 1960s, it was obvious that TL dating would provide an excellent method of detecting whether ceramic works of art were genuine or fake, i.e., were of genuine antiquity or had been made in the few decades prior to the item s arrival on the art market. Fleming (1979) discussed at length the results of numerous examples of both dating of archaeological material and detection of fakes in the art world. The book contains a number of examples where fakes were detected, including recent studies of Chinese ceramics (Fleming et al. 1970), terracotta panels beneath Etruscan wall-paintings (Fleming et al. 1971), Renaissance terracottas (Fleming and Stoneham 1973b) and sand cores from inside bronze heads from Benin (Willett and Fleming 1976). Another example relates to items that, it was claimed, had been excavated at a cemetery in Hacilar, a Neolithic site in Turkey (Aitken et al. 1971). Photographs of these amazing pieces, both genuine and fake, were among my earliest memories of the Research Laboratory at 6 Keble Road, Oxford. Figure 2 shows a double-headed vase that was shown to be a fake. Besides the dating of these ceramics using fine grains, another luminescence-based dating method, the pre-dose method for quartz inclusions (Fleming 1973), was applied to discount the suggestion that the fine-grain TL signal had been removed by the heating claimed to have been carried out after excavation in order to dry the ceramics (Fleming and Stoneham 1971).

6 Fifty years of luminescence dating 281 Figure 2 A Hacilar vase shown by a variety of TL techniques to be made recently (from Aitken et al. 1971). TL measurements of chemically cleaned quartz grains for inclusion dating of pottery showed there to be a strong peak at 110 C when the grains are heated at 20 C s 1 after a laboratory irradiation given in the previous few hours (Fig. 3). The sensitivity of this peak was such that a dose of 0.01 Gy could be used to measure it (Fleming 1973). The 110 C TL peak intensity was shown to increase when heated to temperatures between 450 and 500 C (Fig. 4). The form of this thermal activation curve is typical of many processes in luminescence production. Zimmerman (1971b) showed the sensitization of the 110 C TL peak to be related to the repopulation of luminescence centres with holes. Fleming (1973) showed that the amount of sensitization was linearly dependent on dose, up to about 4 Gy. This led to the pre-dose technique being used for pottery that was up to about 500 years old. It was thus highly suitable for pinning down whether or not ceramic works of art were forgeries by permitting the age of recent manufacture to be obtained directly. Another method was based on the measurement of zircon grains (Sutton and Zimmerman 1976). Zircon grains have very high uranium and thorium contents, and thus the dose rate is dominated by their internal dose rate. Unfortunately they are relatively rare, and the nonuniform distribution of both the U and Th, and zoning of the luminescence sensitivity, as observed during cathodoluminescence measurements (Templer and Walton 1985), means that

7 282 A. G. Wintle Figure 3 TL glow curves for purified quartz from pottery immediately following a laboratory irradiation. The dominant peak at ~325 C was considered malign as it showed significant sensitivity changes, and the TL at ~375 C was preferred for dating. Also visible is the 110 C TL peak that was used for pre-dose dating (redrawn from Fleming 1973). Figure 4 The basis of the pre-dose dating technique, for quartz extracted from pottery. The normalized 110 C peak signal (S T /S 0 ) in response to a small test dose increases with preheat temperature, reaching a level S β that depends upon the dose prior to heating (redrawn from Fleming 1973). zircon dating is rarely applied. In a rare application, the equivalent doses of three zircon grains extracted from the sandy clay core remaining from the manufacture of a bronze horse of Greek or Roman age (Fig. 5) on show in the Metropolitan Museum of Art in New York were found to be several orders of magnitude larger than that obtained from the fine grains making up the bulk of the core (Zimmerman et al. 1974). This confirmed that the age had not been affected by a prolonged exposure to X-rays given in order to investigate the construction of the horse. The dating of zircon grains would also have been of major importance in the archaeological world, had the effects of zoning and anomalous fading been overcome. As these grains have a

8 Fifty years of luminescence dating 283 Figure 5 A bronze horse, the antiquity of which was confirmed by comparing TL signals from fine grains and zircon grains extracted from remnants of the sand core used in the casting (from Zimmerman et al. 1974). very high internal dose rate, they would be scarcely affected by problems with changes in past water content and radioactive disequilibrium in the natural environment. The realization that the equivalent dose for fine-grained polyminerals and the equivalent dose for sand-sized quartz grains were substantially different led Fleming and Stoneham (1973a) to propose a subtraction technique for dating pottery that contained both types of grains. This method effectively removes the need to know the environmental dose rate, an important factor when dating items that had been excavated many years earlier and are now in museum collections. Following tests on pottery collected from an archaeological site of Roman age, the subtraction method was applied to tiles from an excavation at York Minster; here, the tiles were located in hearths, with a complex distribution of surrounding materials giving rise to their environmental dose rate (Fleming and Stoneham 1973a). Sherds from the site of Yotoco Ferry in Colombia, where high water content was an issue (Sampson et al. 1972), were also dated using the subtraction technique (Fleming and Stoneham 1973a). The same approach was also used to authenticate a number of Renaissance terracotta pieces (Fleming and Stoneham 1973b), which were large enough to make it possible to obtain sufficient material to measure both quartz grains and fine grains. Problem behaviour During the 1970s, some of the difficulties that prevented the routine application of TL dating were encountered, and this led Fleming (1979) to comment about the TL method that the

9 284 A. G. Wintle archaeological community has become disillusioned about its practical progress i.e. its ability to produce dates. One major problem was anomalous fading, a phenomenon to which feldspars are particularly prone. It was first observed in feldspar grains extracted from volcanic rocks (Wintle 1973), and it remains a problem to the present day for all luminescence measurements made on feldspars (Huntley and Lian 2006). For such grains, the luminescence signal produced by laboratory radiation can be observed to decay over laboratory time scales when stored at ambient temperature; this loss occurs when calculations based on the physical characteristics of the traps suggest that the signal should be stable over thousands or millions of years. Several experimental methods to overcome this problem were investigated, e.g., the use of phototransfer (Bailiff 1976). Anomalous fading was also found to occur, though to a lesser extent, in the TL signals from fine grains extracted from some ceramics. Using a scanning electron microscope, Singhvi and Zimmerman (1979) were able to see cathodoluminescence from individual grains. Having located these grains, they were then able to use an X-ray microprobe attachment to measure their major element concentrations and thus identify the mineral group to which the grain belonged; for example, silica, K-feldspar, Ca-rich plagioclase feldspar, zircon and apatite. They found that K-feldspar grains were present on fine-grain discs that showed anomalous fading. The attempt to date the formation of volcanic rocks was built on the desire to provide ages for reversals of the Earth s magnetic field in the past 100 ka; the direction of the magnetic field is recorded as the lava cools. Huxtable et al. (1978) obtained an age of 26 ka for one of the lava flows containing a record of anomalous magnetization at Royat in the Massif Central in France by dating the sediment that had been heated by the overlying lava flow. However, the interpretation of the magnetic reversal record in this region is confused by the recording of intermediate directions for the Earth s magnetic field, as at Royat (Plenier et al. 2007). At Laschamp, where the lava shows the field to have been truly reversed, Guérin and Valladas (1980) obtained an average TL age of 32 ka using plagioclase feldspars, though this was substantially less than the age of 40.4 ± 1.0 ka obtained using argon argon dating (Guillou et al. 2004). The exact path of the geomagnetic pole and its strength over the period from 50 to 30 ka remain enigmatic, in spite of even more measurements at other sites in the area (Plenier et al. 2007). One of the reasons for wishing to provide an age for the reversal was to investigate the effect of the raising of the cosmic ray flux during a reversal; this would cause an increase in the production of 14 C in the atmosphere and this would have an effect on the radiocarbon age measured for the material growing at that time. An archaeological site containing a reversal about the same time is Lake Mungo in Australia; here, TL dating was carried out on heated sandstone from fireplaces, an age of 33.5 ± 4.3 ka being obtained (Huxtable and Aitken 1977). A similar age was obtained in a later TL study by Bell (1991). As a site that also has evidence for early human occupation in Australia, Lake Mungo continues to be the object of on-going luminescence studies (e.g., Readhead 1988; Bowler et al. 2003; Olley et al. 2006). A second difficulty in routine application of TL dating was the phenomenon of supralinearity. This term refers to the observation that after grains have been heated, their TL does not grow linearly from the origin. Supralinearity was observed for a number of pottery shards and pieces of burnt flint; it was explained in terms of the different probabilities of charge trapping by the various traps (Bowman 1975; Fleming 1975). Supralinearity was particularly troublesome for TL, since every measurement that was made to determine the form of signal growth up to the level of the natural dose had to be made on an aliquot that had been heated.

10 Fifty years of luminescence dating 285 Applications to non-ceramic materials Some of the most exciting archaeological applications in the 1970s were made on non-ceramic materials. Fleming (1979) produced a summary of all TL ages on archaeological material published up until 1979, including both ceramic and non-ceramic materials, and a review of TL ages on non-pottery materials appeared in the following year (Wintle 1980). Among the non-ceramic materials that have been investigated are burnt quartz (Wintle and Oakley 1972), heated stones (Huxtable et al. 1976) and burnt flints (Göksu and Fremlin 1972). Applications to non-ceramic materials required new methods of sample preparation. Burnt stones were crushed in order to obtain a representative grain size for TL measurement. This approach was applied to stones from a number of stone mounds found on Orkney (Huxtable et al. 1976). The stones were thought to have been pot boilers, i.e., stones heated in fires and then thrown into cooking containers made of stone or skin containing water; such a procedure would have been used prior to the development of bronze cooking vessels to which fire could be applied directly. The TL ages obtained provided evidence that this practice was carried out during the first millennium bc. The study on Orkney was one of the most exciting archaeological studies of its time, and the TL ages were able to provide a comparison with the latest radiocarbon calibration curves obtained on bristlecone pines. Pot boilers of a different type were being used at the same time in the southeastern USA. People of the Poverty Point culture in Mississippi and Louisiana used rounded clay balls in this way; balls excavated from several sites were dated to around 1000 bc using the fine-grain TL procedure (Huxtable et al. 1972). For burnt flint, two approaches have been used. Aitken and Wintle (1977) developed a new approach in which measurements were made on thin slices using a beta source, following a study by Göksu and Fremlin (1972). This study also led to a greater understanding of the dose delivered from a beta source to grains of different sizes laid out in a monolayer on an aluminium disc, and to slices of flint of different thickness (Wintle and Murray 1977). Application of this approach to two heated stone fragments from the cave site of Terra Amata on the Mediterranean coast of France resulted in a mean age of 230 ± 40 ka (Wintle and Aitken 1977), one of the oldest ages obtained using TL on heated material. Dating of slices overcame problems of spurious luminescence induced by crushing of the stones, but a gentle acid wash after crushing also enabled spurious-free TL measurements to be made (Göksu et al. 1974). THE NEXT SIX YEARS, , AND THEIR ON-GOING IMPACT Starting in 1978, the Symposium on Thermoluminescence Dating was instigated by Martin Aitken, who selected the term symposium on the basis of its Greek derivation as a drinking party. Publication of the proceedings of the first three symposia was in PACT, the Journal of the European Study Group on Physical, Chemical, Biological and Mathematical Techniques Applied to Archaeology. Publications of the developments in TL dating then moved away from Archaeometry to PACT and in 1985 the proceedings then moved to the physics journal Nuclear Tracks. After the meeting in 1987, the proceedings were divided and special issues of two journals, Nuclear Tracks and Radiation Measurements (which subsequently became Radiation Measurements), dealing with technical aspects of the TL dating, and Quaternary Science Reviews (see also the more recent new journal Quaternary Geochronology), presenting applications, were published in Papers from these meetings, now called the International Conference on Luminescence and Electron Spin Resonance Dating, continue to be published in this way. Thus from 1978 fewer papers were published in Archaeometry. Also, smaller

11 286 A. G. Wintle Figure 6 Stuart Fleming making TL measurements on equipment from the 1970s (from the cover of his 1979 book). technical contributions to the field are reported in Ancient TL, started by David Zimmerman in Initially this was an informal newsletter, running a couple of issues a year, but Ancient TL became a recognized publication with an ISSN in 1983, and volume 1 was published that year. Ancient TL is now freely available online at In addition, there has been a move since 1980 to the dating of sediments, and although the number of papers on luminescence dating has increased rapidly since 1980 as more laboratories have become involved, fewer have appeared in archaeology-based journals such as Archaeometry. However, until 1979 this journal played a major role in the fundamental studies that lie behind the techniques that we use today, with 43 of the 54 pre-1979 papers cited here being published in Archaeometry. Instrumentation In the years running up to 1979, most instrumentation was home built, as seen on the dust jacket of Fleming s book Thermoluminescence techniques in archaeology (Fleming 1979), reproduced in Figure 6. This shows how all glow curves were collected in analogue form using a chart recorder. This method of glow curve recording required rapid manual changing of scale when measuring the 110 C TL peak in pre-dose dating or the TL peaks of CaF 2 when using these grains as in situ dosimeters. In the mid-1970s commercial TL apparatus became

12 Fifty years of luminescence dating 287 available, and in 1985 one such system, by Littlemore Scientific Engineering Co., that used simple computers to control the heating rate and collect data was advertised in Archaeometry. Two rival TL systems, one manufactured by Daybreak Nuclear and Medical Systems Inc. and one made by Risø National Laboratory (Bøtter-Jensen and Mejdahl 1980), were also commercially available. These two companies have taken over the market and now provide TL/OSL systems that are maintained at the cutting edge of the field. Heated stones During the period from 1979 to 1985, the potential of using TL signals from materials other than pottery were investigated. For example, Plachy and Sutton (1982) used quartz grains from different types of heated rocks from a hearth; e.g., from sandstone and from granite. Because of increasing worries about changing environmental dose rates due to movement of water and possible radon migration, more subtraction techniques were also developed, although they have not been adopted widely. Mejdahl (1983) used grains of quartz, plagioclase feldspars (low in potassium) and grains of potassium-feldspar of different size. Calcite deposits in caves During this period, there was also a brief foray into using TL signals from calcite for dating speleothems (Wintle 1978; Debenham and Aitken 1984). However, although the equivalent dose could be measured and taken as a record of the integrated radiation exposure since formation, the complications of calculating the time-dependent dose rate (owing to 230 Th and daughter products not being present at formation) put off others from similar attempts. In addition, new developments in disequilibrium measurement, using thermal ionization mass spectrometry (TIMS) rather than alpha spectrometry, led to TL dating of speleothem being abandoned because of the greater precision attainable using TIMS (e.g., Vaks et al. 2007). Burnt flint More successful investigations were carried out on burnt flint and several of these were published in Archaeometry. Results were reported for Neolithic flints from the UK (Huxtable and Jacobi 1982) and for burnt flints from Upper Palaeolithic and Neolithic sites in France (Valladas and Valladas 1987), in which study the TL ages were compared with radiocarbon ages. Burnt flints are particularly useful, as they relate directly to human activity. They are well suited to TL dating, as they have similar behaviour to quartz. Also, they are impervious to water, and thus the dose rate is less affected by past environmental changes; only the external dose rate is affected by changes in water content, because the external portion of the grain receiving beta radiation from the surrounding sediment has been removed during sample preparation. Valladas (1992) provides a summary of the methodology that is now widely used. Among the most exciting archaeological applications have been the studies on burnt flint from cave sites in the eastern Mediterranean Levant (e.g., Aitken and Valladas 1992; Richter et al. 2002; Mercier and Valladas 2003; Rink et al. 2003; Richter 2007). Feathers (1996) provided a review of luminescence dating techniques, discussing how they are applied to different materials such as sediments and burnt lithics, and with particular reference to the origins of modern humans, discussing the results obtained at Skhul (Mercier et al. 1993), Tabun (Mercier et al. 1995), Qafzeh (Valladas et al. 1988) and Kebara (Valladas et al. 1987). Besides contributing

13 288 A. G. Wintle to the discussion as to whether modern humans and Neanderthals were contemporaneous in the Levant (Valladas et al. 1999), TL dating has also played a role in defining the period in which Neanderthals occurred in Europe (Mercier et al. 1991; Richter et al. 2000). A review of the TL ages in the Levant obtained before 1997 and comparisons with ages obtained by ESR and U-series dating methods was provided by Roberts (1997). In a more recent study, Mercier et al. (2007b) presented ages for 77 heated flints from Hayonim Cave in Israel, thus providing a detailed chronology for the period from 230 to 140 ka. This permits comparison with the previous chronology based on ESR and U-series dating (Rink et al. 2004) and the age range of ka for the Middle Stone Age site of Rosh Ein Mor in the Negev (Rink et al. 2003). The Mahgreb region of North Africa, from Morocco to Tunisia, contains evidence of Middle Palaeolithic occupation, but the stone tools are often found out of context at the surface. Sites containing Mousterian artefacts (small hand axes and flake tools) are widespread in this region, but at a few sites a different stone tool assemblage (containing tanged pieces) is found above the Mousterian levels. Recently, a number of cave sites with in situ tools have been excavated. At one of these, Rhafas Cave in Morocco, ages were obtained using TL signals from chalcedony (a form of silica) that had been burnt in antiquity to obtain ages ranging from 90 to 60 ka (Mercier et al. 2007a). Using these ages, and others obtained on sediments, the boundary between the two sediments containing the stone tools belonging to the Mousterian and the Aterian cultures could be placed between 80 and 70 ka. This conclusion was consistent with other TL and electron spin resonance (ESR) ages at sites elsewhere in Morocco (e.g., Texier et al. 1988; Wrinn and Rink 2003). TL dating of burnt flint has also had a major impact on Palaeolithic sites in the UK. At West Stow, in the East of England, Preece et al. (2007) reported the dating of five flints, the ages of which were combined to give an age of 414 ± 30 ka, compatible with the attribution of this site to marine isotope stage 11 and the results of the uranium-series dating carried out on carbonate concretions at the site. Such ages are possible because of the low dose rate at this site, and an upper age limit of 500 ka is likely to be achieved at many sites where the sediments surrounding the flints are quartz-rich sands with low radioactive contents. Archaeometry has continued to report on new applications and development in dating of burnt flint. Tribolo et al. (2006) presented a mean age of 74 ± 4 ka obtained using grains extracted from five pieces of burnt silcrete or quartzite excavated from the Still Bay units of the Middle Stone Age (MSA) at Blombos Cave in South Africa; at this site there is a range of artefacts showing that their makers displayed modern human behaviour (Henshilwood et al. 2002). At the inland site of Rose Cottage Cave, Valladas et al. (2005) reported TL ages for burnt lithics from the MSA layers. They obtained a weighted mean age of 70.5 ± 5.0 ka for the earliest pre-howiesons Poort (HP) layer and 50.5 ± 4.6 ka for the earliest post-hp layer. TL ages ranging from 60.4 ± 4.6 to 56.3 ± 4.5 ka were obtained for burnt lithics within the HP layers. TL ages for burnt lithics from layers containing both the Still Bay and HP industries were also reported for the rock shelter site of Diepkloof, on the southwestern coast of South Africa (Rigaud et al. 2006). Also in Archaeometry, Richter and Krbetschek (2006) reported the development of a new measurement technique using only a single aliquot of heated flint to obtain the equivalent dose. This method was made possible by using a different part of the TL spectrum, the red orange emission reported in the extensive spectral studies of Krbetschek et al. (1997) and Richter et al. (1999). Besides dating, TL has been used to determine firing temperatures for pieces of burnt flint (Valladas 1981). Heat treatment of lithic material was important in antiquity for improving its ability to be worked to form a useful tool. Godfrey-Smith et al. (2005) investigated both the

14 Fifty years of luminescence dating 289 high-temperature TL signals and the response of the 110 C TL peak in chert from a palaeoindian site in Ontario. Their measurements on the raw material suggested that the TL signals could be used to determine whether there had been heat treatment in the past, but confirmed that none of the archaeological material at the site had been heated. More recently, Lahaye et al. (2006) presented thermal activation curves for the 110 C TL peak of quartz from ferruginous sandstones (ochres) from an Upper Palaeolithic site near Bordeaux. The stones had been used as hearth stones and Lahaye et al. (2006) showed that both temperature and duration of heating determined the sensitivity of the 110 C TL peak. Sediments When Fleming wrote his book in 1979, he considered thermoluminescence to be an important dating technique in full bloom. However, that was the year in which Wintle and Huntley proposed that grains of quartz and feldspar could be used to date the time elapsed since the grains were last exposed to sunlight, i.e., the time of deposition (Wintle and Huntley 1979). It was time for a second flowering. Although a laboratory in Kiev had been publishing ages in the Russian literature since about 1968, it was not until 1979 that TL dating of sediment was suggested in the West (for a review, see Wintle and Huntley 1982). Given the interest in the 1970s in both deep-sea sediments and loess (windblown dust deposits) as recorders of continuous climate change during the last one million years, it was appropriate that a potential dating method was explored. By 1979, the oxygen isotope record for ocean sediments and its relationship to changes in the Earth s orbit around the Sun had become well established; however, there was a pressing need for chronologies for terrestrial deposits to be constructed. One of the first applications to loess reaching back to the last interglacial was in northern France, but age underestimation was observed when the ages were compared with the age expected for loess beneath the palaeosol from the last interglacial (Wintle et al. 1984). The TL measurements were made on the polymineral fine-grain fraction and it was hypothesized that the TL signal from the feldspars suffered from anomalous fading. Debenham (1985) suggested that measuring the TL signal in a different wavelength region would give ages that would be less affected by fading. A review of applications of TL dating of loess was published in 1990 (Wintle 1990). During the 10 years or so after its proposal, TL dating of sediments was only rarely applied to sediments from archaeological contexts (Chawla and Singhvi 1989). The major reason was the presence in the TL signal of a component that could not be removed by sunlight exposure. For young samples, the uncertainty in determining the magnitude of this signal for each sample was such that dating of pottery or burnt flint was preferred, and then only if there was no suitable material for radiocarbon dating. One application was made to much older sediments at a Lower Palaeolithic site in Siberia, Diring Yuriakh. Several TL ages for loess overlying the stone artefacts indicated that they were older than 260 ka (Waters et al. 1997, 1999). During the period from 1988 to 1996, TL dating had a major impact on the chronology of human occupation of Australia. Here, the timing of human arrival on the continent was close to, or just beyond the limit of, reliable radiocarbon dating. Readhead (1988) was the first to apply TL dating methods to the unheated sediments at Lake Mungo. However, it was several years later when Oyston (1996) obtained an age of 43.3 ± 3.8 ka for the sedimentary layer containing human burial remains. Oyston (1996) used a new TL procedure (Franklin and Hornyak 1990; Prescott and Fox 1990) that involved the selection of the most bleachable part of the quartz TL signal (Spooner et al. 1988); this signal was part of the 325 C TL peak seen

15 290 A. G. Wintle in Figure 3. Finding another way to access this signal, or at least part of it, led to the development of a revolutionary new dating technique that is discussed in the next section. At Malakunanja, in northern Australia, close to where humans may have first set foot on that continent, an age of 61 ± 10 ka was obtained for sediments containing the lowest stone artefacts and ochres, and an age of 45 ± 7 ka for sediments overlying a small pit filled with artefacts (Roberts et al. 1990). A thorough review of this study and the related literature was given by Roberts (1997). THE EARLY OSL YEARS, From 1985, luminescence dating of sediments changed forever. The change was brought about by Huntley s realization that the most appropriate signal for dating sediments would be one in which the stimulation procedure released electrons in the same way as they had been released in nature, i.e., by light (Huntley 1985). This led to the development of dating using optically stimulated luminescence (OSL), termed optical dating (Huntley et al. 1985). Aitken (1989) provided a guide for readers of Archaeometry to the different concepts and terminology used in optical dating, by making a comparison with TL dating of pottery, a method better known to the readers. Aitken (1998) provided an early review of OSL dating and a few applications in his book An introduction to optical dating. A more recent review of OSL dating of sediments was provided by Lian and Roberts (2006). At first, green light from an argon-ion laser was used as the laboratory stimulation source to produce the OSL signals from quartz and feldspars (e.g., Huntley et al. 1985; Smith et al. 1990); subsequently, blue green light from filtered halogen lamps was used as a cheaper alternative (Bøtter-Jensen and Duller 1992). Another advance occurred as the result of Hütt et al. (1988) reporting that stimulating feldspars, but not quartz, in the near-infrared at room temperature resulted in a similar luminescence signal, termed infrared stimulated luminescence (IRSL). This finding enabled techniques for dating feldspars, either as separated potassiumrich feldspars or as mixed feldspars in the fine-grain polymineral fraction of sediments. Feldspar dating then took off, as it was possible to develop stimulation sources based on inexpensive IR emitting diodes (Bøtter-Jensen and Duller 1992). The history of equipment development has been presented by Bøtter-Jensen et al. (2003). The first 12 years of OSL During the first 12 years of OSL dating of sediments, there were many archaeological applications. These were presented in the comprehensive review Luminescence dating in archaeology: from origins to optical by Roberts (1997). This 73-page paper discussed OSL and TL dating alongside the results of 14 C, ESR and U-series dating, and the papers discussed therein will not be mentioned further here. The review by Roberts (1997) provides discussion of the dating results that provide the background to the exciting applications that have occurred since then. In particular, Roberts discusses the impact of dating methods on our views on the history of modern humans in Africa, western Europe, East Asia, Siberia, Australia and the Americas. Single-aliquot measurements IR-emitting diodes, with their stability and ease of turning on and off, were widely used for dating of feldspars using multiple-aliquot measurement protocols. However, in 1991, Duller

16 Fifty years of luminescence dating 291 presented the first procedure for single-aliquot dating. This was an additive dose procedure for potassium feldspars and was widely adopted for dating sands. The single-aliquot approach was then further developed by Galloway (1996) for K-feldspar, and adapted for quartz by Murray et al. (1997) and Murray and Roberts (1998), with application to heated quartz from archaeological sites by Murray and Mejdahl (1999). In these studies on quartz, the sensitivity during the measurement sequences was monitored using the 110 C TL peak. The development of single-aliquot procedures and their advantages over multiple-aliquot methods were reviewed by Wintle (1997). Single-grain measurements Several attempts were made to identify those grains that dominated the luminescence signal from the several thousands of grains that make up a typical aliquot of sand-sized grains. McFee and Tite (1998) used a detection system, based on an imaging photon detector, to map the TL signals from grains in such an aliquot. They found that relatively few grains gave rise to a TL signal. A similarly low percentage of quartz grains giving rise to OSL signals has been widely reported (e.g., Jacobs et al. 2003a). McFee (1998) used the same system, but equipped with IR diodes, to measure IRSL signals from individual feldspar grains from a sedimentary sample. It was also possible to measure OSL signals from single grains when stimulating using blue green light from filtered lamps. Two of the most important papers that have appeared in Archaeometry during the past 22 years have involved such measurements made on grains of quartz. They were written in response to a claim that there was human occupation of a rock shelter in Australia prior to 116 ka. Fullagar et al. (1996) presented TL ages for sediments from Jinmium rock shelter (Fig. 7), but failed to take account of the incorporation of grains Figure 7 Jinmium rock shelter, northern Australia (courtesy of R. G. Roberts).

17 292 A. G. Wintle released from fragments of the sandstone that had broken off the walls of the rock shelter, as pointed out by Spooner (1998) in an examination of the published data. Roberts et al. (1998) carried out OSL measurements on freshly collected sediments from Jinmium. Roberts et al. (1999) used the analytical techniques reported in the companion paper by Galbraith et al. (1999) to obtain ages using both multiple-grain aliquots and single grains. These studies resulted in the occupation at Jinmium being revised to the last 10 ka. As well as demonstrating that it is possible to identify individual grains that have not been sufficiently exposed to light for their inherited geological signal to have been erased at the time of deposition, and their impact on the measurement of single aliquots made up of several thousands of grains, the complex behaviour of the OSL of individual quartz grains was noted. In addition, this was the first use of radial plots as a means of displaying not just the equivalent dose value for an aliquot (or grain), but also the uncertainty associated with each value. Radial plots have now become the standard means of displaying equivalent doses. Figure 8 shows the radial plot for 322 grains from a sample taken at Jinmium. Points on any straight line radiating from the origin have the same dose, which is given on the logarithmic scale for equivalent dose. It can be seen that the values range from < 0.6 Gy to > 18 Gy. The equivalent dose estimates obtained using the central-age model and minimum-age model (Galbraith et al. 1999) are shown by the two dashed lines. The central-age model gave an age of 5.6 ± 0.4 ka (Roberts et al. 1998), much younger than the TL age of 58 ± 7 ka published earlier (Fullagar et al. 1996), but still possibly too old due to the incorporation of data for insufficiently bleached grains. This study demonstrates the use of single grains for identifying both poorly bleached grains and younger grains that have got mixed in as the result of later physical disturbance at the site. Figure 8 A radial plot of equivalent dose values obtained by OSL for single grains of quartz from a sediment sample COOR 8/1 from Jinmium (data courtesy of R. G. Roberts, as used in Roberts et al. 1998).

18 Fifty years of luminescence dating 293 Archaeological sediments and land use Several early applications of optical dating to archaeological sediments were reported in Archaeometry, following the first technical report by Smith et al. (1990) on OSL properties of quartz. The potential of OSL for dating sediments from 300 ka to 1 ka was reported by Rees-Jones and Tite (1997) for sediments from British archaeological sites. Optical dating of sediments from archaeological sites, rather than pottery or burnt flint, has allowed a range of new questions to be answered. These questions do not just relate to when humans were present at a particular geographical location, as shown by the dating of sediments surrounding either human remains or artefacts. Instead, it is possible to provide information on whether humans had an impact on their environment. For example, Lang and Wagner (1996) used IRSL to date colluvial sediments found at the foot of hills in the proximity of a Neolithic site in south-west Germany. The IRSL ages suggested that surface sediment was moved down slope at the same time as the site was occupied. This led to the conclusion that human activities, such as forest clearance and agriculture, resulted in soil erosion. At another site in southern Germany, Lang et al. (1999) were also able to use IRSL signals to date colluvium collected from deep pits dug into loess at Iron Age sites, although in some instances the reworked sediment appeared to contain a mixture of bleached and unbleached grains. An extensive OSL and radiocarbon dating programme was carried out along the Atlantic shores of the Outer Hebrides, Scotland (Gilbertson et al. 1999). In this study the OSL ages were obtained on quartz, and an episodic record of sand deposition was obtained. The OSL dating was carried out using an additive-dose multiple-aliquot technique on all 80 of the sand samples. However, as with most of the early studies, there was no allowance for luminescence sensitivity changes and this may have led to a systematic error in the results. This problem was overcome later by the development of experimental procedures that monitored the sensitivity for each measurement of the OSL signal. This could have been applied to multiple-aliquot dating procedures, but has come into prominence in its use in the determination of equivalent doses for single aliquots (and grains) as discussed in the next section. Major advances MAJOR ADVANCES IN OSL AND APPLICATIONS, Three major breakthroughs occurred in First, inexpensive and compact blue light-emitting diodes (LEDs) were used as a high-power light source for optical stimulation (Bøtter-Jensen et al. 1999), instead of the filtered halogen lamps. LEDs were even easier to build into existing OSL readers. Second, rather than having to mount quartz grains one by one on to individual stainless steel or aluminium discs for OSL measurement, as was done for the Jinmium studies (Roberts et al. 1998, 1999), new instrumentation was developed that allowed several tens of grains (up to 100 grains in the latest system) to be mounted in a regular array on a similar-size disc (Duller et al. 1999). Each hole is 300 μm wide and 300 μm deep, allowing a single sand-sized grain up to 250 μm diameter to fall into the hole when grains are tipped across the surface of the disc (Fig. 9, lower). Once each grain is located in one of the holes, it is possible to stimulate it optically using a laser beam that is focused to a 20 μm diameter spot directed to hit the grain. This instrument can be mounted on to a regular OSL reader made by Risø National Laboratory (Fig. 9, upper). The fast component of the OSL signal is removed in less than 0.1 s under such

19 294 A. G. Wintle Figure 9 An OSL reader and a close-up of a disc used to hold 100 grains of sand-sized quartz for individual stimulation by the focused laser (courtesy of Risø National Laboratory, Roskilde, Denmark). high-power stimulation. The use of such a system allows many thousands of grains to be measured in a relatively short time which is just as well, as a high percentage of quartz grains do not appear to have a fast OSL component (e.g., Jacobs et al. 2003a). Time is also saved with the application of laboratory doses, as all of the grains on a disc are irradiated simultaneously.

20 Fifty years of luminescence dating 295 The third advance was the development of a reliable method for constructing a dose response curve for a single aliquot of quartz, which could also be a single grain. This was the singlealiquot regenerative-dose (SAR) protocol, in which allowance can be made for luminescence sensitivity changes that have occurred during the burial period and during laboratory preheat procedures needed to select thermally stable OSL signals. The method was developed for the fast OSL component of quartz. The technique in its most frequently cited form was put forward by Murray and Wintle (2000), and a full review of the experimental data behind the protocol has been provided by Wintle and Murray (2006). Computer control of both the LED stimulation sources and the laser enabled long SAR measurement sequences to be applied with ease. The development of a reliable method for dating quartz, a mineral that does not suffer from anomalous fading, has moved luminescence dating away from IRSL measurement of potassiumfeldspars. The most up-to-date summary of the optically stimulated luminescence properties of both feldspars and quartz is given in the book by Bøtter-Jensen et al. (2003). Application of SAR to sediments related to human activities Two such studies using OSL of quartz grains have appeared in recent issues of Archaeometry. Dating of sedimentary sequences has provided information on agricultural practices over several millennia. For example, in Scotland Burbidge et al. (2001) reported OSL ages for sediments surrounding the site of Old Scatness, Shetland. Occupation of this site was found to have occurred from the Neolithic through to the post-medieval period. In a different type of study, thick alluvial deposits in southern Greece have also been investigated in order to identify any links between historical farming activities and Holocene soil erosion (Fuchs and Wagner 2005). In this case, the short transport distance of the grains during active erosion processes resulted in not all of the grains having their OSL signal reset. There have also been many other interesting archaeological applications. The recent developments in OSL dating have enabled it to be used right up to the last 100 years, thus encompassing the last few centuries, when radiocarbon dating is unreliable because of changes in the production of 14 C (Duller 2004). Records of sand movement in coastal areas can also be shown to have occurred on an historical time scale (e.g., Bailey et al. 2001) and thus enable soil degradation to be linked to farming in marginal areas. Sand layers also occur within archaeological sites in northern Scotland. Sommerville et al. (2007) carried out an extensive study at two sites on the Orkney island of Sanday. At one site, OSL ages showed that an aeolian sand with an age of 4020 ± 190 years covered the Neolithic soils, and at the other site, an aeolian sand with an age of 3080 ± 170 years was found to overlie soils from the Bronze Age. Sand mobilization at those times may have been related to climatic deterioration, with possible additional impact from volcanic eruptions in Iceland. In China, Roberts et al. (2001a) reported a very high deposition rate for the last 2.5 ka at a section on the Loess Plateau; the deposition rate in this part of the section was much higher than the rates found deeper in the section and at nearby sites where loess deposition during the Holocene was unaffected by human activity. The onset of this period of high deposition was related to an agricultural expansion mentioned in the historical record. In Cambodia, near the ancient city of Angkor Borei, there is an extensive canal network. The canals accumulated sediment shortly after they had been dug, with more rapid accumulation occurring after they had been abandoned. Sanderson et al. (2007) have carried out OSL analysis of canal infill. Their results on the sediment from one canal indicate that it was first used in the first millennium bc.

21 296 A. G. Wintle Combining OSL dating and DNA analysis of sediments In the past few years, it has become possible to extract DNA from plant and animal remains that have been buried in permafrost, and new techniques are being developed that enable plant and animal DNA to be extracted from bulk sediments (Willerslev et al. 2003, 2007). This would mean that the timing of the evolution and demise of now-extinct animals can be determined, provided that there is a suitable independent dating method. In the case of sedimentary DNA relating to the woolly mammoth in Siberia, OSL dating can provide this chronology; however, the low-temperature burial of the quartz grains resulted in charge competition effects that needed to be taken into account (Arnold et al. 2008). The OSL ages for these sediments suggest that the woolly mammoth survived in north-central Siberia well into the mid-holocene. Heated archaeological materials Although luminescence techniques are applied to sediments from many types of archaeological sites, heated materials continue to be dated both by TL and OSL. This is particularly true where it is the heating that is the relevant activity carried out by early humans. TL was used by Haustein et al. (2003) in their study of metallurgical slags resulting from the processing of mineral ore, a process that has been carried out at Early Bronze Age sites in Europe. These materials have been heated to temperatures of around 1200 C, and only when quartz was separated out from the slag was TL dating found to be reliable. TL dating of material relating to smelting during the Early Iron Age in Africa was reported by Godfrey-Smith and Casey (2003). On a different time scale, fired clay bricks have also been dated, not just by TL but more recently by OSL (Bailiff and Holland 2000; Bailiff 2007). Bricks were often reused and it is thus useful to have a method that is able to determine when they were initially fired. In a study of late-medieval and post-medieval buildings, ranging from c. ad 1390 to ad 1740, Bailiff (2007) showed excellent agreement with the independent ages available for that period from both historical records and tree ring measurements on construction timber. This in-depth study brings OSL dating of fired materials to a new level and demonstrates both the complexity and power of OSL dating when applied to such materials. Single-grain OSL measurements Measurements on single grains have been carried out mainly as a means of identifying contaminant grains, those either too young as a result of being carried down through sediment as a result of later biological (including human) activity or too old as a result of not having been exposed to sufficient sunlight. Problems of mixing of grains with different depositional ages as the result of occupation of the site are commonly encountered at sites with low deposition rates. Such problems have been identified by making SAR measurements on many hundreds or thousands of grains for each sediment sample; for example, at Nonda Rock (David et al. 2007) and at Blombos (Jacobs et al. 2003a, 2006). Single-grain OSL measurements are thus made routinely on sediments taken from archaeological sites. At Sibudu, a riverside cave site in South Africa, it was also necessary to measure single grains of quartz in order to overcome microdosimetry problems brought about by the high potassium content found in the wood ash that was abundant at the site (Jacobs et al. 2008a,b). Jacobs and Roberts (2007) have provided a comprehensive review of single-grain dating applications.

22 Fifty years of luminescence dating 297 Single-grain dating has also been applied at a number of archaeological sites in North America; for example, sites on the Southern High Plains (Feathers 2003) and elsewhere in the south-east United States (Bush and Feathers 2003), and at the Cactus Hill site in Virginia (Feathers et al. 2006). However, there is one particular application that must be carried out using single grains the dating of rock art by dating the construction of wasp nests on top of the piece of art (Roberts et al. 1997; Yoshida et al. 2003). Single grains are required because of the small sample size and the formation of the nest from many layers, the outer ones of which will have been exposed to light ever since being placed on the surface by the wasps. CONTRIBUTION TO THE CHRONOLOGY OF THE DISPERSAL OF MODERN HUMANS The most exciting contribution of OSL dating, using chemically cleaned quartz grains and applying the SAR protocol, has been to the dating of sediments containing material related to the presence of modern humans. Many of these sites are cave sites, or rock shelters, and have provided a number of challenges to OSL dating. Figure 10 is a summary map of sites where there are OSL or TL ages that contribute to our knowledge of human occupation of key areas. Once this chronology has been obtained, it is possible to compare it with records of climate change along the possible migration routes. Two such records exist. First, the pollen records Figure 10 A map showing sites relevant to human dispersal at which luminescence dating has been applied, as discussed in the text.