- Email: [email protected]
The fadia method: a new approach in luminescence dating using the analysis of single feldspar grains
Radiation Measurements 32 (2000) 433±438
www.elsevier.com/locate/radmeas
The fadia method: a new approach in luminescence dating using the analysis of single feldspar grains M. Lamothe*, M. Auclair Laboratoire de luminescence Lux, DeÂpartement des sciences de la Terre et de l'AtmospheÁre, Universite du QueÂbec aÁ MontreÂal, C.P. 8888, Succ. Centre Ville, MontreÂal, Que., Canada, H3C 3P8 Received 3 November 1999; received in revised form 16 March 2000; accepted 18 May 2000
Abstract The fadia method has been recently introduced in luminescence as one that may potentially resolve the problem of anomalous fading and age shortfalls in IRSL dating of sediments. This method takes advantage of the dierential fading rates of single feldspar grains and allows one to extrapolate to zero fading. This paper describes step by step the protocol used in the MontreÂal laboratory. The application of the method is shown to be hampered by the occurrence of faintly luminescent feldspar minerals, and/or unbleached grains in the dated sediment samples. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The use of optically stimulated luminescence from single feldspar grains to date sedimentary events was ®rst introduced by Lamothe et al. (1994), and this methodology has been later successfully extended to quartz by Murray and Roberts (1997). The use of single feldspar grains for dating a sedimentary depositional event is hampered both by the degree of bleaching for the grains prior to their burial and, contrary to quartz, by the common occurrence of anomalous fading. For a mineral that fades, the decay of luminescence with time is logarithmic, suggesting that this process is driven by barrier tunneling (Visocekas, 1979; Spooner, 1994; Lamothe and Auclair, 1999). Apparently, no thermal assistance is required to explain the
* Corresponding author. Fax: +1-514-987-7749. E-mail address: [email protected] (M. Lamothe).
decay even though a slight thermal dependence may have been recently detected by Visocekas (2000). Lamothe and Auclair (1999) showed that fading in multiple aliquot, multiple grain samples from dierent parts of North America could be circumvented through the investigation of the dierential fading rates of single feldspar grains. The method introduced (called fadia ) uses the parameter RI, which is de®ned as the ratio of luminescence (L ) following a laboratory dose to that measured before the irradiation RI LNg =LN ; Lamothe and Auclair, 1997). It compares RI(t1), measured shortly after irradiation, to RI(t2), acquired after some time delay. Grains of higher RI exhibit larger fading rates and, for a given grain population, RI(t1) and RI(t2) are linearly correlated (Fig. 1a). Therefore, it is postulated that extrapolation to a point where RI t1 RI t2 yields the R0I for which there would be no fading. The use of the fadia method was found to correctly assess the amount of anomalous fading in four feldspar-bearing sediment samples, three of which are of Pre-Quaternary age. The fourth
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 2 4 - 4
434
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
fadia corrected age of 6827 ka is in agreement with the U±Th dates of ca. 70±80 ka, obtained on corals extracted from the same unit (Norfolk Formation; Wehmiller et al., 1998). The objectives of this paper are to fully describe the methodology and laboratory measurements used in the MontreÂal laboratory and to discuss some complicating factors, the two most important being the scatter in luminescence measurements of single grains and the apparent limited applicability of the method to a unique grain population. Areas of future investigations are outlined. 2. Methodology: the fadia protocol
Fig. 1. (a) Fadia plot for 32 single feldspar grains (150±250 mm) from sample GP3 following the addition of 570 Gy t1 3 h; t2 10 days). (b) Histogram showing the range of Natural/Regenerated (1/RRI) values for the same set; the data under the arrows are shown as open circles in (a) and are not included in the linear regression. (c) Multiple aliquot additive growth curves for the same sample; prompt p 5 days; De 7024 Gy), delayed d 22 months; De 8125 Gy), and fadia corrected ( f; De 13529 Gy). The R0I intercepts (diamonds) obtained following the addition of 325, 570, and 1005 Gy are shown. Annual dose is 2:0020:10 Gy/ka.
sample is a Late Pleistocene marine sand from Virginia (USA; sample GP3), previously dated at ca. 40 ka by the IRSL additive multiple aliquot technique. The
The dierent measurements involved in the fadia method consist of the following steps: (1) Selection of grains; (2) Preheat; (3) IRSL-S0; (4) Gamma dose; (5) Preheat; (6) IRSL-S1; (7) Storage at room temperature for several days; (8) IRSL-S2; (9) Sunlamp bleach; (10) Gamma dose; (11) Preheat; (12) IRSL-S3; (13) Storage; (14) IRSL-S4. This sequence of measurements is summarized in Table 1 and some methodological comments are added below. The grains (usually 30±60) are ®rst selected using a 1 s infrared short shine. They must emit enough luminescence in the blue-UV spectral band, transmitted through a BG39/Corning 7-59 ®lters combination. The rate of success is sample and age dependent. In young samples, it is not uncommon to ®nd only 10% of the grains that emit more than ®ve times the background (bg050 counts/s). The minimum required light level is ®xed at 0300±500 counts/s, considering that the grains will be further preheated before the measurement of the natural signal (S0). For each and every IRSL measurement, four successive 1 s short shines are carried out in order to increase the reproducibility and evaluate the optical erosion. This is found by comparing the two last shines of each measurement with the two ®rst shines. The average optical erosion is 1±2%/1 s shine. To reduce the delay between irradiation and measurement, we selected a short on-plate preheat of 2508C/min as we found that the associated thermal erosion was similar to the 2208C/10 min preheat commonly used for feldspar. The same preheat is used after each irradiation step. The choice of the gamma dose is sample dependent, high doses giving better reproducibility (see below). The prompt luminescence (S1) is measured as soon as possible after the end of the irradiation and preheat t1 1±3 h). The grains are stored at room temperature for a period of 10±15 days (t2) and their IRSL is monitored again (S2). The grains are then bleached overnight under a solar lamp l > 460 nm), gamma regenerated, preheated, measured promptly (S3), and measured again after the
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
same time delay (S4). The RI and RRI are calculated as in Table 1. The reader is a Daybreak 1100 automatic luminescence system that can accommodate 20 samples. Therefore, for each run, a set of 16 grains is measured to which three natural ®ne-grain discs (4±11 mm) are added and one sample position is left empty. Natural ®ne-grain discs are used to monitor changes in reader sensitivity. Additive measurements (S1 and S2) are corrected for thermal erosion of the natural signal, evaluated from another set of natural grains preheated twice. The average value of thermal erosion is used for all the single grains. The delayed measurements (S2 and S4) are further corrected for optical erosion due to previous shines. The error for the RI values is based on the sum of statistical uncertainties at each measurement (N 1/2; Aitken, 1998). A plot of RI(t1) vs. RI(t2) allows the determination of R0I : The stable component of the induced luminescence is then estimated by comparing the R0I intercept with the RI obtained from multiple grain aliquots that have received the same laboratory dose. As an example, in our original work (Lamothe and Auclair, 1999), the R0I intercept for a suite of GP3 single feldspar grains following the addition of 1005 Gy was 5:1820:16: Since the RI from the multiple aliquots that had received the same dose was at 9:5720:29, the stable induced luminescence component was, therefore, calculated to be around 50%. The multiple aliquot growth curve is then corrected accordingly at all the other additive dose points. In order to test this correction, two other R0I dose-points (at 325 and 570 Gy, Fig. 1c) were measured. The R0I for N 570 Gy is found reasonably close to the corrected growth curve. However, the R0I obtained from the addition of 325 Gy plots signi®cantly below, suggesting a much higher fading ratio, of the order of 65%. This apparent discrepancy is discussed in the next section. Finally, one can use regeneration to derive the paleodose, and hence, monitor RRI. An example is found in Balescu et al. (in
435
press) for a penultimate interglacial sediment from Southern Quebec. 3. Discussion: complicating factors Results obtained so far from dierent geological contexts and sediments of dierent depositional ages clearly indicate that the fadia method can potentially circumvent the problem of anomalous fading in feldspar. However, there are several problems that need to be investigated as they can strongly aect both the precision and the accuracy of the R0I intercept. It is the experience of the authors that physical processes exhibited by single feldpar grains may be highly variable. However, at this stage of development, we identify the following as the limiting factors in the application of the method, in order of decreasing annoyance: (1) the high scatter in the natural luminescence signal arising from dim grains; (2) the diculty one has to identify and reject unbleached grains in the fadia analysis; (3) the steep slope of the fadia line with consequent high uncertainty in the R0I intercept; (4) the number of R0I intercepts that are needed to appropriately correct the additive and regeneration growth curves; (5) sensitivity changes following the sunlamp bleach in the determination of a RRI value; (6) possible variable thermal charge transfer and thermal erosion during the successive preheats. Only the ®rst two factors are considered here. 1. In order to document the eect of scatter, we turn again to sample GP3. In Fig. 2, the scatter for the luminescence emission of the grains before and after the addition of a low vs. a high gamma dose, with respect to their RI value, is shown. The relatively higher uncertainty for grains of low intensity will be strongly propagated in the ®nal RI value if dosed at low radiation levels. We believe that the R0I obtained for the N 325 Gy dose-point is, therefore, a result of low luminescence intensity and con-
Table 1 The fadia protocola Steps
Measurements
Ratios
Short shine Preheat; four short shines Dose; preheat; four short shines Storage; four short shines Bleach; dose; preheat; four short shines Storage; four short shines
Selection of grains Natural IRSL signal, S0 Additive prompt, S1 Additive delayed, S2 Regeneration prompt, S3 Regeneration delayed, S4
RI t1 S1=S0 RI t2 S2=S0 RRI t1 S3=S0 RRI t2 S4=S0
a
Short shine: 1 s, infrared, 208C; preheat: 2508C/min.
436
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
sequent lower precision on the determination of the value of R0I : Since low intensity grains must be part of the fadia analysis, particularly for young samples, the only remedy is to develop highly sensitive luminescence readers that could enhance the luminescence signal for single grains by, at least, one order of magnitude. 2. Another major drawback in the application of the method is the common occurrence of unbleached grains. Even in a presumably well-bleached sample, such as GP3, undesired unbleached grains are present. In order to detect those, we use a histogram of values as an indication of grain population homogeneity. Herein, we construct a Natural/Regenerated luminescence histogram (e.g. 1/RRI, see Fig. 1b) and values positively skewed away from the main Gaussian distribution are rejected. The assumption here is that unbleached grains are much less sensitive to a regeneration gamma dose. In each series of GP3 grains analyzed, 5±10% of the grains seem to have been buried without being fully exposed to sunlight (under the arrows in Fig. 1b). The following example demonstrates the combined eect of low luminescence intensity and the occurrence of unbleached grains in a sediment sample. The fadia method was applied to a Lateglacial glaciomarine sandy and clayey silt sample (CES-5), dated by 14C at 10:520:14 ka BP (uncalibrated) and by routine IRSL at ca. 40 ka (see Lamothe and Auclair, 1997 for more details). It was hoped that the approach described above could successfully detect the population of unbleached grains so that the fadia analysis could be carried out using only the population of well-bleached
Fig. 2. Statistical uncertainty (de®ned as N 1/2/N ) for the natural luminescence light emission (A and C from the 325 and 1005 Gy sets) and the natural plus laboratory dose (B and D from the 325 and 1005 Gy sets); the uncertainties are plotted with respect to their corresponding RI(t1) values.
grains. In order to document the behavior of the unbleached grains, a glacio¯uvial sand (CES-2), not expected to be zeroed, was also investigated. A gamma dose of 50 Gy was used for both RI and RRI measurements. Such a low dose was selected to ensure the linearity of the growth of the IRSL signal upon dose for the well-bleached grains. Fig. 3 presents the fadia plots and the respective 1/RRI ratios for both samples. For CES-2, the unbleached grains can clearly be characterized by RI close to 1 and high 1/RRI values. Even though the scatter is high, the R0I comes to 1:1020:11 as expected for an unbleached sediment. However, for the CES-5 sample, the R0I intercept of 1:4820:25 implies an overestimated paleodose of 150 Gy; hence, an IRSL date of, at least, 40±50 ka, similar to the multiple aliquot IRSL date. In this sediment sample, the presumably well-bleached grains, i.e. those with high RI and low 1/RRI values, cannot be discriminated against the unbleached ones and constitute probably less than half of the total grain population. Scatter is uncomfortably high since most of the well-bleached grains are dim. Therefore, low abundance and high scatter associated with this population does not allow one to derive a signi®cant fadia correction. To reduce scatter, one could use higher gamma doses but deviation from linearity would then be a problem. Another possibility is to use more than one dose, much as in the single aliquot method (Duller, 1991). The conclusion, herein, is that the fadia method cannot be applied at the moment to poorly bleached sediment samples. 4. Conclusion The fadia method has been only recently introduced in the luminescence literature, and as such, this approach shall open new avenues of research in the ®eld of sediment dating. An increase in the eciency with which luminescence can be detected from single feldspar grains is seen as an urgent remedy to several problems documented above. New luminescence readers have to be developed in order to increase the reproducibility and reduce the time required to run a large number of single grains, such as the new single grain laser luminescence system described by Bùtter-Jensen et al., 2000. Some underlying assumptions will need to be tested further. For example, one puzzling implication of a unique R0I intercept for a population of grains is that the deduced ``stable'' luminescence sensitivity to dose appears similar for any K-feldspar. This deduction may not be veri®ed for every sample. One critical limitation is the steep slope of the fadia regression line that may result in uncomfortably high errors in the R0I intercept. Routes of acceleration for fading are being
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
437
Fig. 3. Fadia plots and Natural/Regenerated (1/RRI) values for sample CES-2 (a, b) and sample CES-5 (c, d). The additive and regeneration dose is 50 Gy. Each sample set contains 48 grains (150±250 mm). The histogram for CES-5 suggests that it is composed of a signi®cant number of unbleached grains, such as the ones contained in CES-2.
investigated (e.g. extended preheats at lower temperatures) but the phenomenology of anomalous fading makes it unlikely that other factors than time will contribute to the solution of this problem. Finally, evidence is accumulating in this laboratory and elsewhere that sensitivity change occurs for feldspar grains after they have been exposed to a sunlamp bleach. Correction for this unwanted behavior may follow the methods of Murray and Roberts (1998) and Wallinga et al. (2000). Notwithstanding the above, we would argue that as the investigation in the dating of single feldspar grains proceed, the fadia method shall prove to be an appropriate approach to the dating of geological and archaeological events.
Acknowledgements This work is funded through NSERC (Canada) operating grant OGP0037375. The authors wish to thank D.J. Huntley (Physics, SFU) for sharing ideas about the problem of anomalous fading.
References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford (267 pp.). Balescu, S., Lamothe, M., Auclair, M., Shilts, W.W., in press. IRSL dating of Middle Pleistocene interglacial sediments
438
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
from Southern Quebec (Canada) using multiple and single grain aliquots. Quaternary Geochronology. Bùtter-Jensen, L., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 523±528. Duller, G.A.T., 1991. Equivalent dose determination using single aliquots. Nuclear Tracks and Radiation Measurements 18, 371±378. Lamothe, M., Auclair, M., 1997. Assessing the datability of young sediments by IRSL using an intrinsic laboratory protocol. Radiation Measurements 27, 107±118. Lamothe, M., Auclair, M., 1999. A solution to anomalous fading and age shortfalls in optical dating of feldspar minerals. Earth and Planetary Science Letters 171, 319±323. Lamothe, M., Balescu, S., Auclair, M., 1994. Natural IRSL intensities and apparent luminescence ages of single feldspar grains extracted from partially bleached sediments. Radiation Measurements 23, 555±562. Murray, A.S., Roberts, R.G., 1997. Determining the burial time of single grains of quartz using optically stimulated luminescence. Earth and Planetary Science Letters 152, 163±180.
Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose singlealiquot protocol. Radiation Measurements 29, 503±515. Spooner, N.A., 1994. The anomalous fading of infraredstimulated luminescence from feldspars. Radiation Measurements 23, 625±632. Visocekas, R., 1979. Miscellanous aspects of arti®cial TL of calcite: emission spectra athermal detrapping and anomalous fading. PACT 3, 258±265. Visocekas, R., 2000. Monitoring anomalous fading of TL in feldspars using far-red TL emission as a gauge. Radiation Measurements 32, 499±504. Wallinga, J., Murray, G.A.T., Duller, G.A.T., Tornqvist, T.E., 2000. Infrared stimulated luminescence dating of feldspar: the problem of age underestimation. Radiation Measurements. Wehmiller, J.F., Lamothe, M., Noller, J.S., Comparison of approaches to dating Atlantic coastal plain sediments, Virginia Beach, VA. In: Dating and Earthquakes: Review of Quaternary Geochronology and its Application to Paleoseismology, U.S. Nuclear Regulation Commission, NUREG/CR-5562, 1998, pp. 4.3±4.31.
www.elsevier.com/locate/radmeas
The fadia method: a new approach in luminescence dating using the analysis of single feldspar grains M. Lamothe*, M. Auclair Laboratoire de luminescence Lux, DeÂpartement des sciences de la Terre et de l'AtmospheÁre, Universite du QueÂbec aÁ MontreÂal, C.P. 8888, Succ. Centre Ville, MontreÂal, Que., Canada, H3C 3P8 Received 3 November 1999; received in revised form 16 March 2000; accepted 18 May 2000
Abstract The fadia method has been recently introduced in luminescence as one that may potentially resolve the problem of anomalous fading and age shortfalls in IRSL dating of sediments. This method takes advantage of the dierential fading rates of single feldspar grains and allows one to extrapolate to zero fading. This paper describes step by step the protocol used in the MontreÂal laboratory. The application of the method is shown to be hampered by the occurrence of faintly luminescent feldspar minerals, and/or unbleached grains in the dated sediment samples. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The use of optically stimulated luminescence from single feldspar grains to date sedimentary events was ®rst introduced by Lamothe et al. (1994), and this methodology has been later successfully extended to quartz by Murray and Roberts (1997). The use of single feldspar grains for dating a sedimentary depositional event is hampered both by the degree of bleaching for the grains prior to their burial and, contrary to quartz, by the common occurrence of anomalous fading. For a mineral that fades, the decay of luminescence with time is logarithmic, suggesting that this process is driven by barrier tunneling (Visocekas, 1979; Spooner, 1994; Lamothe and Auclair, 1999). Apparently, no thermal assistance is required to explain the
* Corresponding author. Fax: +1-514-987-7749. E-mail address: [email protected] (M. Lamothe).
decay even though a slight thermal dependence may have been recently detected by Visocekas (2000). Lamothe and Auclair (1999) showed that fading in multiple aliquot, multiple grain samples from dierent parts of North America could be circumvented through the investigation of the dierential fading rates of single feldspar grains. The method introduced (called fadia ) uses the parameter RI, which is de®ned as the ratio of luminescence (L ) following a laboratory dose to that measured before the irradiation RI LNg =LN ; Lamothe and Auclair, 1997). It compares RI(t1), measured shortly after irradiation, to RI(t2), acquired after some time delay. Grains of higher RI exhibit larger fading rates and, for a given grain population, RI(t1) and RI(t2) are linearly correlated (Fig. 1a). Therefore, it is postulated that extrapolation to a point where RI t1 RI t2 yields the R0I for which there would be no fading. The use of the fadia method was found to correctly assess the amount of anomalous fading in four feldspar-bearing sediment samples, three of which are of Pre-Quaternary age. The fourth
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 2 4 - 4
434
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
fadia corrected age of 6827 ka is in agreement with the U±Th dates of ca. 70±80 ka, obtained on corals extracted from the same unit (Norfolk Formation; Wehmiller et al., 1998). The objectives of this paper are to fully describe the methodology and laboratory measurements used in the MontreÂal laboratory and to discuss some complicating factors, the two most important being the scatter in luminescence measurements of single grains and the apparent limited applicability of the method to a unique grain population. Areas of future investigations are outlined. 2. Methodology: the fadia protocol
Fig. 1. (a) Fadia plot for 32 single feldspar grains (150±250 mm) from sample GP3 following the addition of 570 Gy t1 3 h; t2 10 days). (b) Histogram showing the range of Natural/Regenerated (1/RRI) values for the same set; the data under the arrows are shown as open circles in (a) and are not included in the linear regression. (c) Multiple aliquot additive growth curves for the same sample; prompt p 5 days; De 7024 Gy), delayed d 22 months; De 8125 Gy), and fadia corrected ( f; De 13529 Gy). The R0I intercepts (diamonds) obtained following the addition of 325, 570, and 1005 Gy are shown. Annual dose is 2:0020:10 Gy/ka.
sample is a Late Pleistocene marine sand from Virginia (USA; sample GP3), previously dated at ca. 40 ka by the IRSL additive multiple aliquot technique. The
The dierent measurements involved in the fadia method consist of the following steps: (1) Selection of grains; (2) Preheat; (3) IRSL-S0; (4) Gamma dose; (5) Preheat; (6) IRSL-S1; (7) Storage at room temperature for several days; (8) IRSL-S2; (9) Sunlamp bleach; (10) Gamma dose; (11) Preheat; (12) IRSL-S3; (13) Storage; (14) IRSL-S4. This sequence of measurements is summarized in Table 1 and some methodological comments are added below. The grains (usually 30±60) are ®rst selected using a 1 s infrared short shine. They must emit enough luminescence in the blue-UV spectral band, transmitted through a BG39/Corning 7-59 ®lters combination. The rate of success is sample and age dependent. In young samples, it is not uncommon to ®nd only 10% of the grains that emit more than ®ve times the background (bg050 counts/s). The minimum required light level is ®xed at 0300±500 counts/s, considering that the grains will be further preheated before the measurement of the natural signal (S0). For each and every IRSL measurement, four successive 1 s short shines are carried out in order to increase the reproducibility and evaluate the optical erosion. This is found by comparing the two last shines of each measurement with the two ®rst shines. The average optical erosion is 1±2%/1 s shine. To reduce the delay between irradiation and measurement, we selected a short on-plate preheat of 2508C/min as we found that the associated thermal erosion was similar to the 2208C/10 min preheat commonly used for feldspar. The same preheat is used after each irradiation step. The choice of the gamma dose is sample dependent, high doses giving better reproducibility (see below). The prompt luminescence (S1) is measured as soon as possible after the end of the irradiation and preheat t1 1±3 h). The grains are stored at room temperature for a period of 10±15 days (t2) and their IRSL is monitored again (S2). The grains are then bleached overnight under a solar lamp l > 460 nm), gamma regenerated, preheated, measured promptly (S3), and measured again after the
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
same time delay (S4). The RI and RRI are calculated as in Table 1. The reader is a Daybreak 1100 automatic luminescence system that can accommodate 20 samples. Therefore, for each run, a set of 16 grains is measured to which three natural ®ne-grain discs (4±11 mm) are added and one sample position is left empty. Natural ®ne-grain discs are used to monitor changes in reader sensitivity. Additive measurements (S1 and S2) are corrected for thermal erosion of the natural signal, evaluated from another set of natural grains preheated twice. The average value of thermal erosion is used for all the single grains. The delayed measurements (S2 and S4) are further corrected for optical erosion due to previous shines. The error for the RI values is based on the sum of statistical uncertainties at each measurement (N 1/2; Aitken, 1998). A plot of RI(t1) vs. RI(t2) allows the determination of R0I : The stable component of the induced luminescence is then estimated by comparing the R0I intercept with the RI obtained from multiple grain aliquots that have received the same laboratory dose. As an example, in our original work (Lamothe and Auclair, 1999), the R0I intercept for a suite of GP3 single feldspar grains following the addition of 1005 Gy was 5:1820:16: Since the RI from the multiple aliquots that had received the same dose was at 9:5720:29, the stable induced luminescence component was, therefore, calculated to be around 50%. The multiple aliquot growth curve is then corrected accordingly at all the other additive dose points. In order to test this correction, two other R0I dose-points (at 325 and 570 Gy, Fig. 1c) were measured. The R0I for N 570 Gy is found reasonably close to the corrected growth curve. However, the R0I obtained from the addition of 325 Gy plots signi®cantly below, suggesting a much higher fading ratio, of the order of 65%. This apparent discrepancy is discussed in the next section. Finally, one can use regeneration to derive the paleodose, and hence, monitor RRI. An example is found in Balescu et al. (in
435
press) for a penultimate interglacial sediment from Southern Quebec. 3. Discussion: complicating factors Results obtained so far from dierent geological contexts and sediments of dierent depositional ages clearly indicate that the fadia method can potentially circumvent the problem of anomalous fading in feldspar. However, there are several problems that need to be investigated as they can strongly aect both the precision and the accuracy of the R0I intercept. It is the experience of the authors that physical processes exhibited by single feldpar grains may be highly variable. However, at this stage of development, we identify the following as the limiting factors in the application of the method, in order of decreasing annoyance: (1) the high scatter in the natural luminescence signal arising from dim grains; (2) the diculty one has to identify and reject unbleached grains in the fadia analysis; (3) the steep slope of the fadia line with consequent high uncertainty in the R0I intercept; (4) the number of R0I intercepts that are needed to appropriately correct the additive and regeneration growth curves; (5) sensitivity changes following the sunlamp bleach in the determination of a RRI value; (6) possible variable thermal charge transfer and thermal erosion during the successive preheats. Only the ®rst two factors are considered here. 1. In order to document the eect of scatter, we turn again to sample GP3. In Fig. 2, the scatter for the luminescence emission of the grains before and after the addition of a low vs. a high gamma dose, with respect to their RI value, is shown. The relatively higher uncertainty for grains of low intensity will be strongly propagated in the ®nal RI value if dosed at low radiation levels. We believe that the R0I obtained for the N 325 Gy dose-point is, therefore, a result of low luminescence intensity and con-
Table 1 The fadia protocola Steps
Measurements
Ratios
Short shine Preheat; four short shines Dose; preheat; four short shines Storage; four short shines Bleach; dose; preheat; four short shines Storage; four short shines
Selection of grains Natural IRSL signal, S0 Additive prompt, S1 Additive delayed, S2 Regeneration prompt, S3 Regeneration delayed, S4
RI t1 S1=S0 RI t2 S2=S0 RRI t1 S3=S0 RRI t2 S4=S0
a
Short shine: 1 s, infrared, 208C; preheat: 2508C/min.
436
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
sequent lower precision on the determination of the value of R0I : Since low intensity grains must be part of the fadia analysis, particularly for young samples, the only remedy is to develop highly sensitive luminescence readers that could enhance the luminescence signal for single grains by, at least, one order of magnitude. 2. Another major drawback in the application of the method is the common occurrence of unbleached grains. Even in a presumably well-bleached sample, such as GP3, undesired unbleached grains are present. In order to detect those, we use a histogram of values as an indication of grain population homogeneity. Herein, we construct a Natural/Regenerated luminescence histogram (e.g. 1/RRI, see Fig. 1b) and values positively skewed away from the main Gaussian distribution are rejected. The assumption here is that unbleached grains are much less sensitive to a regeneration gamma dose. In each series of GP3 grains analyzed, 5±10% of the grains seem to have been buried without being fully exposed to sunlight (under the arrows in Fig. 1b). The following example demonstrates the combined eect of low luminescence intensity and the occurrence of unbleached grains in a sediment sample. The fadia method was applied to a Lateglacial glaciomarine sandy and clayey silt sample (CES-5), dated by 14C at 10:520:14 ka BP (uncalibrated) and by routine IRSL at ca. 40 ka (see Lamothe and Auclair, 1997 for more details). It was hoped that the approach described above could successfully detect the population of unbleached grains so that the fadia analysis could be carried out using only the population of well-bleached
Fig. 2. Statistical uncertainty (de®ned as N 1/2/N ) for the natural luminescence light emission (A and C from the 325 and 1005 Gy sets) and the natural plus laboratory dose (B and D from the 325 and 1005 Gy sets); the uncertainties are plotted with respect to their corresponding RI(t1) values.
grains. In order to document the behavior of the unbleached grains, a glacio¯uvial sand (CES-2), not expected to be zeroed, was also investigated. A gamma dose of 50 Gy was used for both RI and RRI measurements. Such a low dose was selected to ensure the linearity of the growth of the IRSL signal upon dose for the well-bleached grains. Fig. 3 presents the fadia plots and the respective 1/RRI ratios for both samples. For CES-2, the unbleached grains can clearly be characterized by RI close to 1 and high 1/RRI values. Even though the scatter is high, the R0I comes to 1:1020:11 as expected for an unbleached sediment. However, for the CES-5 sample, the R0I intercept of 1:4820:25 implies an overestimated paleodose of 150 Gy; hence, an IRSL date of, at least, 40±50 ka, similar to the multiple aliquot IRSL date. In this sediment sample, the presumably well-bleached grains, i.e. those with high RI and low 1/RRI values, cannot be discriminated against the unbleached ones and constitute probably less than half of the total grain population. Scatter is uncomfortably high since most of the well-bleached grains are dim. Therefore, low abundance and high scatter associated with this population does not allow one to derive a signi®cant fadia correction. To reduce scatter, one could use higher gamma doses but deviation from linearity would then be a problem. Another possibility is to use more than one dose, much as in the single aliquot method (Duller, 1991). The conclusion, herein, is that the fadia method cannot be applied at the moment to poorly bleached sediment samples. 4. Conclusion The fadia method has been only recently introduced in the luminescence literature, and as such, this approach shall open new avenues of research in the ®eld of sediment dating. An increase in the eciency with which luminescence can be detected from single feldspar grains is seen as an urgent remedy to several problems documented above. New luminescence readers have to be developed in order to increase the reproducibility and reduce the time required to run a large number of single grains, such as the new single grain laser luminescence system described by Bùtter-Jensen et al., 2000. Some underlying assumptions will need to be tested further. For example, one puzzling implication of a unique R0I intercept for a population of grains is that the deduced ``stable'' luminescence sensitivity to dose appears similar for any K-feldspar. This deduction may not be veri®ed for every sample. One critical limitation is the steep slope of the fadia regression line that may result in uncomfortably high errors in the R0I intercept. Routes of acceleration for fading are being
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
437
Fig. 3. Fadia plots and Natural/Regenerated (1/RRI) values for sample CES-2 (a, b) and sample CES-5 (c, d). The additive and regeneration dose is 50 Gy. Each sample set contains 48 grains (150±250 mm). The histogram for CES-5 suggests that it is composed of a signi®cant number of unbleached grains, such as the ones contained in CES-2.
investigated (e.g. extended preheats at lower temperatures) but the phenomenology of anomalous fading makes it unlikely that other factors than time will contribute to the solution of this problem. Finally, evidence is accumulating in this laboratory and elsewhere that sensitivity change occurs for feldspar grains after they have been exposed to a sunlamp bleach. Correction for this unwanted behavior may follow the methods of Murray and Roberts (1998) and Wallinga et al. (2000). Notwithstanding the above, we would argue that as the investigation in the dating of single feldspar grains proceed, the fadia method shall prove to be an appropriate approach to the dating of geological and archaeological events.
Acknowledgements This work is funded through NSERC (Canada) operating grant OGP0037375. The authors wish to thank D.J. Huntley (Physics, SFU) for sharing ideas about the problem of anomalous fading.
References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford (267 pp.). Balescu, S., Lamothe, M., Auclair, M., Shilts, W.W., in press. IRSL dating of Middle Pleistocene interglacial sediments
438
M. Lamothe, M. Auclair / Radiation Measurements 32 (2000) 433±438
from Southern Quebec (Canada) using multiple and single grain aliquots. Quaternary Geochronology. Bùtter-Jensen, L., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 523±528. Duller, G.A.T., 1991. Equivalent dose determination using single aliquots. Nuclear Tracks and Radiation Measurements 18, 371±378. Lamothe, M., Auclair, M., 1997. Assessing the datability of young sediments by IRSL using an intrinsic laboratory protocol. Radiation Measurements 27, 107±118. Lamothe, M., Auclair, M., 1999. A solution to anomalous fading and age shortfalls in optical dating of feldspar minerals. Earth and Planetary Science Letters 171, 319±323. Lamothe, M., Balescu, S., Auclair, M., 1994. Natural IRSL intensities and apparent luminescence ages of single feldspar grains extracted from partially bleached sediments. Radiation Measurements 23, 555±562. Murray, A.S., Roberts, R.G., 1997. Determining the burial time of single grains of quartz using optically stimulated luminescence. Earth and Planetary Science Letters 152, 163±180.
Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose singlealiquot protocol. Radiation Measurements 29, 503±515. Spooner, N.A., 1994. The anomalous fading of infraredstimulated luminescence from feldspars. Radiation Measurements 23, 625±632. Visocekas, R., 1979. Miscellanous aspects of arti®cial TL of calcite: emission spectra athermal detrapping and anomalous fading. PACT 3, 258±265. Visocekas, R., 2000. Monitoring anomalous fading of TL in feldspars using far-red TL emission as a gauge. Radiation Measurements 32, 499±504. Wallinga, J., Murray, G.A.T., Duller, G.A.T., Tornqvist, T.E., 2000. Infrared stimulated luminescence dating of feldspar: the problem of age underestimation. Radiation Measurements. Wehmiller, J.F., Lamothe, M., Noller, J.S., Comparison of approaches to dating Atlantic coastal plain sediments, Virginia Beach, VA. In: Dating and Earthquakes: Review of Quaternary Geochronology and its Application to Paleoseismology, U.S. Nuclear Regulation Commission, NUREG/CR-5562, 1998, pp. 4.3±4.31.