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Single-grain OSL dating of sediments from the Southern High Plains, USA
Quaternary Science Reviews 22 (2003) 1035–1042
Single-grain OSL dating of sediments from the Southern High Plains, USA James K. Feathers* Luminescence Dating Laboratory, Department of Anthropology, University of Washington, Box 353100, Seattle, WA 98195-3100, USA
Abstract Single-grain OSL dating is applied to sediments from different depositional settings on the Southern High Plains of western Texas and eastern New Mexico. Criteria of acceptance are used to screen equivalent doses from individual grains and resulting distributions are evaluated in terms of normality. Wide variation is found in proportion of acceptable grains and in the distributions. While some of the latter are normal, many show broadening that may be the result of mixing of different-aged grains and skewness that may result from variant depositional and post-depositional modes. Geological modeling will be required to understand better these distributions, although for most samples means produced ages that agree with independent evidence. The resolution possible with single grains is necessary for best estimates of equivalent dose for other samples. A few samples do not agree with independent evidence, even with a normal distribution of equivalent dose. Sand dunes seem the most difficult to date accurately, probably because of mixing. r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Since advancements in instrumentation (B^tter-Jensen et al., 2000) have made more practical single-grain dating by optically stimulated luminescence (OSL), attention is turning to understanding single-grain equivalent dose (DE ) distributions and the ability to derive accurate ages from them for different kinds of deposits. This paper assesses the accuracy of single-grain dates drawn from various alluvial and aeolian deposits from archaeological sites on the Southern High Plains in western Texas and eastern New Mexico. Many of these sites have independent dating controls from radiocarbon, but others have been dated only by presumed chronological range of projectile point types found in them. Improving the chronology of these points is an overall aim of this study. The Great Plains has long attracted the attention of archaeologists for its evidence of early peoples in North America, but a robust chronology has proven illusive. Charcoal is scarce, so radiocarbon dating has relied on bulk organic matter from soil, often problematic *Corresponding author. Tel.: +1-206-685-1659; fax: +1-206-5433285. E-mail address: [email protected] (J.K. Feathers).
because of difficulties of association with events of interest. Luminescence dating avoids this problem by directly targeting sediment deposition and has seen increasing use in Great Plains research (e.g., Stokes and Swinehart, 1997; Rich and Stokes, 2001). Problems from mixing of different aged sediments or inadequate insolation are addressed by single-aliquot and singlegrain dating (Roberts et al., 1999; Murray and Wintle, 2000). Samples represent a variety of depositional settings, including alluvial draws, sand dunes and ephemeral lakes. Table 1 lists the samples, their depositional contexts, and their expected age based either on associated radiocarbon dates or currently hypothesized age ranges for point types (Meltzer, 1991; Holliday, 1997).
2. Procedures Samples were collected in light tight cylinders. For most samples, 90125 mm quartz was extracted using standard procedures (Aitken, 1985) and measured by OSL. Equivalent dose (DE ) was determined by singlealiquot-regenerative dose (SAR) method (Murray and Wintle, 2000), using both multi-grain aliquots and single
0277-3791/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0277-3791(03)00049-0
J.K. Feathers / Quaternary Science Reviews 22 (2003) 1035–1042
1036 Table 1 Luminescence samples Sample
Site
Depositional contexta
Expected age (ka)b
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Ted Williamson Ted Williamson Milnesand San Jon San Jon Mustang Springs Mustang Springs Shifting Sands Bedford Ranch Bedford Ranch Clovis
Alluvial sand Paludal mud/loam Slopewash, aeolian Aeolian sand Aeolian sand Slopewash, aeolian Lunette Lunette Dune sand Dune sand Dune sand Slopewash, aeolian Slopewash, aeolian Aeolian sand Aeolian sand Dune sand Dune sand Dune sand Aeolian sand
5.8 (14C) 9.45 (14C) 0.5 (14C) 2.0 (14C) 5.65 (14C) 8.6 (14C) 1.3 (14C) 6.6–10 (14C) 11.5 (Plainview points) 11.5 (Plainview points) 11.5 (Milnesand points) o1.7 (14C) o9-13.5 (14C) >2.0 (14C) 7.7 (14C) 12.7 (Folsom Points) 11.1 (Firstview Points) 2.0 (14C) o1.5 (14C)
a b
sand
sand
sand sand
Designations drawn from Holliday (1997), Meltzer (1991). In approximate calendar years (Haas et al., 1986; Holliday, personal communication; 1997; Meltzer, 1991).
Table 2 Single-grain OSL measurement parameters System: Ris^ TL-DA-15, single-grain attachment Excitation: 532 nm laser (90% power) Detection filters: 7.5 mm U340 Preheat: 240 C 10 s Cut heat: 160 C Test dose: 2–5 Gy Shine: 0.8s at 125 C Analysis: 0.06s, background 0.65–0.8 s
grains. Two samples, not listed, were taken from carbonate marls and contained no coarse-grained material. For these, polymineral fine grains (4–11 mm) were prepared and measured by infra-red stimulated luminescence (IRSL). Details on these samples will be presented elsewhere and nothing more will be said here other than to note that the IRSL ages agreed with radiocarbon determinations. Results from the multigrain single aliquots analyses will also not be discussed, other than to mention that results were fully consistent with the single-grain results but with less resolution. Single-grain measurement parameters are given in Table 2. The Ris^ single-grain disks (B^tter-Jensen et al., 2000) contain holes that allow for 180–212 mm grains. The smaller grain sizes used here meant that 2–3 grains fit into each hole, hence single-grain resolution was not obtained entirely, but nearly so given that, at most, only one-third of these aliquots had measurable signals. The SAR sequence included the natural dose, four regenera-
tion doses, a zero dose and a repeated regeneration dose. Best-fit quadratic growth curves were used. A saturating-exponential function was employed where possible, but this was often unsatisfactory. DE distributions were evaluated using radial graphs (Galbraith et al., 1999), histograms and measurements of skewness. Bin width on histograms was taken as the median DE error for individual aliquots (Lepper, 2001). A test for feldspar grains in the quartz samples compared corrected responses on multi-grain aliquots to two identical doses, one of which was followed by an IRSL exposure (Henshilwood et al., 2002). If this ratio differed significantly from the average recycling ratio (Murray and Wintle, 2000) of the multi-grain aliquots, then feldspar grains were judged present. This was the case for only three samples: UW576, UW580, and UW582, but the ages derived for these three samples were consistent with independent evidence, so the effect is considered slight. Dose rate parameters were measured in the laboratory by thick source alpha counting, flame photometry and beta counting, and in the field by CaSO4:Dy dosimeters and portable gamma spectrometry (the latter collected by Shannon Mahan of the US Geological Survey).
3. Dose rate results Dose rates were determined from alpha counting and flame photometry and these results were checked by comparisons with results from other methods. Fig. 1
J.K. Feathers / Quaternary Science Reviews 22 (2003) 1035–1042
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4. Equivalent dose results 4.1. Initial considerations
Fig. 1. Comparison of beta dose rates determined directly by beta counting and indirectly by calculation from alpha counting and flame photometry assuming secular equilibrium. The 1:1 correspondence line is shown. Error bars are 71s here and in the next figure.
Fig. 2. Comparison of external dose rate determined directly in the field by gamma spectrometry and by CaSO4:Dy dosimetry, and indirectly by calculation from alpha counting and flame photometry (assuming secular equilibrium and including cosmic ray dose rate calculations). The 1:1 correspondence line is shown.
compares the beta dose rates determined directly by beta counting (B^tter-Jensen and Mejdahl, 1988) with those calculated from alpha counting and flame photometry assuming decay-series secular equilibrium. Fig. 2 compares external dose rates determined directly in the field by either dosimetry or gamma spectrometry with those calculated from alpha counting, flame photometry and cosmic dose estimations. In both cases the correspondence is close to 1:1 except for some dosimetry measures, which appear overestimated. Errors in estimation of moisture content, based on measurements at the time of collection (February) and textural considerations, are not a likely explanation since the overestimations show no pattern even within single sites. The overall agreement among different methods suggests not only relative secure determinations of current activities but also secular equilibrium in the decay chains (because different methods sample different parts of the chains).
Single-grain analyses produce results of varying quality and precision (e.g., Galbraith et al., 1999). For this work, results were not accepted if: (1) the recycling ratio (Murray and Wintle, 2000) did not fall within 0.8 and 1.2; (2) the error on the test dose signal was greater than 20%; (3) the net natural signal was less than three times the standard deviation of the background signal; (4) the natural signal was significantly higher than the signal from the highest regeneration dose; and (5) the error on the DE was more than 30%, except for the two youngest samples where this criterion was not applied. Criterion 1 eliminates aliquots that do not conform to SAR assumptions. Criteria 2 and 3 eliminate aliquots with little signal. Criterion 4 eliminates the need for extrapolation to determine DE. While a few of the aliquots rejected by this criterion may have been rescued by use of higher regeneration doses, most were aliquots where the regeneration growth curve saturated well below the level of the natural signal (Yoshida et al., 2000). The 30% criterion was adopted to diminish a slight negative correlation between DE and precision and thereby make the use of histograms less biased (Lepper, 2001). This had the effect of removing some low-value, low-precision points, representing near-zero ages that were present in nearly every sample. These were insignificant in number, with some exceptions discussed later, and their removal had little bearing on subsequent analyses (Fig. 3). These criteria are less stringent than has been used by other researchers (e.g., Roberts et al., 1999). The number of grains affected by these criteria varied by an order of magnitude (Table 3) with some samples having acceptance ratios of nearly 0.2 and others having ratios less than 0.02. The working assumption is that well-bleached, unmixed samples should have a normal DE distribution among grains and the mean should give the proper age. Normality was assessed by adjusting radial graphs to maximize the number of grains that fall within two standard deviations of some reference value (Galbraith et al., 1999) and by computing a standardized skewness score of histogram data (Field, 2000). For normal distributions 95% of the DE ’s should fall within twostandard deviations of a mean. In practice a wider distribution is expected because of heterogeneity in beta dose rate (Roberts et al., 1999). The magnitude of this effect, which will vary from sample to sample, is evaluated on UW479, an alluvial sand from the Lubbock Lake site that had a skewness value closer to zero than most other samples. As will be seen, the age computed from the mean of this distribution closely matched radiocarbon determinations. The radial graph in Fig. 3 can be compared with the one in Fig. 4,
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Fig. 3. (a) Radial graph and histogram of single-grain DE ’s from UW479, an alluvial sand from Lubbock Lake. On this and subsequent figures, the precision on the radial graph is the reciprocal of the percent uncertainty and the vertical scale refers to the number of standard deviations away from the mean. The mean is a reference value corresponding to that value where the number of individual DE ’s within two standard deviations is maximized. The bin width on the histogram is equal to the median error of equivalent dose and values shown are midpoints (Table 3). (b) The same data with the removal of those DE values with greater than 30% uncertainty. (c) Relationship between percent error and DE for UW479. The solid line is a linear regression of the points, while the dotted line represents 30% error. The correlation is diminished when the low precision points are removed.
constructed with the same data but leaving out the natural signal and treating the first regeneration point as an unknown dose. This ‘‘recovered’’ dose, which is not affected by beta heterogeneity, is used as the reference value. Ninety-two percent of the computed DE ’s fall within two standard deviations of the recovered dose, where only 84% do around the natural dose. The distribution broadening reflected in this difference plus the skewness values allows a qualitative identification of which samples may deviate from normality (Table 4). Notice that some samples have dose recovery rates as low as 77.8%. Part of the reason for this is sample size. Removing the two smallest samples (for which dose recovery was 100%), a significant positive correlation (R2 ¼ 0:32) obtains between sample size and percent recovery. Aliquots that did not accurately recover the dose covered the full distribution of DE ’s, so their retention had no significant effect on results.
4.2. Results In terms of these normality measures, the samples can be grouped as follows: (1) Four additional samples (UW573, UW577, UW586, UW587), all from sand dunes, showed little distributional broadening beyond that of UW479 (being used as a standard) and no significant skewness, although the last two of these had small sample sizes. The mean is probably the best DE estimate for these. (2) Four samples (UW480, UW570, UW571, and UW576) show little distributional broadening but have negative skewness. Two exhibit bimodality (e.g., Fig. 5). The first three are from the same location at Lubbock Lake and represent various alluvial deposits with aeolian components. UW576 is from a sand dune. The skewness of these samples is
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Table 3 Acceptance criteria for single grains Sample
Total grains
Bright grainsa
Recycle failsb
N>regen.c
>30% errord
Acceptede
Median error (Gy)
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
1000 1000 1000 1000 999 900 999 999 1000 1000 1895 999 1500 1400 1000 1997 2099 1799 1000
410 338 294 180 305 311 224 399 82 147 183 48 145 176 259 88 67 56 37
153 102 98 55 99 122 78 159 31 51 60 27 36 61 77 36 27 20 16
31 56 5 23 63 44 20 40 10 20 31 2 20 29 40 10 5 6 1
38 35 NA 13 29 20 14 48 9 18 33 8 19 18 17 12 7 9 NA
188 145 191 89 114 145 112 152 32 58 59 11 70 68 125 30 28 21 20
1.7 2.5 0.4 1.0 2.6 2.0 1.7 8.9 1.4 1.8 1.7 0.6 7.1 1.3 1.7 1.5 0.8 0.7 0.3
a
Number of grains passing criteria 2 and 3 (see text). Number failing recycle test (criterion 1), but passing criteria 2 and 3. c Number having natural signal significantly larger than highest regeneration signal (criterion 4). d Number having equivalent-dose uncertainty >30% (criterion 5). e Value in column 3 less values in columns 4–6. b
Table 4 Normality evaluation Sample
Fig. 4. Dose recovery distribution data for UW479. The reference point here represents the first regeneration dose and the values represent the ‘‘recovery’’ of that dose from subsequent regeneration doses.
even more pronounced if the low-precision (>30% error), low-value points are included. The source of this skewness is unknown, perhaps representing some turbation process, but if the values from the three lowest bins in the histograms are removed, representing 7–12% of the values, the skewness disappears. The mean of these truncated histograms is assumed to represent the best estimate of DE : (3) Six samples (UW569, UW574, UW579, UW582, UW585, UW588), all but one from dune or other
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
% values o2s from referencea Natural
Dose recovery
84.1 81.8 82.7 76.4 78.1 88.8 84.7 68.7 72.2 78.7 76.7 72.7 78.3 75.4 75.0 81.3 89.7 86.4 85.0
92.0 (176) 88.3 (128) 95.2 (168) 91.4 (81) 82.2 (90) 86.8 (114) 89.8 (98) 86.0 (121) 81.3 (32) 77.8 (54) 84.9 (53) 100 (8) 86.2 (58) 85.5 (62) 94.7 (114) 90.0 (30) 80.0 (25) 89.5 (19) 100 (18)
(189) (143) (191) (89) (114) (125) (111) (150) (36) (61) (60) (11) (69) (69) (124) (32) (29) (22) (20)
Skewnessb
0.08270.177 0.78570.201 0.85370.178 0.09570.255 0.74970.226 0.44070.217 0.02970.229 0.08170.197 0.16470.414 0.35970.314 0.10770.311 0.25870.845 0.52770.287 0.22170.291 0.27270.217 0.16070.427 0.12670.441 0.15370.501 0.03570.512
a The reference value for the natural dose is the one where the percentage of DE’s within 2s is maximized. The reference value for dose recovery is the first regeneration dose. Number of aliquots is given in parentheses. b Skewness was calculated using the SPSS computer software package (see Field, 2000).
aeolian deposits, show significant broadening but no skewness (e.g., Fig. 6). UW579 is an alluvial sand but also has very small sample size. The last
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Fig. 5. DE distribution data for UW480, a paludal mud/loam from Lubbock Lake. The data are negatively skewed and show bimodality.
Fig. 6. DE distribution data for UW574, dune sand from a lunette near Lubbock Lake. The data are significantly broader than might be expected for a normal distribution.
two dune deposits also have small sample sizes. Mixing of grains deposited over a long age range, perhaps most likely in a dune setting, is one explanation. With no other defensible choice, the mean remains the best estimate for DE : (4) Two samples have significant positive skewness, one (UW568) also with significant broadening, and one (UW580) with broadening at about the same magnitude as UW479 (e.g., Fig. 7). Both are slopewash deposits, although with an aeolian component. Others have noted positive skewness in waterlain deposits and have suggested using the grains with the smallest DE as the best estimate for age of deposition (Olley et al., 1999, Lepper, 2001). Lepper’s leading edge algorithm was used to determine DE for these samples. (5) One sample (UW583) showed significant broadening and negative skewness (Fig. 8). This sample, from Mustang Springs, was taken from the base of a massive homogeneous aeolian sand unit, containing no sedimentary structures such as cross-bedding, and bounded by marsh or pond sediments (Meltzer, 1991). The lack of structure suggests a high degree of mixing by various turbation processes. In such a case, the trailing edge (analogous to Lepper’s leading
edge), i.e., the oldest grains, should be the best DE estimate for the base. UW582, also with a broad distribution, came from the top of this unit, where one might expect positive skewness, but no significant skewness was found for this sample.
5. Ages and conclusions The OSL ages compare favorably with expected ages with some notable exceptions (Table 5). The ages from UW573 and UW574, both from the same lunette at Lubbock Lake, are grossly overestimated. UW574 had one of the broadest distributions in DE ; suggesting considerable mixing (although even an age based on the leading edge does not approach the radiocarbon age), but UW573 in terms of the normality measures used was as good as any sample. The only conclusion is that the OSL and radiocarbon are addressing different depositional events. Other contextual information, such as faunal remains, suggests the radiocarbon dates reflect the most recent deposition, implying poor bleaching of the sand grains at this time (Holliday, personal communication).
J.K. Feathers / Quaternary Science Reviews 22 (2003) 1035–1042
Fig. 7. DE distribution data for UW580, slopewash adjacent to lake sediments at San Jon. The data are positively skewed.
UW585 and UW586 are somewhat underestimated and UW587 is badly overestimated. All three are from a still active sand dune field (Andrews Dunes). The sample size for all three was small, so interpretation is difficult. Nevertheless the nearly identical ages of the two samples from Bedford Ranch, one from high and one from low in the stratigraphy, suggests considerable mixing. On the other hand, the sand deposit of the lower sample is characterized by clay lamina, which, at least for this sample, suggests less mixing. The trailing edge date for UW583, while still slightly underestimated, is an improvement over using the mean, which would have been the result if multi-grain aliquots had been used. The two leading edge dates (UW568 and UW580) are reasonable, although dates using the means (0.5870.04 for UW568 and 13.370.9 for UW580) are also not inconsistent with the radiocarbon data. In summary, single-grain dating provided relatively accurate chronological information for the Southern High Plains. While reasonable dates could also be obtained from multi-grain aliquots, some samples such as UW583 required single-grain resolution to make the best estimate. DE distributions were quite variable and difficult to interpret inductively. Where geological evidence suggested leading edge or trailing edge determinations should apply, the dates were consistent
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Fig. 8. DE distribution data for UW583, aeolian sand from Mustang Springs. The data are negatively skewed.
Table 5 Age derivations Sample
OSL (ka)
Expected (ka)
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
5.8170.33 8.4870.44 0.3770.14 2.4170.13 5.6470.25 7.8670.42 5.3770.34 27.971.7 10.870.77 12.970.91 11.670.82 1.1670.17 8.8771.9 4.0470.31 6.5070.57 11.071.1 7.9970.89 8.4871.0 0.9570.11
5.8 9.4 0.5 2.0 5.6 8.6 1.3 6.6–10 12 12 12 o1.7 o9–13.5 >2.0 7.7 13 11 2.0 o1.5
with other chronological information. Such geologically modeling will probably be required to understand better single-grain distributions. Although not true in every case, samples from sand dunes caused the most trouble,
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sometimes imperceptively. Without independent dating evidence, nothing in the DE distribution of UW573 would suggest mixing or poor bleaching.
Acknowledgements This research was funded by the National Science Foundation. Special thanks to Vance Holliday, David Meltzer, Jason LaBelle and Shannon Mahan for helping collect samples and providing their context. Aksel Casson assisted with laboratory work.
References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. B^tter-Jensen, L., Mejdahl, V., 1988. Assessment of beta dose-rate using a GM multicounter system. Nuclear Tracks and Radiation Measurements 14, 187–191. B^tter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 523–528. Field, A., 2000. Discovering Statistics Using SPSS for Windows. Sage Publications, London. Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: part I, experimental design and statistical models. Archaeometry 41, 339–364. Haas, H., Holliday, V.T., Stuckenrath, R., 1986. Dating of Holocene stratigraphy with soluble and insoluble organic fractions at the Lubbock Lake archaeological site, Texas: an ideal case study. Radiocarbon 28, 473–485.
Henshilwood, C.S., d’Errico, F., Yates, R., Jacobs, Z., Tribolo, C., Duller, G.A.T., Mercier, N., Sealy, J.C., Valladas, H., Watts, Wintle, A.G., 2002. Emergence of modern human behavior: Middle stone age engravings from South Africa. Science 295, 1278–1280. Holliday, V.T., 1997. Paleoindian geoarchaeology of the Southern High Plains. University of Texas Press, Austin. Lepper, K., 2001. Development of an objective dose distribution analysis method for OSL dating and pilot studies for planetary applications. Ph.D. Thesis, Oklahoma State University, Stillwater. Meltzer, D.J., 1991. Altithermal archaeology and paleoecology at Mustang Springs, on the southern high plains of Texas. American Antiquity 56, 236–267. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73. Olley, J.M., Caitcheon, G.G., Roberts, R.G., 1999. The origin of dose distributions in fluvial sediments, and the prospect of dating single grains from fluvial deposits using optically stimulated luminescence. Radiation Measurements 30, 207–217. Rich, J., Stokes, S., 2001. Optical dating of geoarchaeologically significant sites from the southern high plains and South Texas, USA. Quaternary Science Reviews 20, 949–959. Roberts, R.G., Galbraith, R.F., Olley, J.M., Yoshida, H., Laslett, G.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: part II, results and implications. Archaeometry 41, 365–395. Stokes, S., Swinehart, J.B., 1997. Middle- and late-Holocene dune reactivation in the Nebraska Sand Hills, USA. The Holocene 7, 263–272. Yoshida, H., Roberts, R.G., Olley, J.M., Laslett, G.M., Galbraith, R.F., 2000. Extending the age range of optical dating using single ‘supergrains’ of quartz. Radiation Measurements 32, 439–446.
Single-grain OSL dating of sediments from the Southern High Plains, USA James K. Feathers* Luminescence Dating Laboratory, Department of Anthropology, University of Washington, Box 353100, Seattle, WA 98195-3100, USA
Abstract Single-grain OSL dating is applied to sediments from different depositional settings on the Southern High Plains of western Texas and eastern New Mexico. Criteria of acceptance are used to screen equivalent doses from individual grains and resulting distributions are evaluated in terms of normality. Wide variation is found in proportion of acceptable grains and in the distributions. While some of the latter are normal, many show broadening that may be the result of mixing of different-aged grains and skewness that may result from variant depositional and post-depositional modes. Geological modeling will be required to understand better these distributions, although for most samples means produced ages that agree with independent evidence. The resolution possible with single grains is necessary for best estimates of equivalent dose for other samples. A few samples do not agree with independent evidence, even with a normal distribution of equivalent dose. Sand dunes seem the most difficult to date accurately, probably because of mixing. r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Since advancements in instrumentation (B^tter-Jensen et al., 2000) have made more practical single-grain dating by optically stimulated luminescence (OSL), attention is turning to understanding single-grain equivalent dose (DE ) distributions and the ability to derive accurate ages from them for different kinds of deposits. This paper assesses the accuracy of single-grain dates drawn from various alluvial and aeolian deposits from archaeological sites on the Southern High Plains in western Texas and eastern New Mexico. Many of these sites have independent dating controls from radiocarbon, but others have been dated only by presumed chronological range of projectile point types found in them. Improving the chronology of these points is an overall aim of this study. The Great Plains has long attracted the attention of archaeologists for its evidence of early peoples in North America, but a robust chronology has proven illusive. Charcoal is scarce, so radiocarbon dating has relied on bulk organic matter from soil, often problematic *Corresponding author. Tel.: +1-206-685-1659; fax: +1-206-5433285. E-mail address: [email protected] (J.K. Feathers).
because of difficulties of association with events of interest. Luminescence dating avoids this problem by directly targeting sediment deposition and has seen increasing use in Great Plains research (e.g., Stokes and Swinehart, 1997; Rich and Stokes, 2001). Problems from mixing of different aged sediments or inadequate insolation are addressed by single-aliquot and singlegrain dating (Roberts et al., 1999; Murray and Wintle, 2000). Samples represent a variety of depositional settings, including alluvial draws, sand dunes and ephemeral lakes. Table 1 lists the samples, their depositional contexts, and their expected age based either on associated radiocarbon dates or currently hypothesized age ranges for point types (Meltzer, 1991; Holliday, 1997).
2. Procedures Samples were collected in light tight cylinders. For most samples, 90125 mm quartz was extracted using standard procedures (Aitken, 1985) and measured by OSL. Equivalent dose (DE ) was determined by singlealiquot-regenerative dose (SAR) method (Murray and Wintle, 2000), using both multi-grain aliquots and single
0277-3791/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0277-3791(03)00049-0
J.K. Feathers / Quaternary Science Reviews 22 (2003) 1035–1042
1036 Table 1 Luminescence samples Sample
Site
Depositional contexta
Expected age (ka)b
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Lubbock Lake Ted Williamson Ted Williamson Milnesand San Jon San Jon Mustang Springs Mustang Springs Shifting Sands Bedford Ranch Bedford Ranch Clovis
Alluvial sand Paludal mud/loam Slopewash, aeolian Aeolian sand Aeolian sand Slopewash, aeolian Lunette Lunette Dune sand Dune sand Dune sand Slopewash, aeolian Slopewash, aeolian Aeolian sand Aeolian sand Dune sand Dune sand Dune sand Aeolian sand
5.8 (14C) 9.45 (14C) 0.5 (14C) 2.0 (14C) 5.65 (14C) 8.6 (14C) 1.3 (14C) 6.6–10 (14C) 11.5 (Plainview points) 11.5 (Plainview points) 11.5 (Milnesand points) o1.7 (14C) o9-13.5 (14C) >2.0 (14C) 7.7 (14C) 12.7 (Folsom Points) 11.1 (Firstview Points) 2.0 (14C) o1.5 (14C)
a b
sand
sand
sand sand
Designations drawn from Holliday (1997), Meltzer (1991). In approximate calendar years (Haas et al., 1986; Holliday, personal communication; 1997; Meltzer, 1991).
Table 2 Single-grain OSL measurement parameters System: Ris^ TL-DA-15, single-grain attachment Excitation: 532 nm laser (90% power) Detection filters: 7.5 mm U340 Preheat: 240 C 10 s Cut heat: 160 C Test dose: 2–5 Gy Shine: 0.8s at 125 C Analysis: 0.06s, background 0.65–0.8 s
grains. Two samples, not listed, were taken from carbonate marls and contained no coarse-grained material. For these, polymineral fine grains (4–11 mm) were prepared and measured by infra-red stimulated luminescence (IRSL). Details on these samples will be presented elsewhere and nothing more will be said here other than to note that the IRSL ages agreed with radiocarbon determinations. Results from the multigrain single aliquots analyses will also not be discussed, other than to mention that results were fully consistent with the single-grain results but with less resolution. Single-grain measurement parameters are given in Table 2. The Ris^ single-grain disks (B^tter-Jensen et al., 2000) contain holes that allow for 180–212 mm grains. The smaller grain sizes used here meant that 2–3 grains fit into each hole, hence single-grain resolution was not obtained entirely, but nearly so given that, at most, only one-third of these aliquots had measurable signals. The SAR sequence included the natural dose, four regenera-
tion doses, a zero dose and a repeated regeneration dose. Best-fit quadratic growth curves were used. A saturating-exponential function was employed where possible, but this was often unsatisfactory. DE distributions were evaluated using radial graphs (Galbraith et al., 1999), histograms and measurements of skewness. Bin width on histograms was taken as the median DE error for individual aliquots (Lepper, 2001). A test for feldspar grains in the quartz samples compared corrected responses on multi-grain aliquots to two identical doses, one of which was followed by an IRSL exposure (Henshilwood et al., 2002). If this ratio differed significantly from the average recycling ratio (Murray and Wintle, 2000) of the multi-grain aliquots, then feldspar grains were judged present. This was the case for only three samples: UW576, UW580, and UW582, but the ages derived for these three samples were consistent with independent evidence, so the effect is considered slight. Dose rate parameters were measured in the laboratory by thick source alpha counting, flame photometry and beta counting, and in the field by CaSO4:Dy dosimeters and portable gamma spectrometry (the latter collected by Shannon Mahan of the US Geological Survey).
3. Dose rate results Dose rates were determined from alpha counting and flame photometry and these results were checked by comparisons with results from other methods. Fig. 1
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4. Equivalent dose results 4.1. Initial considerations
Fig. 1. Comparison of beta dose rates determined directly by beta counting and indirectly by calculation from alpha counting and flame photometry assuming secular equilibrium. The 1:1 correspondence line is shown. Error bars are 71s here and in the next figure.
Fig. 2. Comparison of external dose rate determined directly in the field by gamma spectrometry and by CaSO4:Dy dosimetry, and indirectly by calculation from alpha counting and flame photometry (assuming secular equilibrium and including cosmic ray dose rate calculations). The 1:1 correspondence line is shown.
compares the beta dose rates determined directly by beta counting (B^tter-Jensen and Mejdahl, 1988) with those calculated from alpha counting and flame photometry assuming decay-series secular equilibrium. Fig. 2 compares external dose rates determined directly in the field by either dosimetry or gamma spectrometry with those calculated from alpha counting, flame photometry and cosmic dose estimations. In both cases the correspondence is close to 1:1 except for some dosimetry measures, which appear overestimated. Errors in estimation of moisture content, based on measurements at the time of collection (February) and textural considerations, are not a likely explanation since the overestimations show no pattern even within single sites. The overall agreement among different methods suggests not only relative secure determinations of current activities but also secular equilibrium in the decay chains (because different methods sample different parts of the chains).
Single-grain analyses produce results of varying quality and precision (e.g., Galbraith et al., 1999). For this work, results were not accepted if: (1) the recycling ratio (Murray and Wintle, 2000) did not fall within 0.8 and 1.2; (2) the error on the test dose signal was greater than 20%; (3) the net natural signal was less than three times the standard deviation of the background signal; (4) the natural signal was significantly higher than the signal from the highest regeneration dose; and (5) the error on the DE was more than 30%, except for the two youngest samples where this criterion was not applied. Criterion 1 eliminates aliquots that do not conform to SAR assumptions. Criteria 2 and 3 eliminate aliquots with little signal. Criterion 4 eliminates the need for extrapolation to determine DE. While a few of the aliquots rejected by this criterion may have been rescued by use of higher regeneration doses, most were aliquots where the regeneration growth curve saturated well below the level of the natural signal (Yoshida et al., 2000). The 30% criterion was adopted to diminish a slight negative correlation between DE and precision and thereby make the use of histograms less biased (Lepper, 2001). This had the effect of removing some low-value, low-precision points, representing near-zero ages that were present in nearly every sample. These were insignificant in number, with some exceptions discussed later, and their removal had little bearing on subsequent analyses (Fig. 3). These criteria are less stringent than has been used by other researchers (e.g., Roberts et al., 1999). The number of grains affected by these criteria varied by an order of magnitude (Table 3) with some samples having acceptance ratios of nearly 0.2 and others having ratios less than 0.02. The working assumption is that well-bleached, unmixed samples should have a normal DE distribution among grains and the mean should give the proper age. Normality was assessed by adjusting radial graphs to maximize the number of grains that fall within two standard deviations of some reference value (Galbraith et al., 1999) and by computing a standardized skewness score of histogram data (Field, 2000). For normal distributions 95% of the DE ’s should fall within twostandard deviations of a mean. In practice a wider distribution is expected because of heterogeneity in beta dose rate (Roberts et al., 1999). The magnitude of this effect, which will vary from sample to sample, is evaluated on UW479, an alluvial sand from the Lubbock Lake site that had a skewness value closer to zero than most other samples. As will be seen, the age computed from the mean of this distribution closely matched radiocarbon determinations. The radial graph in Fig. 3 can be compared with the one in Fig. 4,
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Fig. 3. (a) Radial graph and histogram of single-grain DE ’s from UW479, an alluvial sand from Lubbock Lake. On this and subsequent figures, the precision on the radial graph is the reciprocal of the percent uncertainty and the vertical scale refers to the number of standard deviations away from the mean. The mean is a reference value corresponding to that value where the number of individual DE ’s within two standard deviations is maximized. The bin width on the histogram is equal to the median error of equivalent dose and values shown are midpoints (Table 3). (b) The same data with the removal of those DE values with greater than 30% uncertainty. (c) Relationship between percent error and DE for UW479. The solid line is a linear regression of the points, while the dotted line represents 30% error. The correlation is diminished when the low precision points are removed.
constructed with the same data but leaving out the natural signal and treating the first regeneration point as an unknown dose. This ‘‘recovered’’ dose, which is not affected by beta heterogeneity, is used as the reference value. Ninety-two percent of the computed DE ’s fall within two standard deviations of the recovered dose, where only 84% do around the natural dose. The distribution broadening reflected in this difference plus the skewness values allows a qualitative identification of which samples may deviate from normality (Table 4). Notice that some samples have dose recovery rates as low as 77.8%. Part of the reason for this is sample size. Removing the two smallest samples (for which dose recovery was 100%), a significant positive correlation (R2 ¼ 0:32) obtains between sample size and percent recovery. Aliquots that did not accurately recover the dose covered the full distribution of DE ’s, so their retention had no significant effect on results.
4.2. Results In terms of these normality measures, the samples can be grouped as follows: (1) Four additional samples (UW573, UW577, UW586, UW587), all from sand dunes, showed little distributional broadening beyond that of UW479 (being used as a standard) and no significant skewness, although the last two of these had small sample sizes. The mean is probably the best DE estimate for these. (2) Four samples (UW480, UW570, UW571, and UW576) show little distributional broadening but have negative skewness. Two exhibit bimodality (e.g., Fig. 5). The first three are from the same location at Lubbock Lake and represent various alluvial deposits with aeolian components. UW576 is from a sand dune. The skewness of these samples is
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Table 3 Acceptance criteria for single grains Sample
Total grains
Bright grainsa
Recycle failsb
N>regen.c
>30% errord
Acceptede
Median error (Gy)
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
1000 1000 1000 1000 999 900 999 999 1000 1000 1895 999 1500 1400 1000 1997 2099 1799 1000
410 338 294 180 305 311 224 399 82 147 183 48 145 176 259 88 67 56 37
153 102 98 55 99 122 78 159 31 51 60 27 36 61 77 36 27 20 16
31 56 5 23 63 44 20 40 10 20 31 2 20 29 40 10 5 6 1
38 35 NA 13 29 20 14 48 9 18 33 8 19 18 17 12 7 9 NA
188 145 191 89 114 145 112 152 32 58 59 11 70 68 125 30 28 21 20
1.7 2.5 0.4 1.0 2.6 2.0 1.7 8.9 1.4 1.8 1.7 0.6 7.1 1.3 1.7 1.5 0.8 0.7 0.3
a
Number of grains passing criteria 2 and 3 (see text). Number failing recycle test (criterion 1), but passing criteria 2 and 3. c Number having natural signal significantly larger than highest regeneration signal (criterion 4). d Number having equivalent-dose uncertainty >30% (criterion 5). e Value in column 3 less values in columns 4–6. b
Table 4 Normality evaluation Sample
Fig. 4. Dose recovery distribution data for UW479. The reference point here represents the first regeneration dose and the values represent the ‘‘recovery’’ of that dose from subsequent regeneration doses.
even more pronounced if the low-precision (>30% error), low-value points are included. The source of this skewness is unknown, perhaps representing some turbation process, but if the values from the three lowest bins in the histograms are removed, representing 7–12% of the values, the skewness disappears. The mean of these truncated histograms is assumed to represent the best estimate of DE : (3) Six samples (UW569, UW574, UW579, UW582, UW585, UW588), all but one from dune or other
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
% values o2s from referencea Natural
Dose recovery
84.1 81.8 82.7 76.4 78.1 88.8 84.7 68.7 72.2 78.7 76.7 72.7 78.3 75.4 75.0 81.3 89.7 86.4 85.0
92.0 (176) 88.3 (128) 95.2 (168) 91.4 (81) 82.2 (90) 86.8 (114) 89.8 (98) 86.0 (121) 81.3 (32) 77.8 (54) 84.9 (53) 100 (8) 86.2 (58) 85.5 (62) 94.7 (114) 90.0 (30) 80.0 (25) 89.5 (19) 100 (18)
(189) (143) (191) (89) (114) (125) (111) (150) (36) (61) (60) (11) (69) (69) (124) (32) (29) (22) (20)
Skewnessb
0.08270.177 0.78570.201 0.85370.178 0.09570.255 0.74970.226 0.44070.217 0.02970.229 0.08170.197 0.16470.414 0.35970.314 0.10770.311 0.25870.845 0.52770.287 0.22170.291 0.27270.217 0.16070.427 0.12670.441 0.15370.501 0.03570.512
a The reference value for the natural dose is the one where the percentage of DE’s within 2s is maximized. The reference value for dose recovery is the first regeneration dose. Number of aliquots is given in parentheses. b Skewness was calculated using the SPSS computer software package (see Field, 2000).
aeolian deposits, show significant broadening but no skewness (e.g., Fig. 6). UW579 is an alluvial sand but also has very small sample size. The last
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Fig. 5. DE distribution data for UW480, a paludal mud/loam from Lubbock Lake. The data are negatively skewed and show bimodality.
Fig. 6. DE distribution data for UW574, dune sand from a lunette near Lubbock Lake. The data are significantly broader than might be expected for a normal distribution.
two dune deposits also have small sample sizes. Mixing of grains deposited over a long age range, perhaps most likely in a dune setting, is one explanation. With no other defensible choice, the mean remains the best estimate for DE : (4) Two samples have significant positive skewness, one (UW568) also with significant broadening, and one (UW580) with broadening at about the same magnitude as UW479 (e.g., Fig. 7). Both are slopewash deposits, although with an aeolian component. Others have noted positive skewness in waterlain deposits and have suggested using the grains with the smallest DE as the best estimate for age of deposition (Olley et al., 1999, Lepper, 2001). Lepper’s leading edge algorithm was used to determine DE for these samples. (5) One sample (UW583) showed significant broadening and negative skewness (Fig. 8). This sample, from Mustang Springs, was taken from the base of a massive homogeneous aeolian sand unit, containing no sedimentary structures such as cross-bedding, and bounded by marsh or pond sediments (Meltzer, 1991). The lack of structure suggests a high degree of mixing by various turbation processes. In such a case, the trailing edge (analogous to Lepper’s leading
edge), i.e., the oldest grains, should be the best DE estimate for the base. UW582, also with a broad distribution, came from the top of this unit, where one might expect positive skewness, but no significant skewness was found for this sample.
5. Ages and conclusions The OSL ages compare favorably with expected ages with some notable exceptions (Table 5). The ages from UW573 and UW574, both from the same lunette at Lubbock Lake, are grossly overestimated. UW574 had one of the broadest distributions in DE ; suggesting considerable mixing (although even an age based on the leading edge does not approach the radiocarbon age), but UW573 in terms of the normality measures used was as good as any sample. The only conclusion is that the OSL and radiocarbon are addressing different depositional events. Other contextual information, such as faunal remains, suggests the radiocarbon dates reflect the most recent deposition, implying poor bleaching of the sand grains at this time (Holliday, personal communication).
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Fig. 7. DE distribution data for UW580, slopewash adjacent to lake sediments at San Jon. The data are positively skewed.
UW585 and UW586 are somewhat underestimated and UW587 is badly overestimated. All three are from a still active sand dune field (Andrews Dunes). The sample size for all three was small, so interpretation is difficult. Nevertheless the nearly identical ages of the two samples from Bedford Ranch, one from high and one from low in the stratigraphy, suggests considerable mixing. On the other hand, the sand deposit of the lower sample is characterized by clay lamina, which, at least for this sample, suggests less mixing. The trailing edge date for UW583, while still slightly underestimated, is an improvement over using the mean, which would have been the result if multi-grain aliquots had been used. The two leading edge dates (UW568 and UW580) are reasonable, although dates using the means (0.5870.04 for UW568 and 13.370.9 for UW580) are also not inconsistent with the radiocarbon data. In summary, single-grain dating provided relatively accurate chronological information for the Southern High Plains. While reasonable dates could also be obtained from multi-grain aliquots, some samples such as UW583 required single-grain resolution to make the best estimate. DE distributions were quite variable and difficult to interpret inductively. Where geological evidence suggested leading edge or trailing edge determinations should apply, the dates were consistent
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Fig. 8. DE distribution data for UW583, aeolian sand from Mustang Springs. The data are negatively skewed.
Table 5 Age derivations Sample
OSL (ka)
Expected (ka)
UW479 UW480 UW568 UW569 UW570 UW571 UW573 UW574 UW575 UW576 UW577 UW579 UW580 UW582 UW583 UW585 UW586 UW587 UW588
5.8170.33 8.4870.44 0.3770.14 2.4170.13 5.6470.25 7.8670.42 5.3770.34 27.971.7 10.870.77 12.970.91 11.670.82 1.1670.17 8.8771.9 4.0470.31 6.5070.57 11.071.1 7.9970.89 8.4871.0 0.9570.11
5.8 9.4 0.5 2.0 5.6 8.6 1.3 6.6–10 12 12 12 o1.7 o9–13.5 >2.0 7.7 13 11 2.0 o1.5
with other chronological information. Such geologically modeling will probably be required to understand better single-grain distributions. Although not true in every case, samples from sand dunes caused the most trouble,
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sometimes imperceptively. Without independent dating evidence, nothing in the DE distribution of UW573 would suggest mixing or poor bleaching.
Acknowledgements This research was funded by the National Science Foundation. Special thanks to Vance Holliday, David Meltzer, Jason LaBelle and Shannon Mahan for helping collect samples and providing their context. Aksel Casson assisted with laboratory work.
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