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Optical dating studies of quartz and feldspar sediment extracts
Quaternary Science Reviews, Vol. 7, pp. 373-380, 1988. Printed in Great Britain. All rights reserved.
0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc
OPTICAL DATING STUDIES OF QUARTZ AND FELDSPAR SEDIMENT EXTRACTS D.I. Godfrey-Smith,* D.J. Huntley* and W.-H. Chent *Department of Physics, Simon Fraser University, Burnaby, B.C., V5A 1S6, Canada t lnstitute of Electro-optical Engineering, National Chiao Tung University, Hsin-Chu, Taiwan, 30049, R.O.C. Continuing developmental work into optical dating has led to some clarification of the optical behaviour of quartz and feldspar sediment extracts. We have verified the key assumption intrinsic to the method - - that the optical signal is zeroed completely upon exposure to daylight. We also show that the rate of zeroing in optical dating is much more rapid than it is in TL dating. Studies concerned with the extraction of the thermally stable signal from laboratory-irradiated samples showed that problems may arise from medium-to-high-temperature preheating of young samples. Optical alternatives to thermal stabilization were tested using dye and krypton lasers. Using a krypton laser we have succeeded in obtaining an equivalent dose of 0.0 + 0.7 Gy for the quartz from a modern intertidal sand, and an average dose of 120 + 22 Gy for the feldspar extract of a - 7 0 ka sand unit. Both were obtained using six wavelengths, from the infrared to the violet, and no dependence on wavelength was found. We found that the photon energies of infrared and deep red light can stimulate luminescence in unirradiated quartz and feldspar extracts of sediments. This was not expected, and suggests that the process of trap emptying is more complex than we had envisioned, but implies a far greater than expected degree of technical flexibility for the optical dating method.
INTRODUCTION Optical dating is a new, and as yet experimental, dating technique for the direct dating of sediments; it was introduced 3 years ago by Huntley et al. (1985). In this method the past radiation dose accumulated in quartz and feldspar mineral grains since their deposition is measured, and therefore the technique is in the same class as the thermoluminescence (TL) and electron spin resonance (ESR) dating techniques. As in the TL dating of sediments, the zeroing mechanism in optical dating is exposure to light. The major difference between the two methods is that in TL the past radiation dose absorbed is measured by heating the mineral, while in optical dating the dose is measured by exposing the minerals to monochromatic light; so far the 514 nm green wavelength emitted by an argon ion laser has normally been used. This difference in techniques is critical, since the radiation dose can be thought to result in electrons in both light-sensitive and in light-insensitive electron traps in a mineral. In a TL measurement, the heating process yields a TL signal from electrons in both types of traps and therefore a sediment sample freshly exposed to sunlight still yields a TL signal. In order to obtain the correct age it is necessary to determine this TL signal, and various methods have been devised for doing this. In contrast, in optical dating, since light is used to measure the trapped electron population, only the electrons accumulated in light-sensitive electron traps, those assumed to have been completely emptied during the process of deposition, are measured, while those electrons accumulated in light-insensitive traps are not. In this discussion we have implied a clear distinction between the two kinds of electron traps. In practice, however, such a sharp distinction is not evident, since light-sensitive traps exist with sensitivities that vary
from seconds to days of sunlight being required for their emptying. In the experiments discussed below we were addressing the following questions: (1) How much sunlight is required during the deposition of a sediment to effectively zero it? How is this affected by less than ideal conditions? How different is it for quartz and feldspar? How does this compare with the exposure times required for TL dating of sediments? (2) In order to determine the equivalent dose by using laboratory irradiated samples, it is thought to be necessary to empty the thermally unstable lightsensitive traps before doing the measurements. In principle, this can be done either thermally or optically. What are the advantages and disadvantages of each method? (3) With optical trap emptying, it should be possible to sample successively deeper, and hence more stable, traps by using optical excitation with monochromatic light of successively greater photon energies. Can we then construct a graph of equivalent dose vs. photon energy, which should start at zero at low photon energy, and rise to a plateau at higher energies? If so, this should be a good stability test, similar to the plateau test used in TL dating. SAMPLES
The samples used in the measurements described below are quartz or feldspar extracts of the natural sediments described in Table 1. The quartz extracts were prepared using the standard quartz extraction procedure used in TL, including heavy liquid separation in an aqueous solution of sodium polytungstate at a density of 2.75 g/ml. Following the HF treatment and prior to magnetic separation, the quartz extracts were
373
374
D.I. Godfrey-Smith et al. 600
TABLE 1. Sediments studied
A: QUARTZ
Sample WSBS
PGQS
SNMS
SPLS
Description
Geological reference .
Whaling Station Bay Sand, modern, intertidal. Hornby Island, British Columbia (49°32'N 124°37'W) Point Grey Quadra Sand, 25.5 ka, Vancouver, British Columbia (49°17'N 123°15'W)
,"t" Ll.l r, ,m Z
~ - ~ _
300
0
Ciague, 1977
St Nicolas Marine Sand, 10.0 ka St Nicolas, Qu6bec (46°42'N 71°23'W)
Gaddet al., 1972 Hillaire-Marcel, 1981
St Pierre Lower Sand, --70 ka, Pierreville, Qu6bec (46°03'N 72°47'W)
Lamothe, 1984, 1985
SESA-43 Southeast South Australia, -500 ka (?), oxidized, noncalcareous sand unit, Woolumbool Range. Map: Bool Lagoon, grid ref. 564 033
,
0 LU 03 ,n," W 0.. O3 I-Z
o 0 Z
0
15,000 I a..
Schwebel,1978 Idnurm and Cook, 1980
treated with warm AI20 3 or HC1, and wet-screened through a 75 Ixm screen; this ensured that poorly dissolved feldspar grains, precipitates, and badly broken quartz grains were removed from the sample. The grains were sprinkled in a monolayer onto A1 disks; sample masses were in the range of 3-7 mg. Feldspar extracts were prepared as above, with the exception that the 40 min HF treatment, the subsequent chemical rinse, and the wet-screening steps were omitted. This resulted in a mixed quartz and feldspar extract; however, since the proportion of feldspar to quartz in these sediments is high (about 4:1 by weight for PGQS), and feldspar has an optical signal which is much brighter than that of quartz (see below), the optical signal measured from such a mixed extract may be considered to be due entirely to its feldspar component. The proportion of plagioclases to K-feldspars in PGQS is about 6:1; this was determined by weighing fractions of different densities, separated as suggested by Mejdahl (1985). Typical quartz and feldspar luminescence curves, illustrating their relative luminescence intensities and rates of signal decay under 10 mW/cm 2 of Ar laser light, are shown for sample PGQS in Fig. 1. In this sample, the feldspar is 50 times brighter than the quartz; in contrast, the feldspar extract of sample SPLS was 240 times brighter than its quartz extract. With the exception of two samples, the extracts were prepared from 90-125 I~m grains, separated from the bulk sediment by sieving; sample WSBS was prepared from 150-180 Ixm grains, and SNMS from 125-150 ~m grains. The feldspar extract of sample SPLS was given a 40-min etch in 10% H F to remove the or-dosed outer layer on the grains.
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4'0 Laser OFF
FIG. 1. Typical luminescence curves of (a) quartz, and (b) feldspar, extracts from sample PGQS, using 514 nm excitation. Background (12 cps per rag), was not subtracted. Note the differences in signal intensity and rate of signal decay between the quartz and the feldspar extracts.
APPARATUS The apparatus used was essentially that described by Huntley et al. (1985), with minor changes. A sliding sample holder has been constructed which allows the samples to be rapidly changed without moving the photomultiplier tube or turning off the high voltage to it. The chamber has been anodized black. Input and output filters were selected according to the exciting wavelength of the laser beam, and are listed in Table 2. Sharp-cut, long-pass filters were used on the input beam to prevent wavelengths shorter than those of the laser beam from entering the chamber; in all cases the luminescence from the sample was detected in a region of higher photon energy than that of the exciting beam. Data were collected and processed by an IBM PC-AT microcomputer equipped with a multichannel-analyzer board. This arrangement allowed samples to be measured at a rate of one every two minutes.
OPTICAL SIGNAL BLEACHING BY SUNLIGHT The key assumption in the optical dating method is that the optical signal is effectively completely bleached, given a sufficiently long exposure to sunlight. On the basis of the luminescence curve under optical stimulation, it is possible to estimate the rate of bleaching of the optical signal by natural sunlight, and therefore the duration of exposure to sun necessary to ensure a completely zeroed sediment. For quartz, we had predicted earlier that 90% of the natural optical signal should be erased following a 10 sec exposure to sunlight.
375
Optical Dating Studies TABLE 2. Details of the input and output filters and their transmission characteristics for the various laser wavelengths used Laser line (nm)
Input filters
Transmittance (%) at laser line
799 & 753 647.1 568.2 530.9 514.5 468
RG610 RG610 3-69 3-70 + 3-71 + 4-96 3-70 + 3-71 + 4-96 GG435 + 4-96
90 90 91 45 45 83
413
3-74, 4-96, 3-75, 4-96, 0-51
11
Output filters 2x 2x 3x 4x 4x 3x 3x 4x
Output transmittance peak (nm)
Peak transmittance (%)
400 400 380 380 380 320 320 320
32 42 56 48 48 37 52 41
7-59 + BG38 + 2x 4-96 7-59 + BG38 + 4-96 7-59 + BG38 7-59 + BG38 7-59 + BG38 U G l l + 2x 7-54 (WSBS) U G l l (other samples) UGll
Notes: (1) Where necessary, for feldspar extracts of samples PGQS, SNMS, and SPLS, neutral density filters were added to the output filters. (2) The BG38 filter was used to suppress the red emission of quartz, and the infrared emission of feldspar, which have been demonstrated in the TL of these minerals (Huntley et al., 1988; Huntley et al., 1988). (3) Filters with numeric designations are Corning colour glass filters; those with alphabetical prefixes are Schott colour glass filters. (4) All lines are from a krypton laser, except for the 514.5 nm line, which is from an argon laser.
Experimental confirmation of the completeness of bleaching may be obtained from measurement of the optical signal intensity of an old sediment extract after it has been exposed to sunlight, and from measurement of the natural optical signal intensity of modern sediments. The sunlight bleaching experiments were carried out on two samples, a - 7 0 ka feldspar extract and a -500 ka quartz extract. These were chosen because they give the most intense luminescence signals of any feldspar or quartz extracts studied by us so far. The sample disks were exposed to bright sunlight, on clear days, in May to August (spring/summer), at Simon Fraser University (365 m above sea level). For comparison, TL bleaching curves were measured for the same samples. The results are shown in Fig. 2. They demonstrate that, for quartz, the assumption of rapid and complete bleaching of the optical signal is essentially correct: the optical signal decayed from a 'natural' unbleached value of 1.15 x 10 6 cps/mg to only 1% of this after 10 sec of sun, and to <150 cps/mg after 30 min of sun. The TL response of this quartz to sunlight was very different: there was no change from its unbleached value for the first 1.5 min of sun, and after 20 hours the TL decreased to 17% of its unbleached value. It may be concluded from the above that the optical signal in quartz is very much more sensitive to light than the TL of quartz. Thus, the zeroing requirements for optical dating should be very much less stringent than those for TL. The difference appears to be due to the presence of electron traps which are extremely sensitive to sunlight; these give a large optical signal, but make a negligible contribution to the TL. The feldspar optical response to sunlight, also shown in Fig. 2, is also very rapid, but not as rapid as that of the quartz. This was surprising, since, on the basis of the initial optical signal decay under laser stimulation, it was expected that a feldspar should bleach much more quickly than quartz. Nevertheless, this sample's optical signal decreases from an initial value of 3 x 105
10 r • -¢,,
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1 0 3.
t 02
* Feldspar Optical o Feldspar TL
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DURATION OF SUNLIGHT EXPOSURE (seconds)
FIG. 2. The effect of sunlight exposure on the optical and TL signals of quartz and feldspar. The unbleached values are shown along the vertical axis; each point represents the average of 4 samples, each normalized to its mass. The optical signal is that for one second immediately after the laser is switched on, measured at 10 mW/cm 2 for quartz, and at 15 mW/cm 2 for feldspar, using 514 nm as the exciting wavelength. Backgrounds were subtracted. The TL for quartz is at a well-defined peak at 320-330°C, and for feldspar at 310-320°C; these were obtained using a 5°C/sec heating rate, and a 558 filter. Samples are: quartz = SESA-43; feldspar = SPLS.
cps/mg to 1% of this value in 9 min, and to 400 cps/mg after 20 hours of sun, with no sign of having reached a residual level. The expectation is that, given sufficient time, the luminescence of this feldspar should also decrease to a negligibly low level. The TL response of this sample is much less light-sensitive. We also verified that the short exposure to 514 nm laser light during an optical measurement did not affect the natural TL of either the quartz or the feldspar. On the basis of the above comparisons, we conclude that both optical and TL dating studies may be done on the same sample, provided that the optical measurements are done first, and that typical light exposures of <500 mJ/cm2 are used during optical measurements. For samples well bleached at deposition, this allows a
376
D.I. Godfrey-Smith et al.
sediment's past radiation dose to be determined by two different methods. We made use of the above principle in obtaining the data shown in Fig. 2; here the TL measurements were made on the same sample disks after the laser measurements were done. Since most sediments do not undergo bleaching under the ideal conditions of continuous bright sunlight, the experiment on quartz was repeated using daylight on an overcast day. The result, shown in Fig. 3, demonstrates that quartz is zeroed in the absence of direct sunlight. This suggests that bleaching of the optically sensitive traps in quartz can be accomplished by the mid-range and long wavelengths of visible light, and is not due solely to the UV component of sunlight. Optical bleaching will therefore proceed under naturally low-light and restricted-light conditions, although at a proportionately slower rate. Confirmation of complete zeroing of quartz under natural conditions has been obtained from the quartz extract of WSBS, a modern intertidal sand. This sample, which showed a normal quartz TL emission, yielded no measurable signal under laser stimulation by any of the wavelengths listed in Table 2. When the sample was given radiation doses of 7 to 45 Gy, an optical signal proportional to the radiation dose was observed. This result is described below.
THERMAL STABILITY CONSIDERATIONS The most significant drawback in optical dating is that an analogue of TL dating's 'plateau test' is difficult to obtain. One method, suggested by Huntley et al. (1985), is to preheat a large number of natural and laboratory-irradiated samples to different temperatures, and to determine the past dose for each preheat temperature separately. A plot of the estimated dose as a function of preheat temperature should, in principle,
reach a plateau level when a thermally stable range of optical traps is reached. The difficulty with preheating prior to laser stimulation is revealed when young or modern samples are heated to temperatures greater than 200°C. The effect of preheating an unirradiated modern quartz, WSBS, to temperatures ranging from 75-425°C, is that an unwanted luminescence signal is created; the magnitude of the effect is shown in Fig. 4. A similar result, on a single partially bleached quartz sample, successively heated and exposed to Ar laser light, was obtained by Smith et al. (1986). The magnitude of the effect can be quantified in terms of the natural luminescence from the quartz extract of sample PGQS, since both sediments were sampled in the Gulf of Georgia area, and so both quartz extracts are likely of the same geological origin. The highest value for the undelayed measurement corresponds to 30% of the natural luminescence of PGQS, which is equivalent to 7.6 ka, while the highest value following a 48-hour delay is 15% of PGQS, or the equivalent of 3.8 ka. We suggest that the effect results from thermal excitation of electrons initially occupying thermally stable light-insensitive traps. Most of these electrons recombine and yield the natural TL glow curve typical of unirradiated zero-age quartz extracts. A small fraction of the electrons do not recombine but are retrapped in shallow and deep light-sensitive traps, from which they can be excited optically, to yield the 2,500 [] No Delay •
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PREHEAT TEMPERATURE (°C)
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2h
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0 1 10 10a 10~ 104 105 DURATION OF DAYLIGHT EXPOSURE (seconds)
FIG. 3. The effect of daylight exposure on the optical signal of quartz. The light intensity on the overcast day was 10% of that on a clear day; the data show that the sample was bleached 10 times more slowly on the overcast day than on a clear day, demonstrating that the rate of bleaching is proportional to the ambient light level.
FIG. 4. Optical signal created in a zero-age quartz by preheating. The intensity of this signal as a function of preheat temperature is shown. T h e sample is WSBS, undosed. Fresh disks were used for each preheat temperature. Preheating was done in a conventional T L oven, u n d e r high-purity A r gas, at a heating rate of 5°C/sec. The heater was switched off when the temperature was reached, and the samples cooled to <50°C prior to opening the glow oven. The data shown are mass-normalized, integrated over the first 20 sec of stimulation by a 10 m W / c m 2 A r laser beam. U p p e r curve (squares, one m e a s u r e m e n t per point): data m e a s u r e d immediately after preheating. Lower curve (circles, average of two m e a s u r e m e n t s per point): data m e a s u r e d following a 48-hour delay. A dark count of 14.5 cps was subtracted from all points; the signal above dark count from an unheated sample was equal to that from a blank AI disk.
377
Optical Dating Studies
curves in Fig. 4. The difference in intensities between immediate and delayed optical measurements is most LU likely due to room temperature recombination of those Z electrons which had been retrapped in the shallow, and therefore thermally unstable, light-sensitive traps. We ~5 now feel that our 250°C preheat was at least partly responsible for the dose overestimates in CBBS and HaRk-ID in our initial work. X < Some possible alternatives to high-temperature preheating in order to empty the electrons from thermally 0 unstable traps following laboratory irradiation include 300 700 560 long-term post-irradiation delays of weeks to months, a WAVELENGTH (nm) prolonged low-temperature treatment, or the use of optical rather than thermal means of hastening electron FIG. 5. The lasing spectrum of a krypton ion laser (Coherent Radiation, model CR-2000K). Heavy bars indicate the lines used. eviction. Similar approaches have been demonstrated The infrared lines 799 nm and 753 nm were used together; all other to be successful in removing the anomalous fading lines were used in monochromatic mode. component in the TL of irradiated zircons and pottery samples (Templer, 1985; Bowman, 1988). The last of light from a blank AI disk, the brass sample holder, and these carries with it the advantage that the use of light an unirradiated sample of WSBS. All three showed no of different photon energies should allow electrons in decrease with time, and were similar to each other. The traps of different depths to be sampled; the past dose only exception to the above was that when the violet can then be estimated as a function of the energy of the (413 nm) laser line was used, a strong response (about exciting photons. One way to accomplish this is to 8 times that of the scattered light level off brass) was perform a continuous scan through the whole or a part observed from a blank AI disk. The responses from a of the visible spectrum, while observing the sample's freshly cleaned, HF-etched A1 disk and a stainless steel optical response. In a preliminary experiment, we used disk, were equal to that from brass. Wiping of the AI a dye laser slowly tuned from 684 nm to 615 nm, and disk did not decrease the signal, implying that the effect obtained measurable luminescence in the blue-violet was not due to an accidentally-deposited contaminant, region for both the natural and irradiated quartz and but to some process involving the A1 surface. A sample feldspar extracts of sample PGQS, as well as for an of WSBS showed much less optical response than a older quartz. None of the samples showed any structure blank disk; this was clearly due to a shielding of the AI in their excitation spectra over this range; instead, a surface by the non-luminescent quartz grains. Theregradual increase in luminescence intensity per mW of fore, if very short wavelengths are to be used in optical incident light was seen as the energy of the exciting dating studies, we recommend that stainless steel, photons increased. An intense source of white light, brass, gold-plated, or rhodium-plated substrates should coupled with a monochromator, could be used to be used in order to avoid this 'spurious' luminescence perform such a scan through a wider portion of the phenomenon. visible spectrum. The krypton laser measurements yielded two interAn alternative method is to select a small number of esting results. The first, and unexpected, result was that discrete wavelengths, and to use these to sample the measurable luminescence was obtained from all natural optically-sensitive traps. We chose to employ seven of samples except WSBS under IR (799 + 753 nm) stimuthe lasing wavelengths of a krypton ion laser, shown lation. Two growth curves under IR stimulation are schematically in Fig. 5, to estimate the past doses of shown in Fig. 6a for the feldspar extract of sample four quartz and three feldspar extracts, of samples SPLS. The past dose estimates for this sample at WSBS, SNMS, PGQS, and SPLS. The two IR wave- t = 0-2.5 sec under all six stimulating wavelengths are lengths, 799 nm and 753 nm, were used together. Sets summarized in Fig. 7a. The average past dose estimate of 30 A1 disks of each extract were prepared; each of is 120 + 22 Gy at t = 0-2.5 sec, and it rises to 145 + 28 these were divided into five subsets, of which one was Gy after 21 sec of laser light, as shown in Fig. 7b. Using left unirradiated, and four were given various labora- the results of K20 analysis, a-counting, and a water tory radiation doses up to 3 times the expected natural content estimate of A = 0.21 + 0.25 (average of in situ dose. Optical measurements commenced following a and saturation values for sample SP-2 of Lamothe, 31/2-month post-irradiation delay. The laser was oper- 1985), a dose rate of 2.6 Gy/ka due to 13 and ated at 80% of the rated maximum single-line power contributions was calculated. For the mean past dose output; before adjusting for the transmission charac- estimates given above, this yields age estimates of teristics of the input filters listed in Table 2, 10% of this 46 + 14 ka and 55 + 16 ka, respectively. These values would have reached the sample. Each sample was are somewhat lower than the known age of this exposed to each wavelength for 45 sec, starting with sediment, as can be observed by comparison with the infrared, and ending with violet light. 14C and TL dates summarized in Table 3. Since our Background levels were monitored at each wave- experiments on the fading of the optical signal in length and for each sample, by measuring the scattered feldspar extracts which had been stored at room
I,II
378
D.I. Godfrey-Smith et al. TABLE 3. A summary of chronometric results relevant to SPLS
A: SPLS
Method
8xl 0 s
14C OC 4xl W O.
0n
Sample
Age
Reference
peat
66.5 _+ 1.6 ka
GrN-1711, Vogel and Waterbolk (1972)
peat
74.7 + 2.7 ka 2.0
QL-198, Stuiver et al. (1978)
-
/ ¢v W O_
i,,Y 0
-200
I
i
200
400
TL, on silt sample RS overlying the peat:
I-Z
o Z
o
~/SBS 6O o
-r
•
20
i
I ¢ ~
115-17.5s
0 20 '40 APPLIED GAMMA DOSE (Gy)
FIG. 6. Growth curves for samples SPLS and WSBS, shown for t = 0-2.5 sec and t = 15-17.5 sec after the laser was switched on. A: growth curves for feldspar sample SPLS, under infrared (799 and 753 nm) excitation. The equivalent doses at this photon energy, determined by exponential fitting, are 136 + 11 Gy for t = 0-2.5 sec, and 166 + 18 Gy for t = 15-17.5 sec. The background is negligible in comparison with the sample's luminescence, and was not subtracted. B: growth curves for modern quartz sample WSBS under red (647 nm) excitation. The equivalent doses at this photon energy, determined by linear fitting, are 1.7 + 0.6 Gy for t = 0-2.5 sec, and 0.2 + 0.8 Gy for t = 15-17.5 sec. A mean background value of 12.8 counts per 2.5 sec was subtracted from all points. All fits are least squares with weighting inversely proportional to the luminescence intensity before the background was subtracted.
temperature following laboratory irradiation showed that fading does not reach a limit after 68 days of storage, we feel that the ages obtained above for SPLS should be considered as minimum ages only. The second interesting result was the complete lack of a luminescent response from the modern quartz of sample WSBS, regardless of the stimulating wavelength, unless it had first been irradiated. Two typical growth curves for this sample under excitation by the
A
}
~ 200 z~u~ Mean
W
quartz (RS-Q)
Lamothe (1984, 1985)
61.1 + 11.1 ka
red wavelength are shown in Fig. 6b. The sample's response at zero dose did not change with time, which is consistent with scattered light and not luminescence, whereas the luminescence from irradiated samples decreased with time as shown. Equivalent doses were calculated for five of the stimulating wavelengths, using linear least squares fitting. Due to the problems encountered during background measurements under violet light stimulation, equivalent doses were not determined for this wavelength. The equivalent dose estimates are shown for t = 0-2.5 sec in Fig. 8a; their resultant mean is 0.0 + 0.7 Gy. The change in the mean dose with time after the laser was switched on is also shown in Fig. 8b. Growth curves for the other non-zero-age samples extrapolated to order-of-magnitude dose estimates which were also consistent with the expected ages. However, the intrinsic scatter, especially in the quartz samples, prevented us from obtaining meaningful numerical results at this time. Normalization methods
A 150
Lamothe (1984, 1985)
Notes: (1) Sample SPLS was collected 30-40 cm below the peat; sample RS was collected 30 cm above the peat. (2) The lower sand unit (from which SPLS was collected) is 3 m thick, the peat is highly compressed and is 30 cm thick, and the upper silt unit is 4 m thick. (3) The three units make up a single lithostratigraphic unit known as the St-Pierre Sediments, within which all stratigraphic relationships are conformable. (4) Contacts with the underlying and overlying lithostratigraphic units are unconformable (Lamothe, 1984),
40
0 -10
polymineral 61.2 + 9.2 ka (RS-2000)
t}B
o o lOO Z w
~, l o o
5 0
0
tt tt ttt
_> 5O
Z w
01.5
210
215
PHOTON ENERGY (eV)
310
o
0
'
2'0
'
40
TIME (seconds)
FIG. 7. A: the optical 'plateau test' for the feldspar extract of SPLS. All six equivalent doses shown are for t = 0-2.5 sec after the laser was switched on. The average of these dose estimates is 120 + 22 Gy. B: the change with time after switching the laser on in the mean equivalent dose estimate, averaged over all six wavelengths.
379
Optical Dating Studies
based on criteria other than sample mass need to be applied to these results. Finally, we have observed in the old quartz but not the feldspar samples, differences in the shape of the luminescence curves as the photon energy of the incident laser beam was increased. Under IR excitation, all showed the typical luminescence shown in Fig. la, with varying degrees of intensity. Under green, blue, or violet excitation, some samples began to show luminescence that increased in intensity during the first one to three seconds after the laser was switched on, and subsequently decreased. This occurred under green, blue, and violet excitation for sample SNMS and is shown in Fig. 9. The effect was also observed under blue and violet excitation in SPLS, and possibly, but not clearly, under violet excitation in PGQS.
pected. Firstly, Mott and Gurney (1984) showed that optical trap depths are expected to be greater than thermal trap depths by a factor approximately equal to the ratio of the static and optical dielectric constants. For quartz these are 4.3 and 2.4, respectively, and yield a ratio of --1.8. Secondly, in a test of the effect of the 514 nm wavelength on the TL of quartz, Smith et al. (1986) observed that the 325°C peak is bleached, but the 375°C peak is not. Since the energy of a 514 nm photon is 2.41 eV, and the trap depth of the 325°C peak is - 1 . 7 eV (estimated on the basis of data presented in Table E. 1 in Aitken, 1985), this yields an experimental optical:thermal trap depth ratio of 1.4. On the basis of the above estimates, we would expect 753 nm photons, which have an energy of 1.65 eV, to excite electrons from traps with thermal depths up to about 1.2 eV only. From TL studies it is known that such trapped electrons are not thermally stable at environmental temperatures over the time scale of interest (Aitken, 1985). Thus, we did not expect that IR excitation would result in luminescence from any
DISCUSSION The discovery that infrared light was able to evict electrons from thermally stable states was not ex-
A 2 LU
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B
O a
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~-2 LU
-41.5
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3 ,.0
-4 0 ~
20
10
TIME(seconds)
(eV)
FIG. 8. A: the optical 'plateau test' for the quartz extract of WSBS. The five equivalent doses shown were determined for t = 0-2.5 sec after the laser was switched on. The average of these dose estimates is 0.0 _+ 0.7 Gy. B: the change with time in the mean equivalent dose estimate, averaged over five wavelengths.
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0
20
40
TIME (seconds) FIG. 9. Luminescence curves for quartz sample SNMS, undosed, under excitation by the various krypton laser lines. Backgrounds, in order of decreasing wavelength, were: 0.3, 0.5, 3.7, 5.2, 0.5, and 10 counts per 0.1 sec per mg; they were not subtracted from the curves shown. Measurements were made successively on the same disk, in order of decreasing wavelength. Each curve shown is an average of 4 measurements.
D.I. Godfrey-Smith et al.
380
natural samples. We have verified that it does with measurements of luminescence from natural quartz and feldspar samples using ~5 mW from an infrared lightemitting diode with an emission peak at 880 nm (FWHM = 100 nm). Thus, the simple model presented above is clearly incorrect, and must be abandoned. It appears that the relationship between the optical and the TL behaviour of minerals is quite complex. The association of the optical signal with the 325°C TL peak (Smith et al., 1986) must be viewed with caution, since we have found that heating of quartz samples beyond this temperature does not result in erasure of the optical signal. We feel that the photon energy of the exciting wavelength cannot be directly related to an electron trap depth or to a particular emission peak in TL, but that electron eviction from a light-sensitive trap into the conduction band is a complex, multi-step, multi-photon process. An alternative explanation is that the optically-evicted electrons are assisted into the conduction band thermally as suggested by Hiitt et al. (1988). CONCLUSIONS The results of bleaching by natural light of the brightest of our feldspar and quartz samples support our initial hypothesis of complete bleaching of the optical signal in sediments. Terrestrial sediments such as loess and beach deposits, sand dunes, and other sediments with a subaerial mode of deposition are therefore highly suitable for this method. The results of bleaching under overcast conditions imply that sediments deposited under a UV blocker, such as a water layer, will also be completely zeroed, although a proportionately longer bleaching period will be required. We therefore conclude that shallow marine, lacustrine, and estuarine deposits should also be suitable for optical dating, assuming that the deposition is sufficiently slow. We found that preheating a sample which yielded no natural luminescence created an unwanted luminescence signal. This suggests that caution and a lot of empirical work are needed before optical dating can be applied to heated materials such as pottery, tephra, or metamorphosed sediments. Measurements of a modern quartz extract demonstrated that it has been zeroed completely in nature, since it gives no natural luminescence under any wavelength of incident beam employed. An equivalent dose of 0.0 + 0.7 Gy was obtained for it. We do not wish to place undue weight on a result from a single modern quartz sediment; clearly, similar tests on other modern sediments, preferably from other geological areas to ensure a variety of sources of the quartz, are necessary to confirm its generality. The results of our experiments lead us to continued optimism for the optical dating method. The discovery
that even infrared photons can be used to stimulate natural luminescence from a sediment implies a greater degree of technical flexibility than we had previously thought possible for the method, but that the mechanism of optical excitation is more complex than had initially been expected. ACKNOWLEDGEMENTS We would like to thank K.E. Rieckhoff for the use of his dye and krypton lasers; M. Lamothe for his guidance, assistance, and very, welcome advice in the collection of the QuEbec samples; J.T. Hutton and J.R. Prescott for collection of the Australian sample; and O. Lian for much assistance in the laboratory. This research was supported by the Natural Sciences and Engineering Research Council, and the Department of Energy, Mines and Resources of Canada.
REFERENCES Aitken, M.J. (1985). Thermoluminescence Dating, Academic Press, London. Bowman, S.G.E. (1988). Observations of anomalous fading in Maiolica. Nuclear Tracks and Radiation Measurements, 14, 131-137. Clague, J.J. (1977). Quadra Sand: A study of the Late Pleistocene geology and geomorphic history of coastal southwest British Columbia. Geological Survey of Canada, Paper 77-17. Gadd, N.R., LaSalle, P., MacDonald, B.C., Shilts, W.W. and Dionne, J.C. (1972). GEologie et gEormorphologie du Quaternaire dans le Sud du QuEbec. Livret-guide d' excursion C-44, 24e Congrds International de G~ologie, MontrEal, 74 pp. Hillaire-Marcel, C. (1981). Paleo-oceanographie isotopique des mers post-glaciaires du QuEbec. Palaeogeography, Palaeoclimatology, Palaeoecology, 35, 63-119. Huntley, D.J., Godfrey-Smith, D.I. and Thewalt, M.L.W. (1985). Optical dating of sediments. Nature, 313, 105-107. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W. and Berger, G.W. (1988). Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating. Journal of Luminescence, 39, 123-136. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., Prescott, J.R. and Hutton, J.T. (1988). Some quartz thermoluminescence spectra relevant to thermoluminescence dating. Nuclear Tracks and Radiation Measurements, 14, 27-33. Htitt, G., Jaek, I. and Tchonka, J. (1988). Optical dating: Kfeldspars optical response stimulation spectra. Quaternary Science Reviews, 7, 381-385. Idnurm, M. and Cook, P.J. (1980). Palaeomagnetism of beach ridges in South Australia and the Milankovitch theory of ice ages. Nature, 286, 699-702. Lamothe, M. (1984). Apparent thermoluminescence ages of StPierre Sediments at Pierreville, Quebec, and the problem of anomalous fading. Canadian Journal of Earth Sciences, 21, 1406-1409. Lamothe, M. (1985). Lithostratigraphy and geochronology of the Quaternary deposits of the Pierreville and St-Pierre Les Becquets areas, Quebec. PhD thesis, University of Western Ontario, 227 pp. Mejdahl, V. (1985). Thermoluminescence dating of partially bleached sediments. Nuclear Tracks, 10, 711-715. Mott, N.F. and Gurney, R.W. (1948). Electronic Processes in Ionic Crystals, 2nd edition, Oxford University Press, London. Schwebel, D.A. (1978). Quaternary stratigraphy of the southeast of South Australia. PhD thesis, Flinders University of South Australia, 495 pp. Smith, B.W., Aitken, M.J., Rhodes, E.J., Robinson, P.D. and Geldard, D.M. (1986). Optical Dating: Methodological Aspects. Radiation Protection Dosimetry, 17, 229-233. Stuiver, M., Heusser, C.J. and Yang, I.C. (1978). North American glacial history extended to 75,000 years ago. Science, 200, 16-21. Templer, R.H. (1985). The removal of anomalous fading in zircon. Nuclear Tracks, 10, 531-537. Vogel, J.C. and Waterbolk, H.T. (1972). Groningen Radiocarbon Dates X. Radiocarbon, 14, 6-110.
0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc
OPTICAL DATING STUDIES OF QUARTZ AND FELDSPAR SEDIMENT EXTRACTS D.I. Godfrey-Smith,* D.J. Huntley* and W.-H. Chent *Department of Physics, Simon Fraser University, Burnaby, B.C., V5A 1S6, Canada t lnstitute of Electro-optical Engineering, National Chiao Tung University, Hsin-Chu, Taiwan, 30049, R.O.C. Continuing developmental work into optical dating has led to some clarification of the optical behaviour of quartz and feldspar sediment extracts. We have verified the key assumption intrinsic to the method - - that the optical signal is zeroed completely upon exposure to daylight. We also show that the rate of zeroing in optical dating is much more rapid than it is in TL dating. Studies concerned with the extraction of the thermally stable signal from laboratory-irradiated samples showed that problems may arise from medium-to-high-temperature preheating of young samples. Optical alternatives to thermal stabilization were tested using dye and krypton lasers. Using a krypton laser we have succeeded in obtaining an equivalent dose of 0.0 + 0.7 Gy for the quartz from a modern intertidal sand, and an average dose of 120 + 22 Gy for the feldspar extract of a - 7 0 ka sand unit. Both were obtained using six wavelengths, from the infrared to the violet, and no dependence on wavelength was found. We found that the photon energies of infrared and deep red light can stimulate luminescence in unirradiated quartz and feldspar extracts of sediments. This was not expected, and suggests that the process of trap emptying is more complex than we had envisioned, but implies a far greater than expected degree of technical flexibility for the optical dating method.
INTRODUCTION Optical dating is a new, and as yet experimental, dating technique for the direct dating of sediments; it was introduced 3 years ago by Huntley et al. (1985). In this method the past radiation dose accumulated in quartz and feldspar mineral grains since their deposition is measured, and therefore the technique is in the same class as the thermoluminescence (TL) and electron spin resonance (ESR) dating techniques. As in the TL dating of sediments, the zeroing mechanism in optical dating is exposure to light. The major difference between the two methods is that in TL the past radiation dose absorbed is measured by heating the mineral, while in optical dating the dose is measured by exposing the minerals to monochromatic light; so far the 514 nm green wavelength emitted by an argon ion laser has normally been used. This difference in techniques is critical, since the radiation dose can be thought to result in electrons in both light-sensitive and in light-insensitive electron traps in a mineral. In a TL measurement, the heating process yields a TL signal from electrons in both types of traps and therefore a sediment sample freshly exposed to sunlight still yields a TL signal. In order to obtain the correct age it is necessary to determine this TL signal, and various methods have been devised for doing this. In contrast, in optical dating, since light is used to measure the trapped electron population, only the electrons accumulated in light-sensitive electron traps, those assumed to have been completely emptied during the process of deposition, are measured, while those electrons accumulated in light-insensitive traps are not. In this discussion we have implied a clear distinction between the two kinds of electron traps. In practice, however, such a sharp distinction is not evident, since light-sensitive traps exist with sensitivities that vary
from seconds to days of sunlight being required for their emptying. In the experiments discussed below we were addressing the following questions: (1) How much sunlight is required during the deposition of a sediment to effectively zero it? How is this affected by less than ideal conditions? How different is it for quartz and feldspar? How does this compare with the exposure times required for TL dating of sediments? (2) In order to determine the equivalent dose by using laboratory irradiated samples, it is thought to be necessary to empty the thermally unstable lightsensitive traps before doing the measurements. In principle, this can be done either thermally or optically. What are the advantages and disadvantages of each method? (3) With optical trap emptying, it should be possible to sample successively deeper, and hence more stable, traps by using optical excitation with monochromatic light of successively greater photon energies. Can we then construct a graph of equivalent dose vs. photon energy, which should start at zero at low photon energy, and rise to a plateau at higher energies? If so, this should be a good stability test, similar to the plateau test used in TL dating. SAMPLES
The samples used in the measurements described below are quartz or feldspar extracts of the natural sediments described in Table 1. The quartz extracts were prepared using the standard quartz extraction procedure used in TL, including heavy liquid separation in an aqueous solution of sodium polytungstate at a density of 2.75 g/ml. Following the HF treatment and prior to magnetic separation, the quartz extracts were
373
374
D.I. Godfrey-Smith et al. 600
TABLE 1. Sediments studied
A: QUARTZ
Sample WSBS
PGQS
SNMS
SPLS
Description
Geological reference .
Whaling Station Bay Sand, modern, intertidal. Hornby Island, British Columbia (49°32'N 124°37'W) Point Grey Quadra Sand, 25.5 ka, Vancouver, British Columbia (49°17'N 123°15'W)
,"t" Ll.l r, ,m Z
~ - ~ _
300
0
Ciague, 1977
St Nicolas Marine Sand, 10.0 ka St Nicolas, Qu6bec (46°42'N 71°23'W)
Gaddet al., 1972 Hillaire-Marcel, 1981
St Pierre Lower Sand, --70 ka, Pierreville, Qu6bec (46°03'N 72°47'W)
Lamothe, 1984, 1985
SESA-43 Southeast South Australia, -500 ka (?), oxidized, noncalcareous sand unit, Woolumbool Range. Map: Bool Lagoon, grid ref. 564 033
,
0 LU 03 ,n," W 0.. O3 I-Z
o 0 Z
0
15,000 I a..
Schwebel,1978 Idnurm and Cook, 1980
treated with warm AI20 3 or HC1, and wet-screened through a 75 Ixm screen; this ensured that poorly dissolved feldspar grains, precipitates, and badly broken quartz grains were removed from the sample. The grains were sprinkled in a monolayer onto A1 disks; sample masses were in the range of 3-7 mg. Feldspar extracts were prepared as above, with the exception that the 40 min HF treatment, the subsequent chemical rinse, and the wet-screening steps were omitted. This resulted in a mixed quartz and feldspar extract; however, since the proportion of feldspar to quartz in these sediments is high (about 4:1 by weight for PGQS), and feldspar has an optical signal which is much brighter than that of quartz (see below), the optical signal measured from such a mixed extract may be considered to be due entirely to its feldspar component. The proportion of plagioclases to K-feldspars in PGQS is about 6:1; this was determined by weighing fractions of different densities, separated as suggested by Mejdahl (1985). Typical quartz and feldspar luminescence curves, illustrating their relative luminescence intensities and rates of signal decay under 10 mW/cm 2 of Ar laser light, are shown for sample PGQS in Fig. 1. In this sample, the feldspar is 50 times brighter than the quartz; in contrast, the feldspar extract of sample SPLS was 240 times brighter than its quartz extract. With the exception of two samples, the extracts were prepared from 90-125 I~m grains, separated from the bulk sediment by sieving; sample WSBS was prepared from 150-180 Ixm grains, and SNMS from 125-150 ~m grains. The feldspar extract of sample SPLS was given a 40-min etch in 10% H F to remove the or-dosed outer layer on the grains.
I
0
I
I
I
L
30,000
L_
°0 Laser ON
'
B: FELDSPAR
2'0
'
TIME (seconds)
4'0 Laser OFF
FIG. 1. Typical luminescence curves of (a) quartz, and (b) feldspar, extracts from sample PGQS, using 514 nm excitation. Background (12 cps per rag), was not subtracted. Note the differences in signal intensity and rate of signal decay between the quartz and the feldspar extracts.
APPARATUS The apparatus used was essentially that described by Huntley et al. (1985), with minor changes. A sliding sample holder has been constructed which allows the samples to be rapidly changed without moving the photomultiplier tube or turning off the high voltage to it. The chamber has been anodized black. Input and output filters were selected according to the exciting wavelength of the laser beam, and are listed in Table 2. Sharp-cut, long-pass filters were used on the input beam to prevent wavelengths shorter than those of the laser beam from entering the chamber; in all cases the luminescence from the sample was detected in a region of higher photon energy than that of the exciting beam. Data were collected and processed by an IBM PC-AT microcomputer equipped with a multichannel-analyzer board. This arrangement allowed samples to be measured at a rate of one every two minutes.
OPTICAL SIGNAL BLEACHING BY SUNLIGHT The key assumption in the optical dating method is that the optical signal is effectively completely bleached, given a sufficiently long exposure to sunlight. On the basis of the luminescence curve under optical stimulation, it is possible to estimate the rate of bleaching of the optical signal by natural sunlight, and therefore the duration of exposure to sun necessary to ensure a completely zeroed sediment. For quartz, we had predicted earlier that 90% of the natural optical signal should be erased following a 10 sec exposure to sunlight.
375
Optical Dating Studies TABLE 2. Details of the input and output filters and their transmission characteristics for the various laser wavelengths used Laser line (nm)
Input filters
Transmittance (%) at laser line
799 & 753 647.1 568.2 530.9 514.5 468
RG610 RG610 3-69 3-70 + 3-71 + 4-96 3-70 + 3-71 + 4-96 GG435 + 4-96
90 90 91 45 45 83
413
3-74, 4-96, 3-75, 4-96, 0-51
11
Output filters 2x 2x 3x 4x 4x 3x 3x 4x
Output transmittance peak (nm)
Peak transmittance (%)
400 400 380 380 380 320 320 320
32 42 56 48 48 37 52 41
7-59 + BG38 + 2x 4-96 7-59 + BG38 + 4-96 7-59 + BG38 7-59 + BG38 7-59 + BG38 U G l l + 2x 7-54 (WSBS) U G l l (other samples) UGll
Notes: (1) Where necessary, for feldspar extracts of samples PGQS, SNMS, and SPLS, neutral density filters were added to the output filters. (2) The BG38 filter was used to suppress the red emission of quartz, and the infrared emission of feldspar, which have been demonstrated in the TL of these minerals (Huntley et al., 1988; Huntley et al., 1988). (3) Filters with numeric designations are Corning colour glass filters; those with alphabetical prefixes are Schott colour glass filters. (4) All lines are from a krypton laser, except for the 514.5 nm line, which is from an argon laser.
Experimental confirmation of the completeness of bleaching may be obtained from measurement of the optical signal intensity of an old sediment extract after it has been exposed to sunlight, and from measurement of the natural optical signal intensity of modern sediments. The sunlight bleaching experiments were carried out on two samples, a - 7 0 ka feldspar extract and a -500 ka quartz extract. These were chosen because they give the most intense luminescence signals of any feldspar or quartz extracts studied by us so far. The sample disks were exposed to bright sunlight, on clear days, in May to August (spring/summer), at Simon Fraser University (365 m above sea level). For comparison, TL bleaching curves were measured for the same samples. The results are shown in Fig. 2. They demonstrate that, for quartz, the assumption of rapid and complete bleaching of the optical signal is essentially correct: the optical signal decayed from a 'natural' unbleached value of 1.15 x 10 6 cps/mg to only 1% of this after 10 sec of sun, and to <150 cps/mg after 30 min of sun. The TL response of this quartz to sunlight was very different: there was no change from its unbleached value for the first 1.5 min of sun, and after 20 hours the TL decreased to 17% of its unbleached value. It may be concluded from the above that the optical signal in quartz is very much more sensitive to light than the TL of quartz. Thus, the zeroing requirements for optical dating should be very much less stringent than those for TL. The difference appears to be due to the presence of electron traps which are extremely sensitive to sunlight; these give a large optical signal, but make a negligible contribution to the TL. The feldspar optical response to sunlight, also shown in Fig. 2, is also very rapid, but not as rapid as that of the quartz. This was surprising, since, on the basis of the initial optical signal decay under laser stimulation, it was expected that a feldspar should bleach much more quickly than quartz. Nevertheless, this sample's optical signal decreases from an initial value of 3 x 105
10 r • -¢,,
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t 02
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I
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I
10'
DURATION OF SUNLIGHT EXPOSURE (seconds)
FIG. 2. The effect of sunlight exposure on the optical and TL signals of quartz and feldspar. The unbleached values are shown along the vertical axis; each point represents the average of 4 samples, each normalized to its mass. The optical signal is that for one second immediately after the laser is switched on, measured at 10 mW/cm 2 for quartz, and at 15 mW/cm 2 for feldspar, using 514 nm as the exciting wavelength. Backgrounds were subtracted. The TL for quartz is at a well-defined peak at 320-330°C, and for feldspar at 310-320°C; these were obtained using a 5°C/sec heating rate, and a 558 filter. Samples are: quartz = SESA-43; feldspar = SPLS.
cps/mg to 1% of this value in 9 min, and to 400 cps/mg after 20 hours of sun, with no sign of having reached a residual level. The expectation is that, given sufficient time, the luminescence of this feldspar should also decrease to a negligibly low level. The TL response of this sample is much less light-sensitive. We also verified that the short exposure to 514 nm laser light during an optical measurement did not affect the natural TL of either the quartz or the feldspar. On the basis of the above comparisons, we conclude that both optical and TL dating studies may be done on the same sample, provided that the optical measurements are done first, and that typical light exposures of <500 mJ/cm2 are used during optical measurements. For samples well bleached at deposition, this allows a
376
D.I. Godfrey-Smith et al.
sediment's past radiation dose to be determined by two different methods. We made use of the above principle in obtaining the data shown in Fig. 2; here the TL measurements were made on the same sample disks after the laser measurements were done. Since most sediments do not undergo bleaching under the ideal conditions of continuous bright sunlight, the experiment on quartz was repeated using daylight on an overcast day. The result, shown in Fig. 3, demonstrates that quartz is zeroed in the absence of direct sunlight. This suggests that bleaching of the optically sensitive traps in quartz can be accomplished by the mid-range and long wavelengths of visible light, and is not due solely to the UV component of sunlight. Optical bleaching will therefore proceed under naturally low-light and restricted-light conditions, although at a proportionately slower rate. Confirmation of complete zeroing of quartz under natural conditions has been obtained from the quartz extract of WSBS, a modern intertidal sand. This sample, which showed a normal quartz TL emission, yielded no measurable signal under laser stimulation by any of the wavelengths listed in Table 2. When the sample was given radiation doses of 7 to 45 Gy, an optical signal proportional to the radiation dose was observed. This result is described below.
THERMAL STABILITY CONSIDERATIONS The most significant drawback in optical dating is that an analogue of TL dating's 'plateau test' is difficult to obtain. One method, suggested by Huntley et al. (1985), is to preheat a large number of natural and laboratory-irradiated samples to different temperatures, and to determine the past dose for each preheat temperature separately. A plot of the estimated dose as a function of preheat temperature should, in principle,
reach a plateau level when a thermally stable range of optical traps is reached. The difficulty with preheating prior to laser stimulation is revealed when young or modern samples are heated to temperatures greater than 200°C. The effect of preheating an unirradiated modern quartz, WSBS, to temperatures ranging from 75-425°C, is that an unwanted luminescence signal is created; the magnitude of the effect is shown in Fig. 4. A similar result, on a single partially bleached quartz sample, successively heated and exposed to Ar laser light, was obtained by Smith et al. (1986). The magnitude of the effect can be quantified in terms of the natural luminescence from the quartz extract of sample PGQS, since both sediments were sampled in the Gulf of Georgia area, and so both quartz extracts are likely of the same geological origin. The highest value for the undelayed measurement corresponds to 30% of the natural luminescence of PGQS, which is equivalent to 7.6 ka, while the highest value following a 48-hour delay is 15% of PGQS, or the equivalent of 3.8 ka. We suggest that the effect results from thermal excitation of electrons initially occupying thermally stable light-insensitive traps. Most of these electrons recombine and yield the natural TL glow curve typical of unirradiated zero-age quartz extracts. A small fraction of the electrons do not recombine but are retrapped in shallow and deep light-sensitive traps, from which they can be excited optically, to yield the 2,500 [] No Delay •
•
o
48-Hr Delay
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o
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0
104
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.
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i
i
200
300
400
PREHEAT TEMPERATURE (°C)
Z
103 o"r" O_
102 2 S 5S
lo-,,
I
100
,
,
.....
20S
15 m
......
,
. . . 30m . . 6m
1
.......
I
.........
2h
6h I ~ l
I }....~2°h
0 1 10 10a 10~ 104 105 DURATION OF DAYLIGHT EXPOSURE (seconds)
FIG. 3. The effect of daylight exposure on the optical signal of quartz. The light intensity on the overcast day was 10% of that on a clear day; the data show that the sample was bleached 10 times more slowly on the overcast day than on a clear day, demonstrating that the rate of bleaching is proportional to the ambient light level.
FIG. 4. Optical signal created in a zero-age quartz by preheating. The intensity of this signal as a function of preheat temperature is shown. T h e sample is WSBS, undosed. Fresh disks were used for each preheat temperature. Preheating was done in a conventional T L oven, u n d e r high-purity A r gas, at a heating rate of 5°C/sec. The heater was switched off when the temperature was reached, and the samples cooled to <50°C prior to opening the glow oven. The data shown are mass-normalized, integrated over the first 20 sec of stimulation by a 10 m W / c m 2 A r laser beam. U p p e r curve (squares, one m e a s u r e m e n t per point): data m e a s u r e d immediately after preheating. Lower curve (circles, average of two m e a s u r e m e n t s per point): data m e a s u r e d following a 48-hour delay. A dark count of 14.5 cps was subtracted from all points; the signal above dark count from an unheated sample was equal to that from a blank AI disk.
377
Optical Dating Studies
curves in Fig. 4. The difference in intensities between immediate and delayed optical measurements is most LU likely due to room temperature recombination of those Z electrons which had been retrapped in the shallow, and therefore thermally unstable, light-sensitive traps. We ~5 now feel that our 250°C preheat was at least partly responsible for the dose overestimates in CBBS and HaRk-ID in our initial work. X < Some possible alternatives to high-temperature preheating in order to empty the electrons from thermally 0 unstable traps following laboratory irradiation include 300 700 560 long-term post-irradiation delays of weeks to months, a WAVELENGTH (nm) prolonged low-temperature treatment, or the use of optical rather than thermal means of hastening electron FIG. 5. The lasing spectrum of a krypton ion laser (Coherent Radiation, model CR-2000K). Heavy bars indicate the lines used. eviction. Similar approaches have been demonstrated The infrared lines 799 nm and 753 nm were used together; all other to be successful in removing the anomalous fading lines were used in monochromatic mode. component in the TL of irradiated zircons and pottery samples (Templer, 1985; Bowman, 1988). The last of light from a blank AI disk, the brass sample holder, and these carries with it the advantage that the use of light an unirradiated sample of WSBS. All three showed no of different photon energies should allow electrons in decrease with time, and were similar to each other. The traps of different depths to be sampled; the past dose only exception to the above was that when the violet can then be estimated as a function of the energy of the (413 nm) laser line was used, a strong response (about exciting photons. One way to accomplish this is to 8 times that of the scattered light level off brass) was perform a continuous scan through the whole or a part observed from a blank AI disk. The responses from a of the visible spectrum, while observing the sample's freshly cleaned, HF-etched A1 disk and a stainless steel optical response. In a preliminary experiment, we used disk, were equal to that from brass. Wiping of the AI a dye laser slowly tuned from 684 nm to 615 nm, and disk did not decrease the signal, implying that the effect obtained measurable luminescence in the blue-violet was not due to an accidentally-deposited contaminant, region for both the natural and irradiated quartz and but to some process involving the A1 surface. A sample feldspar extracts of sample PGQS, as well as for an of WSBS showed much less optical response than a older quartz. None of the samples showed any structure blank disk; this was clearly due to a shielding of the AI in their excitation spectra over this range; instead, a surface by the non-luminescent quartz grains. Theregradual increase in luminescence intensity per mW of fore, if very short wavelengths are to be used in optical incident light was seen as the energy of the exciting dating studies, we recommend that stainless steel, photons increased. An intense source of white light, brass, gold-plated, or rhodium-plated substrates should coupled with a monochromator, could be used to be used in order to avoid this 'spurious' luminescence perform such a scan through a wider portion of the phenomenon. visible spectrum. The krypton laser measurements yielded two interAn alternative method is to select a small number of esting results. The first, and unexpected, result was that discrete wavelengths, and to use these to sample the measurable luminescence was obtained from all natural optically-sensitive traps. We chose to employ seven of samples except WSBS under IR (799 + 753 nm) stimuthe lasing wavelengths of a krypton ion laser, shown lation. Two growth curves under IR stimulation are schematically in Fig. 5, to estimate the past doses of shown in Fig. 6a for the feldspar extract of sample four quartz and three feldspar extracts, of samples SPLS. The past dose estimates for this sample at WSBS, SNMS, PGQS, and SPLS. The two IR wave- t = 0-2.5 sec under all six stimulating wavelengths are lengths, 799 nm and 753 nm, were used together. Sets summarized in Fig. 7a. The average past dose estimate of 30 A1 disks of each extract were prepared; each of is 120 + 22 Gy at t = 0-2.5 sec, and it rises to 145 + 28 these were divided into five subsets, of which one was Gy after 21 sec of laser light, as shown in Fig. 7b. Using left unirradiated, and four were given various labora- the results of K20 analysis, a-counting, and a water tory radiation doses up to 3 times the expected natural content estimate of A = 0.21 + 0.25 (average of in situ dose. Optical measurements commenced following a and saturation values for sample SP-2 of Lamothe, 31/2-month post-irradiation delay. The laser was oper- 1985), a dose rate of 2.6 Gy/ka due to 13 and ated at 80% of the rated maximum single-line power contributions was calculated. For the mean past dose output; before adjusting for the transmission charac- estimates given above, this yields age estimates of teristics of the input filters listed in Table 2, 10% of this 46 + 14 ka and 55 + 16 ka, respectively. These values would have reached the sample. Each sample was are somewhat lower than the known age of this exposed to each wavelength for 45 sec, starting with sediment, as can be observed by comparison with the infrared, and ending with violet light. 14C and TL dates summarized in Table 3. Since our Background levels were monitored at each wave- experiments on the fading of the optical signal in length and for each sample, by measuring the scattered feldspar extracts which had been stored at room
I,II
378
D.I. Godfrey-Smith et al. TABLE 3. A summary of chronometric results relevant to SPLS
A: SPLS
Method
8xl 0 s
14C OC 4xl W O.
0n
Sample
Age
Reference
peat
66.5 _+ 1.6 ka
GrN-1711, Vogel and Waterbolk (1972)
peat
74.7 + 2.7 ka 2.0
QL-198, Stuiver et al. (1978)
-
/ ¢v W O_
i,,Y 0
-200
I
i
200
400
TL, on silt sample RS overlying the peat:
I-Z
o Z
o
~/SBS 6O o
-r
•
20
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FIG. 6. Growth curves for samples SPLS and WSBS, shown for t = 0-2.5 sec and t = 15-17.5 sec after the laser was switched on. A: growth curves for feldspar sample SPLS, under infrared (799 and 753 nm) excitation. The equivalent doses at this photon energy, determined by exponential fitting, are 136 + 11 Gy for t = 0-2.5 sec, and 166 + 18 Gy for t = 15-17.5 sec. The background is negligible in comparison with the sample's luminescence, and was not subtracted. B: growth curves for modern quartz sample WSBS under red (647 nm) excitation. The equivalent doses at this photon energy, determined by linear fitting, are 1.7 + 0.6 Gy for t = 0-2.5 sec, and 0.2 + 0.8 Gy for t = 15-17.5 sec. A mean background value of 12.8 counts per 2.5 sec was subtracted from all points. All fits are least squares with weighting inversely proportional to the luminescence intensity before the background was subtracted.
temperature following laboratory irradiation showed that fading does not reach a limit after 68 days of storage, we feel that the ages obtained above for SPLS should be considered as minimum ages only. The second interesting result was the complete lack of a luminescent response from the modern quartz of sample WSBS, regardless of the stimulating wavelength, unless it had first been irradiated. Two typical growth curves for this sample under excitation by the
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Lamothe (1984, 1985)
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red wavelength are shown in Fig. 6b. The sample's response at zero dose did not change with time, which is consistent with scattered light and not luminescence, whereas the luminescence from irradiated samples decreased with time as shown. Equivalent doses were calculated for five of the stimulating wavelengths, using linear least squares fitting. Due to the problems encountered during background measurements under violet light stimulation, equivalent doses were not determined for this wavelength. The equivalent dose estimates are shown for t = 0-2.5 sec in Fig. 8a; their resultant mean is 0.0 + 0.7 Gy. The change in the mean dose with time after the laser was switched on is also shown in Fig. 8b. Growth curves for the other non-zero-age samples extrapolated to order-of-magnitude dose estimates which were also consistent with the expected ages. However, the intrinsic scatter, especially in the quartz samples, prevented us from obtaining meaningful numerical results at this time. Normalization methods
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Lamothe (1984, 1985)
Notes: (1) Sample SPLS was collected 30-40 cm below the peat; sample RS was collected 30 cm above the peat. (2) The lower sand unit (from which SPLS was collected) is 3 m thick, the peat is highly compressed and is 30 cm thick, and the upper silt unit is 4 m thick. (3) The three units make up a single lithostratigraphic unit known as the St-Pierre Sediments, within which all stratigraphic relationships are conformable. (4) Contacts with the underlying and overlying lithostratigraphic units are unconformable (Lamothe, 1984),
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379
Optical Dating Studies
based on criteria other than sample mass need to be applied to these results. Finally, we have observed in the old quartz but not the feldspar samples, differences in the shape of the luminescence curves as the photon energy of the incident laser beam was increased. Under IR excitation, all showed the typical luminescence shown in Fig. la, with varying degrees of intensity. Under green, blue, or violet excitation, some samples began to show luminescence that increased in intensity during the first one to three seconds after the laser was switched on, and subsequently decreased. This occurred under green, blue, and violet excitation for sample SNMS and is shown in Fig. 9. The effect was also observed under blue and violet excitation in SPLS, and possibly, but not clearly, under violet excitation in PGQS.
pected. Firstly, Mott and Gurney (1984) showed that optical trap depths are expected to be greater than thermal trap depths by a factor approximately equal to the ratio of the static and optical dielectric constants. For quartz these are 4.3 and 2.4, respectively, and yield a ratio of --1.8. Secondly, in a test of the effect of the 514 nm wavelength on the TL of quartz, Smith et al. (1986) observed that the 325°C peak is bleached, but the 375°C peak is not. Since the energy of a 514 nm photon is 2.41 eV, and the trap depth of the 325°C peak is - 1 . 7 eV (estimated on the basis of data presented in Table E. 1 in Aitken, 1985), this yields an experimental optical:thermal trap depth ratio of 1.4. On the basis of the above estimates, we would expect 753 nm photons, which have an energy of 1.65 eV, to excite electrons from traps with thermal depths up to about 1.2 eV only. From TL studies it is known that such trapped electrons are not thermally stable at environmental temperatures over the time scale of interest (Aitken, 1985). Thus, we did not expect that IR excitation would result in luminescence from any
DISCUSSION The discovery that infrared light was able to evict electrons from thermally stable states was not ex-
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TIME (seconds) FIG. 9. Luminescence curves for quartz sample SNMS, undosed, under excitation by the various krypton laser lines. Backgrounds, in order of decreasing wavelength, were: 0.3, 0.5, 3.7, 5.2, 0.5, and 10 counts per 0.1 sec per mg; they were not subtracted from the curves shown. Measurements were made successively on the same disk, in order of decreasing wavelength. Each curve shown is an average of 4 measurements.
D.I. Godfrey-Smith et al.
380
natural samples. We have verified that it does with measurements of luminescence from natural quartz and feldspar samples using ~5 mW from an infrared lightemitting diode with an emission peak at 880 nm (FWHM = 100 nm). Thus, the simple model presented above is clearly incorrect, and must be abandoned. It appears that the relationship between the optical and the TL behaviour of minerals is quite complex. The association of the optical signal with the 325°C TL peak (Smith et al., 1986) must be viewed with caution, since we have found that heating of quartz samples beyond this temperature does not result in erasure of the optical signal. We feel that the photon energy of the exciting wavelength cannot be directly related to an electron trap depth or to a particular emission peak in TL, but that electron eviction from a light-sensitive trap into the conduction band is a complex, multi-step, multi-photon process. An alternative explanation is that the optically-evicted electrons are assisted into the conduction band thermally as suggested by Hiitt et al. (1988). CONCLUSIONS The results of bleaching by natural light of the brightest of our feldspar and quartz samples support our initial hypothesis of complete bleaching of the optical signal in sediments. Terrestrial sediments such as loess and beach deposits, sand dunes, and other sediments with a subaerial mode of deposition are therefore highly suitable for this method. The results of bleaching under overcast conditions imply that sediments deposited under a UV blocker, such as a water layer, will also be completely zeroed, although a proportionately longer bleaching period will be required. We therefore conclude that shallow marine, lacustrine, and estuarine deposits should also be suitable for optical dating, assuming that the deposition is sufficiently slow. We found that preheating a sample which yielded no natural luminescence created an unwanted luminescence signal. This suggests that caution and a lot of empirical work are needed before optical dating can be applied to heated materials such as pottery, tephra, or metamorphosed sediments. Measurements of a modern quartz extract demonstrated that it has been zeroed completely in nature, since it gives no natural luminescence under any wavelength of incident beam employed. An equivalent dose of 0.0 + 0.7 Gy was obtained for it. We do not wish to place undue weight on a result from a single modern quartz sediment; clearly, similar tests on other modern sediments, preferably from other geological areas to ensure a variety of sources of the quartz, are necessary to confirm its generality. The results of our experiments lead us to continued optimism for the optical dating method. The discovery
that even infrared photons can be used to stimulate natural luminescence from a sediment implies a greater degree of technical flexibility than we had previously thought possible for the method, but that the mechanism of optical excitation is more complex than had initially been expected. ACKNOWLEDGEMENTS We would like to thank K.E. Rieckhoff for the use of his dye and krypton lasers; M. Lamothe for his guidance, assistance, and very, welcome advice in the collection of the QuEbec samples; J.T. Hutton and J.R. Prescott for collection of the Australian sample; and O. Lian for much assistance in the laboratory. This research was supported by the Natural Sciences and Engineering Research Council, and the Department of Energy, Mines and Resources of Canada.
REFERENCES Aitken, M.J. (1985). Thermoluminescence Dating, Academic Press, London. Bowman, S.G.E. (1988). Observations of anomalous fading in Maiolica. Nuclear Tracks and Radiation Measurements, 14, 131-137. Clague, J.J. (1977). Quadra Sand: A study of the Late Pleistocene geology and geomorphic history of coastal southwest British Columbia. Geological Survey of Canada, Paper 77-17. Gadd, N.R., LaSalle, P., MacDonald, B.C., Shilts, W.W. and Dionne, J.C. (1972). GEologie et gEormorphologie du Quaternaire dans le Sud du QuEbec. Livret-guide d' excursion C-44, 24e Congrds International de G~ologie, MontrEal, 74 pp. Hillaire-Marcel, C. (1981). Paleo-oceanographie isotopique des mers post-glaciaires du QuEbec. Palaeogeography, Palaeoclimatology, Palaeoecology, 35, 63-119. Huntley, D.J., Godfrey-Smith, D.I. and Thewalt, M.L.W. (1985). Optical dating of sediments. Nature, 313, 105-107. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W. and Berger, G.W. (1988). Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating. Journal of Luminescence, 39, 123-136. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., Prescott, J.R. and Hutton, J.T. (1988). Some quartz thermoluminescence spectra relevant to thermoluminescence dating. Nuclear Tracks and Radiation Measurements, 14, 27-33. Htitt, G., Jaek, I. and Tchonka, J. (1988). Optical dating: Kfeldspars optical response stimulation spectra. Quaternary Science Reviews, 7, 381-385. Idnurm, M. and Cook, P.J. (1980). Palaeomagnetism of beach ridges in South Australia and the Milankovitch theory of ice ages. Nature, 286, 699-702. Lamothe, M. (1984). Apparent thermoluminescence ages of StPierre Sediments at Pierreville, Quebec, and the problem of anomalous fading. Canadian Journal of Earth Sciences, 21, 1406-1409. Lamothe, M. (1985). Lithostratigraphy and geochronology of the Quaternary deposits of the Pierreville and St-Pierre Les Becquets areas, Quebec. PhD thesis, University of Western Ontario, 227 pp. Mejdahl, V. (1985). Thermoluminescence dating of partially bleached sediments. Nuclear Tracks, 10, 711-715. Mott, N.F. and Gurney, R.W. (1948). Electronic Processes in Ionic Crystals, 2nd edition, Oxford University Press, London. Schwebel, D.A. (1978). Quaternary stratigraphy of the southeast of South Australia. PhD thesis, Flinders University of South Australia, 495 pp. Smith, B.W., Aitken, M.J., Rhodes, E.J., Robinson, P.D. and Geldard, D.M. (1986). Optical Dating: Methodological Aspects. Radiation Protection Dosimetry, 17, 229-233. Stuiver, M., Heusser, C.J. and Yang, I.C. (1978). North American glacial history extended to 75,000 years ago. Science, 200, 16-21. Templer, R.H. (1985). The removal of anomalous fading in zircon. Nuclear Tracks, 10, 531-537. Vogel, J.C. and Waterbolk, H.T. (1972). Groningen Radiocarbon Dates X. Radiocarbon, 14, 6-110.