Optical dating studies of postglacial aeolian deposits from the south-central interior of British Columbia, Canada

Optical dating studies of postglacial aeolian deposits from the south-central interior of British Columbia, Canada

Quaternary Science Reviews 18 (1999) 1453 } 1466 Optical dating studies of postglacial aeolian deposits from the south-central interior of British Co...

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Quaternary Science Reviews 18 (1999) 1453 } 1466

Optical dating studies of postglacial aeolian deposits from the south-central interior of British Columbia, Canada Olav B. Lian!,*,1, D. J. Huntley" !Department of Earth Sciences, The University of Western Ontario, London, ON, N6A 5B7, Canada "Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada

Abstract Tests of the validity of optical dating of the "ne-grained (4}11 lm) component of aeolian deposits from six sites in south-central British Columbia, using 1.4 eV (infrared) excitation of potassium feldspars, were made making use of known-age tephra beds. At one site on the western side of the Fraser Valley, samples bracketing the Mazama (7.5}7.6 ka, cal. yrs.) and Bridge River tephras (2.3 ka, cal. yrs.) yielded optical ages in accordance with the known ages when a correction for thermal-transfer was used. The same method was applied to aeolian deposits on the Fraser Plateau, known to have been deposited between 14 and 2.3 ka (cal. yrs). The ages obtained from "ve samples, from four separate exposures spanning 10 km, varied from 30 to 94 ka, clearly indicating that this sediment was not exposed to su$cient sunlight prior to burial. As a result of a detailed investigation it is suggested that the reason is that some grains had been transported over short distances ((100 m) while shielded within carbonate-cemented clusters. ( 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The timing of deglaciation, and early postglacial sedimentation, in the relatively low-lying interior of southcentral British Columbia is poorly de"ned. This is largely due to a paucity of suitable organic material for radiocarbon dating (Ryder, 1971; Clague et al., 1990). The radiocarbon ages that do exist indicate that parts of the interior were ice-free by about 11 ka (13 ka, cal.yrs; Hughen et al., 1998), although less reliable ages suggest that deglaciation could have occurred as early as 12 ka (14 ka, cal. yrs.; Hughen et al., 1998) near the boundary of British Columbia and Washington State (Clague, 1981). In south-central British Columbia glacigenic deposits on the plateaux, on the valley sides, and on the dissected Pleistocene sediment "lls that occupy the valley bottoms, are in many places capped with up to several metres of aeolian sediments. These sediments are found to cap till, to rest directly on outwash, or lie on (or within) paraglacial deposits (Ryder, 1971), that were laid down during, or shortly after, deglaciation. The surfaces of these * Corresponding author. 1Present address: School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. E-mail: olav.lian @vuw.ac.nz.

aeolian deposits are generally vegetated and stable; actively eroding or accreting surfaces are rare. It is therefore likely that formation of these deposits commenced immediately after glacial meltwater activity had essentially ended, but before a signi"cant amount of stabilising vegetation had become established. In this region therefore, aeolian deposits are expected to represent early postglacial sedimentation. Because aeolian sediments are usually expected to have received extended exposure to direct sunlight prior to deposition, they are considered to be one of the most suitable deposits for luminescence dating. Thus, within the interior of British Columbia, luminescence dating of aeolian sediments has the potential for supplying limiting ages on the time of deglaciation, and on the formation of early postglacial landforms. In this paper we report on the "rst research into the utility of optical dating of these postglacial aeolian deposits. The only previous work was a pilot study conducted by Godfrey-Smith (1991) using samples collected in the Kamloops region (Fig. 1 inset), about 100 km to the southeast of our sites. Godfrey-Smith used 2.4 eV (green, 514 nm) excitation from an argon ion laser to produce equivalent doses from two Holocene-aged aeolian samples. One was consistent with what was expected, but the other was not; optical ages were not calculated.

0277-3791/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 8 ) 0 0 0 8 5 - 7

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Fig. 1. Map showing the location of features referred to in the text. The lightest shade represents elevations over 1200 m, while the immediately darker shade shows areas over 1800 m. Lakes are shown by the darkest shade. The Fraser River #ows at about 300 m in this region.

2. Lithostratigraphic sections and sample locations The sites selected for this study are on the western side of the Fraser Valley and on the Fraser Plateau to the east (Fig. 1). These sites were chosen because the sediments at these localities contain one, or both, of the two Holocene-aged tephra beds found in this region, or are of &zero age' (i.e., presently being formed), which permit us to test the accuracy of our technique.

2.1. Fraser Valley sites: Chisholm canyon and Watson Bar Creek The Chisholm Canyon site (site 51, Fig. 1) is on the western side of the Fraser Valley, near the top of the

valley "ll. The site can be reached from the Big Bar ferry landing by travelling about 6.5 km south along Slok Creek Road to an unnamed road that descends toward the Fraser River. The site, which is on private property, is on the northern side of the road at an elevation of about 680 m, almost 400 m above the present position of the Fraser River. The section is approximately 3.8 m thick and can be divided into two units (Figs. 2 and 3). The lower unit (unit 51-1) consists of poorly sorted pebble gravel and sand. The pebbles are rounded to angular and consist mainly of local volcanic lithologies, but there is also a minor amount of far-travelled granitic stones. At this site only 60 cm of unit 51-1 is exposed; it is more than 5 m thick at adjacent sections. Unit 51-1 is the remnant of a steeply dipping fan.

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Fig. 2. The Chisholm Canyon site (site 51). A ca. 3.2-m-thick section of aeolian sandy silt is exposed; it contains Mazama tephra near the base, and Bridge River tephra near the top, both tephras being visible as near-horizontal white lines. The aeolian deposit rests on paraglacial alluvial fan gravel and sand deposited after the last glaciation. The location where sample CCL1 was collected can be seen immediately left of the person wearing white, while the locations of samples CCL3 and CCL4 can be seen at the top left-hand corner of the exposure. A section diagram is shown in Fig. 3.

Fig. 3. Section diagram of the Chisholm Canyon exposure (drawn to scale) showing the sample locations, and the optical ages. Unit 51-2 is paraglacial alluvial gravel and sand, while unit 51-2 is aeolian sandy silt.

Unit 51-1 is sharply overlain with about 3.2 m of normally consolidated massive sandy silt containing rare pebbles (unit 51-2); the surface is vegetated and stable. Included in this unit are two tephra beds. The lower

tephra, which occurs about 40 cm above the top of unit 51-1, consists of silt-sized material. The upper tephra bed occurs about 1.7 m higher and is composed of granulesized grains, about 1}3 mm in diameter.

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Unit 51-1 is alluvial fan gravel and sand. The presence of distantly derived stones suggests that it formed from glacial sediments higher on the valley sides. Unit 51-1 is therefore interpreted to be part of a paraglacial fan that was deposited soon after ice had left the area. Unit 51-2 is interpreted to be aeolian. Chemical analysis of glassencased magnetite grains by electron microprobe (Lian 1997, pp. 449}454) using the method of Brewster and Barnet (1979) con"rmed that the upper tephra bed is Bridge River, which is known to have been deposited 2.3$0.2 ka (cal. yr, Clague et al., 1995; Leonard, 1995), and that the lower bed is Mazama tephra, deposited between 7.5 and 7.6 ka (cal. yr, Hallett et al., 1997). Four samples for optical dating were collected from unit 51-2. They consist, on average, of 38% sand, 55% silt, and 7% clay. Samples CCL1 and CCL2 bracket Mazama tephra, while samples CCL3 and CCL4 bracket Bridge River tephra. The sample locations are shown in Fig. 3. The Watson Bar Creek site (site 69, Fig. 1) comprises an aeolian dune "eld located on a ca. 60-m-high #uvial terrace near the con#uence of Watson Bar Creek and Fraser River. The dunes are generally less than 1 m high and have formed behind sage brush and bunch grass. The dunes consist of a moist stable interior and a dry and mobile rippled surface (Fig. 4). Two samples were obtained from an active dune: sample WBDS1 (96% sand, 2% silt, and 2% clay) was collected from the moist interior, about 3 cm below the contact with the dry surface layer, while sample WBDS2 comprised sediment skimmed from the surface of an active ripple (Fig. 4). 2.2. Fraser Plateau sites The Fraser Plateau, at an elevation of &1000 m, is located immediately east of the Marble Range (Fig. 1). Five sites were studied; their locations are shown in Fig. 1. Site 45 is in a roadcut located on the south side of Big Bar Lake Road, 13.3 km from its junction with Highway 97 (Fig. 1). About 4 m of normally consolidated sandy silt is exposed. A lower contact was not observed, but hand augering indicated that these sediments continue beneath the surface for at least another metre. The sandy silt is nearly massive, except for a ca. 40 cm-thick sand-rich zone near the centre of the exposure consisting of climbing ripples. The rippled zone is directly overlain with an approximately 3-cm-thick bed of laminated clayey silt. The section is capped with&30 cm of soil; the surface is vegetated and stable (Fig. 5). There was no evidence of bioturbation below the soil. Test pits showed that at this site the aeolian sediments were deposited in a swale between two glacio#uvial hummocks. The climbing ripples near the centre of this unit represent a mobile surface, while the thin laminated bed was probably deposited in standing surface water, or

Fig. 4. Close-up view of sediments comprising the small dune "eld at site 69, near Watson Bar Creek. These sediments consist of a moist and stable interior (dark grey sediment), and a dry and mobile surface (light grey sediment). Sample WBDS1 was collected from the moist interior, while sample WBDS2 was skimmed from the surface of an active ripple. The knife is 20 cm long.

perhaps by translocation onto an impermeable substrate. A sample for optical dating (sample BLRL2; 48% sand, 50% silt, 2% clay) was collected about 55 cm below the surface, beneath the active soil while another (sample BLRL1; 8% sand, 90% silt, 2% clay) was taken 30 cm lower, immediately above the bed of laminated clayey silt. At a similar exposure 400 m to the west, Bridge River tephra (quantitatively identi"ed at site 52, see below) occurs near the surface indicating that aeolian sediments at these sites were deposited before 2.4 ka. Site 46 is located 300 m west of site 45. The sediments at site 46 are similar to those at site 45, except that they show weak horizontal bedding, and are sharply overlain with about 60 cm of poorly sorted pebble gravel and sand, which is in turn capped with about 20 cm of soil. Hand augering showed that the gravel bed near the surface can be traced to an adjoining, and slightly higher, hummock composed of pebble gravel, sand, and silt. The sediments at site 46 are interpreted to be aeolian, overlain by slopewash. The latter appears to have originated from the adjoining hummock. A sample for optical dating (sample BLRL4; 31% sand, 65% silt, 4% clay) was collected from this aeolian deposit, about 60 cm below the base of the gravel unit. The aeolian unit at site 46 can be traced to an adjacent exposure (site 56; Fig. 1) where it is found to rest directly on outwash gravel and sand. Site 52 is located immediately south of Big Bar Lake Road, 24.7 km from its junction with Highway 97. At Site 52, a gravel pit excavation had exposed the interior of a hummock that is about 7 m high and &45 m in diameter. At the base, the hummock consists of about 3 m of horizontally and cross bedded sand, which is overlain by about 4 m of cross-bedded cobble } pebble gravel and sand, rich in carbonate precipitate; this precipitate is

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Fig. 5. Photograph and section log of the sediments exposed at site 45 on the Fraser Plateau. Shown to the right is the variation with depth of luminescence response to 1.4 eV excitation. Each point on the graph is the average of three separate measurements; the error bars are 1p. Note that the expected systematic increase of the natural luminescence with depth is not observed here. One set (solid points) have been shifted to the right by 200 units for clarity. Samples BLRL1 (right) and BLRL2 (left) were taken from blocks that can be identi"ed by round holes augered for c-ray spectrometry.

found as cement in the sand and silt, and as a coating on the gravel (Fig. 6). The hummock is capped with 30}50 cm of normally consolidated massive sandy silt which thickens to&1.5 m in the swales. Near the surface of the sandy-silt deposit, and interbedded with the active soil, is a 10}20-cm-thick discontinuous bed of tephra, the grains of which reach diameters of 2}3 mm. Another discontinuous bed of "ne-grained tephra, of variable thickness, is found near the base (Fig. 6). The sand and gravel units that make up most of the hummock at site 52 are interpreted to be glacio#uvial, while the sandy silt that caps the hummocks, and has accumulated in the swales, is interpreted to be aeolian. A sample for optical dating was collected from the northern swale, 75 cm below the surface (sample BLRL3; 37% sand, 61% silt, and 2% clay). Chemical analysis of glassencased magnetite grains by electron microprobe showed

that the uppermost tephra is Bridge River (Lian, 1997). The tephra that occurs near the base of the aeolian unit was not analysed, but is probably Mazama. Site 58 is located near the south side of Big Bar Lake Road, 26.9 km from its junction with Highway 97. Site 58 is one of several hummocks, all about 5 m thick, that occur in the immediate area. They are all covered with vegetation and are stable. Test pits indicate that they are composed of sandy silt. At site 58 a detailed investigation revealed that the sediments are normally consolidated, and show weak horizontal to subhorizontal bedding. The sediments at site 58, and in the vicinity, are thought to have been deposited as aeolian dunes. Nearby gravel pit excavations suggest that these dunes probably formed directly on outwash. At site 58, a sample for optical dating (sample BLRL5; 37% sand, 59% silt, 4% clay) was collected 1.7 m below the ground surface.

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photon energies which can be readily separated and measured in the laboratory. The intensity of this measured light is therefore related to the number of trapped electrons, and hence the time elapsed since the sample was last exposed to su$cient sunlight. Through laboratory measurements, the radiation dose that produced the trapped electron population can be determined, and, with a calculation of the radiation dose-rate, the time elapsed since burial can be estimated. The task is therefore to estimate the radiation dose the sample has received since burial (the equivalent dose), and the environmental dose rate. Detailed and varied reviews of optical dating can be found in Wintle (1993), Aitken (1998), Berger (1995), Prescott and Robertson (1997) and Huntley and Lian (in press). 3.1. Sample collection and preparation

Fig. 6. Oblique view of the upper part of the section at site 52. Shown is about 50 cm of aeolian sandy silt (the large holes are animal burrows) overlying outwash gravel and sand rich in carbonate precipitate. White carbonate coatings on the pebbles, mostly dark-grey basalt, can clearly be seen. Sample BLRL3 was collected about 75 m north of this location (to the left in the photograph).

3. Optical dating Optical dating (Huntley et al., 1985) works on the principle that common minerals such as quartz and feldspars contain impurities, some of which can act as traps for free electrons. Free electrons are produced when minerals are subjected to a, b, and c radiation, from the decay of U, Th, and 40K in the sediment matrix, and cosmic-ray radiation. If, for example, a sedimentary deposit is eroded and the mineral grains exposed to sunlight, light-sensitive electron traps will be emptied. After subsequent burial, these traps will gradually "ll again. In the laboratory, aliquots of speci"c grains are exposed to light of a speci"c photon energy, or energy range, thus releasing electrons from traps. Some of these electrons recombine with holes at other defects, the excess energy being released as light of characteristic

Each sample was collected by carving out blocks from cleaned faces in natural daylight. In the laboratory, under subdued "ltered light, about 5 mm of the outer exposed surfaces were removed and discarded. For each sample, approximately 150 g of sediment was removed from near the centre of the block. For dosimetry, a representative fraction of this (&30 g) was subsampled, dried, and milled to a "ne powder. The remainder was placed in 10% HCl for 24 h to dissolve the carbonates, and then in 20% H O for 24 h to remove organic matter. The sample was 2 2 then rinsed with distilled water, and most of the water separated by centrifuge. To de#occulate the clay fraction, dispersing solution (1 g sodium hexametaphosphate per litre of distilled water) was added to produce a thick slurry, which was mechanically agitated using a wristshaker for 1 h (Norrish and Tiller, 1976). The sediment was then transferred to glass cylinders, and the 4}11 lm grain-size range obtained by making use of di!erent settling times in distilled water; this procedure was repeated several times to assure a clean separation. The sediment separates were rinsed several times with methanol to remove the water. Finally, 50 or more &1 mg aliquots were deposited from an acetone suspension onto 1-cm-diameter aluminium discs. 3.2. Equivalent dose determination Luminescence was measured using the automated 50sample apparatus described by Godfrey-Smith (1991), "tted with an array of 31 light-emitting diodes (Hu, 1994) that together delivered about 20 mW cm~2 of 1.4 eV photons to the sample. The 3.1 eV (400 nm) emission characteristic of potassium feldspars (Huntley et al. 1991; Krbetschek et al., 1997) was detected using an EMI 9365Q photomultiplier tube behind Schott BG-39 and Kopp 5-57 optical "lters, which together absorb scattered 1.4 eV photons, and block the 2.2 eV (570 nm) emission of plagioclase feldspars.

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To later correct for the variability between aliquots, each was initially given a short (20}100 mJ cm~2) exposure to 1.4 eV photons. Normalisation factors were calculated as the luminescence intensity of the individual aliquots divided by the average luminescence intensity of all the aliquots in the set. Laboratory irradiations were from a 60Co source which delivered &0.45 Gy min~1 of c radiation to a sample. For the purpose of determining the a-e$ciency (b-value) some of the aliquots were exposed to 241Am sources that delivered a particles at about 0.38 lm~2 min~1. Aliquots that were used to construct thermal-transfer correction curves, and the regeneration curves, were given a 3 h infrared bleach using a quartzhalogen lamp adapted with a Schott RG-715 optical "lter that absorbs ultraviolet and most visible light. A natural sunlight bleach was not used to avoid phototransfer that may not have been present during sediment transport, such as that found by Huntley and Clague (1996). The laboratory bleach used for this work decreased the measured luminescence to within 3}4% of that of a similar unbleached aliquot. Laboratory irradiation populates electron traps that are thermally unstable over geological time, as well as those that are thermally stable. To assure that only thermally stable electron traps are sampled during the "nal measurement, all of the aliquots were "rst heated (preheated) in air. For this study 1603C for 4 h was used; a discussion of the rationale behind this preheat combination, and a test, can be found in Lian (1997). For some of the samples preheating caused a slight colour change in the sediment. The cause of this e!ect is presently unknown. For these samples employment of the normalisation factors increased the scatter in the measured luminescence, and for that reason normalisation factors were not used for them (see Table 3). The equivalent dose (D ) was generally determined %2 from a series of dose-intercept values, each re#ecting the sample's response in the form of luminescence intensity to progressingly increasing radiation doses, as a function of time after the 1.4 eV excitation was switched on. For all the samples the additive-dose method was used, with a correction for the dose-dependent thermal transfer that results from the preheat procedure. For most of the samples the regeneration method, using the procedure described by Huntley et al. (1993) and Huntley and Lian (in press), was also employed. Dose}response curves were constructed using maximum likelihood "ts. The data were "tted with straight lines for samples CCL1, CCL3, CCL4, WBDS1 and WBDS2, whereas saturating exponential "ts were required for the remainder. In all cases straight lines were "tted to the thermal transfer correction data; examples are shown in Fig. 7. For the regeneration method, an intensity scaling parameter was included

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in the "t, and in each case it was within 1p of unity which indicates that the infrared bleach did not signi"cantly change the sample sensitivity; the equivalent doses reported are those with this scaling factor "xed at one. Dose-intercept values were determined for successive 5 or 10 s intervals over the duration of the excitation time, which was for 45 or 95 s. Dose intercepts determined by the additive-dose method with the thermal transfer correction are shown plotted as a function of excitation time for all but one of the samples in Fig. 8, and in each case they are constant. For each sample, the data were re-analysed in the same manner using the total number of photon counts measured over the entire excitation time; the D 's were taken as the dose-intercept %2 values, with a correction for the decay that resulted from the normalisation exposure. These D 's are listed in %2 Table 3 together with those found from the regeneration method, the latter being corrected for incomplete laboratory bleaching; in all cases the results from the two methods are consistent.

4. Dosimetry 4.1. Methods The dose delivered to the samples comes mainly from a, b, and c radiation emitted by 238U, 235U, 232Th, and their daughter products, and 40K; there is also a contribution from cosmic-ray radiation. For this study a milled subsample of that which was used for dating was analysed for K content by atomic absorption spectroscopy, while U and Th concentrations were determined by a variety of methods. The results are shown in Tables 1 and 2. For the 238U and 232Th decay chains, delayed neutron analysis (DNA) and neutron activation analysis (NAA) give the concentration of the parents, respectively. Thick-source a counting (TSAC) gives U and Th equivalent concentrations (U and Th , respectively) by count% % ing all the a particles in each chain (Huntley et al. 1986), while in-situ c-ray spectrometry (IGRS) allows the calculation of U and Th equivalent concentrations by measurement of the c-rays from 214Bi, a late daughter in the 238U chain, and 208Tl, a late daughter in the 232Th chain. The purpose of multiple analyses is to obtain limits or measures of the possible extent of radioactive disequilibrium in the U and Th decay chains, and to allow for inhomogeneities in the sediment matrix. 4.2. Results and evaluation of the dose rate For most of the samples the K, U, and Th concentrations obtained by laboratory analyses are consistent with

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Fig. 7. Luminescence decay measured under 1.4 eV excitation for some of the samples. The insets show c-dose response curves from the "rst 5 or 10 s of excitation. The luminescence intensity from a &natural' and &bleached' aliquot that have not been preheated, are shown as Nnp and Bnp, respectively; these measurements were not made for sample CCL2.

those obtained by IGRS after correction for water content, and the dose rates were therefore calculated using di!erent combinations of the analytical results mentioned above. The total dose rates given in Table 3 are suitable averages, with uncertainties re#ecting the possibility of minor disequilibrium, and inhomogeneity. For the four CCL samples there is a tendency for the apparent U content as determined by IGRS to be signi"cantly higher than that determined from other analyses. This is most evident for samples CCL3 and CCL4. Because the same e!ect is not observed in the K and Th results, we do not attribute this to an inhomogeneous sediment matrix, but we interpret it as evidence for disequilibrium in the U decay chain as a result of leaching. Since we do not know when the leaching occurred, dose rates were calculated for the case in which all the leaching

occurred immediately after deposition, and for the case in which the U content decreased linearly in time; the value used is the average of these with an uncertainty that encompasses both at $2p. The uncertainty in when the leaching occurred contributes less than 1% to the uncertainty in the optical age. The cosmic-ray dose rate for all these samples is significant, making up to 15% of the total dose rate. While near the surface, these samples would have received 0.2}0.3 Gy ka~1, but during that time the c dose rate would have been up to 50% less than at present. Once the overburden had reached a thickness of about 50 cm, the cosmic-ray dose rate would have reduced exponentially with burial depth as shown by Prescott and Hutton (1994). However, since there is no way of ascertaining the rate of sedimentation, or to account for possible periods

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water contents would have been a!ected only by relatively minor wet and dry periods. The water contents used for the dose-rate calculations (Table 1) were therefore the as collected values, with an uncertainty that should encompass the extremes (dry and wet periods) at $2p.

5. Optical ages The optical ages, calculated by dividing the equivalent doses obtained from the additive-dose method by the dose rates, are shown in Table 3. Those for the Chisholm Canyon section (samples CCL1-4) are also shown in Fig. 3 where they are seen to be in good agreement with the ages expected on the basis of the known age of the tephra beds. The modern samples from the Watson Bar Creek site (samples WBDS1, 2) produced D 's that are not %2 consistent with zero. The optical ages obtained from the Fraser Plateau sites (samples BLRL1}5) ranged from 30$2 to 94$17 ka and are, in contrast, all very much older than the anticipated ages which are expected to be between 14 and 2.3 ka (cal. 14C yr). Indeed, at site 45 the apparent optical age of sample BLRL2 is older than that of underlying sample BLRL1 by more than a factor of two. This inspired a detailed study to determine the cause of these excessive, and stratigraphically inverted, ages.

6. Detailed study of the Fraser Plateau and Watson Bar Creek samples

Fig. 8. Dose-intercept versus excitation time plots for all of the samples except WBDS2, as determined from the additive-dose method with thermal-transfer correction. In each case, the D used in the age %2 calculation was the dose intercept calculated from the total number of photon counts measured over the entire excitation time.

of erosion, the present burial depths were used to estimate the cosmic-ray dose rates (Table 3). Water content estimates were based on what is known about the history of the deposits. It was assumed that since all our samples occurred near the surface, and at well-drained locations, and since they were all deposited during post-glacial time, that the long-term

The highly variable optical &ages' obtained from samples collected on the Fraser Plateau prompted an experiment to see how the natural luminescence varied with depth. At site 45 (the location of samples BLRL1 and BLRL2) two sets of samples were collected every 10 cm, starting at the base of the surface soil, to a depth of 1.8 m (Fig. 5). In the laboratory, unexposed sediment was removed from each sample container and mixed with a small amount of clear plastic sealer to keep the grains immobile. The mixture was then pressed into 1 cm diameter planchets and allowed to dry. Three aliquots were made from each sample. All the aliquots were measured under 1.4 eV excitation using the same apparatus and "lter combination as was used for the original samples. After measurement, all the aliquots received an&20 h infrared bleach, and then a c dose of 50 Gy. After a delay of several days, the aliquots were measured again, and this luminescence was used to correct the "rst measurements for the variable sensitivity of the aliquots. The results, shown in Fig. 5, show that there is a large variation in natural luminescence and that the expected systematic increase with depth is not observed. Sample

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Table 1 Thick-source a count (TSAC) rates!, equivalent U and Th concentrations, and water contents. TSAC

Water content"

Sample

Total count rate (cm~2 ks~1)

Th count rate (cm~2 ks~1)

Th % (lg g~1)

U % (lg g~1)

*8 !#

*8 4!5

*8

CCL1 CCL2 CCL3 CCL4 BLRL1 BLRL2 BLRL3 BLRL4 BLRL5 WBDS1 WBDS2

0.320$0.005 0.342$0.004 0.236$0.005 0.246$0.004 0.171$0.004 * 0.173$0.005 0.148$0.004 0.169$0.004 0.315$0.005 *

0.118$0.013 0.177$0.014 0.089$0.010 0.109$0.011 0.080$0.010 * 0.083$0.011 0.071$0.011 0.090$0.011 0.158$0.015 *

3.16$0.36 4.76$0.38 2.40$0.27 2.94$0.31 2.15$0.28 * 2.23$0.31 1.91$0.31 2.42$0.31 4.24$0.40 *

1.58$0.11 1.29$0.12 1.14$0.09 1.07$0.09 0.71$0.09 * 0.70$0.10 0.60$0.09 0.62$0.10 1.23$0.12 *

0.028 0.020 0.016 0.015 0.066 0.060 0.037 0.047 0.078 0.066

0.279 0.237 0.271 0.344 0.249 0.240 0.200 0.242 0.260 0.263

0.030$0.007 0.020$0.005 0.020$0.005 0.015$0.003 0.07$0.01 0.06$0.01 0.04$0.01 0.05$0.01 0.08$0.02 0.07$0.01 *

*

*

!See Huntley et al. (1986). Count rates have been corrected for sample re#ectivity using the model of Huntley and Wintle (1978). " *8 and *8 , are the as collected and saturated water contents, respectively; *8 is the water content used for the dose rate calculation. Water contents !# 4!5 are (water mass)/(mineral mass).

Table 2 K, U, and Th concentrations determined from laboratory analyses and in situ c-ray spectroscopy From in situ c-ray spectroscopy Sample

K (%)!

U (lg g~1)"

Th (lg g~1)#

K (%)

U (lg g~1)$ %

Th (lg g~1)$ %

Apparent c dose rate (Gy ka~1)%

CCL1 CCL2 CCL3 CCL4 BLRL1 BLRL2 BLRL3 BLRL4 BLRL5 WBDS1 WBDS2

1.18$0.06 1.11$0.05 1.10$0.05 1.05$0.05 0.93$0.05 0.89$0.04 1.17$0.06 0.78$0.04 0.79$0.04 1.09$0.05 *

1.64$0.12 1.55$0.06 1.08$0.09 1.14$0.09 0.94$0.09 0.67$0.06 0.97$0.07 0.76$0.05 1.06$0.05 1.42$0.06 *

4.2$0.1 4.15$0.05 2.9$0.1 2.9$0.1 1.8$0.1 1.7$0.1 2.3$0.1 1.40$0.03 1.7$0.1 2.4$0.2 *

1.13$0.03 1.23$0.03 1.34$0.03 1.04$0.03 0.89$0.03 0.91$0.03 0.83$0.03 0.83$0.03 * * *

1.89$0.16 1.89$0.16 1.60$0.15 1.54$0.14 0.96$0.11 0.88$0.11 0.97$0.11 0.97$0.11 * * *

3.75$0.23 4.23$0.25 3.14$0.24 2.83$0.21 1.41$0.16 1.67$0.17 1.61$0.16 1.61$0.16 * * *

0.685$0.023 0.734$0.024 0.670$0.023 0.574$0.020 0.400$0.016 0.408$0.016 0.504$0.019 0.395$0.016 * * *

!From atomic absorption spectroscopy. "From delayed neutron analysis. #From neutron activation analysis. $Equivalent concentrations calculated from the activities of 214Bi for U and 208Tl for Th, assuming secular equilibrium. %From counts in the energy range 0.8}2.6 MeV, corrected for the relative proportions of K, U, and Th.

BLRL2 gave a D which is about 2.2 times greater than %2 that of sample BLRL1, and the natural luminescence measured at these two sites (Fig. 5) corresponds roughly to this ratio. If this relationship between D and natural %2 luminescence holds for the remainder of the samples, as shown in Fig. 5, then it is clear that it is not possible to obtain an acceptable D (and optical age) along this %2 pro"le. The samples from the zone of rippled sediments gave more natural luminescence than sediments from the massive, and presumably more-rapidly buried, zone dir-

ectly above. This was unexpected because this zone of rippled sediment presumably represents a former mobile surface in which the sediment grains would have been continually reworked at the surface, until "nally buried. To check how readily these sediments are bleached, several aliquots of laboratory-cleaned and separated grains were exposed for various durations of natural sunlight (under clear mid-day skies in June, at a latitude of&493N and an elevation of&370 m). The results show that the natural luminescence signal is halved by an

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Table 3 Equivalent doses (D ), b values, dose rates, and apparent optical ages %2 Sample

CCL1 CCL2 CCL3 CCL4 BLRL1 BLRL2 BLRL3 BLRL4 BLRL5 WBDS1 WBDS2

Normalisation factors used? No No Yes Yes Yes Yes Yes Yes No $ $

D (Gy) %2 additive-dose

regeneration

b value (Gy lm2)!

DQ c (Gy ka~1)"

DQ T (Gy ka~1)#

Optical age (ka)

18.6$1.2 28.9$2.1 6.25$0.41 5.97$0.32 73$6 153$27 55$8 108$19 44$3 1.2$0.3 1.7$0.4

16$5 * 5.6$0.8 5.3$0.7 67$4 * 46$6 103$6 42$2 * *

0.93$0.04 0.93$0.05 1.01$0.04 1.12$0.03 0.91$0.04 * 0.85$0.07 1.27$0.10 1.00$0.04 1.00$0.20 *

0.15 0.15 0.20 0.20 0.18 0.20 0.18 0.18 0.18 0.30 0.30

2.50$0.05 2.53$0.05 2.48$0.12 2.28$0.12 1.63$0.06 1.62$0.06 1.95$0.06 1.31$0.06 1.45$0.06 1.55$0.42 1.55$0.42

7.4$0.5 11$1 2.5$0.2 2.6$0.2 45$4 94$17 28$4 82$15 30$2 0.8$0.3 1.1$0.4

!b value as de"ned by Huntley et al. (1988). The b value for sample BLRL1 was used for sample BLRL2, and that for sample WBDS1 was used for sample WBDS2. "DQ : dose rate due to cosmic rays (Prescott and Hutton, 1994). # #DQ : total dose rate (that due to cosmic rays plus that due to c, b, and a radiation). T $Normalisation measurements were not made on these samples.

Fig. 10. Microscope photograph (black and white) showing some of the carbonate-cemented grain clusters that remained after a 5 min cleaning with dispersing solution and mechanical agitation in an ultrasonic bath. The silt and clay fraction have been removed for clarity. The large grain cluster near the bottom of the photograph (arrowed) is about 200 lm in diameter. Fig. 9. Luminescence response to 1.4 eV excitation as a function of prior natural sunlight exposure time. The data are for laboratory cleaned and separated 4}11 lm-sized grains from sample BLRL1. Measurement conditions were the same as for all the other measurements.

exposure of only 1 s, and reduced to 6% after 1 min. (Fig. 9). Thus the incorrect ages at this site clearly indicate that a signi"cant proportion of the 4}11 lm-sized silt grains were not exposed to su$cient sunlight during transport. The anomalously old ages from the other sites suggest that this e!ect was widespread over this part of the Fraser Plateau.

Bulk extracts from all the Fraser Plateau samples were subsequently viewed under a binocular microscope. In each case, the sediment was found to consist of many loose grains, but there were also many grain clusters. The extracts were then put in dispersing solution, and mechanically agitated while in an ultrasonic bath. After drying, it was found that most of these clusters remained intact (Fig. 10). However after a 5 min exposure to 10% HCl, and a "nal rinse in dispersing solution, all of the clusters had become disaggregated. It seems probable, therefore, that these carbonate-cemented clusters contained grains that had remained shielded from

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sunlight during transport. During routine sample preparation, grains that had remained shielded within clusters would have been liberated, and mixed with others that had been exposed to sunlight prior to burial. The Chisholm Canyon samples were also examined in this manner, but these appeared to be free of concretions. The modern aeolian sediment collected from near the mouth of Watson Bar Creek was also examined for concretions, but none could be identi"ed. However, when laboratory-cleaned and separated 4}11 lm-sized silt from sample WBDS2, an active ripple, was exposed to sunlight it was found that the natural luminescence could be further reduced, by about 25% after 5 h, to a level comparable with that measured after a 3 h infrared laboratory bleach (Fig. 11). This indicates that at least some of the grains had remained unexposed to sunlight at this location. It is interesting to note that the intensities after the 3 h infrared bleach and the 5 h sunlight bleach are similar, which is in contrast to that observed by Huntley and Clague (1996, their Fig. 11).

7. Discussion The optical ages obtained for the Chisholm Canyon (CCL) samples are in excellent agreement with the known age of the deposit, and this lends support to our procedures. It should be emphasised, however, that had we not used the thermal-transfer correction procedure, the D 's (and ages) obtained for samples CCL3 and %2 CCL4, and the Watson Bar Creek (WBDS) samples would be too high by nearly a factor of two (see, for example, Fig. 7, samples CCL4 and WBDS1). Although tests for the presence of anomalous fading were not performed during this study, the ages obtained for the Chisholm Canyon samples suggest that it was not a signi"cant source of error for these samples. The optical ages from the Fraser Plateau (BLRL) samples are, in contrast, much too old. In light of the ages obtained from the Chisholm Canyon samples, we can only conclude that these anomalous ages are a result of the sedimentological circumstances rather than our dating procedures. Most of the sediments that comprise the Fraser Plateau sections from which we have sampled are thought to have been de#ated from carbonate-rich outwash deposits (Fig. 6) no more than 150 m away. Carbonate-cemented grain clusters, as shown in Fig. 10, are abundant in these outwash deposits, and it is therefore probable that at least some of these survived transport, in some cases re-working, and loading as the deposit thickened. The sediment comprising the Chisholm Canyon samples, on the other hand, was derived mainly from glaciolacustrine mud, which readily separates into loose grains. The &modern' surface samples collected near Watson Bar Creek gave D 's that are not consistent with zero at %2

Fig. 11. Luminescence response to 1.4 eV excitation as a function of prior natural sunlight exposure time. These data are for laboratory cleaned and separated 4}11 lm-sized grains from modern (surface) sample WBDS2. Measurement conditions were the same as for all the other measurements. Blank disc refers to an aluminium disc with no sediment cover. The Dark count was measured with the 1.4 eV lightemitting diodes turned o!. Laboratory infrared bleach was from a quartz-halogen lamp behind a Schott RG-715 optical "lter. Photon counts are those integrated over the "rst 50 s of excitation. The blank disc gave a higher photon count than the disc with bleached sediment on it because the former was more e$cient at re#ecting photons (probably of energies '1.4 eV from the light-emitting diodes) back to the photomultiplier tube.

2p. It is therefore apparent that here too some grains had remained shielded from sunlight during transport. This is again probably due to short transport distances ((10 m) for some of the grains, and the fact that the Watson Bar Creek samples came from a relatively high-energy deposit (96% sand), implying rapid transport and deposition. It is also possible that some silt separated from these sample (comprising only 2% of the bulk sample) travelled as shielded coatings on the sand grains, but further work is needed to substantiate this. There have been many studies on the dating of aeolian deposits (see, for example, Wintle (1990) and Prescott and Robertson (1997) for reviews); very few of these report on the possible shielding of wind-blown grains during transport. Berger (1990) reported on a modern aeolian deposit that yielded an anomalous thermoluminescence age of about 1 ka, while Wintle (1990) obtained similar results from another aeolian deposit. Both Berger and Wintle attributed their excessive thermoluminescence ages to the presence of grain aggregates. However, the e!ects reported by these authors are insigni"cant when compared to those observed in our Fraser Plateau samples.

8. Conclusions We have demonstrated that our optical dating procedures have the potential for providing valuable

O.B. Lian, D.J. Huntley / Quaternary Science Reviews 18 (1999) 1453}1466

information on the timing of the formation these aeolian deposits, and hence the timing of deglaciation, in southcentral British Columbia. In this region, aeolian sediments are found to be preserved within some paraglacial fans where there is generally a paucity of organic material for radiocarbon dating, and aeolian deposits commonly occur on the surfaces of degradational terraces in the larger valleys, such as the Fraser and Thompson. Optical dating therefore has the potential for providing important information on the timing of postglacial #uvial incision in these regions. In light of this study, however, care must be taken to evaluate the sedimentological circumstances before optical dating is attempted. This can be achieved by visual examination of the sediment grains, and by the use of multiple samples from di!erent sections. Sunlight bleaching tests of unprepared samples may also be desirable. Our study has also emphasised the importance of including thermal-transfer correction curves when evaluating dose-intercepts. This method also makes the necessary allowance for scattered excitation light and photomultiplier dark counts. Had we not used this correction, several of our optical ages would have been too old, some by nearly a factor of two. Unfortunately, the inclusion of thermal-transfer correction curves is still not common practice amongst luminescence dating practitioners. Finally, for the Fraser Plateau samples, our dose-intercept versus time plots show no intrinsic evidence of insu$cient bleaching prior to burial. An intercept that is rising with time can be interpreted as showing the presence of grains that were partially bleached before burial. There is no indication of this for these samples. In contrast, a mixture well-bleached grains and grains not bleached at all, deposited together at about the time of burial, would be expected to yield a #at intercept versus time plot, as observed in this study, and hence this is our interpretation.

Acknowledgements Assistance in the "eld was supplied by D.G. McPhee and E.C. Little. We thank T. Hancock for giving us permission to work on his property, and K. Lange for accommodation at Kelly Lake Ranch. S.R. Hicock is thanked for valuable discussion in the "eld. Electron microprobe analyses of the tephras was provided by Y. Thibault (UWO), and N. Butter"eld (UWO) kindly allowed the use of his microscope and camera. R.G. Roberts (La Trobe University) critically reviewed and edited an earlier version of the manuscript. G.W. Berger and another reviewer provided comments that improved the manuscript. Financial support for this research was provided through Natural Science and Engin-

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eering Research Council of Canada grants to D.J. Huntley and S.R. Hicock.

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