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FOCUS: Luminescence Dating of Coastal Sands: Overcoming Changes in Environmental Dose Rate
Journal of Archaeological Science (1999) 26, 729–733 Article No. jasc.1999.0450, available online at http://www.idealibrary.com on
FOCUS: Luminescence Dating of Coastal Sands: Overcoming Changes in Environmental Dose Rate J. C. Vogel Quaternary Dating Research Unit, CSIR, P.O. Box 395, Pretoria, South Africa
A. G Wintle Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, U.K.
S. M. Woodborne Quaternary Dating Research Unit, CSIR, P.O. Box 395, Pretoria, South Africa (Received 1 February 1999, revised manuscript accepted 26 March 1999) The luminescence dating of Pleistocene deposits produces erroneous ages if the radioactivity of the surroundings has changed over time. A subtraction procedure is described by which this problem can be overcome, if both the quartz and potassium feldspar minerals are analysed separately. The method is best used in cases where the external radiation is similar in magnitude to the internal dose rate of the feldspar. This approach has been applied to four sand deposits on the coast of South Africa that date to the beginning of the Late Pleistocene. 1999 Academic Press Keywords: LUMINESCENCE DATING, COASTAL SANDS, QUARTZ, FELDSPAR.
Introduction
1995), when moisture percolates through the deposit. To a lesser degree, the radiation experienced by the grains is affected by the amount of water in the deposit which could have varied considerably in the past. We intend to show that the problem of changing dose rates can, under certain circumstances, be overcome by separately analysing both the quartz and potassium feldspar minerals from the same sediment sample. Pure quartz grains contain negligible internal radioactivity and the radiation they receive is solely from sources external to the grains. Feldspar grains receive additional radiation from the radioactive potassium within the mineral itself. By subtracting the external dose measured on the quartz from the total dose measured for the feldspar, the accumulated dose from the internal potassium radiation is obtained; this source of radiation does not change over time. Combining this with the internal radiation dose rate of the potassium in the feldspars gives the true age since last exposure to sunlight. A subtraction procedure for TL signals from quartz and alkali feldspars extracted from pottery was previously proposed by McKerrell & Mejdahl in 1976 and published 5 years later (1981). It was not adopted since adequate amounts of large grains were not available from typical pottery and the external dose rates were
L
uminescence dating techniques are being used increasingly for the dating of archaeological sites that are beyond the range of radiocarbon dating. Early studies involved thermoluminescence (TL) dating of pottery, and more recently it has been applied to burned flint (Mercier et al., 1995) and cave deposits (Woodborne & Vogel, 1997; Feathers, 1997). The technique is based on the fact that the radiation from radioactivity in the sediment produces excitation of electrons in the mineral grains, and that the build-up of these excited electrons over time can be measured by appropriate stimulation of the minerals in the laboratory (Wintle, 1997). The accumulated radiation dose that the grains have received, De, divided by the average mineral dose rate gives the time elapsed since the sediment was laid down. Any previously acquired excited electrons are removed by exposure to sunlight during deposition, especially in the case of aeolian sediments. A problem with this dating technique is that in geochemically active environments the radioactivity of the sediment may have changed, due mainly to the potential mobility of the radioactive elements, uranium (Olley et al., 1997) and potassium (Mercier et al., 729 0305–4403/99/070729+05 $30.00/0
1999 Academic Press
730
J. C. Vogel et al.
generally quite high. This approach can, however, be used effectively for dating deposits in cases where the external radiation dose rate is relatively small and comparable with the internal dose rate of the potassium feldspar.
Material and Methods The samples analysed were sand deposits collected along the coast of South Africa. In all cases the feldspar content was very low, but sufficient amounts could be recovered for single aliquot IRSL regeneration analysis. The first sample comes from Nahoon Point near East London. Here the coastal aeolian sand dunes consolidated into a calcarenite through the reprecipitation of the carbonate that is ubiquitous in sands of marine origin. The calcarenite formation has subsequently been eroded by wave action to produce cliffs that reveal a section through the deposit. This exposure has previously been studied by Mountain (1966) and a radiocarbon date on shell fragments of 29 ka (SR-83) was reported by Deacon (1966). A large block of the sediment was transported intact to the Pretoria laboratory for luminescence analysis; it has a carbonate content of 51%. The second sample was collected from a near-coastal dune in the Wilderness area of the southern Cape (Illenberger, 1996). The dune is well vegetated and it forms a linear barrier between the present coastline and a series of coastal lakes. The sample was collected with a coring device and transported to the laboratory inside the coring tube to avoid any risk of exposure to light. The sample contained 14% carbonate and was not cemented. The third sample was taken from a Middle Stone Age occupation level of an archaeological site called Blombos Cave. The site is located in a sea cliff on the southern Cape coast near the town of Still Bay. Radiocarbon dating of charcoal and shell had yielded ages close to 40 ka for the upper Middle Stone Age (MSA) levels and these dates have been interpreted as being minimum ages (Henshilwood & Sealy, 1997). The material for luminescence dating was collected in a coring tube from the uppermost MSA level in the cave. There is evidence of water percolation through the cave deposits which could result in uranium mobilization. The carbonate content was 48%, but the sediment was not cemented. The fourth sample was sandy infill from an ancient hyaena lair that is the subject of an ongoing palaeontological investigation. The burrow is in a sand deposit on the lee side of a granite outcrop called Basaansklip, north of the town of Saldanha Bay on the Cape west coast. The carbonate content of 62% indicates that the sand is of marine origin, with the most likely source being Saldanha Bay several kilometres away. The walls of the lair were cemented, probably before it was filled.
The infill is not consolidated and there is no evidence of further carbonate migration. In addition, a sample from Rose Cottage Cave near Ladybrand has been included for comparison. It contains 2% carbonate and has an associated calibrated radiocarbon age of 10·5 ka (Woodborne & Vogel, 1997). Laboratory procedures included the isolation of quartz and potassium-rich feldspar grains by magnetic separation and flotation in heavy liquid at specific gravities of 2·58 and 2·62. The grain size used was selected from within the range of 150 to 300 ìm. Neither of the minerals were treated with hydrofluoric acid, which is usually used to remove the small contribution of the alpha particles to the luminescence signal. The TL signal of the quartz was used to derive its accumulated dose, while that of the feldspar was obtained by measuring the infra-red stimulated luminescence (IRSL) using the regeneration method. In both cases, the grains were bleached for 40 h (or more) in bright sunlight and were preheated after each beta irradiation for 16 h at 150C. An IRSL single aliquot approach, similar to that of Duller (1991), was used with a 0·4 s IRSL measurement, with correction being applied for the cumulative effects of successive preheats on each regenerated signal. For these samples the use of a sunlight bleach, rather than an infra-red bleach as used by Duller (1991), did not cause a sensitivity change. Figure 1 shows the results obtained for the Basaansklip sediment. The other samples produced similar curves. No significant anomalous fading was detected in the regenerated feldspar after 4 to 13 months.
Results and Discussion The analytical data pertaining to the samples analysed are given in Table 1. The calculated apparent ages for the quartz and feldspar grains from the first two samples differ considerably. It is unlikely that the difference is due to insufficient bleaching of the quartz because the grains were derived from the nearby beach and thus would have received sufficient exposure to sunlight before deposition. Instead it can be interpreted as being due to the external dose rate having been higher in the past than it is today. This could be the result of either the removal of radioactivity by percolating water or the recent precipitation of carbonate which dilutes the external dose rate. The depositional ages of 75·49·5 and 67·38·9 ka obtained on the near-coastal dunes by the subtraction technique are independent of changes in the external dose rate, and as expected are considerably younger than both the individual ages. The TL and IRSL ages of the other two samples are similar, but in the case of the Blombos sample, the subtraction technique gives a slightly older age of
FOCUS: Luminescence Dating of Coastal Sands 731 200
Thermoluminescence (c/s/mg)
(a)
150
100
50
BL + 268.2 Gy BL + 178.8 Gy Natural BL + 89.4 Gy BL + 44.7 Gy BL
0 200
300 350 Temperature (°C)
250
400
450
8000 (b) Thermoluminescence (c/mg)
7000 6000 5000 4000 3000 2000 1000
0
50
100
50
100
150 Dose (Gy)
200
250
300
70 000 (c) 60 000
IRSL (c/0.1 s)
50 000 40 000 30 000 20 000 10 000
300 150 200 250 Dose (Gy) Figure 1. Luminescence results obtained for the Basaansklip aeolianite. (a) Natural, bleached and regenerated TL glow curves for quartz grains using a heating rate of 2·7C/s. Each curve is the average of five measurements. (b) Quartz TL growth curve showing individual measurements of the signal integrated between 260 and 400C. (c) IRSL growth curve for potassium-rich feldspars, constructed using five discs which are each given three successive beta doses. All data points are shown normalized to the natural IRSL. 0
732
J. C. Vogel et al. Table 1. Dose and dose rate information for age calculations Bulk analyses Accumulated dose U De (Gy) (ppm) Nahoon Point TL IRSL Ä Wilderness 20 TL IRSL Ä Blombos 2 TL IRSL Ä Basaansklip D9 TL IRSL Ä Rose Cottage Cave 4 TL IRSL Ä
Dose rate
Th (ppm)
K (%)
External (ìGy/a)
Internal (ìGy/a)
Total (ìGy/a)
Age (ka)
103·53·2 147·82·6 44·34·1
0·83
1·46
0·01
43919 43919 0
0 58815 58815
43919 102751 58815
23612 1447·5 75·49·4
80·61·8 131·73·3 51·13·7
0·60
2·34
0·01
50319 50319 0
0 76084 76084
50319 126382 76084
1607 1047 67·38·9
71·61·5 149·65·1 785·3
2·24
1·15
0·10
82133 82133 0
0 76051 76051
82133 158158 76051
87·23·9 94·44·7 1039·8
100·33·1 175·03·4 74·74·6
0·57
2·50
0·44
84344 84344 0
0 64198 64198
84344 1484102 64198
1197 1188 11719
25·71·2 29·50·7 3·821·4
1·01
3·87
1·37
193664 193664 0
0 34179 34179
193664 227772 34179
13·30·8 12·90·5 11·24·9
1. Dose rates derived from thick source alpha counting and XRF analysis of the potassium content, using conversion factors of Nambi & Aitken (1986). 2. Water content of 82% (wt water/ wt wet sediment) assumed for all samples, except Rose Cottage Cave for which 102% was used. 3. Cosmic dose rate calculated from burial depth using bulk density of 1·5 g cm 3 and knowledge of overburden (Prescott & Hutton, 1994).
1039·8 ka. This implies that the dose rate measured today is higher than the long-term average. Samples lower in the section show an increase in dose rate with depth due to the uptake of uranium from percolating ground water. All three dates for the Basaansklip sample are so similar that a closed system is indicated. Besides the data set for the coastal dune sands, Table 1 contains the same information on a sample from Rose Cottage Cave. In this case the TL and IRSL ages of 13·3 and 12·9 ka are somewhat older than the calibrated radiocarbon age of 10·5 ka, while the subtraction age is 11·24·9 ka. Unfortunately, the subtraction technique always results in larger uncertainties in the calculated age. The subtraction dates can be compared with other chronological information. For the first two samples, their location suggests formation during the onset of a marine regression. Within the error limits, an age relating to the oxygen isotope stage 5a/4 boundary seems indicated. The subtraction date of 1039·8 ka for the sand from the uppermost MSA level at Blombos can be correlated with an earlier part of stage 5, namely substage 5c. At this time the sea was no more than 100 m from the cave (Henshilwood & Sealy, 1997) and the exposed shore would have provided a source for the grains found in the deposit. At Basaansklip the age of 118 ka indicates a Last Interglacial (5e) date for sand accumulation.
Conclusions The results obtained on sand samples from four coastal sites in South Africa demonstrate the applicability of a subtraction technique using luminescence signals from both quartz and potassium feldspar grains. The technique totally avoids problems resulting from uncertainty in environmental dose rate due to changes in radioactivity and water content, calcite precipitation and removal of fine grains by percolating ground water. Measurement of luminescence signals from two different minerals is imperative when there is any possibility of changes in dose rate through time. If the dose rate has not changed with time, the same ages will be obtained for quartz and feldspar, and the subtraction age will automatically give the same result. The subtraction technique is appropriate when the internal dose rate makes up a substantial portion (>40%) of the total dose rate to the potassium feldspars, as measured in the laboratory today. This technique can be improved by selecting paired samples in different grain sizes, an option available for coastal sands. This would permit construction of an isochron plot (line of equal age), analogous to that suggested by Mejdahl (1983) for Viking pottery. The slope of a plot of the difference in De versus the internal dose rate for the feldspars defines the age; in addition, a non-zero intercept on the dose axis would
FOCUS: Luminescence Dating of Coastal Sands 733
indicate insufficient initial bleaching of the quartz grains and could enable the elimination of a further uncertainty in luminescence dating.
Acknowledgements We would like to thank Dave Roberts, Werner Illenberger, Chris Henshilwood, Hymne Loubser and Lyn Wadley for providing the samples used in this study. We are greatly indebted to Gill Collett for her meticulous preparation and measurement of these samples. We wish to thank the FRD for financial support, including a travel grant to AGW.
References Deacon, H. J. (1966). The dating of the Nahoon footprints. South African Journal of Science 62, 111–113. Duller, G. A. T. (1991). Equivalent dose determination using single aliquots. Nuclear Tracks and Radiation Measurements 18, 371– 378. Feathers, J. K. (1997). Luminescence dating of sediment samples from White Paintings Rockshelter, Botswana. Quaternary Science Reviews (Quaternary Geochronology) 16, 321–331. Henshilwood, C. & Sealy, J. (1997). Bone artefacts from the Middle Stone Age at Blombos Cave, southern Cape, South Africa. Current Anthropology 38, 890–895.
Illenberger, W. K. (1996). The geomorphological evolution of the Wilderness dune cordons, South Africa. Quaternary International 33, 11–20. McKerrell, H. & Mejdahl, V. (1981). Progress and problems with automated TL dating. Proceedings of the 16th International Symposium of Archaeometry, National Museum of Antiquities of Scotland, Edinburgh, and Risø Report M-2265, 36 pp. Mejdahl, V. (1983). Feldspar inclusion dating of ceramics and burnt stones. PACT (Journal of the European Study Group on Physical, Chemical and Mathematical Techniques Applied to Archaeology, Strasbourg) 9, 351–364. Mercier, N., Valladas, H., Joron, J. L., Schiegl, S., Bar Yosef, O. & Weiner, S. (1995). Thermoluminescence dating and the problem of geochemical evolution of sediments—a case study: the Mousterian levels at Hayonim. Israel Journal of Chemistry 35, 137–141. Mountain, E. D. (1966). Footprints in calcareous sandstone at Nahoon Point. South African Journal of Science 62, 103–111. Nambi, K. S. V. & Aitken, M. J. (1986). Annual dose conversion factors for TL and ESR dating. Archaeometry 28, 202–205. Olley, J. M., Roberts, R. G. & Murray, A. S. (1997). Disequilibria in the uranium decay series in sedimentary deposits at Allen’s Cave, Nullarbor Plain, Australia: implications for dose rate determinations. Radiation Measurements 27, 433–443. Prescott, J. R. & Hutton, J. T. (1994). Cosmic ray and gamma ray dosimetry for TL and ESR. Nuclear Tracks and Radiation Measurements 14, 223–227. Wintle, A. G. (1997). Luminescence dating: laboratory procedures and protocols. Radiation Measurements 27, 769–817. Woodborne, S. & Vogel, J. C. (1997). Luminescence dating at Rose Cottage Cave: a progress report. South African Journal of Science 93, 476–478.
FOCUS: Luminescence Dating of Coastal Sands: Overcoming Changes in Environmental Dose Rate J. C. Vogel Quaternary Dating Research Unit, CSIR, P.O. Box 395, Pretoria, South Africa
A. G Wintle Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, U.K.
S. M. Woodborne Quaternary Dating Research Unit, CSIR, P.O. Box 395, Pretoria, South Africa (Received 1 February 1999, revised manuscript accepted 26 March 1999) The luminescence dating of Pleistocene deposits produces erroneous ages if the radioactivity of the surroundings has changed over time. A subtraction procedure is described by which this problem can be overcome, if both the quartz and potassium feldspar minerals are analysed separately. The method is best used in cases where the external radiation is similar in magnitude to the internal dose rate of the feldspar. This approach has been applied to four sand deposits on the coast of South Africa that date to the beginning of the Late Pleistocene. 1999 Academic Press Keywords: LUMINESCENCE DATING, COASTAL SANDS, QUARTZ, FELDSPAR.
Introduction
1995), when moisture percolates through the deposit. To a lesser degree, the radiation experienced by the grains is affected by the amount of water in the deposit which could have varied considerably in the past. We intend to show that the problem of changing dose rates can, under certain circumstances, be overcome by separately analysing both the quartz and potassium feldspar minerals from the same sediment sample. Pure quartz grains contain negligible internal radioactivity and the radiation they receive is solely from sources external to the grains. Feldspar grains receive additional radiation from the radioactive potassium within the mineral itself. By subtracting the external dose measured on the quartz from the total dose measured for the feldspar, the accumulated dose from the internal potassium radiation is obtained; this source of radiation does not change over time. Combining this with the internal radiation dose rate of the potassium in the feldspars gives the true age since last exposure to sunlight. A subtraction procedure for TL signals from quartz and alkali feldspars extracted from pottery was previously proposed by McKerrell & Mejdahl in 1976 and published 5 years later (1981). It was not adopted since adequate amounts of large grains were not available from typical pottery and the external dose rates were
L
uminescence dating techniques are being used increasingly for the dating of archaeological sites that are beyond the range of radiocarbon dating. Early studies involved thermoluminescence (TL) dating of pottery, and more recently it has been applied to burned flint (Mercier et al., 1995) and cave deposits (Woodborne & Vogel, 1997; Feathers, 1997). The technique is based on the fact that the radiation from radioactivity in the sediment produces excitation of electrons in the mineral grains, and that the build-up of these excited electrons over time can be measured by appropriate stimulation of the minerals in the laboratory (Wintle, 1997). The accumulated radiation dose that the grains have received, De, divided by the average mineral dose rate gives the time elapsed since the sediment was laid down. Any previously acquired excited electrons are removed by exposure to sunlight during deposition, especially in the case of aeolian sediments. A problem with this dating technique is that in geochemically active environments the radioactivity of the sediment may have changed, due mainly to the potential mobility of the radioactive elements, uranium (Olley et al., 1997) and potassium (Mercier et al., 729 0305–4403/99/070729+05 $30.00/0
1999 Academic Press
730
J. C. Vogel et al.
generally quite high. This approach can, however, be used effectively for dating deposits in cases where the external radiation dose rate is relatively small and comparable with the internal dose rate of the potassium feldspar.
Material and Methods The samples analysed were sand deposits collected along the coast of South Africa. In all cases the feldspar content was very low, but sufficient amounts could be recovered for single aliquot IRSL regeneration analysis. The first sample comes from Nahoon Point near East London. Here the coastal aeolian sand dunes consolidated into a calcarenite through the reprecipitation of the carbonate that is ubiquitous in sands of marine origin. The calcarenite formation has subsequently been eroded by wave action to produce cliffs that reveal a section through the deposit. This exposure has previously been studied by Mountain (1966) and a radiocarbon date on shell fragments of 29 ka (SR-83) was reported by Deacon (1966). A large block of the sediment was transported intact to the Pretoria laboratory for luminescence analysis; it has a carbonate content of 51%. The second sample was collected from a near-coastal dune in the Wilderness area of the southern Cape (Illenberger, 1996). The dune is well vegetated and it forms a linear barrier between the present coastline and a series of coastal lakes. The sample was collected with a coring device and transported to the laboratory inside the coring tube to avoid any risk of exposure to light. The sample contained 14% carbonate and was not cemented. The third sample was taken from a Middle Stone Age occupation level of an archaeological site called Blombos Cave. The site is located in a sea cliff on the southern Cape coast near the town of Still Bay. Radiocarbon dating of charcoal and shell had yielded ages close to 40 ka for the upper Middle Stone Age (MSA) levels and these dates have been interpreted as being minimum ages (Henshilwood & Sealy, 1997). The material for luminescence dating was collected in a coring tube from the uppermost MSA level in the cave. There is evidence of water percolation through the cave deposits which could result in uranium mobilization. The carbonate content was 48%, but the sediment was not cemented. The fourth sample was sandy infill from an ancient hyaena lair that is the subject of an ongoing palaeontological investigation. The burrow is in a sand deposit on the lee side of a granite outcrop called Basaansklip, north of the town of Saldanha Bay on the Cape west coast. The carbonate content of 62% indicates that the sand is of marine origin, with the most likely source being Saldanha Bay several kilometres away. The walls of the lair were cemented, probably before it was filled.
The infill is not consolidated and there is no evidence of further carbonate migration. In addition, a sample from Rose Cottage Cave near Ladybrand has been included for comparison. It contains 2% carbonate and has an associated calibrated radiocarbon age of 10·5 ka (Woodborne & Vogel, 1997). Laboratory procedures included the isolation of quartz and potassium-rich feldspar grains by magnetic separation and flotation in heavy liquid at specific gravities of 2·58 and 2·62. The grain size used was selected from within the range of 150 to 300 ìm. Neither of the minerals were treated with hydrofluoric acid, which is usually used to remove the small contribution of the alpha particles to the luminescence signal. The TL signal of the quartz was used to derive its accumulated dose, while that of the feldspar was obtained by measuring the infra-red stimulated luminescence (IRSL) using the regeneration method. In both cases, the grains were bleached for 40 h (or more) in bright sunlight and were preheated after each beta irradiation for 16 h at 150C. An IRSL single aliquot approach, similar to that of Duller (1991), was used with a 0·4 s IRSL measurement, with correction being applied for the cumulative effects of successive preheats on each regenerated signal. For these samples the use of a sunlight bleach, rather than an infra-red bleach as used by Duller (1991), did not cause a sensitivity change. Figure 1 shows the results obtained for the Basaansklip sediment. The other samples produced similar curves. No significant anomalous fading was detected in the regenerated feldspar after 4 to 13 months.
Results and Discussion The analytical data pertaining to the samples analysed are given in Table 1. The calculated apparent ages for the quartz and feldspar grains from the first two samples differ considerably. It is unlikely that the difference is due to insufficient bleaching of the quartz because the grains were derived from the nearby beach and thus would have received sufficient exposure to sunlight before deposition. Instead it can be interpreted as being due to the external dose rate having been higher in the past than it is today. This could be the result of either the removal of radioactivity by percolating water or the recent precipitation of carbonate which dilutes the external dose rate. The depositional ages of 75·49·5 and 67·38·9 ka obtained on the near-coastal dunes by the subtraction technique are independent of changes in the external dose rate, and as expected are considerably younger than both the individual ages. The TL and IRSL ages of the other two samples are similar, but in the case of the Blombos sample, the subtraction technique gives a slightly older age of
FOCUS: Luminescence Dating of Coastal Sands 731 200
Thermoluminescence (c/s/mg)
(a)
150
100
50
BL + 268.2 Gy BL + 178.8 Gy Natural BL + 89.4 Gy BL + 44.7 Gy BL
0 200
300 350 Temperature (°C)
250
400
450
8000 (b) Thermoluminescence (c/mg)
7000 6000 5000 4000 3000 2000 1000
0
50
100
50
100
150 Dose (Gy)
200
250
300
70 000 (c) 60 000
IRSL (c/0.1 s)
50 000 40 000 30 000 20 000 10 000
300 150 200 250 Dose (Gy) Figure 1. Luminescence results obtained for the Basaansklip aeolianite. (a) Natural, bleached and regenerated TL glow curves for quartz grains using a heating rate of 2·7C/s. Each curve is the average of five measurements. (b) Quartz TL growth curve showing individual measurements of the signal integrated between 260 and 400C. (c) IRSL growth curve for potassium-rich feldspars, constructed using five discs which are each given three successive beta doses. All data points are shown normalized to the natural IRSL. 0
732
J. C. Vogel et al. Table 1. Dose and dose rate information for age calculations Bulk analyses Accumulated dose U De (Gy) (ppm) Nahoon Point TL IRSL Ä Wilderness 20 TL IRSL Ä Blombos 2 TL IRSL Ä Basaansklip D9 TL IRSL Ä Rose Cottage Cave 4 TL IRSL Ä
Dose rate
Th (ppm)
K (%)
External (ìGy/a)
Internal (ìGy/a)
Total (ìGy/a)
Age (ka)
103·53·2 147·82·6 44·34·1
0·83
1·46
0·01
43919 43919 0
0 58815 58815
43919 102751 58815
23612 1447·5 75·49·4
80·61·8 131·73·3 51·13·7
0·60
2·34
0·01
50319 50319 0
0 76084 76084
50319 126382 76084
1607 1047 67·38·9
71·61·5 149·65·1 785·3
2·24
1·15
0·10
82133 82133 0
0 76051 76051
82133 158158 76051
87·23·9 94·44·7 1039·8
100·33·1 175·03·4 74·74·6
0·57
2·50
0·44
84344 84344 0
0 64198 64198
84344 1484102 64198
1197 1188 11719
25·71·2 29·50·7 3·821·4
1·01
3·87
1·37
193664 193664 0
0 34179 34179
193664 227772 34179
13·30·8 12·90·5 11·24·9
1. Dose rates derived from thick source alpha counting and XRF analysis of the potassium content, using conversion factors of Nambi & Aitken (1986). 2. Water content of 82% (wt water/ wt wet sediment) assumed for all samples, except Rose Cottage Cave for which 102% was used. 3. Cosmic dose rate calculated from burial depth using bulk density of 1·5 g cm 3 and knowledge of overburden (Prescott & Hutton, 1994).
1039·8 ka. This implies that the dose rate measured today is higher than the long-term average. Samples lower in the section show an increase in dose rate with depth due to the uptake of uranium from percolating ground water. All three dates for the Basaansklip sample are so similar that a closed system is indicated. Besides the data set for the coastal dune sands, Table 1 contains the same information on a sample from Rose Cottage Cave. In this case the TL and IRSL ages of 13·3 and 12·9 ka are somewhat older than the calibrated radiocarbon age of 10·5 ka, while the subtraction age is 11·24·9 ka. Unfortunately, the subtraction technique always results in larger uncertainties in the calculated age. The subtraction dates can be compared with other chronological information. For the first two samples, their location suggests formation during the onset of a marine regression. Within the error limits, an age relating to the oxygen isotope stage 5a/4 boundary seems indicated. The subtraction date of 1039·8 ka for the sand from the uppermost MSA level at Blombos can be correlated with an earlier part of stage 5, namely substage 5c. At this time the sea was no more than 100 m from the cave (Henshilwood & Sealy, 1997) and the exposed shore would have provided a source for the grains found in the deposit. At Basaansklip the age of 118 ka indicates a Last Interglacial (5e) date for sand accumulation.
Conclusions The results obtained on sand samples from four coastal sites in South Africa demonstrate the applicability of a subtraction technique using luminescence signals from both quartz and potassium feldspar grains. The technique totally avoids problems resulting from uncertainty in environmental dose rate due to changes in radioactivity and water content, calcite precipitation and removal of fine grains by percolating ground water. Measurement of luminescence signals from two different minerals is imperative when there is any possibility of changes in dose rate through time. If the dose rate has not changed with time, the same ages will be obtained for quartz and feldspar, and the subtraction age will automatically give the same result. The subtraction technique is appropriate when the internal dose rate makes up a substantial portion (>40%) of the total dose rate to the potassium feldspars, as measured in the laboratory today. This technique can be improved by selecting paired samples in different grain sizes, an option available for coastal sands. This would permit construction of an isochron plot (line of equal age), analogous to that suggested by Mejdahl (1983) for Viking pottery. The slope of a plot of the difference in De versus the internal dose rate for the feldspars defines the age; in addition, a non-zero intercept on the dose axis would
FOCUS: Luminescence Dating of Coastal Sands 733
indicate insufficient initial bleaching of the quartz grains and could enable the elimination of a further uncertainty in luminescence dating.
Acknowledgements We would like to thank Dave Roberts, Werner Illenberger, Chris Henshilwood, Hymne Loubser and Lyn Wadley for providing the samples used in this study. We are greatly indebted to Gill Collett for her meticulous preparation and measurement of these samples. We wish to thank the FRD for financial support, including a travel grant to AGW.
References Deacon, H. J. (1966). The dating of the Nahoon footprints. South African Journal of Science 62, 111–113. Duller, G. A. T. (1991). Equivalent dose determination using single aliquots. Nuclear Tracks and Radiation Measurements 18, 371– 378. Feathers, J. K. (1997). Luminescence dating of sediment samples from White Paintings Rockshelter, Botswana. Quaternary Science Reviews (Quaternary Geochronology) 16, 321–331. Henshilwood, C. & Sealy, J. (1997). Bone artefacts from the Middle Stone Age at Blombos Cave, southern Cape, South Africa. Current Anthropology 38, 890–895.
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