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Comment on ``Human footprints in Central Mexico older than 40,000 years'' by S. González, D. Huddart, M.R. Bennett and A. González-Huesca
... Dates have been reported for footprints of 120 ka in Africa (Jacobs et al. 2009), of 20 ka in Australia (Webb et al. 2006), and of 36 ka in Mexico (Gonzales et al. 2006). The last is rather controversial and the results have been disputed (Duller 2006). Burbidge et al. (2007) recommended ''luminescence profiling'' for complex situations, as, e.g., mixing, poor bleaching, or unsuitable signals. ...
Half a century after the publication of the first Thermoluminescence (TL) ages, the field of Luminescence Dating has reached a level of maturity. Both research and applications from all fields of archaeological science, from archaeological materials to anthropology and geoarchaeology, now routinely employ luminescence dating. The advent of optically stimulated luminescence (OSL) techniques and the potential for exploring a spectrum from mono-minerallic single grains to polymineral multi-aliquots enhanced the applicability, accuracy and the precision of luminescence dating. The present contribution reviews the physical basis, mechanisms and methodological aspects of luminescence dating; discusses advances in instrumentations and facilities, improvements in analytical procedures, and statistical treatment of data along with some examples of applications across continents. The case studies review the dating of heated and solar bleached archaeological material (artefacts, sediments, rocks, rock art and buildings) that cover all periods from Middle Palaeolithic to Medieval Eras and both Old and New World archaeology. They also include interdisciplinary applications that contribute to palaeo-landscape reconstruction.
... Dates have been reported for footprints of 120 ka in Africa (Jacobs et al. 2009), of 20 ka in Australia (Webb et al. 2006), and of 36 ka in Mexico (Gonzales et al. 2006). The last is rather controversial and the results have been disputed (Duller 2006). Burbidge et al. (2007) recommended ''luminescence profiling'' for complex situations, as, e.g., mixing, poor bleaching, or unsuitable signals. ...
Half a century after the publication of the first Thermoluminescence (TL) ages, the field of Luminescence Dating has reached a level of maturity. Both research and applications from all fields of archaeological science, from archaeological materials to anthropology and geoarchaeology, now routinely employ luminescence dating. The advent of optically stimulated luminescence (OSL) techniques and the potential for exploring a spectrum from mono-minerallic single grains to polymineral multi-aliquots enhanced the applicability, accuracy and the precision of luminescence dating. The present contribution reviews the physical basis, mechanisms and methodological aspects of luminescence dating; discusses advances in instrumentations and facilities, improvements in analytical procedures, and statistical treatment of data along with some examples of applications across continents. The case studies review the dating of heated and solar bleached archaeological material (artefacts, sediments, rocks, rock art and buildings) that cover all periods from Middle Palaeolithic to Medieval Eras and both Old and New World archaeology. They also include interdisciplinary applications that contribute to palaeo-landscape reconstruction.
... Dates have been reported for footprints of 120 ka in Africa (Jacobs et al. 2009), of 20 ka in Australia (Webb et al. 2006), and of 36 ka in Mexico (Gonzales et al. 2006). The last is rather controversial and the results have been disputed (Duller 2006). Burbidge et al. (2007) recommended ''luminescence profiling'' for complex situations, as, e.g., mixing, poor bleaching, or unsuitable signals. ...
Half a century after the publication of the first Thermoluminescence (TL) ages, the field of Luminescence Dating has reached a level of maturity. Both research and applications from all fields of archaeological science, from archaeological materials to anthropology and geoarchaeology, now routinely employ luminescence dating. The advent of optically stimulated luminescence (OSL) techniques and the potential for exploring a spectrum from mono-minerallic single grains to polymineral multi-aliquots enhanced the applicability, accuracy and the precision of luminescence dating. The present contribution reviews the physical basis, mechanisms and methodological aspects of luminescence dating; discusses advances in instrumentations and facilities, improvements in analytical procedures, and statistical treatment of data along with some examples of applications across continents. The case studies review the dating of heated and solar bleached archaeological material (artefacts, sediments, rocks, rock art and buildings) that cover all periods from Middle Palaeolithic to Medieval Eras and both Old and New World archaeology. They also include interdisciplinary applications that contribute to palaeo-landscape reconstruction.
... Dates have been reported for footprints of 120 ka in Africa (Jacobs et al. 2009), of 20 ka in Australia (Webb et al. 2006), and of 36 ka in Mexico (Gonzales et al. 2006). The last is rather controversial and the results have been disputed (Duller 2006). Burbidge et al. (2007) recommended ''luminescence profiling'' for complex situations, as, e.g., mixing, poor bleaching, or unsuitable signals. ...
Half a century after the publication of the first Thermoluminescence (TL) ages, the field of Luminescence Dating has reached a level of maturity. Both research and applications from all fields of archaeological science, from archaeological materials to anthropology and geoarchaeology, now routinely employ luminescence dating. The advent of optically stimulated luminescence (OSL) techniques and the potential for exploring a spectrum from mono-minerallic single grains to polymineral multi-aliquots enhanced the applicability, accuracy and the precision of luminescence dating. The present contribution reviews the physical basis, mechanisms and methodological aspects of luminescence dating; discusses advances in instrumentations and facilities, improvements in analytical procedures, and statistical treatment of data along with some examples of applications across continents. The case studies review the dating of heated and solar bleached archaeological material (artefacts, sediments, rocks, rock art and buildings) that cover all periods from Middle Palaeolithic to Medieval Eras and both Old and New World archaeology. They also include interdisciplinary applications that contribute to palaeo-landscape reconstruction.
... Dates have been reported for footprints of 120 ka in Africa (Jacobs et al. 2009), of 20 ka in Australia (Webb et al. 2006), and of 36 ka in Mexico (Gonzales et al. 2006). The last is rather controversial and the results have been disputed (Duller 2006). Burbidge et al. (2007) recommended ''luminescence profiling'' for complex situations, as, e.g., mixing, poor bleaching, or unsuitable signals. ...
Half a century after the publication of the first Thermoluminescence (TL) ages, the field of Luminescence Dating has reached a level of maturity. Both research and applications from all fields of archaeological science, from archaeological materials to anthropology and geoarchaeology, now routinely employ luminescence dating. The advent of optically stimulated luminescence (OSL) techniques and the potential for exploring a spectrum from mono-minerallic single grains to polymineral multi-aliquots enhanced the applicability, accuracy and the precision of luminescence dating. The present contribution reviews the physical basis, mechanisms and methodological aspects of luminescence dating; discusses advances in instrumentations and facilities, improvements in analytical procedures, and statistical treatment of data along with some examples of applications across continents. The case studies review the dating of heated and solar bleached archaeological material (artefacts, sediments, rocks, rock art and buildings) that cover all periods from Middle Palaeolithic to Medieval Eras and both Old and New World archaeology. They also include interdisciplinary applications that contribute to palaeo-landscape reconstruction.
... 40 ka age for the Xalnene Tuff is an OSL date obtained from a single quartzofeldspathic xenolith in the tuff (González et al., 2006aGonzález et al., , 2006b). The validity of this OSL age has since been questioned (Duller, 2006; Schwenninger et al., 2006). The latest contribution to the ongoing debate about the age of the Xalnene Tuff is a report by Gogichaishvili et al. (2007) of transitional paleomagnetic directions from the Xalnene Tuff, and of anomalously weak geomagnetic paleo intensity recorded by lavas from the eruptive center for the tuff (Cerro Toluquilla), collectively interpreted to support the Laschamp age suggested by González et al. (2006a González et al. ( , 2006b). ...
Impressions in a basaltic tuff located around Valsequillo Reservoir near Puebla, Mexico, have been interpreted as human and animal footprints along an ancient lakeshore, and are cited as evidence of the presence of humans in North America at 40 ka B.P. In this paper, we present new data that challenge this interpretation. Paleomagnetic analyses of the Xalnene Tuff, and lavas from the volcano from which it erupted, yield fully reversed magnetic polarities, indicating that the tuff was deposited prior to the last geomagnetic reversal (the Brunhes-Matuyama ca. 790 ka). 40Ar/39Ar dating of Xalnene lapilli and lava from the source volcano yields indistinguishable ages of ∼1.3 Ma, consistent with a period of reversed magnetic polarity (C1r.2r). Additional paleomagnetic measurements of individual millimeter-size lapilli indicate that the pyroclastic grains within the Xalnene Tuff have not been disturbed or rotated since their initial deposition, thereby ruling out the possibility that the tuff was reworked by wave action along the shores of an ancient lacustrine environment. This and other evidence indicate that the marks observed in the stone quarry site are not human ichnofossils.
... Subsequently, there has been support for both sides of the debate. The OSL age presented by Gonzalez et al. (2006a) was questioned (Duller, 2006) and reconfirmed (Schwenninger et al., 2006), and interpretations of palaeomagnetic data support both deposition of the Xalnene Ash during the LGE (Gogichaishvili et al., 2007Gogichaishvili et al., , 2009) and at 1.3 Ma (Feinberg et al., 2009). In attempt to resolve the controversy surrounding the age of the Xalnene Ash, we report new 40 Ar/ 39 Ar data for the volcanic rocks of the Valsequillo Basin (Fig. 1A). ...
Seven hominin ichnosites in aeolianites on the Cape south coast of South Africa have been dated using Optically Stimulated Luminescence, yielding age estimates from Marine Isotope 6 through Marine Isotope Stage 4. All rock outcrops containing these sites are situated on the modern coastline. The new ages are consistent with geomorphological expectations, and with other numerical dating results from the wider southern Cape coastline. Seen in a global ichnological context, the cluster of South African sites (including two previously dated sites) contains nine of the twenty-three sites older than 70 ka from which hominin tracks have been reported. With a single exception they are also the only sites older than 40 ka that have been attributed to Homo sapiens, and include the oldest tracks (153 ± 10 ka) thus far attributed to our species. The South African coastline contains an archaeological and palaeoanthropological record of global significance, to which the hominin ichnological record, preserved on aeolianite palaeosurfaces and now chronologically constrained, can make a substantial contribution.
More than 12,000 years ago, in one of the greatest triumphs of prehistory, humans colonized North America, a continent that was then truly a new world. Just when and how they did so has been one of the most perplexing and controversial questions in archaeology. This dazzling, cutting-edge synthesis, written for a wide audience by an archaeologist who has long been at the center of these debates, tells the scientific story of the first Americans: where they came from, when they arrived, and how they met the challenges of moving across the vast, unknown landscapes of Ice Age North America. David J. Meltzer pulls together the latest ideas from archaeology, geology, linguistics, skeletal biology, genetics, and other fields to trace the breakthroughs that have revolutionized our understanding in recent years. Among many other topics, he explores disputes over the hemisphere's oldest and most controversial sites and considers how the first Americans coped with changing global climates. He also confronts some radical claims: that the Americas were colonized from Europe or that a crashing comet obliterated the Pleistocene megafauna. Full of entertaining descriptions of on-site encounters, personalities, and controversies, this is a compelling behind-the-scenes account of how science is illuminating our past.
La estratigrafía de la secuencia volcano-sedimentaria en la cuenca de Valsequillo, Puebla es investigada por medio de sondeos eléctricos de resistividad. Los depósitos en la cuenca preservan registros paleontológicos, paleoclimáticos y paleoambientales, que incluyen evidencias sobre restos fósiles de megafauna asociados con artefactos humanos. El reporte de posibles huellas de humanos preservadas en la tefra Xalnene, con una edad de alrededor de 38 mil años, ha atraído nuevamente interés en analizar la estratigrafía en Valsequillo. Este estudio se realizó en la zona cubierta por la tefra Xalnene, en la planicie situada al noroeste del volcán Toluquilla, empleando sondeos eléctricos verticales (SEVs). Los modelos de inversión de las curvas SEVs muestran una secuencia caracterizada por unidades de alta y baja resistividad aparente, que son asociadas a las tefras, sedimentos lacustres y fluviales y rocas volcánicas. Las secciones compuestas de resistividad documentan tres unidades, con las tefras y sedimentos formando una sección de unos 30 m sobre las rocas volcánicas. Los datos relacionan las tefras con el volcán monogenético, las cuales se emplazan en un ambiente lacustre somero. Las secciones de resistividad documentan las variaciones de espesores de los depósitos. Observaciones durante los estudios de campo y adquisición de datos proporcionan evidencias adicionales sobre el posible origen de las huellas aparentes, que estarían asociadas a las excavaciones de las tefras como materiales de construcción y los procesos de erosión. Estudios de detalle y experimentos en laboratorio proporcionarían mayor evidencia para entender la morfología y rasgos en las tefras. El retiro y actividades de corte están afectando significativamente al depósito por lo que se requieren de medidas de protección para proteger la zona dado su valor científico.
doi: https://doi.org/10.1016/S0016-7169(14)71492-8
Previous age estimates for SP Crater in the San Francisco volcanic field in northern Arizona, USA, suggested it erupted 70 ka; however its non-weathered geomorphic character places it as one of the youngest volcanic vents in the region. Samples of quartz xenocrysts were extracted from the SP Crater basalt flows and dated using single-grain dating. Analysis of the luminescence signals indicate they were dominated by medium- to slow-decaying OSL components, which can produce age underestimates if not removed. In order to reduce their contribution, the fast component was deconvolved from the initial OSL signal, a high-temperature bleach (blue-green LED exposure at 280 °C) was added at the end of each measurement cycle and elevated cutheat temperatures were used. Fading tests and crushing experiments indicate no dependence on recovered equivalent dose values. Resultant single-grain quartz xenocryst OSL ages are consistent with the geomorphic character of SP Crater and suggest it erupted 5.5–6 ka. Revision of the age of SP Crater has implications for the eruption recurrence interval in the region and assessment of volcanic hazards in nearby communities.
Middle and Upper Quaternary volcanic events are often difficult to date, in particular when minerals suitable for 40Ar/39Ar-dating are missing. Here, we present first tests of a newly developed technique to use the thermoluminescence (TL) of maar tephra and crustal xenoliths for dating the eruption event. We take into consideration that resetting of the latent geological TL of country rock fragments during eruption may be incomplete. We therefore develop the “partial heat – longest plateau” (PHLP) technique to approach the inherited TL signal at eruption. This technique can overcome TL age overestimates due to incomplete zeroing, as is demonstrated for some eruptions in the Quaternary Eifel Volcanic Field, Germany, in the time range from 11 to 300 ka old. Although we avoid the often observed strong anomalous fading of volcanic feldspars and other volcanic minerals by using heated country rock xenoliths, we still face the problem of longer-term anomalous fading which may be responsible for apparent age underestimates. The orange-red TL emissions (RTL) of pure fine-grained quartz extracts from crustal xenoliths are also tested from two samples. This approach needs, however, further systematic investigations into the TL characteristics of RTL.
Three hominid footprints preserved as casts were found in 1964 in Pleistocene aeolianites of the Nahoon Formation near East London, South Africa. Dating of the footprint-bearing aeolianite proved a lengthy and problematic task. Previous age estimates included a radiocarbon age of 29,090−390+410 years BP [Deacon, H.J., 1966. The dating of the Nahoon footprints. South African Journal of Science 62, 111–113], an infrared stimulated luminescence (IRSL) age of 144±4ka, a thermoluminescence age of 236±8ka (TL) and a luminescence subtraction age based on the IRSL and TL estimates of 75±9ka [Vogel, J.C., Wintle, A.G., Woodborne, S.M., 1999. Luminescence dating of coastal sands: overcoming changes in environmental dose rate. Journal of Archaeological Science 26, 729–733]. Thus there was no consistency among the ages derived using different dating methods. Here we describe optically stimulated luminescence (OSL) dating of quartz grains from a new set of samples collected from the footprint-bearing aeolianite and from associated geological formations. We also re-assessed the dose rate parameters for the sample for which TL and IRSL ages were reported and analysed the OSL signal from this sample. The new OSL dating yielded a coherent series of ages consistent with the stratigraphy as well as global and local sea level history. The indicated footprint age of 124±4ka thus relates to an important period of modern human evolution.
Verification of human footprints within the geological record provides critical evidence of presence as well as information on the biomechanics of the individuals who made those prints. Consequently, the correct identification of human footprints is important, but is something for which critical and objective criteria do not exist. The current paper attempts to address this issue by presenting a new statistically based approach to the verification of human footprints. The importance of this is illustrated by the recent controversy surrounding a series of marks identified as human prints in the Valsequillo Basin in Central Mexico dated originally to 40 000 years ago. The dating of these marks remains highly controversial with some teams placing their age at 1.3 million years old. Irrespective of this debate the crucial question that must be addressed is whether or not they represent evidence of human presence. Using an objective statistically based methodology developed here, these controversial marks are re-examined and found to be of questionable origin, as they are inconsistent with a suite of other, known human and hominin prints. Consequently, we argue that they should be removed as evidence in the ongoing controversy surrounding the colonization of the Americas.
Abstract: Abstract: Abstract: Optically stimulated luminescence (OSL) dating of light-exposed sediments is used increasingly as a mean of establishing a sediment deposition chronology in a wide variety of late Quaternary studies. There has been considerable technological development in the last few years ñ in instrumentation, in the preferred mineral, and in various measurement proto- cols. New approaches to the latter, especially with the introduction of the single-aliquot regenerative-dose (SAR) protocol, have given rise to an increasing number of ages in the li- terature based on the OSL signals from quartz. This paper examines the reliability of these results by reviewing both published and unpublished SAR quartz ages for which some independent age control exists. It first discusses studies of modern (zero age) sediments, and the implications of these results for the importance of incomplete bleaching, especially in water-lain sediments, i.e. sediments for which the initial light exposure is expected to have been insufficient to reduce the apparent dose at deposition to a negligible fraction of the final burial dose. It then compares OSL and independent ages derived from various types of sedi- ments, including aeolian, fluvial/lacustrine, marine and glacio-fluvial/lacustrine. It is concluded that, in general, the ages are accurate, in that there is no evidence for systematic errors over an age range from the last century to at least 350 ka. Nevertheless, the published uncertain- ties of a small fraction of OSL ages are probably underestimated. We conclude that OSL dating of quartz is a reliable chronological tool; this conclusion is reflected in its growing popularity in Quaternary studies.
The Jinmium rock shelter is located in the Kimberley region of northern Australia. Claims for ancient rock art and an early human presence at this site were based on thermoluminescence ages of 50-75 thousand years (kyr) for quartz sands associated with buried circular engravings (pecked cupules) and on thermoluminescence ages of 116-176 kyr for the underlying artefactbearing deposits. Here we report substantially younger optical ages for quartz sand, and ages based on measurements of radioactive carbon in charcoal fragments, from the occupation deposit. Using conventional (multiple-grain) optical dating methods, we estimate that the base of the deposit is 22 kyr. However, dating of individual grains shows that some have been buried more recently. The single-grain optical ages indicate that the Jinmium deposit is younger than 10 kyr. This result is in agreement with the late-Holocene ages obtained for the upper two-thirds of the deposit from radiocarbon measurements. We suggest that some grains have older optical ages because they received insufficient exposure to sunlight before burial. The presence of such grains in a sample will cause age overestimates using multiple-grain methods, whether using thermoluminescence or optical dating.
A report of human footprints preserved in 40,000-year-old volcanic ash near Puebla, Mexico (http://www.royalsoc.ac.uk/exhibit.asp?id=3616&tip=1), was the subject of a press conference that stirred international media attention. If the claims (http://www.mexicanfootprints.co.uk) of Gonzalez et al. are valid, prevailing theories about the timing of human migration into the Americas would need significant revision. Here we show by 40Ar/39Ar dating and corroborating palaeomagnetic data that the basaltic tuff on which the purported footprints are found is 1.30+/-0.03 million years old. We conclude that either hominid migration into the Americas occurred very much earlier than previously believed, or that the features in question were not made by humans on recently erupted ash.
The timing, route and origin of the first colonization to the Americas remains one of the most contentious topics in human evolution. A number of migration routes have been suggested and there are different views as to the antiquity of the earliest human occupation. Some believe that settlement happened as early as 30ka BP, but most of the currently accepted early sites in North America date to the latest Pleistocene, related to the expansion of the Clovis culture, while the oldest directly radiocarbon dated human remains are 11.5ka BP. In this context new evidence is presented in this paper, in the form of human footprints preserved in indurated volcanic ash, to suggest that Central Mexico was inhabited as early as over 40ka BP.Human and animal footprints have been found within the upper bedding surfaces of the Xalnene volcanic ash layer that outcrops in the Valsequillo Basin, south of Puebla, Mexico. This ash layer was produced by a subaqueous monogenetic volcano erupting within a palaeo-lake, dammed by lava within the Valsequillo Basin during the Pleistocene. The footprints were formed during low stands in lake level along the former shorelines and indicate the presence of humans, deer, canids, big felids, and probably camels and bovids. The footprints were buried by ash and lake sediments as lake levels rose and transgressed across the site. The ash has been dated to at least 40ka BP by OSL dating of incorporated, baked lake sediments.
We have investigated the application of luminescence dating to the palaeosol underlying the Cape Riva ignimbrite from the volcanic island of Santorini (36.40°N, 25.40°E) in the Aegean Sea. The history of volcanism in this area is of intrinsic interest, and is also of importance to archaeology in the eastern Mediterranean — a flourishing late Bronze Age settlement was buried under the volcanic products of the Minoan Eruption (1640 BC). Volcanic provinces are notoriously difficult to date using luminescence techniques, because volcanic feldspars almost invariably show anomalous fading (which can lead to age underestimates of more than 50%). Quartz does not show anomalous fading, but unfortunately it is rare or non-existent in volcanic sediments, and so it is usually not considered as a potential chronometer. We have studied the luminescence characteristics of material extracted from the Cape Riva palaeosol, with good independent age control. All samples contain detectable quartz, but usually
Luminescence dating is unique amongst geochronological methods in dating the deposition of sediments based upon the properties of the constituent minerals. For Quaternary sediments the event being dated is the last exposure of the grains to daylight, and an implicit assumption is that this exposure was sufficient to remove any pre-existing signal. Until recently it has been difficult to test this assumption, other than by dating samples from a given depositional context with an age that is known from independent methods. There have been a series of technological and methodological developments in the past 5–10 yr that make it possible to undertake many replicate measurements of the luminescence from a single sample and hence to explicitly test whether for a specific sample all the grains had their luminescence signal reset at deposition. This allows the reliability of the luminescence age to be assessed. Where all the grains in a sample were not exposed to sufficient daylight to reset their luminescence signal, the apparent age will be an average value of the luminescence from all the grains measured at one time. Where many grains are measured simultaneously, this will overestimate the depositional age. The distribution of apparent age can be made clear by reducing the number of grains being measured in a given experiment, ultimately to the point of measuring individual sand-sized mineral grains. Copyright © 2004 John Wiley & Sons, Ltd.
Single aliquot protocols are now widely used as a means of measuring the equivalent dose (De) in quartz and feldspar optical stimulated luminescence (OSL) dating of both heated and sedimentary materials. The most recent of these is the single-aliquot regenerative-dose (SAR) protocol, first suggested by Murray and Roberts (Radiation Measurements 29, 503–515, 1998). In this approach, each natural or regenerated dose OSL measurement is corrected for changes in sensitivity using the OSL response to a subsequent test dose (10–20% of De). If the sensitivity correction is adequate, then the corrected OSL response should be independent of prior dose and thermal/optical treatment, i.e. there should be no change in the sensitivity-corrected dose–response curve on remeasurement. Here we examine the interpretation of the sensitivity corrected growth curve as a function of dose, and the effect of changing measurement conditions (e.g. preheat temperature, size of test dose, stimulation temperature) on the estimation of De. The dependence of the dose response on prior treatment is tested explicitly, and the significance of thermal transfer discussed. It is concluded that a robust SAR protocol is now available for quartz, and that it is applicable to a wide range of heated and unheated materials.
A common problem in fission track dating is to estimate the component ages, particularly the youngest age, when a sample contains grains of different true ages. In this situation it is sometimes reasonable to suppose that the grains are a random sample from a population in which a proportion π1 have age τ1 and a proportion π2 have age τ2, where π1 + π2 = 1 and where π1, τ1, π2, τ2 are all unknown. We give formulae for estimating these parameters along with their relative standard errors, confidence intervals and appropriate diagnostics when the grains are dated by the external detector method.The generalisation to three or more component ages is indicated.The calculations are experimentally tested using data obtained from synthetic mixtures of grains, so that the estimates can be compared with known values and also in order to demonstrate the applicability of the method.
The date at which people entered Australia has important implications for the debate on modern human origins. Thermoluminescence dates of 50–60 ka, reported for initial occupation of the Malakunanja II site in northern Australia, have been used as a means of calibrating the rate of mitochondrial and nuclear DNA evolution in modern humans. Optical dating of unburnt quartzose sediments from a rock shelter site (Nauwalabila I, Lindner Site) in Deaf Adder Gorge, 70 km south of Malakunanja II, provides new evidence for the timing of the colonisation of the continent. Optical dates were determined for several stratigraphic levels within a 3 m deep excavation, in which flaked stone artefacts and ground pigments were found in primary depositional setting. The lowest human occupation levels are bracketed by dates of 53.4 ± 5.4 ka and 60.3 ± 6.7 ka, while the upper levels show good agreement between optical and calibrated 14C age estimates. High-quality haematite with ground facets and striations is associated directly with the 53 ka level and indicates the use of pigments by these early Australians. The optical dates independently confirm evidence for the colonisation of northern Australia shortly after 60 ka and should be seen in the context of this region as having been a likely entry route for the first human movements into Sahul.
Many natural deposits contain grains that have different burial histories, but reliable procedures to extract the component doses from mixed-dose samples have not been developed in optical dating. Here we present results for synthetic two- and three-component mixtures of data derived from laboratory-dosed sedimentary quartz using a single-aliquot regenerative-dose protocol and statistical models for finite mixtures. Composite sets of data were created from the doses measured for individual grains that had been bleached by sunlight and then given a beta dose of 5, 10 or 20 Gy. We found that the correct number of dose components, corresponding doses, and relative proportions of each component may be estimated if the within-component dispersion is small or is known. We also found, however, that recuperation of the OSL signal in sun-bleached quartz may produce artificially high dose estimates for some grains, an effect that should be taken into account when modelling natural mixtures.
Recent development of an instrument for measuring the optically stimulated luminescence signal from individual mineral grains has made it practicable to measure the equivalent dose (De) from many hundreds or thousands of single mineral grains from a sample. Such measurements can potentially be used to address issues such as sample integrity, and to make it possible to obtain ages from samples that consist of mixtures of grains, enlarging the range of materials to which luminescence dating can be applied. However, for reliable ages to be obtained, the characteristics of the equipment and the sample being analysed need to be understood.Using sensitised sedimentary quartz grains, the instrumental uncertainty in repeated optically stimulated luminescence measurements made using a single grain laser luminescence unit attached to a conventional luminescence reader was evaluated; a value of 1.2% was obtained. Grains from this sample were then used to investigate the uncertainty in a measured dose distribution obtained using the single aliquot measurement protocol on each grain that had previously received a known laboratory dose; after systematic rejection of grains that did not pass defined acceptance criteria, overdispersion of 7% was found.Additional spread in data was found when uniform aeolian sands were examined, resulting in overdispersion of ∼12%; this was attributed to a combination of factors relating to differences in field and laboratory conditions. A similar value was found for an archaeological horizon below this sand. For another sample from the same section, a significantly larger value was found, ∼29%; on this basis the finite mixture model was applied to obtain the likely dose components. The paper demonstrates the importance of correct assessment of error terms when analysing single grain De distributions and a number of rejection criteria that are vital to avoid the inclusion of data that could lead to misinterpretation of the degree of scatter present.
Different single-aliquot regenerative-dose (SAR) protocols have now been applied for some years to quartz grains for determining the equivalent dose, in both dating and retrospective dosimetry studies. This paper begins by laying out the basic principles of the SAR procedure on which the various SAR protocols are based. A general model is examined and three simple performance tests are laid out. Recent experimental characterisation of the optically stimulated luminescence signals from quartz grains is reviewed, giving particular regard to implications for SAR protocols. Various published SAR protocols are presented and discussed, particularly with regard to changes in experimental procedure and methods of data analysis in order to isolate the fast component of the OSL signal. It is concluded that the SAR procedure has proved to be a very powerful approach for determining the equivalent dose using quartz OSL signals dominated by the fast component. The presence of other components complicates the simplest protocols and further quantification of their behaviour is required.
A sequence of optically stimulated luminescence measurements was made on each of 8,961 grains from three sand samples from Blombos on the southern Cape coast. One sand unit overlay Middle Stone Age deposits in Blombos Cave. The measurement sequence, the single aliquot regenerative dose protocol, was used to obtain values for the total effective radiation dose to which each grain had been exposed since burial. A series of checks was carried out on each grain to ensure that the luminescence signals were reproducible, and that they were derived from quartz. This led to acceptance of less than 5% of the grains. An estimate of the radiation dose for the sand unit was obtained by combining the values using the central age model. In order to use a larger number of grains that might be representative of the sand unit, the radiation dose was also estimated by using the signal from the above grains, combined with the signals from those grains that had lower signals, but nonetheless contributed to the total light sum; this utilised between 9 and 18% of the grains. This enables us to obtain estimates of the ages as 67.3+/-3.8 ka, 65.6+/-2.8 ka and 68.8+/-3.0 ka for the three samples. These values agree with ages obtained using the single aliquot regenerative dose protocol for aliquots composed of several hundred grains.
Optically stimulated luminescence (OSL) measurements are reported for both single aliquots (of two different sizes) and single grains of quartz from deposits within Blombos Cave. Ages have been obtained for six sediments from the Middle Stone Age (MSA) occupation levels and for two sterile sands, one underlying the archaeological sediment and one overlying the Later Stone Age occupation levels. The ages for the archaeological sediments were obtained from single-grain measurements that enabled unrepresentative grains to be rejected. The MSA occupation levels have ages that, within error limits, are in stratigraphic order and fall between the OSL age for the oldest dune sand (143.2+/-5.5 ka) and a previously published OSL age for the sterile sand ( approximately 70 ka) that separates the Middle and Later Stone Age deposits. The earliest MSA archaeological phase, M3, from where fragments of ochre were found as well as human teeth, is dated to 98.9+/-4.5 ka, coinciding with the sea-level high of oxygen isotope substage 5c. The cave then appears to be unoccupied until oxygen isotope substage 5a on the basis of four OSL ages for archaeological phase M2, ranging from 84.6+/-5.8 to 76.8+/-3.1 ka; these levels contained large hearths and bone tools. An age of 72.7+/-3.1 ka was obtained for the final MSA archaeological phase, M1, from which deliberately engraved ochre and shell beads were recovered along with bifacial stone points. We conclude that the periods of occupation were determined by changes in sea level, with abundant sources of seafood available in times of high sea level and with the cave being closed by the accumulation of large dunes during periods of low sea level, such as during oxygen isotope stages 4 and 6.
Duller Institute of Geography and Earth Sciences
- G A T 3db
- Uk Wales
G.A.T. Duller Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, Wales, UK E-mail address: ggd@aber.ac.uk (G.A.T. Duller) 23 March 2006; accepted 4 June 2006 ARTICLE IN PRESS QSR Correspondence / Quaternary Science Reviews 25 (2006) 3074–3076