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Luminescence dating of aeolian sands from archaeological sites in Northern Britain: a preliminary study
Quaternary Science Reviews 20 (2001) 913}919
Luminescence dating of aeolian sands from archaeological sites in Northern Britain: a preliminary study夽 A.A. Sommerville *, D.C.W. Sanderson , J.D. Hansom, R.A. Housley Scottish Universities Research and Reactor Centre, Rankin Ave., East Kilbride, Scotland G75 0QF, UK Department of Geography and Topographic Science, University of Glasgow, Glasgow G12 8QQ, UK Department of Archaeology, University of Glasgow, Glasgow G12 8QQ, UK
Abstract Luminescence dating of aeolian sands from archaeological sites has potential to contribute to regional chronologies for sediment deposition and to provide a greater understanding of climatic in#uences on early communities. The Northern and Western Isles of Scotland provide important opportunities for sampling archaeologically intercalated sands for these purposes, and to provide constrained samples for method validation. A wide range of modern beaches have been sampled in the Western and Orkney Isles of Scotland to examine regional variations in luminescence sensitivity, residuals and ease of bleaching. These modern sands have negligible residuals for infra-red stimulated luminescence (IRSL), small optically stimulated luminescence (OSL) residuals and signi"cant thermoluminescence residuals. The relationship between these signals and laboratory bleaching results may indicate the initial depositional environment, and hence lead to a means of identifying well-bleached dating samples. Both sensitivities and residuals show regional di!erences, re#ecting local geology. Preliminary ages obtained from aeolian sands associated with archaeological sites at Amble (Northumbria) and Tofts Ness (Sanday, Orkney) using regenerative blue OSL techniques on extracted quartz are broadly consistent with external age controls from the "rst and third millennium BC. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The Western and Orkney Isles of Scotland (Fig. 1) are characterised by treeless windswept dune systems and large sections of the coast are dominated by long sandy beaches backed by extensive aeolian depositional systems. Large numbers of well-preserved archaeological sites exist within these dune areas, but because of a lack of organic material, the use of C dating is often precluded. An alternative dating solution in these situations is luminescence dating of the aeolian sands, which are stratigraphically intercalated between independently dated cultural horizons in archaeological sites. Several issues must be resolved before accurate optically stimulated luminescence (OSL) ages for aeolian sands are generated from archaeological sites. It is important to establish the following sediment characteristics: the degree of solar re-setting of luminescence at the
time of deposition; the degree of sensitivity to irradiation; the ability to record a stored dose free from systematic errors related to dose-response e!ects or fading; and a satisfactory dosimetry. In this paper we examine the residual signals of modern beach sands from the study area, the sensitivity of these sands to irradiation, and the susceptibility of modern sands to bleaching using arti"cial daylight lamps in a light box. Preliminary results from sands collected from two archaeological sites are presented. Regenerative OSL procedures, developed from those suggested by Murray and Mejdahl (1999) and Murray and Roberts (1998), have been used to determine estimated palaeodoses and preliminary ages of some archaeologically intercalated sands.
2. Samples 2.1. Modern beach sands
夽 Paper published in December 2000. * Corresponding author. Scottish Universities Research and Reactor Centre, Rankine Ave., East Kilbride, Scotland G75 0QF, UK. E-mail address: [email protected] (A.A. Sommerville).
The beaches within the study area have di!ering mineralogies, re#ecting the sediment provenance and underlying bedrock geology. These beach systems may in some
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 1 5 - 9
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Fig. 1. Location of modern beach samples in the Western Isles and Orkney and the archaeological sites at Amble and Tofts Ness.
cases represent the primary provenance of the aeolian sands that collect both in the dunes behind and in association with archaeological sites (Schwenninger, 1996; Hansom, 1999; Gilbertson et al., 1999). In order to establish the source characteristics of the aeolian sands, present-day beach samples were collected from the back edges of many of the beaches in the Western Isles and Orkney (Fig. 1). The sand was collected using a trowel, skimmed across the surface to a depth of approximately 1 cm. Further surface samples were taken using adhesive packing tape to ensure that only the surface layer was collected. All of the samples were immediately placed in sealed black bags to prevent further exposure to light. 2.2. Archaeologically intercalated sands The archaeologically intercalated sands were collected from vertical exposed sections or pits using 30 cm long copper pipes with a diameter of 19 mm. Before the pipes were inserted horizontally into the section, the section surface was cleaned to remove contaminants and bleached sediments. Once collected, both ends of the tube were sealed with black tape to prevent water loss and exposure
to light. The surface ends of the tubes were marked and were immediately placed in black plastic bags. 2.3. Amble, northeast England In February 1999, three samples of aeolian sands were collected from an exposed section adjacent to an archaeological site at Amble, currently being eroded by the sea. Gamma dose rates were measured in situ and surface samples were taken from the dunes and beach. The Amble site was originally excavated by Dr. C. Bonsall (University of Edinburgh) and consists of two cairns containing human remains which have yielded Bronze Age radiocarbon dates (C. Bonsall, pers. comm.), and which were subsequently enclosed by wind-blown sands (Bonsall, 1984) (Fig. 2a). Five hundred metres north of the archaeological site, the stratigraphic sequence consists of a basal till overlain by peat, which in turn is overlain by aeolian sands (Frank, 1982). Radiocarbon dates on the peat indicate deposition between 3770}3540 cal BC (SRR 1422: 4890$50 BP) and 1050}840 cal BC (SRR 1420: 2810$40 BP) (Frank, 1982). The second date gives a terminus post quem for initiation of sand deposition of 1200}1000 cal BC,
A.A. Sommerville et al. / Quaternary Science Reviews 20 (2001) 913}919
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Fig. 2. Working stratigraphy and assumed chronology for sands sampled from (a) Amble and (b) Tofts Ness.
consistent with the archaeological estimate for initiation of aeolian sand deposition associated with the cairns. 2.4. Tofts Ness, Sanday, Orkney Tofts Ness is located on a low-lying peninsula in the northeast of the island of Sanday. Excavations by S.J. Dockrill (Bradford University) demonstrated Late Neolithic and Bronze/Iron Age occupation with associated palaeosols buried by wind-blown sands (Simpson et al., 1998). Radiocarbon and thermoluminescence (TL) dates from organic material, hearths and ceramics together give a consistent chronology for human occupation of the site. Luminescence evidence puts the occupation of two Late Neolithic structures to between 2600$240 and 2250$290 BC. The Late Neolithic and Bronze Age settlements appear to have been interleaved with abandonment, tentatively associated with the onset of aeolian sands at the end of the third millennium BC and in the middle of the "rst millennium BC. There then followed a major sequence of structures associated with the later roundhouse. Luminescence ages on three hearths indicate occupation from 1040$80 to 410$290 BC. Much of the area lies below 5 m asl and shelly deposits of aeolian sands have buried the landscape at various times, either as individual blow events or by dune encroachment (Simpson and Dockrill, 1996; Dockrill et al., 1994). The two palaeosols are separated by aeolian sands (Fig. 2b), the upper of which lies above the Bronze/Iron Age soil and structures and is therefore assumed to be younger than the luminescence date of 410$290 BC which dates the "nal occupation of the site. The lower sand layer lies below the Bronze Age soil and is therefore presumed to pre-date 1520}1320 cal BC (SRR 5247: 3140$40 BP) (Simpson et al., 1998). This sand layer also lies above a lower anthropogenic soil which is tentatively ascribed to the Late Neolithic (GU 2210: 3260}2925 cal BC) (S.J. Dockrill, pers. comm.). The relationship between the remaining structural sequence and the environmental test pits will be discussed further at a later date.
Further excavation by Erica Guttmann (University of Stirling) and AOC Scotland in 1999 to the east of the Tofts Ness settlements aimed to investigate the palaeosols and their associated sedimentary sequence. Luminescence dating samples and associated gamma spectrometry readings were taken from each pit representing the upper, and where present, lower sands. Test Pit 6 (Fig. 2b) contained both sand layers and is discussed below.
3. Methodology A comparative study of the Western Isles and the Orkney beach samples was conducted using TL, blue OSL and IRSL. The modern beach sands were tested using 125}250 lm polymineral samples treated with 10% hydrochloric acid for 30 min to remove calcite. Three discs per sample were measured to allow a comparison of TL, blue OSL and infra-red stimulated luminescence (IRSL). Three runs were completed on each disc: the natural signal, 5 Gy, and 5 Gy # 1 h of bleaching in a light box. The discs were irradiated using a 1.85 GBq Strontium-90 source and bleached with four 40 W arti"cial daylight lamps in a light box. TL measurements were made using a manual reader "tted with a 7-59 and KG1 "lter. The heater plate was ramped from 0}4003C at a rate of 53C/s. The blue OSL measurements were made using an SURRC-built PSL reader using gallium-nitride LEDs with a peak wavelength of 470 nm for sample stimulation (Sanderson et al., 2000). Each OSL disc was given a 30 s preheat at 2603C and measured at 1253C for 100 s. The IRSL measurements were conducted using an SURRC pulsed PSL reader using 180 mW IR diodes with a peak wavelength of 880 nm and a 50 mm diameter measurement chamber. Each disc was measured for 100 s. The archaeologically intercalated sands were tested using the 125}250 lm quartz fraction. All the samples were treated with 10% hydrochloric acid for 30 min to remove calcite,
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and 40% hydro#uoric acid for 40 min to remove feldspar grains and etch the quartz grains (Aitken, 1985). A further 30 min of 10% HCl ensured removal of #uorides. A regenerative OSL procedure was used to determine the estimated palaeodoses of the archaeologically intercalated sands. The approach was based on suggestions by Murray and Mejdahl (1999) and Murray and Roberts (1998), namely to interleave a sequence of measurements of natural and regenerated luminescence signals with measurements of the response to small test doses to monitor sensitivity changes as a result of readout and thermal cycling. Stored doses were interpolated using sensitivity normalised regenerative dose-response curves (Murray and Mejdahl, 1999); however, two changes of methodology were introduced. Firstly, non-linear doseresponse curves were observed in earlier studies of Thai quartz (Houston, 1999; Sanderson et al., 2001); therefore, a greater range of regenerative doses was used than originally proposed. Secondly, the OSL sensitivity of some samples was extremely low in the archaeological dose range investigated. Since this situation is unlikely to be unique to Scottish sands a new methodology was established which would have application beyond the present study. 3.1. Regenerative procedures to cope with low-sensitivity samples Rather than normalise integrated OSL signals to individual sensitivity measurements, a statistical trend was established to show the sensitivity corrections related to the dose cycle. Measured data were normalised to the mean sensitivity change determined for the appropriate cycle point from the complete set of data from each disc. Each disc was preheated at 2603C for 30 s and measurements were carried out at 1253C using the SURRC blue OSL system. Regenerative dose-response curves were constructed using doses of 0, 2, 4, 6, 8 and 10 Gy, and test doses of 1 Gy were read between each dose to allow the samples to be normalised, and to check for sensitivity changes. A further 2 Gy was then added and read at the end of the run as a further sensitivity check. The methodology worked well and appears to be a robust approach for coping with low-sensitivity samples.
4. Results 4.1. Modern beach sands All of the beach samples contain both quartz and feldspars and indicate sensitivity to the TL, blue OSL and IRSL techniques. The samples were taken from the surface of the beaches where the natural residual signal should be low. Fig. 3 shows the residual ratio (natural/5 Gy) plotted against number of observations for the
Fig. 3. Comparison of residual signals from modern beaches in the Western Isles and Orkney using TL, blue OSL and IRSL.
three techniques. TL results for the Western Isles and Orkney show a high residual signal, implying residual doses of between 0.5 and 10 Gy, although the Western Isles samples tend to have slightly lower values. Blue OSL residuals are much lower (0.2}2 Gy) but there is a clear distinction between Orkney and the much lower residuals of the Western Isles. The IRSL results show a small residual signal (0.01}0.1 Gy) for both sample sets, but the Orkney and Western Isles samples remain distinct from each other. Fig. 4 shows the sensitivity distribution in photons/Gy derived from the observed response to a 5 Gy dose. The results indicate that for each technique the Western Isles samples are more sensitive than those from Orkney. The highest sensitivities of the Western Isles samples are similar for the three techniques, however the range of sensitivities increases, with TL having the narrowest range and IRSL the greatest. The Orkney samples have lower sensitivities, with similar sensitivity ranges to the Western Isles. Regional patterns of sensitivity emerge with 2}3 orders of magnitude in inter-regional variation, and more than one order of magnitude in intra-regional variation. All samples show su$cient TL sensitivity to permit quanti"cation of archaeological doses of the order of 1}2 Gy; however, the residuals remain a signi"cant obstacle to accurate dating. The majority of OSL and IRSL sensitivities should be adequate for recording doses in this region; however it should be noted that some of the samples lack su$cient sensitivity for high precision.
A.A. Sommerville et al. / Quaternary Science Reviews 20 (2001) 913}919
Fig. 4. Comparison of luminescence sensitivities from modern beaches in the Western Isles and Orkney using TL, blue OSL and IRSL.
The sensitivity to bleaching was assessed by illuminating each sample to approximately 25 J/cm in a light box (Thorn Lighting arti"cial daylight #uorescent tubes, main spectral range 330}750 nm), following irradiation, and measuring the remaining signal. Fig. 5 shows the bleachability (ratio of the unbleached to the bleached response) to vary by one order of magnitude within each stimulation method, and by two orders of magnitude between methods. Moreover, there is a strong linear relationship between the residual blue OSL and IRSL levels, and the response to laboratory bleaching. Whereas both blue OSL and IRSL form part of the same trend lines, the TL residuals are generally higher than would have been expected on the basis of laboratory bleaching experiments. This implies a di!erence between environmental and laboratory bleaching conditions, most probably as a result of di!erences in spectral composition between the two cases. Thus, the relationship between TL, blue OSL and IRSL residuals may provide information about the quality of the original bleaching event. 4.2. Archaeologically intercalated sands Three samples of aeolian sands at Amble together with one sample each from the upper and lower Tofts Ness sands were subjected to regenerative blue OSL dose determination and preliminary age estimation based on four discs per sample at this stage (Table 1). The blue
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Fig. 5. Relationship between residual luminescence and sensitivity to laboratory bleaching for modern beaches in the Western Isles and Orkney.
OSL sensitivities of all three Amble sands, and the Tofts Ness sand were between 10 and 10 photons/Gy, amongst the least sensitive measured. The OSL system used has a background count rate of 10}10 cps; therefore, the statistical precision in net intensities from these low-sensitivity samples was limited. It was necessary to use the overall sensitivity trend, from test-dose responses rather than individual test dose results, as described above, to produce normalised OSL dose-response curves for these samples. The lower Tofts Ness sand showed approximately two orders of magnitude greater OSL sensitivity compared to the upper sand, implying di!erences in sediment composition. Sensitivity changes during regenerative procedures were in general within 50% of initial values, and showed no abrupt changes. The test dose-response slope (cycle\) is also tabulated (Table 1) and is the fractional change per measurement cycle. Most are well below 10% change per cycle, which is comparable with the precision available from such low-intensity OSL response to the 1 Gy test dose. Recycling checks and the recycling ratio (last 2 Gy/"rst 2 Gy) (Table 1) con"rmed that the 2 Gy response could be reproduced, within error, at the end of the cycle in all cases. The initial age estimates, based on measured matrix beta dose rates, in situ gamma dose rates and estimated cosmic dose rates, are broadly consistent with archaeological constraints. Improved precision would be needed to examine possible systematic di!erences, and
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Table 1 Table of results Sample
Stored dose (Gy)
Test dose response slope (cycle\)
Recycling ratio
E!ective dose rate (mGy/a)\
Age (ka)
Date
Amble 503 (upper) Amble 502 Amble 501 (lower) Amble average Tofts Ness 614 (upper) Tofts Ness 614 (lower)
2.6$0.6 4.2$0.6 1.9$0.3
!0.11 0.04 0.02
2.3$0.9 0.9$0.5 0.55$0.24
0.87$0.11 0.86$0.11 1.02$0.09
1.6$0.6 1.2$0.8
0.58$0.11 1.06$0.1
2.94$0.34 4.90$0.9 1.87$0.34 3.24$0.72 3.37$1.35 4.92$0.72
940$800 BC 2900$900 BC 130$720 BC 1240$720 BC 1370$1350 BC 2980$710 BC
1.97$0.7 5.24$0.6
0.12 0.05
will receive attention in the next stage of this project. At Amble, however, the mean age of 1240$720 BC is broadly consistent with both the ages of the archaeological cairns, and with the onset of aeolian sand deposition at the nearby environmental section. At Tofts Ness, the lower sand gives a preliminary estimate in the early third millennium BC, several centuries earlier than the end of the Late Neolithic occupation, but con"rming the tentative attribution of the underlying palaeosol with its ard digging tool marks, to the Neolithic period. The preliminary age estimate for the upper sand at Tofts Ness is also in the correct stratigraphic sequence, and is compatible with 900}790 cal BC (SRR 5256: 2665$40 BP), the radiocarbon age of the upper palaeosol. It is, however, several centuries older than the abandonment of the Bronze/Iron Age settlement. The relationship between structural and environmental chronologies will receive further attention in the future; however, the initial results are highly encouraging.
5. Discussion and conclusions Accurate luminescence dating of sediments assumes that upon deposition grains have received su$cient light exposure to have little, if any, residual signal. Thus, in order to date aeolian sands with the precision needed for archaeological purposes, it is relevant to examine the residual levels of modern source sands. The results from the Western Isles and Orkney indicate that residuals vary depending on the technique being used. The TL residuals are relatively high and generally preclude accurate dating of young samples. Whether this is the result of lightinsensitive traps being included in the signal (Aitken, 1998), or a consequence of bleaching conditions is unclear. However, it appears that there are environmental factors in#uencing the relative extent to which TL and OSL/IRSL are bleached. There is a clear distinction between the Western Isles and Orkney samples when blue OSL is used and this may be the result of di!erent sediment sources or depositional processes (Rink, 1999). The residual signals measured using IRSL are all low suggesting that the feldspars in the samples were well
bleached. It is notable that the IRSL residuals are typically one order of magnitude lower than OSL residuals. This is perhaps surprising given the published values for relative bleaching (see, for example, Aitken, 1998, Table 6.1), and further investigation of the mineralogical content of these residuals is therefore warranted. The sand collected from the modern beaches has undoubtedly been recycled several times, and it is possible that the grains have not been exposed for a long enough period to reduce the luminescence signal. In addition, the sediment collected on the beaches may represent sand recycled from recently eroded dunes or from the nearshore as a result of storms. In the Western and Northern Isles o!shore sources dominate the beach and dune sand provenance (Mather and Ritchie, 1977; Hansom, 1999). Results from the beach samples indicate that the blue OSL and IRSL o!er the greatest potential for dating wind-blown sands from archaeological sites. The implications from these results is that OSL residuals, and to a lesser extent IRSL residuals may, to a minor extent, in#uence the dates derived from aeolian sands from archaeological sites. Although the environmental conditions in the past may not have been the same as today, the signals from modern source sands should be recognised and a robust methodology developed for dealing with residuals of equivalent magnitude in dating procedures. The preliminary OSL dates from Amble and Tofts Ness, obtained using regenerative OSL quartz procedures, are highly encouraging. The samples examined are experimentally demanding in terms of sensitivity and required precision. Although more work is clearly needed to improve precision, the results obtained so far are broadly consistent with archaeological expectations. However, in all cases the initial age estimates may appear to be slightly older than expected ages based on archaeological and radiocarbon evidence. While this should be taken cautiously at this stage, it is notable that the residual levels observed from modern beach sands are approximately of the same magnitude as these age di!erences. More work is needed to improve precision for lowsensitivity quartz, to examine the systematic running of
A.A. Sommerville et al. / Quaternary Science Reviews 20 (2001) 913}919
the regenerative OSL procedure and particularly to investigate the dependence of results on the preheating regime. It is also intended to compare IRSL and OSL results from di!erent mineral suites from these and other archaeological sites, with a view to establishing a regional chronology for aeolian sand depositional events.
Acknowledgements Drs. C. Bonsall, G. Coles and I. Simpson, and S.J. Dockrill are gratefully thanked for their invaluable help and interest in the "eld and in providing excavation details. Drs. P. Ashmore, S. Foster, I. Ralston and L. Carmichael, and E. Guttmann, I. Anthony and I. Houston are also thanked for their input. Carnegie Trust, BGRG and the RSGS funded the "eldwork, and Anne Sommerville is supported by SURRC, the Physical Science Planning Unit and the Department of Archaeology at the University of Glasgow.
References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1998. An Introduction to Optical Dating * the Dating of Quaternary Sediments by the use of Photo-stimulated Luminescence. Oxford University Press, New York. Bonsall, C., 1984. Low Hauxley, Northumberland. Proceedings of the Prehistoric Society 50, 398. Dockrill, S.J., Bond, J.M., Milles, A., Simpson, I., Ambers, J., 1994. Tofts Ness, Sanday, Orkney. An integrated study of a buried Orcadian landscape. In: Lu!, R., Rowley-Conwy, P. (Eds.), Whither Environmental Archaeology. Oxbow Monography, Vol. 38, pp. 115}132.
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Frank, R., 1982. A Holocene peat and dune-sand sequence on the coast of Northeast England * a preliminary report. Quaternary Newsletter 36, 24}32. Gilbertson, D.D., Schwenninger, J.-L., Kemp, R.A., Rhodes, E.J., 1999. Sand-drift and soil formation along an exposed North Atlantic coastline: 14,000 years of diverse geomorphological, climatic and human impacts. Journal of Archaeological Science 26, 439}469. Hansom, J.D., 1999. The coastal geomorphology of Scotland: understanding sediment budgets for e!ective coastal management. In: Baxter, J.M., Duncan, K., Atkins, S.M., Lees, G. (Eds.), Scotland's Living Coastline. HMSO, Edinburgh, pp. 34}44. Houston, I., 1999. The application of stimulated luminescence to the dating of South East Asian quartz. B.Sc. (Hons.) Dissertation, Department of Physics, University of Strathclyde, Glasgow. Mather, A.S., Ritchie, W., 1977. The Beaches of the Highlands and Islands of Scotland. CCS, Perth, 201 pp. Murray, A.S., Mejdahl, V., 1999. Comparison of regenerative-dose single-aliquot and multiple-aliquot (SARA) protocols using heated quartz from archaeological sites. Quaternary Geochronology 18, 223}229. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single aliquot protocol. Radiation Measurements 29 (5), 503}515. Rink, W.J., 1999. Quartz luminescence as a light-sensitive indicator of sediment transport in coastal processes. Journal of Coastal Research 15 (1), 148}154. Sanderson, D.C.W., Bishop, P., Houston, I., , Boonsener, M., 2001. Luminescence characteristics of quartz-rich cover sands from NE Thailand. Quaternary Science Reviews (Quaternary Geochronology) 20, 893}900. Schwenninger, J.-L., 1996. The evolution of coastal sand dunes in the Southern Isles of the Outer Hebrides of Scotland. Unpublished Ph.D. Dissertation, University of London. Simpson, I.A., Dockrill, S.J., 1996. Early cultivated soils at Tofts Ness, Sanday, Orkney. In: Hall, A.M. (Ed.), The Quaternary of Orkney Field Guide. Quaternary Research Association, London, pp. 130}143. Simpson, I.A., Dockrill, S.J., Bull, S.J., Evershed, R.P., 1998. Early anthropogenic soil formation at Tofts Ness, Sanday, Orkney. Journal of Archaeological Science 25, 729}746.
Luminescence dating of aeolian sands from archaeological sites in Northern Britain: a preliminary study夽 A.A. Sommerville *, D.C.W. Sanderson , J.D. Hansom, R.A. Housley Scottish Universities Research and Reactor Centre, Rankin Ave., East Kilbride, Scotland G75 0QF, UK Department of Geography and Topographic Science, University of Glasgow, Glasgow G12 8QQ, UK Department of Archaeology, University of Glasgow, Glasgow G12 8QQ, UK
Abstract Luminescence dating of aeolian sands from archaeological sites has potential to contribute to regional chronologies for sediment deposition and to provide a greater understanding of climatic in#uences on early communities. The Northern and Western Isles of Scotland provide important opportunities for sampling archaeologically intercalated sands for these purposes, and to provide constrained samples for method validation. A wide range of modern beaches have been sampled in the Western and Orkney Isles of Scotland to examine regional variations in luminescence sensitivity, residuals and ease of bleaching. These modern sands have negligible residuals for infra-red stimulated luminescence (IRSL), small optically stimulated luminescence (OSL) residuals and signi"cant thermoluminescence residuals. The relationship between these signals and laboratory bleaching results may indicate the initial depositional environment, and hence lead to a means of identifying well-bleached dating samples. Both sensitivities and residuals show regional di!erences, re#ecting local geology. Preliminary ages obtained from aeolian sands associated with archaeological sites at Amble (Northumbria) and Tofts Ness (Sanday, Orkney) using regenerative blue OSL techniques on extracted quartz are broadly consistent with external age controls from the "rst and third millennium BC. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The Western and Orkney Isles of Scotland (Fig. 1) are characterised by treeless windswept dune systems and large sections of the coast are dominated by long sandy beaches backed by extensive aeolian depositional systems. Large numbers of well-preserved archaeological sites exist within these dune areas, but because of a lack of organic material, the use of C dating is often precluded. An alternative dating solution in these situations is luminescence dating of the aeolian sands, which are stratigraphically intercalated between independently dated cultural horizons in archaeological sites. Several issues must be resolved before accurate optically stimulated luminescence (OSL) ages for aeolian sands are generated from archaeological sites. It is important to establish the following sediment characteristics: the degree of solar re-setting of luminescence at the
time of deposition; the degree of sensitivity to irradiation; the ability to record a stored dose free from systematic errors related to dose-response e!ects or fading; and a satisfactory dosimetry. In this paper we examine the residual signals of modern beach sands from the study area, the sensitivity of these sands to irradiation, and the susceptibility of modern sands to bleaching using arti"cial daylight lamps in a light box. Preliminary results from sands collected from two archaeological sites are presented. Regenerative OSL procedures, developed from those suggested by Murray and Mejdahl (1999) and Murray and Roberts (1998), have been used to determine estimated palaeodoses and preliminary ages of some archaeologically intercalated sands.
2. Samples 2.1. Modern beach sands
夽 Paper published in December 2000. * Corresponding author. Scottish Universities Research and Reactor Centre, Rankine Ave., East Kilbride, Scotland G75 0QF, UK. E-mail address: [email protected] (A.A. Sommerville).
The beaches within the study area have di!ering mineralogies, re#ecting the sediment provenance and underlying bedrock geology. These beach systems may in some
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 1 5 - 9
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Fig. 1. Location of modern beach samples in the Western Isles and Orkney and the archaeological sites at Amble and Tofts Ness.
cases represent the primary provenance of the aeolian sands that collect both in the dunes behind and in association with archaeological sites (Schwenninger, 1996; Hansom, 1999; Gilbertson et al., 1999). In order to establish the source characteristics of the aeolian sands, present-day beach samples were collected from the back edges of many of the beaches in the Western Isles and Orkney (Fig. 1). The sand was collected using a trowel, skimmed across the surface to a depth of approximately 1 cm. Further surface samples were taken using adhesive packing tape to ensure that only the surface layer was collected. All of the samples were immediately placed in sealed black bags to prevent further exposure to light. 2.2. Archaeologically intercalated sands The archaeologically intercalated sands were collected from vertical exposed sections or pits using 30 cm long copper pipes with a diameter of 19 mm. Before the pipes were inserted horizontally into the section, the section surface was cleaned to remove contaminants and bleached sediments. Once collected, both ends of the tube were sealed with black tape to prevent water loss and exposure
to light. The surface ends of the tubes were marked and were immediately placed in black plastic bags. 2.3. Amble, northeast England In February 1999, three samples of aeolian sands were collected from an exposed section adjacent to an archaeological site at Amble, currently being eroded by the sea. Gamma dose rates were measured in situ and surface samples were taken from the dunes and beach. The Amble site was originally excavated by Dr. C. Bonsall (University of Edinburgh) and consists of two cairns containing human remains which have yielded Bronze Age radiocarbon dates (C. Bonsall, pers. comm.), and which were subsequently enclosed by wind-blown sands (Bonsall, 1984) (Fig. 2a). Five hundred metres north of the archaeological site, the stratigraphic sequence consists of a basal till overlain by peat, which in turn is overlain by aeolian sands (Frank, 1982). Radiocarbon dates on the peat indicate deposition between 3770}3540 cal BC (SRR 1422: 4890$50 BP) and 1050}840 cal BC (SRR 1420: 2810$40 BP) (Frank, 1982). The second date gives a terminus post quem for initiation of sand deposition of 1200}1000 cal BC,
A.A. Sommerville et al. / Quaternary Science Reviews 20 (2001) 913}919
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Fig. 2. Working stratigraphy and assumed chronology for sands sampled from (a) Amble and (b) Tofts Ness.
consistent with the archaeological estimate for initiation of aeolian sand deposition associated with the cairns. 2.4. Tofts Ness, Sanday, Orkney Tofts Ness is located on a low-lying peninsula in the northeast of the island of Sanday. Excavations by S.J. Dockrill (Bradford University) demonstrated Late Neolithic and Bronze/Iron Age occupation with associated palaeosols buried by wind-blown sands (Simpson et al., 1998). Radiocarbon and thermoluminescence (TL) dates from organic material, hearths and ceramics together give a consistent chronology for human occupation of the site. Luminescence evidence puts the occupation of two Late Neolithic structures to between 2600$240 and 2250$290 BC. The Late Neolithic and Bronze Age settlements appear to have been interleaved with abandonment, tentatively associated with the onset of aeolian sands at the end of the third millennium BC and in the middle of the "rst millennium BC. There then followed a major sequence of structures associated with the later roundhouse. Luminescence ages on three hearths indicate occupation from 1040$80 to 410$290 BC. Much of the area lies below 5 m asl and shelly deposits of aeolian sands have buried the landscape at various times, either as individual blow events or by dune encroachment (Simpson and Dockrill, 1996; Dockrill et al., 1994). The two palaeosols are separated by aeolian sands (Fig. 2b), the upper of which lies above the Bronze/Iron Age soil and structures and is therefore assumed to be younger than the luminescence date of 410$290 BC which dates the "nal occupation of the site. The lower sand layer lies below the Bronze Age soil and is therefore presumed to pre-date 1520}1320 cal BC (SRR 5247: 3140$40 BP) (Simpson et al., 1998). This sand layer also lies above a lower anthropogenic soil which is tentatively ascribed to the Late Neolithic (GU 2210: 3260}2925 cal BC) (S.J. Dockrill, pers. comm.). The relationship between the remaining structural sequence and the environmental test pits will be discussed further at a later date.
Further excavation by Erica Guttmann (University of Stirling) and AOC Scotland in 1999 to the east of the Tofts Ness settlements aimed to investigate the palaeosols and their associated sedimentary sequence. Luminescence dating samples and associated gamma spectrometry readings were taken from each pit representing the upper, and where present, lower sands. Test Pit 6 (Fig. 2b) contained both sand layers and is discussed below.
3. Methodology A comparative study of the Western Isles and the Orkney beach samples was conducted using TL, blue OSL and IRSL. The modern beach sands were tested using 125}250 lm polymineral samples treated with 10% hydrochloric acid for 30 min to remove calcite. Three discs per sample were measured to allow a comparison of TL, blue OSL and infra-red stimulated luminescence (IRSL). Three runs were completed on each disc: the natural signal, 5 Gy, and 5 Gy # 1 h of bleaching in a light box. The discs were irradiated using a 1.85 GBq Strontium-90 source and bleached with four 40 W arti"cial daylight lamps in a light box. TL measurements were made using a manual reader "tted with a 7-59 and KG1 "lter. The heater plate was ramped from 0}4003C at a rate of 53C/s. The blue OSL measurements were made using an SURRC-built PSL reader using gallium-nitride LEDs with a peak wavelength of 470 nm for sample stimulation (Sanderson et al., 2000). Each OSL disc was given a 30 s preheat at 2603C and measured at 1253C for 100 s. The IRSL measurements were conducted using an SURRC pulsed PSL reader using 180 mW IR diodes with a peak wavelength of 880 nm and a 50 mm diameter measurement chamber. Each disc was measured for 100 s. The archaeologically intercalated sands were tested using the 125}250 lm quartz fraction. All the samples were treated with 10% hydrochloric acid for 30 min to remove calcite,
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and 40% hydro#uoric acid for 40 min to remove feldspar grains and etch the quartz grains (Aitken, 1985). A further 30 min of 10% HCl ensured removal of #uorides. A regenerative OSL procedure was used to determine the estimated palaeodoses of the archaeologically intercalated sands. The approach was based on suggestions by Murray and Mejdahl (1999) and Murray and Roberts (1998), namely to interleave a sequence of measurements of natural and regenerated luminescence signals with measurements of the response to small test doses to monitor sensitivity changes as a result of readout and thermal cycling. Stored doses were interpolated using sensitivity normalised regenerative dose-response curves (Murray and Mejdahl, 1999); however, two changes of methodology were introduced. Firstly, non-linear doseresponse curves were observed in earlier studies of Thai quartz (Houston, 1999; Sanderson et al., 2001); therefore, a greater range of regenerative doses was used than originally proposed. Secondly, the OSL sensitivity of some samples was extremely low in the archaeological dose range investigated. Since this situation is unlikely to be unique to Scottish sands a new methodology was established which would have application beyond the present study. 3.1. Regenerative procedures to cope with low-sensitivity samples Rather than normalise integrated OSL signals to individual sensitivity measurements, a statistical trend was established to show the sensitivity corrections related to the dose cycle. Measured data were normalised to the mean sensitivity change determined for the appropriate cycle point from the complete set of data from each disc. Each disc was preheated at 2603C for 30 s and measurements were carried out at 1253C using the SURRC blue OSL system. Regenerative dose-response curves were constructed using doses of 0, 2, 4, 6, 8 and 10 Gy, and test doses of 1 Gy were read between each dose to allow the samples to be normalised, and to check for sensitivity changes. A further 2 Gy was then added and read at the end of the run as a further sensitivity check. The methodology worked well and appears to be a robust approach for coping with low-sensitivity samples.
4. Results 4.1. Modern beach sands All of the beach samples contain both quartz and feldspars and indicate sensitivity to the TL, blue OSL and IRSL techniques. The samples were taken from the surface of the beaches where the natural residual signal should be low. Fig. 3 shows the residual ratio (natural/5 Gy) plotted against number of observations for the
Fig. 3. Comparison of residual signals from modern beaches in the Western Isles and Orkney using TL, blue OSL and IRSL.
three techniques. TL results for the Western Isles and Orkney show a high residual signal, implying residual doses of between 0.5 and 10 Gy, although the Western Isles samples tend to have slightly lower values. Blue OSL residuals are much lower (0.2}2 Gy) but there is a clear distinction between Orkney and the much lower residuals of the Western Isles. The IRSL results show a small residual signal (0.01}0.1 Gy) for both sample sets, but the Orkney and Western Isles samples remain distinct from each other. Fig. 4 shows the sensitivity distribution in photons/Gy derived from the observed response to a 5 Gy dose. The results indicate that for each technique the Western Isles samples are more sensitive than those from Orkney. The highest sensitivities of the Western Isles samples are similar for the three techniques, however the range of sensitivities increases, with TL having the narrowest range and IRSL the greatest. The Orkney samples have lower sensitivities, with similar sensitivity ranges to the Western Isles. Regional patterns of sensitivity emerge with 2}3 orders of magnitude in inter-regional variation, and more than one order of magnitude in intra-regional variation. All samples show su$cient TL sensitivity to permit quanti"cation of archaeological doses of the order of 1}2 Gy; however, the residuals remain a signi"cant obstacle to accurate dating. The majority of OSL and IRSL sensitivities should be adequate for recording doses in this region; however it should be noted that some of the samples lack su$cient sensitivity for high precision.
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Fig. 4. Comparison of luminescence sensitivities from modern beaches in the Western Isles and Orkney using TL, blue OSL and IRSL.
The sensitivity to bleaching was assessed by illuminating each sample to approximately 25 J/cm in a light box (Thorn Lighting arti"cial daylight #uorescent tubes, main spectral range 330}750 nm), following irradiation, and measuring the remaining signal. Fig. 5 shows the bleachability (ratio of the unbleached to the bleached response) to vary by one order of magnitude within each stimulation method, and by two orders of magnitude between methods. Moreover, there is a strong linear relationship between the residual blue OSL and IRSL levels, and the response to laboratory bleaching. Whereas both blue OSL and IRSL form part of the same trend lines, the TL residuals are generally higher than would have been expected on the basis of laboratory bleaching experiments. This implies a di!erence between environmental and laboratory bleaching conditions, most probably as a result of di!erences in spectral composition between the two cases. Thus, the relationship between TL, blue OSL and IRSL residuals may provide information about the quality of the original bleaching event. 4.2. Archaeologically intercalated sands Three samples of aeolian sands at Amble together with one sample each from the upper and lower Tofts Ness sands were subjected to regenerative blue OSL dose determination and preliminary age estimation based on four discs per sample at this stage (Table 1). The blue
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Fig. 5. Relationship between residual luminescence and sensitivity to laboratory bleaching for modern beaches in the Western Isles and Orkney.
OSL sensitivities of all three Amble sands, and the Tofts Ness sand were between 10 and 10 photons/Gy, amongst the least sensitive measured. The OSL system used has a background count rate of 10}10 cps; therefore, the statistical precision in net intensities from these low-sensitivity samples was limited. It was necessary to use the overall sensitivity trend, from test-dose responses rather than individual test dose results, as described above, to produce normalised OSL dose-response curves for these samples. The lower Tofts Ness sand showed approximately two orders of magnitude greater OSL sensitivity compared to the upper sand, implying di!erences in sediment composition. Sensitivity changes during regenerative procedures were in general within 50% of initial values, and showed no abrupt changes. The test dose-response slope (cycle\) is also tabulated (Table 1) and is the fractional change per measurement cycle. Most are well below 10% change per cycle, which is comparable with the precision available from such low-intensity OSL response to the 1 Gy test dose. Recycling checks and the recycling ratio (last 2 Gy/"rst 2 Gy) (Table 1) con"rmed that the 2 Gy response could be reproduced, within error, at the end of the cycle in all cases. The initial age estimates, based on measured matrix beta dose rates, in situ gamma dose rates and estimated cosmic dose rates, are broadly consistent with archaeological constraints. Improved precision would be needed to examine possible systematic di!erences, and
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Table 1 Table of results Sample
Stored dose (Gy)
Test dose response slope (cycle\)
Recycling ratio
E!ective dose rate (mGy/a)\
Age (ka)
Date
Amble 503 (upper) Amble 502 Amble 501 (lower) Amble average Tofts Ness 614 (upper) Tofts Ness 614 (lower)
2.6$0.6 4.2$0.6 1.9$0.3
!0.11 0.04 0.02
2.3$0.9 0.9$0.5 0.55$0.24
0.87$0.11 0.86$0.11 1.02$0.09
1.6$0.6 1.2$0.8
0.58$0.11 1.06$0.1
2.94$0.34 4.90$0.9 1.87$0.34 3.24$0.72 3.37$1.35 4.92$0.72
940$800 BC 2900$900 BC 130$720 BC 1240$720 BC 1370$1350 BC 2980$710 BC
1.97$0.7 5.24$0.6
0.12 0.05
will receive attention in the next stage of this project. At Amble, however, the mean age of 1240$720 BC is broadly consistent with both the ages of the archaeological cairns, and with the onset of aeolian sand deposition at the nearby environmental section. At Tofts Ness, the lower sand gives a preliminary estimate in the early third millennium BC, several centuries earlier than the end of the Late Neolithic occupation, but con"rming the tentative attribution of the underlying palaeosol with its ard digging tool marks, to the Neolithic period. The preliminary age estimate for the upper sand at Tofts Ness is also in the correct stratigraphic sequence, and is compatible with 900}790 cal BC (SRR 5256: 2665$40 BP), the radiocarbon age of the upper palaeosol. It is, however, several centuries older than the abandonment of the Bronze/Iron Age settlement. The relationship between structural and environmental chronologies will receive further attention in the future; however, the initial results are highly encouraging.
5. Discussion and conclusions Accurate luminescence dating of sediments assumes that upon deposition grains have received su$cient light exposure to have little, if any, residual signal. Thus, in order to date aeolian sands with the precision needed for archaeological purposes, it is relevant to examine the residual levels of modern source sands. The results from the Western Isles and Orkney indicate that residuals vary depending on the technique being used. The TL residuals are relatively high and generally preclude accurate dating of young samples. Whether this is the result of lightinsensitive traps being included in the signal (Aitken, 1998), or a consequence of bleaching conditions is unclear. However, it appears that there are environmental factors in#uencing the relative extent to which TL and OSL/IRSL are bleached. There is a clear distinction between the Western Isles and Orkney samples when blue OSL is used and this may be the result of di!erent sediment sources or depositional processes (Rink, 1999). The residual signals measured using IRSL are all low suggesting that the feldspars in the samples were well
bleached. It is notable that the IRSL residuals are typically one order of magnitude lower than OSL residuals. This is perhaps surprising given the published values for relative bleaching (see, for example, Aitken, 1998, Table 6.1), and further investigation of the mineralogical content of these residuals is therefore warranted. The sand collected from the modern beaches has undoubtedly been recycled several times, and it is possible that the grains have not been exposed for a long enough period to reduce the luminescence signal. In addition, the sediment collected on the beaches may represent sand recycled from recently eroded dunes or from the nearshore as a result of storms. In the Western and Northern Isles o!shore sources dominate the beach and dune sand provenance (Mather and Ritchie, 1977; Hansom, 1999). Results from the beach samples indicate that the blue OSL and IRSL o!er the greatest potential for dating wind-blown sands from archaeological sites. The implications from these results is that OSL residuals, and to a lesser extent IRSL residuals may, to a minor extent, in#uence the dates derived from aeolian sands from archaeological sites. Although the environmental conditions in the past may not have been the same as today, the signals from modern source sands should be recognised and a robust methodology developed for dealing with residuals of equivalent magnitude in dating procedures. The preliminary OSL dates from Amble and Tofts Ness, obtained using regenerative OSL quartz procedures, are highly encouraging. The samples examined are experimentally demanding in terms of sensitivity and required precision. Although more work is clearly needed to improve precision, the results obtained so far are broadly consistent with archaeological expectations. However, in all cases the initial age estimates may appear to be slightly older than expected ages based on archaeological and radiocarbon evidence. While this should be taken cautiously at this stage, it is notable that the residual levels observed from modern beach sands are approximately of the same magnitude as these age di!erences. More work is needed to improve precision for lowsensitivity quartz, to examine the systematic running of
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the regenerative OSL procedure and particularly to investigate the dependence of results on the preheating regime. It is also intended to compare IRSL and OSL results from di!erent mineral suites from these and other archaeological sites, with a view to establishing a regional chronology for aeolian sand depositional events.
Acknowledgements Drs. C. Bonsall, G. Coles and I. Simpson, and S.J. Dockrill are gratefully thanked for their invaluable help and interest in the "eld and in providing excavation details. Drs. P. Ashmore, S. Foster, I. Ralston and L. Carmichael, and E. Guttmann, I. Anthony and I. Houston are also thanked for their input. Carnegie Trust, BGRG and the RSGS funded the "eldwork, and Anne Sommerville is supported by SURRC, the Physical Science Planning Unit and the Department of Archaeology at the University of Glasgow.
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Frank, R., 1982. A Holocene peat and dune-sand sequence on the coast of Northeast England * a preliminary report. Quaternary Newsletter 36, 24}32. Gilbertson, D.D., Schwenninger, J.-L., Kemp, R.A., Rhodes, E.J., 1999. Sand-drift and soil formation along an exposed North Atlantic coastline: 14,000 years of diverse geomorphological, climatic and human impacts. Journal of Archaeological Science 26, 439}469. Hansom, J.D., 1999. The coastal geomorphology of Scotland: understanding sediment budgets for e!ective coastal management. In: Baxter, J.M., Duncan, K., Atkins, S.M., Lees, G. (Eds.), Scotland's Living Coastline. HMSO, Edinburgh, pp. 34}44. Houston, I., 1999. The application of stimulated luminescence to the dating of South East Asian quartz. B.Sc. (Hons.) Dissertation, Department of Physics, University of Strathclyde, Glasgow. Mather, A.S., Ritchie, W., 1977. The Beaches of the Highlands and Islands of Scotland. CCS, Perth, 201 pp. Murray, A.S., Mejdahl, V., 1999. Comparison of regenerative-dose single-aliquot and multiple-aliquot (SARA) protocols using heated quartz from archaeological sites. Quaternary Geochronology 18, 223}229. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single aliquot protocol. Radiation Measurements 29 (5), 503}515. Rink, W.J., 1999. Quartz luminescence as a light-sensitive indicator of sediment transport in coastal processes. Journal of Coastal Research 15 (1), 148}154. Sanderson, D.C.W., Bishop, P., Houston, I., , Boonsener, M., 2001. Luminescence characteristics of quartz-rich cover sands from NE Thailand. Quaternary Science Reviews (Quaternary Geochronology) 20, 893}900. Schwenninger, J.-L., 1996. The evolution of coastal sand dunes in the Southern Isles of the Outer Hebrides of Scotland. Unpublished Ph.D. Dissertation, University of London. Simpson, I.A., Dockrill, S.J., 1996. Early cultivated soils at Tofts Ness, Sanday, Orkney. In: Hall, A.M. (Ed.), The Quaternary of Orkney Field Guide. Quaternary Research Association, London, pp. 130}143. Simpson, I.A., Dockrill, S.J., Bull, S.J., Evershed, R.P., 1998. Early anthropogenic soil formation at Tofts Ness, Sanday, Orkney. Journal of Archaeological Science 25, 729}746.