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Luminescence characterisation of quartz-rich cover sands from NE Thailand
Quaternary Science Reviews 20 (2001) 893}900
Luminescence characterisation of quartz-rich cover sands from NE Thailand夽 D.C.W. Sanderson *, P. Bishop, I. Houston , M. Boonsener Scottish Universities Research and Reactor Centre, East Kilbride, Scotland G75 OQF, UK Department of Geography & Topographic Science, University of Glasgow G12 8QQ, UK Department of Geotechnology, Khon Kaen University, Thailand
Abstract The genesis of a several-meter-thick cover sand layer, which is widely distributed in mainland SE Asia has been the subject of much debate, being considered by some to be an aeolian mantle of late Pleistocene/early Holocene origin, and by others to represent a Holocene biomantle. Samples were collected from three sites in NW Thailand in 1998 to assess the potential of luminescence studies for characterisation and chronometric studies of the layer. The material is a quartz-rich, iron-stained sand, with high-luminescence sensitivities, a pronounced 3253C TL peak, and high sensitivity to phototransfer and OSL. A number of methods for estimating stored dose were assessed, of which additive TL and regenerative OSL procedures produced good laboratory results.These methods have been used to produce stored dose and apparent age pro"les for one of the sites. The data are consistent with a late Pleistocene/Holocene aeolian deposition process, possibly modi"ed in the upper layers by bioturbation. However, the possibility that the recorded pro"les are the result of heterogeneous mixing of bleached and unbleached grains within the biomantle model cannot at this stage be excluded. The material has excellent luminescence properties, and it is likely that further luminescence studies will be able to distinguish between the formation models, and contribute signi"cantly to understanding of these regionally signi"cant sediments. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Three sites in NE Thailand exhibiting 2}3 m deep cover sands were sampled in 1998 by Paul Bishop with the assistance of Dr. Montree Boonsener, to investigate the potential for using luminescence to evaluate formation hypotheses and rates of sediment turnover resulting from bioturbation. This paper presents an overview of the luminescence characteristics of the material, a discussion of luminescence pro"les from two sites, and constraints for formation models derived from the luminescence results.
2. Background A conspicuous feature of the sur"cial geology of continental and insular southeast Asia is a surface mantle or &cover layer' of generally sandy material which 夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (D.C.W. Sanderson).
may be up to 5 m or more in thickness. The layer is reported to be extensively distributed throughout Vietnam and in Cambodia, in NE and N Thailand; it is also reported from the uplands of Malaysia and Myanmar, and even from the Punjab in India (Hoang Ngoc Ky., 1989, 1994). Over the last decade or so, the characteristics and genesis of the cover layer, particularly in Vietnam and NE Thailand, have received signi"cant attention in the local Quaternary geological literature. Models range from interpretation as aeolian (loess-like) mantle of Late Pleistocene to Holocene age (Boonsener and Tassanasorn, 1983; Sonsuk and Hastings, 1984; Boonsener, 1987, 1991; Hoang Ngoc Ky., 1989, 1994; Udomchoke, 1989; S[ ibrava, 1993) to lacustrine (Dheeradilok, 1987), marine (Nguyen Duc Tam, 1994), and #uvial (De Dapper, 1987) origins. Bioturbational processes are also relevant (cf. Williams, 1978; Bishop et al., 1980; Johnson, 1993). Indeed, LoK %er and Kubiniok (1991, 1996) have argued that bioturbation is the key to the genesis of the cover layer, which they interpret as a Late Tertiary &biomantle', emplaced as a result of termite activity and subsequent degradation of termite mounds.
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 4 - 7
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This range of interpretations, the regional extent of the cover layer and its potential for palaeoenvironmental interpretations and for intra- and inter-regional correlations mean that clari"cation of the cover layer's age and mode of emplacement is a signi"cant issue (S[ ibrava, 1993). In this paper, we provide a preliminary report on the luminescence characteristics of the cover layer in NE Thailand with a view to elucidating the emplacement and, if possible, the age of the unit.
3. Field studies and sampling The "eld sites are located in the Khon Kaen area in NE Thailand (Boonsener, 1987, 1991, Boonsener and Tassanasorn, 1983), an undulating, low-elevation plateau characterised by a very dry cool season, followed by a very hot and dry season prior to the wet season. Samples of the cover layer were collected in a large gravel quarry at Ban Phang, on the northern side of the Chum Phae Road, 24 km west of Khon Kaen (site 1: TL98/1), and in a small quarry on the Khon Kaen airport road in Khon Kaen (site 3:TL98/3). A small dune south of the Chum Phae Road at the western edge of Khon kaen was also sampled (site 2: TL98/2). The thicknesses of section and the number of samples collected in a vertical sequence at each site are: TL98/1: 200 cm, 6 samples; TL98/2: 120 cm, 3 samples; TL98/3: 320 cm, 8 samples. Bulk samples of sediment were collected for gamma spectrometry from all three sites. Luminescence samples were collected in 30 cm lengths of 19 mm diameter water pipe, and sealed at both ends to prevent light exposure. 3.1. Sample preparation Samples were extruded from the tubes in the laboratory under safelight conditions, and actual and saturated water contents determined. Matrix beta dose rates were measured by thick source beta counting (Sanderson, 1988). Samples were sieved, and the 90}150 lm polymineral fraction used both for initial sensitivity tests and pro"ling, and as a source for mineral separation. Mineral extracts were then obtained by sequential centrifugation in aqueous sodium polytungstate at densities of 2520, 2580, 2620, 2740 kg m\, and appropriate acid treatments. All densities were subjected to 1 M HCl for 30 min, 15% HF for 10 min, followed by repeated HCl treatment to remove any undissolved #uorides. The 2620}2740 kg m\ fraction was also etched in 42% HF for 40 min, followed by further HCl treatment. All fractions were rinsed several times in deionised water, and dried from acetone. Samples were dispensed onto 0.25 mm thick, 10 mm diameter, stainless-steel discs sprayed with Electrolube SCO200D silicone grease.
4. Measurement methods Initial measurements were undertaken to identify the mineralogy and general luminescence pro"les of the samples. Thereafter, a series of tests of dose quanti"cation procedures for TL, PTTL and OSL was conducted using the quartz fractions of the central sample from site 3. Additive TL methods and regenerative OSL methods were then selected to determine the dose and age pro"les of the sites. TL data were recorded at 53C s\ from room temperature to 5003C using an SURRC TL reader with 7/59 and KG3 "lters. Samples were irradiated using a 1.85 GBq 90Sr source at a rate of 3.3 Gy/min\. PTTL measurements were conducted using a simple cryogenic approach. Samples on disc were placed on copper blocks, cooled to !553C using a dry-ice/acetone mixture, and covered by a 5 mm BG39 "lter. Murray (1996) noted the strong relationship between OSL and phototransferred TL (PTTL) in quartzes from young river sediments, and suggested that PTTL to the 1103C TL peak might be an e!ective alternative to OSL for dating such sediments. However, the short mean life at room temperature for the acceptor peak introduces practical constraints. As noted elsewhere (Anthony et al., 2001) we have overcome this in a simple manner, which could be readily implemented in laboratories with single-sample TL equipment and no OSL facilities. Phototransfer was achieved by illumination in an arti"cial daylight lightbox giving a full spectral output of 7 mW cm\ followed by TL readout to 2003C. The area in photon counts of the 1103C TL peak was used as a measure of PTTL. This procedure is extremely rapid and simple. IRSL measurements were conducted using a pulsed IR stimulation system originally developed for detection of irradiated foods (Sanderson et al., 1995). Samples coated on 50 mm diameter petri dishes, were stimulated with 180 mW of IR at 880$40 nm with a pulse frequency of 33 kHz, and luminescence detected synchronously using a 5 mm BG39 detection "lter. OSL measurements were conducted using a system based on 9 GaN blue LED's behind GG420 long pass "lters, and a 9883QB PMT with 5 mm UG11. Stimulation was again pulsed at 33 kHz and luminescence detected by synchronous photon counting, samples being measured on disc on a standard TL heater plate. The decay curve can be well-"tted by two exponential components and a constant. Fig. 1 shows examples of TL, PTTL and OSL data recorded from one of the Thai samples using these techniques. 4.1. Additive and regenerative dose determination approaches Dose quanti"cation was investigated by both additive and regenerative approaches. Validation tests were
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Fig. 1. Examples of (a) TL, (b) PTTL and (c) OSL decay curves from sample TL480, site 3. In (b) curve 1 is the phototransferred signal into the 1103C trap following illumination with arti"cial daylight through a 5 mm BG 39 "lter. Curve 2 is the TL recorded prior to tranfer, and curve 3 is the residual signal following complete transfer
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and Wintle, 1998). Additive dose determinations were carried out using the same irradiation sequence as the TL run described, followed by a 2003C ramp at 53C s\, 900 s phototransfer at !553C, TL readout of the 1103C signal, and thermal resetting of the system during subsequent TL ramp to 5003C. The net area of the 1103C was used for dose estimation. OSL additive runs were conducted again following the irradiation sequence described above, followed by 600 s pre-heating at 2203C, and 100 s stimulation at ambient temperature. Samples were bleached for 300 s in arti"cial daylight following OSL readout, to remove residual signals prior to the next cycle. OSL decay curves were analysed over di!erent decay times, and also with or without `late lighta subtraction; integrated data again being analysed using exponential "tting procedures. Regenerative measurements were made using a procedure based on that suggested by Murray and Roberts (1998), using the OSL test dose}response to monitor sensitivity changes during laboratory measurements. Samples were subjected to a sequence of pre-heating, readout, bleaching (to remove any residual OSL), and irradiation. Readings from `naturala and regenerative irradiations were interleaved by readings of test-dose response to monitor sensitivity changes. Samples were pre-heated at 2603C for 30 s prior to stimulation at 1203C for 100 s per measurement. Bleaching between cycles was conducted for 30 min in an arti"cial daylight lightbox at 7 mW cm\. Stored doses were determined by interpolating natural intensities onto a regenerative OSL dose response plot, all signals being normalised to the subsequent test dose response. Exponential "tting was used as the basis for interpolation.
5. Results conducted using samples from the middle of site 3. Discs of etched quartz from sample TL480, site 3, were bleached out and then irradiated to a known dose of 10 Gy, stored for 24 h in darkness, and then treated as an unknown sample. Equal pre-dose additive measurements using eight dose points from 3 to 24 Gy were implemented for TL, PTTL and OSL methods. Sixteen discs per run were used, with added doses applied in ascending order prior to "rst glow, in descending order prior to the second glow, and dose normalisation based on the response to a "xed 5 Gy dose in third glow. Samples were pre-heated for 300 s at 2103C prior to TL readout, and the main high-temperature peak (typically observed at around 3253C), integrated to construct dose response curves. The dependence of PTTL on illumination time was "rst investigated, and shown to be a multipleexponential function of time (Houston, 1999). It is interesting to note that this is the integral form of the multiple-exponential OSL decays curves observed by several authors (Bailey et al., 1997; Bailey, 1998; Murray
5.1. Initial investigations Initial investigations con"rmed that the material comprises coarse-grained iron-stained quartz with a "ne clay wash. TL glow curves from bulk samples and separated quartz showed predominantly quartz-derived signals, with pronounced 3253C peaks in both natural and laboratory-induced signals, and well-developed lowtemperature peaks including the 1103C signal. High phototransfer and OSL sensitivities were also observed. Interestingly only modest pre-dose sensitisation occured on heating to 5003C, which in combination with a high sensitivity at 1103C, implies that the sample had undergone environmental activation of the pre-dose sensitivity enhancement due to the natural signal, perhaps in the manner suggested by Wintle and Murray (1999). IRSL sensitivity from bulk material was low, but nonetheless measureable from large area samples. Fig. 2 shows depth pro"les for TL, PTTL, IRSL and matrix beta dose rate
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Fig. 2. Initial luminescence and beta dose rate pro"les from site 1.
for site 1. Both TL and PTTL pro"les are similar for both sites, and show progression with depth. By contrast IRSL response } presumed to be derived from the "ne clay wash } shows low intensities and, for site 3 particularly, a subsurface maximum, perhaps implying that weathering in deeper layers limits signal accumulation. Nonuniformities of beta activity were also observed. Given the nature of the material it was decided to investigate dose quanti"cation procedures appropriate to quartz, as the major component of these samples. 5.2. Validation tests for dose quantixcation Fig. 3 shows additive dose results for TL, PTTL and blue OSL, in tests with a known 10 Gy beta dose. The approach to curve "tting was to simultaneously "t both "rst and second growth lines to a coupled pair of saturating exponential functions, with common coe$cient of saturation, but variable amplitudes, determining the extrapolated dose along with the other three coe$cients by least-squares "tting. Linear "tting resulted in overestimation of the 10 Gy dose by up to 50%. There is no evidence for supralinearity in any of the regenerated data sets. It is also notable that there is more scatter in the "rst growth lines from all three techniques, when compared with second growth response. Such behaviour has been observed frequently in TL additive dating runs (e.g. Scott and Sanderson, 1988) where it has been assumed that the excess variation in natural plus added dose response is due to a combination of heterogeneity in the natural radiation "eld and spurious TL associated with the "rst heating of the sample. It is therefore interesting to see the same phenomenon in TL, PTTL and OSL from quartz which had been given a simple laboratory beta dose merely 24 h before conducting the measurements. Neither of the conventional explanations apply to these data,
Fig. 3. Additive dose results with a known 10 Gy dose by TL, PTTL and OSL. Both "rst- and second}dose}response curves were "tted simultaneously to a pair of saturating exponentials with a common coe$cient of saturation, and variable amplitudes, the stored dose being determined as the o!set between the two lines.
since the dose distribution should be identical between "rst and second glows, and it is unlikely that `spuriousa emission would a!ect the optical measurements. Additive TL runs recovered the expected 10 Gy dose within measurement errors (results 9.0$2, 9.2$2 Gy), but in the experiments conducted the PTTL data produced underestimation (5.7$2 Gy), and the OSL additive data overestimated dose (15.1$2). Analysis of the dependence of the OSL result on decay time (Houston, 1999) showed that while the extrapolated result did depend slightly on the part of the decay curve taken, all periods resulted in overestimation of the stored dose. Regenerative OSL data produced highly coherent data sets, again with a saturating exponential dose response, as illustrated in Fig. 4. The OSL were analysed with and without late light subtraction (cf. Aitken, 1998), and also with and without corrections for a small sensitivity change in the instrument. In all cases the results were consistent with the expected dose. Including the minor sensitivity corrections, the results were 10.4$0.5 without late light subtraction, and 9.8$0.5 Gy with late light subtraction, highly consistent with the known 10 Gy dose. 5.3. Dose proxles and apparent ages for site 1 Both additive TL and regenerative OSL procedures were applied to the six samples from site 1 to investigate stored dose pro"les and the relationship between
D.C.W. Sanderson et al. / Quaternary Science Reviews 20 (2001) 893}900
TL and OSL doses and apparent ages as a function of depth. Sets of 16 discs were used for TL determinations, with eight pairs of dose points from 3 to 24 Gy. The natural doses for the four deepest samples exceed the maximum dose added (24 Gy). This results in poorly constrained exponential "ts, and therefore } notwithstanding the conclusions of the previous section } the linear approximation to the dose}response curve was used for dose estimation. It is possible that more precise results could be obtained using a larger range of added dose (e.g. up to 48 Gy, with exponential regression). However, the dose estimates obtained are believed to be adequate for the initial discussion. OSL regenerative runs followed the procedure outlined in the previous section. Pre-heating at 2603C for 30 s and stimulation at 1203C were used for OSL readout. A test dose of 1 Gy test was used, and normalised dose}response curves constructed from 2 to 50 Gy in six steps. Four discs per sample were measured. The extent to which individual dose}response curves conformed to the average dose}response curve from all samples was examined. Despite the overall similarity, the
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dose-depth pro"le obtained using individual curves for each sample was more coherent than that obtained from a mean curve, suggesting that there are minor mineralogical di!erences between samples originating from di!erent depths. Dose estimates and apparent ages by TL and OSL are given in Table 1, and Fig. 5. Close to the surface the TL estimates are signi"cantly greater than OSL estimates, suggesting that surface layers may be partially bleached, although the possibility of extrapolation errors in the TL data should also be borne in mind. Below 90 cm depth, however, both data sets are consistent with each other, perhaps suggesting that bleaching is more homogeneous at lower levels. The OSL results generally show greater coherence with depth, as a result of the higher precision obtainable by regenerative procedures. The apparent age range of the section is from approximately 8}40 ka. Whether these results can be considered to represent accurate dates is dependent on the extent to which the formation processes have resulted in meaningful bleaching events for either TL or OSL. This is discussed further in the next section. It is however of interest to examine the net accumulation rates implied by the luminescence ages, since these will depend more on the relative constancy of bleaching conditions, rather than whether bleaching could be considered to be complete. These are shown in Table 2, from which it can be seen that whereas the surface layers have the lowest accumulation rates, possibly re#ecting partial bleaching of the upper layers, particularly for TL, relatively constant rates are observed for the majority of depth intervals, of approximately 4 cm ka\. Exceptions to this however are in the 21}46 and 120}156 cm depth layers where signi"cantly higher accumulation rates are inferred.
6. Discussion and conclusions
Fig. 4. Regenerative OSL dose}response normalised to a 1 Gy test dose}response. The normalised response from a known 10 Gy dose}interpolates to the correct value.
The cover sand layers in NE Thailand have quartzdominated luminescence characteristics, with high sensitivity to phototransfer and OSL, providing a promising tool for palaeoenvironmental tracer work in the region.
Table 1 Stored dose and apparent age estimates for site 1 by TL and OSL TL
OSL
Depth (Z/cm)
Dose rate (mGy a\)
Stored dose (Gy)
Apparent age (ka)
Stored dose (Gy)
Apparent age (ka)
21 46 90 120 156 200
0.88$0.1 0.87$0.09 0.94$0.09 1.04$0.1 0.96$0.1 0.98$0.1
12.1$1 13.5$1 23.2$2 32.6$3 27.9$3 38.4$4
14.5$1.6 15.5$1.5 24.6$2.5 31.3$3 29.0$3 39.2$4
7.8$.5 12.3$.5 20.1$1 27.3$1.5 33.3$1.5 34.7$2
8.9$0.9 14.1$1 20.5$2 6.5$2.5 34.7$3 35.4$2
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Dose quanti"cation studies have shown the material to have well-reproduced luminescence dose}response characteristics with a saturating exponential form. Additive TL procedures and regenerative OSL procedures produced excellent results in tests to recover a known laboratory dose, but under the conditions investigated this was not so for additive PTTL or additive OSL procedures. The reasons for this are not yet clear, and therefore more work would be needed to apply PTTL and OSL additive procedures with con"dence to this material. It is possible that the discrepencies are an artefact of the thermal treatments used in the procedure, as argued by Houston (1999), in which case it may be possible to obtain accurate results by varying the preheating regimes. However, sensitivity to pre-heating raises questions about the interactions between the di!erent centres in quartz, and the extent to which laboratory irradiation and thermal treatment can realistically simulate the natural dose cycle. Another explanation of these results is that they are a manifestation of the di$culties of making accurate extrapolations to non-linear dose}response functions, compounded by the excess variance
Fig. 5. TL and OSL stored dose pro"les for site 1.
observed in "rst dose}response compared with second dose}response. Moreover, as noted above this excess variation cannot be explained simply in terms of heterogeneous dose distributions, or in terms of spurious luminescence. It is perhaps suggestive of a re-arrangement of the trapped charge distribution immediately following irradiation, but in a manner which is heterogeneously distributed between the grains making up the sample. Whether this is a simple thermal process, or involves tunneling mechanisms, or even re-arrangement of clusters of centres within the quartz following irradiation is at this stage unclear. The overall performance of added dose determinations on the other hand is good in comparison with other quartz samples which we have studied, suggesting a reasonably homogeneous natural dose distribution within the grains which contribute to the TL signal. The regenerative OSL results were very encouraging. Precision is signi"cantly better than obtained with additive approaches. The results from tests with known laboratory doses were excellent, as was the agreement between additive TL and regenerative OSL-stored dose estimates from in site 1. The procedure used is relatively easy to implement. Providing that the sensitivity changes which can be monitored, for all cycles following readout of the natural signal,are also representative of the natural dose cycle, the technique should yield results which are both precise and accurate. In a general sense this will depend on the timing of sensitivity changes and their dependence on preheating (Stokes, 1994; McKeever et al., 1997; Wintle and Murray, 1998), which are potentially variable between quartzes of di!erent origins. Nonetheless the procedure seems highly promising and well suited to this material. The dose pro"les, apparent ages and accumulation rates from site 1 are broadly consistent between both techniques. Whereas there are suggestions of partial bleaching in upper layers, the dose estimates further down the site, and their associated accumulation rates tell a consistent story. In the introductory section the main models for formation of these cover layers were discussed, the extreme cases being (a) a largely aeolian
Table 2 Net accumulation rates for site 1 from both TL and OSL data TL
OSL
Depth interval dZ (cm)
Age interval dt (ka)
Accumulation rate dZ/dt (cm ka\)
Age interval dt (ka)
Accumulation rate dZ/dt (cm ka\)
0}21 21}46 46}90 90}120 120}156 156}200
0}14.5 14.5}15.5 15.5}24.6 24.6}31.3 31.3}29.0 29.0}39.2
1.4 16.7 4.8 4.5 * 4.5
0}8.9 8.9}14.1 14.1}20.5 20.5}26.5 26.5}34.7 34.6}35.4
2.36 4.8 6.9 5 4.4 55
D.C.W. Sanderson et al. / Quaternary Science Reviews 20 (2001) 893}900
process which would be expected to be operating at higher rates during the colder periods of the last glacial cycle, and (b) the biogenic models which see the material as having accumulated by re-distribution processes associated with termite activity. Taken at face value the results from site 1 are what would be expected from the "rst model. Under this interpretation the similarities of stored dose between OSL and TL for lower layers in the site would be taken as an indication of good bleaching conditions, as expected from aeolian processes. The coherent dose}response curves might be regarded as an indication of homogeneous materials, and the luminescence chronology, with its implied higher rates of accumulation during the last glacial cycle could be taken as evidence in support of this model. However, under the biomantle model, the dose-depth pro"les would be taken to represent mixed distributions of unbleached and wellbleached grains within the sedimentary column, with an increasing proportion of unbleached grains at greater depth in the site. The dose pro"les and their implied chronology would be regarded as an artefact of the mixing processes. Our data so far appear to be consistent with the "rst formation model, although the divergence between TL and OSL results in the upper layer may be the result of bioturbation processes. The biomantle model would imply highly heterogeneous dose distributions from grain to grain, which would be expected to lead to poorly reproduced dose estimates. The biomantle model cannot however be ruled out at this stage. Further work will be undertaken to assess whether the luminescence properties could in fact be explained in terms of bioturbation, and to examine the single-grain characteristics of the material. In any event the luminescence properties of the cover layer are well suited to geochronological studies. Thus luminescence methods have considerable potential for in helping to understand the formation processes and chronologies of these deposits in SE Asia. Acknowledgements The authors are grateful to R. McNaughton, Dr.L.A. Carmichael and Dr. S. Fisk for their contribution to parts of the laboratory work reported here. References Aitken, M.J., 1998. An introduction to Optical Dating. Oxford University Press, Oxford. Anthony, I.M.C, Sanderson, D.C.W., Cook, G.T., Abernethy D., Housley, R.A., 2001. Dating a burnt mound from Kilmartin, Argyll, Quaternary Science Reviews (Quaternary Geochronology) 20, 921}925. Bailey, R.M., 1998. The form of the optically stimulated luminescence signal of quartz: implications for dating. Ph.D. Thesis, Royal Holloway, University of London.
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Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in quartz. Radiation Measurements 29 (1), 65}79. Nguyen Duc Tam, 1994. Review of Quaternary geological studies in Vietnam, in ESCAP Sectretariat, Quaternary Stratigraphy of Asia and the Paci"c, IGCP 296. United Nations Mineral Resources Development Series 63, 137}140. Scott, E.M., Sanderson, D.C.W., 1988. Statistics and the additive dose method in TL dating. Radiation Measurements 14, 345}354. Sanderson, D.C.W., Carmichael, L.A., Naylor, J.D., 1995. Photostimulated luminescence and thermoluminescence techniques for the detection of irradiated food. Food Science and Technology Today 9 (3), 150}154. Sanderson, D.C.W., 1988. Thick source beta counting: a rapid method for measuring beta dose rates. Radiation Measurements 14 (1-2), 203}207. S[ ibrava, V., 1993. Quaternary sequences in Southeast Asia and their regional correlation, in ESCAP Secretariat, Quaternary Stratigraphy of Asia and the Paci"c. IGCP 296. ESCAP Secretariat, Quaternary Stratigraphy of Asia and the Paci"c. IGCP 296. ESCAP Atlas of Stratigraphy XII (Mineral Resources Development Series 62), pp. 1}29.
Sonsuk, M., Hastings, P., 1984. An age for the Yasothon soils series in the Sakhon Nakhon basin. Journal of the Geological Society of Thailand VII, 1}11. Stokes, S., 1994. The timing of OSL sensitivity changes in a natural quartz. Radiation Measurements 23 (2}3), 621}624. Udomchoke, V., 1989. Quaternary stratigraphy of the Khorat Plateau area northeastern Thailand. In: Thiramongkol, N. (Ed.), Proceedings of the Workshop on Correlations of Quaternary Successions in South, East and Southeast Asia (November 21}24, 1988), International Geological Correlation Programme, Chulalongkorn University Dept of Geology, UNESCO Regional O$ce for Science and Technology SE Asia, ESCAP, 69}94. Williams, M.A.J., 1978. Termites, soils and landscape equilibrium in the Northern Territory of Australia. In: Davies, J.L., Williams, M.A.J. (Eds.), Landform Evolution in Australasia. ANU Press, Canberra, pp. 128}141. Wintle, A.G., Murray, A.S., 1998. Towards the development of a preheat procedure for OSL dating of quartz. Radiation Measurements 29 (1), 81}94. Wintle, A.G., Murray, A.S., 1999. Luminescence sensitivity changes in quartz. Radiation Measurements 30 (1), 107}118.
Luminescence characterisation of quartz-rich cover sands from NE Thailand夽 D.C.W. Sanderson *, P. Bishop, I. Houston , M. Boonsener Scottish Universities Research and Reactor Centre, East Kilbride, Scotland G75 OQF, UK Department of Geography & Topographic Science, University of Glasgow G12 8QQ, UK Department of Geotechnology, Khon Kaen University, Thailand
Abstract The genesis of a several-meter-thick cover sand layer, which is widely distributed in mainland SE Asia has been the subject of much debate, being considered by some to be an aeolian mantle of late Pleistocene/early Holocene origin, and by others to represent a Holocene biomantle. Samples were collected from three sites in NW Thailand in 1998 to assess the potential of luminescence studies for characterisation and chronometric studies of the layer. The material is a quartz-rich, iron-stained sand, with high-luminescence sensitivities, a pronounced 3253C TL peak, and high sensitivity to phototransfer and OSL. A number of methods for estimating stored dose were assessed, of which additive TL and regenerative OSL procedures produced good laboratory results.These methods have been used to produce stored dose and apparent age pro"les for one of the sites. The data are consistent with a late Pleistocene/Holocene aeolian deposition process, possibly modi"ed in the upper layers by bioturbation. However, the possibility that the recorded pro"les are the result of heterogeneous mixing of bleached and unbleached grains within the biomantle model cannot at this stage be excluded. The material has excellent luminescence properties, and it is likely that further luminescence studies will be able to distinguish between the formation models, and contribute signi"cantly to understanding of these regionally signi"cant sediments. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Three sites in NE Thailand exhibiting 2}3 m deep cover sands were sampled in 1998 by Paul Bishop with the assistance of Dr. Montree Boonsener, to investigate the potential for using luminescence to evaluate formation hypotheses and rates of sediment turnover resulting from bioturbation. This paper presents an overview of the luminescence characteristics of the material, a discussion of luminescence pro"les from two sites, and constraints for formation models derived from the luminescence results.
2. Background A conspicuous feature of the sur"cial geology of continental and insular southeast Asia is a surface mantle or &cover layer' of generally sandy material which 夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (D.C.W. Sanderson).
may be up to 5 m or more in thickness. The layer is reported to be extensively distributed throughout Vietnam and in Cambodia, in NE and N Thailand; it is also reported from the uplands of Malaysia and Myanmar, and even from the Punjab in India (Hoang Ngoc Ky., 1989, 1994). Over the last decade or so, the characteristics and genesis of the cover layer, particularly in Vietnam and NE Thailand, have received signi"cant attention in the local Quaternary geological literature. Models range from interpretation as aeolian (loess-like) mantle of Late Pleistocene to Holocene age (Boonsener and Tassanasorn, 1983; Sonsuk and Hastings, 1984; Boonsener, 1987, 1991; Hoang Ngoc Ky., 1989, 1994; Udomchoke, 1989; S[ ibrava, 1993) to lacustrine (Dheeradilok, 1987), marine (Nguyen Duc Tam, 1994), and #uvial (De Dapper, 1987) origins. Bioturbational processes are also relevant (cf. Williams, 1978; Bishop et al., 1980; Johnson, 1993). Indeed, LoK %er and Kubiniok (1991, 1996) have argued that bioturbation is the key to the genesis of the cover layer, which they interpret as a Late Tertiary &biomantle', emplaced as a result of termite activity and subsequent degradation of termite mounds.
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 4 - 7
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This range of interpretations, the regional extent of the cover layer and its potential for palaeoenvironmental interpretations and for intra- and inter-regional correlations mean that clari"cation of the cover layer's age and mode of emplacement is a signi"cant issue (S[ ibrava, 1993). In this paper, we provide a preliminary report on the luminescence characteristics of the cover layer in NE Thailand with a view to elucidating the emplacement and, if possible, the age of the unit.
3. Field studies and sampling The "eld sites are located in the Khon Kaen area in NE Thailand (Boonsener, 1987, 1991, Boonsener and Tassanasorn, 1983), an undulating, low-elevation plateau characterised by a very dry cool season, followed by a very hot and dry season prior to the wet season. Samples of the cover layer were collected in a large gravel quarry at Ban Phang, on the northern side of the Chum Phae Road, 24 km west of Khon Kaen (site 1: TL98/1), and in a small quarry on the Khon Kaen airport road in Khon Kaen (site 3:TL98/3). A small dune south of the Chum Phae Road at the western edge of Khon kaen was also sampled (site 2: TL98/2). The thicknesses of section and the number of samples collected in a vertical sequence at each site are: TL98/1: 200 cm, 6 samples; TL98/2: 120 cm, 3 samples; TL98/3: 320 cm, 8 samples. Bulk samples of sediment were collected for gamma spectrometry from all three sites. Luminescence samples were collected in 30 cm lengths of 19 mm diameter water pipe, and sealed at both ends to prevent light exposure. 3.1. Sample preparation Samples were extruded from the tubes in the laboratory under safelight conditions, and actual and saturated water contents determined. Matrix beta dose rates were measured by thick source beta counting (Sanderson, 1988). Samples were sieved, and the 90}150 lm polymineral fraction used both for initial sensitivity tests and pro"ling, and as a source for mineral separation. Mineral extracts were then obtained by sequential centrifugation in aqueous sodium polytungstate at densities of 2520, 2580, 2620, 2740 kg m\, and appropriate acid treatments. All densities were subjected to 1 M HCl for 30 min, 15% HF for 10 min, followed by repeated HCl treatment to remove any undissolved #uorides. The 2620}2740 kg m\ fraction was also etched in 42% HF for 40 min, followed by further HCl treatment. All fractions were rinsed several times in deionised water, and dried from acetone. Samples were dispensed onto 0.25 mm thick, 10 mm diameter, stainless-steel discs sprayed with Electrolube SCO200D silicone grease.
4. Measurement methods Initial measurements were undertaken to identify the mineralogy and general luminescence pro"les of the samples. Thereafter, a series of tests of dose quanti"cation procedures for TL, PTTL and OSL was conducted using the quartz fractions of the central sample from site 3. Additive TL methods and regenerative OSL methods were then selected to determine the dose and age pro"les of the sites. TL data were recorded at 53C s\ from room temperature to 5003C using an SURRC TL reader with 7/59 and KG3 "lters. Samples were irradiated using a 1.85 GBq 90Sr source at a rate of 3.3 Gy/min\. PTTL measurements were conducted using a simple cryogenic approach. Samples on disc were placed on copper blocks, cooled to !553C using a dry-ice/acetone mixture, and covered by a 5 mm BG39 "lter. Murray (1996) noted the strong relationship between OSL and phototransferred TL (PTTL) in quartzes from young river sediments, and suggested that PTTL to the 1103C TL peak might be an e!ective alternative to OSL for dating such sediments. However, the short mean life at room temperature for the acceptor peak introduces practical constraints. As noted elsewhere (Anthony et al., 2001) we have overcome this in a simple manner, which could be readily implemented in laboratories with single-sample TL equipment and no OSL facilities. Phototransfer was achieved by illumination in an arti"cial daylight lightbox giving a full spectral output of 7 mW cm\ followed by TL readout to 2003C. The area in photon counts of the 1103C TL peak was used as a measure of PTTL. This procedure is extremely rapid and simple. IRSL measurements were conducted using a pulsed IR stimulation system originally developed for detection of irradiated foods (Sanderson et al., 1995). Samples coated on 50 mm diameter petri dishes, were stimulated with 180 mW of IR at 880$40 nm with a pulse frequency of 33 kHz, and luminescence detected synchronously using a 5 mm BG39 detection "lter. OSL measurements were conducted using a system based on 9 GaN blue LED's behind GG420 long pass "lters, and a 9883QB PMT with 5 mm UG11. Stimulation was again pulsed at 33 kHz and luminescence detected by synchronous photon counting, samples being measured on disc on a standard TL heater plate. The decay curve can be well-"tted by two exponential components and a constant. Fig. 1 shows examples of TL, PTTL and OSL data recorded from one of the Thai samples using these techniques. 4.1. Additive and regenerative dose determination approaches Dose quanti"cation was investigated by both additive and regenerative approaches. Validation tests were
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Fig. 1. Examples of (a) TL, (b) PTTL and (c) OSL decay curves from sample TL480, site 3. In (b) curve 1 is the phototransferred signal into the 1103C trap following illumination with arti"cial daylight through a 5 mm BG 39 "lter. Curve 2 is the TL recorded prior to tranfer, and curve 3 is the residual signal following complete transfer
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and Wintle, 1998). Additive dose determinations were carried out using the same irradiation sequence as the TL run described, followed by a 2003C ramp at 53C s\, 900 s phototransfer at !553C, TL readout of the 1103C signal, and thermal resetting of the system during subsequent TL ramp to 5003C. The net area of the 1103C was used for dose estimation. OSL additive runs were conducted again following the irradiation sequence described above, followed by 600 s pre-heating at 2203C, and 100 s stimulation at ambient temperature. Samples were bleached for 300 s in arti"cial daylight following OSL readout, to remove residual signals prior to the next cycle. OSL decay curves were analysed over di!erent decay times, and also with or without `late lighta subtraction; integrated data again being analysed using exponential "tting procedures. Regenerative measurements were made using a procedure based on that suggested by Murray and Roberts (1998), using the OSL test dose}response to monitor sensitivity changes during laboratory measurements. Samples were subjected to a sequence of pre-heating, readout, bleaching (to remove any residual OSL), and irradiation. Readings from `naturala and regenerative irradiations were interleaved by readings of test-dose response to monitor sensitivity changes. Samples were pre-heated at 2603C for 30 s prior to stimulation at 1203C for 100 s per measurement. Bleaching between cycles was conducted for 30 min in an arti"cial daylight lightbox at 7 mW cm\. Stored doses were determined by interpolating natural intensities onto a regenerative OSL dose response plot, all signals being normalised to the subsequent test dose response. Exponential "tting was used as the basis for interpolation.
5. Results conducted using samples from the middle of site 3. Discs of etched quartz from sample TL480, site 3, were bleached out and then irradiated to a known dose of 10 Gy, stored for 24 h in darkness, and then treated as an unknown sample. Equal pre-dose additive measurements using eight dose points from 3 to 24 Gy were implemented for TL, PTTL and OSL methods. Sixteen discs per run were used, with added doses applied in ascending order prior to "rst glow, in descending order prior to the second glow, and dose normalisation based on the response to a "xed 5 Gy dose in third glow. Samples were pre-heated for 300 s at 2103C prior to TL readout, and the main high-temperature peak (typically observed at around 3253C), integrated to construct dose response curves. The dependence of PTTL on illumination time was "rst investigated, and shown to be a multipleexponential function of time (Houston, 1999). It is interesting to note that this is the integral form of the multiple-exponential OSL decays curves observed by several authors (Bailey et al., 1997; Bailey, 1998; Murray
5.1. Initial investigations Initial investigations con"rmed that the material comprises coarse-grained iron-stained quartz with a "ne clay wash. TL glow curves from bulk samples and separated quartz showed predominantly quartz-derived signals, with pronounced 3253C peaks in both natural and laboratory-induced signals, and well-developed lowtemperature peaks including the 1103C signal. High phototransfer and OSL sensitivities were also observed. Interestingly only modest pre-dose sensitisation occured on heating to 5003C, which in combination with a high sensitivity at 1103C, implies that the sample had undergone environmental activation of the pre-dose sensitivity enhancement due to the natural signal, perhaps in the manner suggested by Wintle and Murray (1999). IRSL sensitivity from bulk material was low, but nonetheless measureable from large area samples. Fig. 2 shows depth pro"les for TL, PTTL, IRSL and matrix beta dose rate
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Fig. 2. Initial luminescence and beta dose rate pro"les from site 1.
for site 1. Both TL and PTTL pro"les are similar for both sites, and show progression with depth. By contrast IRSL response } presumed to be derived from the "ne clay wash } shows low intensities and, for site 3 particularly, a subsurface maximum, perhaps implying that weathering in deeper layers limits signal accumulation. Nonuniformities of beta activity were also observed. Given the nature of the material it was decided to investigate dose quanti"cation procedures appropriate to quartz, as the major component of these samples. 5.2. Validation tests for dose quantixcation Fig. 3 shows additive dose results for TL, PTTL and blue OSL, in tests with a known 10 Gy beta dose. The approach to curve "tting was to simultaneously "t both "rst and second growth lines to a coupled pair of saturating exponential functions, with common coe$cient of saturation, but variable amplitudes, determining the extrapolated dose along with the other three coe$cients by least-squares "tting. Linear "tting resulted in overestimation of the 10 Gy dose by up to 50%. There is no evidence for supralinearity in any of the regenerated data sets. It is also notable that there is more scatter in the "rst growth lines from all three techniques, when compared with second growth response. Such behaviour has been observed frequently in TL additive dating runs (e.g. Scott and Sanderson, 1988) where it has been assumed that the excess variation in natural plus added dose response is due to a combination of heterogeneity in the natural radiation "eld and spurious TL associated with the "rst heating of the sample. It is therefore interesting to see the same phenomenon in TL, PTTL and OSL from quartz which had been given a simple laboratory beta dose merely 24 h before conducting the measurements. Neither of the conventional explanations apply to these data,
Fig. 3. Additive dose results with a known 10 Gy dose by TL, PTTL and OSL. Both "rst- and second}dose}response curves were "tted simultaneously to a pair of saturating exponentials with a common coe$cient of saturation, and variable amplitudes, the stored dose being determined as the o!set between the two lines.
since the dose distribution should be identical between "rst and second glows, and it is unlikely that `spuriousa emission would a!ect the optical measurements. Additive TL runs recovered the expected 10 Gy dose within measurement errors (results 9.0$2, 9.2$2 Gy), but in the experiments conducted the PTTL data produced underestimation (5.7$2 Gy), and the OSL additive data overestimated dose (15.1$2). Analysis of the dependence of the OSL result on decay time (Houston, 1999) showed that while the extrapolated result did depend slightly on the part of the decay curve taken, all periods resulted in overestimation of the stored dose. Regenerative OSL data produced highly coherent data sets, again with a saturating exponential dose response, as illustrated in Fig. 4. The OSL were analysed with and without late light subtraction (cf. Aitken, 1998), and also with and without corrections for a small sensitivity change in the instrument. In all cases the results were consistent with the expected dose. Including the minor sensitivity corrections, the results were 10.4$0.5 without late light subtraction, and 9.8$0.5 Gy with late light subtraction, highly consistent with the known 10 Gy dose. 5.3. Dose proxles and apparent ages for site 1 Both additive TL and regenerative OSL procedures were applied to the six samples from site 1 to investigate stored dose pro"les and the relationship between
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TL and OSL doses and apparent ages as a function of depth. Sets of 16 discs were used for TL determinations, with eight pairs of dose points from 3 to 24 Gy. The natural doses for the four deepest samples exceed the maximum dose added (24 Gy). This results in poorly constrained exponential "ts, and therefore } notwithstanding the conclusions of the previous section } the linear approximation to the dose}response curve was used for dose estimation. It is possible that more precise results could be obtained using a larger range of added dose (e.g. up to 48 Gy, with exponential regression). However, the dose estimates obtained are believed to be adequate for the initial discussion. OSL regenerative runs followed the procedure outlined in the previous section. Pre-heating at 2603C for 30 s and stimulation at 1203C were used for OSL readout. A test dose of 1 Gy test was used, and normalised dose}response curves constructed from 2 to 50 Gy in six steps. Four discs per sample were measured. The extent to which individual dose}response curves conformed to the average dose}response curve from all samples was examined. Despite the overall similarity, the
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dose-depth pro"le obtained using individual curves for each sample was more coherent than that obtained from a mean curve, suggesting that there are minor mineralogical di!erences between samples originating from di!erent depths. Dose estimates and apparent ages by TL and OSL are given in Table 1, and Fig. 5. Close to the surface the TL estimates are signi"cantly greater than OSL estimates, suggesting that surface layers may be partially bleached, although the possibility of extrapolation errors in the TL data should also be borne in mind. Below 90 cm depth, however, both data sets are consistent with each other, perhaps suggesting that bleaching is more homogeneous at lower levels. The OSL results generally show greater coherence with depth, as a result of the higher precision obtainable by regenerative procedures. The apparent age range of the section is from approximately 8}40 ka. Whether these results can be considered to represent accurate dates is dependent on the extent to which the formation processes have resulted in meaningful bleaching events for either TL or OSL. This is discussed further in the next section. It is however of interest to examine the net accumulation rates implied by the luminescence ages, since these will depend more on the relative constancy of bleaching conditions, rather than whether bleaching could be considered to be complete. These are shown in Table 2, from which it can be seen that whereas the surface layers have the lowest accumulation rates, possibly re#ecting partial bleaching of the upper layers, particularly for TL, relatively constant rates are observed for the majority of depth intervals, of approximately 4 cm ka\. Exceptions to this however are in the 21}46 and 120}156 cm depth layers where signi"cantly higher accumulation rates are inferred.
6. Discussion and conclusions
Fig. 4. Regenerative OSL dose}response normalised to a 1 Gy test dose}response. The normalised response from a known 10 Gy dose}interpolates to the correct value.
The cover sand layers in NE Thailand have quartzdominated luminescence characteristics, with high sensitivity to phototransfer and OSL, providing a promising tool for palaeoenvironmental tracer work in the region.
Table 1 Stored dose and apparent age estimates for site 1 by TL and OSL TL
OSL
Depth (Z/cm)
Dose rate (mGy a\)
Stored dose (Gy)
Apparent age (ka)
Stored dose (Gy)
Apparent age (ka)
21 46 90 120 156 200
0.88$0.1 0.87$0.09 0.94$0.09 1.04$0.1 0.96$0.1 0.98$0.1
12.1$1 13.5$1 23.2$2 32.6$3 27.9$3 38.4$4
14.5$1.6 15.5$1.5 24.6$2.5 31.3$3 29.0$3 39.2$4
7.8$.5 12.3$.5 20.1$1 27.3$1.5 33.3$1.5 34.7$2
8.9$0.9 14.1$1 20.5$2 6.5$2.5 34.7$3 35.4$2
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Dose quanti"cation studies have shown the material to have well-reproduced luminescence dose}response characteristics with a saturating exponential form. Additive TL procedures and regenerative OSL procedures produced excellent results in tests to recover a known laboratory dose, but under the conditions investigated this was not so for additive PTTL or additive OSL procedures. The reasons for this are not yet clear, and therefore more work would be needed to apply PTTL and OSL additive procedures with con"dence to this material. It is possible that the discrepencies are an artefact of the thermal treatments used in the procedure, as argued by Houston (1999), in which case it may be possible to obtain accurate results by varying the preheating regimes. However, sensitivity to pre-heating raises questions about the interactions between the di!erent centres in quartz, and the extent to which laboratory irradiation and thermal treatment can realistically simulate the natural dose cycle. Another explanation of these results is that they are a manifestation of the di$culties of making accurate extrapolations to non-linear dose}response functions, compounded by the excess variance
Fig. 5. TL and OSL stored dose pro"les for site 1.
observed in "rst dose}response compared with second dose}response. Moreover, as noted above this excess variation cannot be explained simply in terms of heterogeneous dose distributions, or in terms of spurious luminescence. It is perhaps suggestive of a re-arrangement of the trapped charge distribution immediately following irradiation, but in a manner which is heterogeneously distributed between the grains making up the sample. Whether this is a simple thermal process, or involves tunneling mechanisms, or even re-arrangement of clusters of centres within the quartz following irradiation is at this stage unclear. The overall performance of added dose determinations on the other hand is good in comparison with other quartz samples which we have studied, suggesting a reasonably homogeneous natural dose distribution within the grains which contribute to the TL signal. The regenerative OSL results were very encouraging. Precision is signi"cantly better than obtained with additive approaches. The results from tests with known laboratory doses were excellent, as was the agreement between additive TL and regenerative OSL-stored dose estimates from in site 1. The procedure used is relatively easy to implement. Providing that the sensitivity changes which can be monitored, for all cycles following readout of the natural signal,are also representative of the natural dose cycle, the technique should yield results which are both precise and accurate. In a general sense this will depend on the timing of sensitivity changes and their dependence on preheating (Stokes, 1994; McKeever et al., 1997; Wintle and Murray, 1998), which are potentially variable between quartzes of di!erent origins. Nonetheless the procedure seems highly promising and well suited to this material. The dose pro"les, apparent ages and accumulation rates from site 1 are broadly consistent between both techniques. Whereas there are suggestions of partial bleaching in upper layers, the dose estimates further down the site, and their associated accumulation rates tell a consistent story. In the introductory section the main models for formation of these cover layers were discussed, the extreme cases being (a) a largely aeolian
Table 2 Net accumulation rates for site 1 from both TL and OSL data TL
OSL
Depth interval dZ (cm)
Age interval dt (ka)
Accumulation rate dZ/dt (cm ka\)
Age interval dt (ka)
Accumulation rate dZ/dt (cm ka\)
0}21 21}46 46}90 90}120 120}156 156}200
0}14.5 14.5}15.5 15.5}24.6 24.6}31.3 31.3}29.0 29.0}39.2
1.4 16.7 4.8 4.5 * 4.5
0}8.9 8.9}14.1 14.1}20.5 20.5}26.5 26.5}34.7 34.6}35.4
2.36 4.8 6.9 5 4.4 55
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process which would be expected to be operating at higher rates during the colder periods of the last glacial cycle, and (b) the biogenic models which see the material as having accumulated by re-distribution processes associated with termite activity. Taken at face value the results from site 1 are what would be expected from the "rst model. Under this interpretation the similarities of stored dose between OSL and TL for lower layers in the site would be taken as an indication of good bleaching conditions, as expected from aeolian processes. The coherent dose}response curves might be regarded as an indication of homogeneous materials, and the luminescence chronology, with its implied higher rates of accumulation during the last glacial cycle could be taken as evidence in support of this model. However, under the biomantle model, the dose-depth pro"les would be taken to represent mixed distributions of unbleached and wellbleached grains within the sedimentary column, with an increasing proportion of unbleached grains at greater depth in the site. The dose pro"les and their implied chronology would be regarded as an artefact of the mixing processes. Our data so far appear to be consistent with the "rst formation model, although the divergence between TL and OSL results in the upper layer may be the result of bioturbation processes. The biomantle model would imply highly heterogeneous dose distributions from grain to grain, which would be expected to lead to poorly reproduced dose estimates. The biomantle model cannot however be ruled out at this stage. Further work will be undertaken to assess whether the luminescence properties could in fact be explained in terms of bioturbation, and to examine the single-grain characteristics of the material. In any event the luminescence properties of the cover layer are well suited to geochronological studies. Thus luminescence methods have considerable potential for in helping to understand the formation processes and chronologies of these deposits in SE Asia. Acknowledgements The authors are grateful to R. McNaughton, Dr.L.A. Carmichael and Dr. S. Fisk for their contribution to parts of the laboratory work reported here. References Aitken, M.J., 1998. An introduction to Optical Dating. Oxford University Press, Oxford. Anthony, I.M.C, Sanderson, D.C.W., Cook, G.T., Abernethy D., Housley, R.A., 2001. Dating a burnt mound from Kilmartin, Argyll, Quaternary Science Reviews (Quaternary Geochronology) 20, 921}925. Bailey, R.M., 1998. The form of the optically stimulated luminescence signal of quartz: implications for dating. Ph.D. Thesis, Royal Holloway, University of London.
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Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiation Measurements 27 (2), 123}136. Bishop, P., Mitchell, P., Paton, T.R., 1980. The formation of duplex soils on hillslopes in the Sydney Basin, Australia. Geoderma 23, 175}189. Boonsener, M., 1987. Quaternary geology of Khon Kaen Province, northeastern Thailand. In: Thiramongkol, N. (Ed.), Proceedings of the Workshop on Economic Geology. Tectonics, Sedimentary Processes and Environment of the Quaternary in Southeast Asia (3}7 February, 1986, Haad Yai, Thailand), International Geological Correlations Programme, Dept of Geology Chulalongkorn University, Association of Geoscientists for International Development, pp. 75}85. Boonsener, M., 1991. The Quaternary stratigraphy of northeast Thailand. Journal of Thai Geosciences 1, 23}32. Boonsener, M., Tassanasorn, A., 1983. The geomorphology of Quaternary deposits in Khon Kaen Province, northeastern Thailand. In: Thiramongkol, N., Pisutha-Arnond, V. (Eds), Proceedings of the First Symposium on Geomorphology and Quaternary Geology of Thailand (October 28}29, 1983), Deptartment of Geology Chulalongkorn University, Department of Mineral Resources, and Geological Society of Thailand, pp. 106}111. De Dapper, M., 1987. Landform development during the Late Quaternary in the upland of peninsular Malaysia. In: Wezel, F.W., Rau, J.L. (Eds), Progress in Quaternary Geology of East and Southeast Asia. Proceedings of the CCOP Symposium on Developments in Quaternary Geological Research in E and SE Asia During the Last Decade, (27}30 October, 1986, Bangkok, Thailand), CCOP Technical Publication Vol. 18, pp. 109}134. Dheeradilok, P., 1987. Review of Quaternary geological mapping and research in Thailand. In: Wezel, F.W., Rau, J.L., (Eds), Proceedings of the CCOP Symposium on Developments in Quaternary Geological Research in East and Southeast Asia During the Last Decade, 27}30 October, 1986, Bankok, Thailand. Hoang Ngoc Ky., 1989. The Thu Duc loess formation, a typical eolian deposit of tropical regions, in ESCAP Secretariat, Quaternary Stratigraphy of Asia and the Paci"c. IGCP 296 (1989). ESCAP Atlas of Stratigraphy X (Mineral Resources Development Series 60), pp. 100}104. Hoang Ngoc Ky., 1994. Stratigraphic correlation of Quaternary transgressional and regressional deposits in Viet Nam and adjacent countries. In: ESCAP Secretariat, Quaternary Stratigraphy of Asia and the Paci"c. IGCP 296. ESCAP Atlas of Stratigraphy XIII Mineral Resources Development Series Vol. 63, pp. 141}149. Houston, I., 1999. The application of stimulated luminescence to the dating of South East Asian Quartz. B.Sc Dissertation, Physics Department, University of Strathclyde, 61 pp. Johnson, D.L., 1993. Dynamic denudational evolution of tropical, subtropical and temperate landscapes with three tiered soils: toward a general theory of landscape evolution. Quaternary International 17, 67}78. LoK %er, E., Kubiniok, J., 1991. The age and origin of the Yasothon soils and associated gravel deposits. Journal of the Geologcal Society of Thailand 1, 69}74. LoK %er, E., Kubiniok, J., 1996. landform development and bioturbation on the Khorat plateau, northeast Thailand. Natural History Bulletin of the Siam Society 44, 199}216. McKeever, S.W.S., Agersnap Larsen, N., Botter Jensen, L., Mejdahl, V., 1997. OSL sensitivity changes during single aliquot procedures: computer simulations. Radiation Measurements 27 (2), 75}82. Murray, A.S., 1996. Developments in optically stimulated and phototransferred thermoluminescence dating of young sediments: application to a 2000 year sequence of #ood deposits. Geochimica et Cosmochimica Acta 60 (4), 656}676. 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.
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