Examining the potential of high sampling resolution OSL dating of Chinese loess

ARTICLE IN PRESS

Quaternary Geochronology 2 (2007) 15–22 www.elsevier.com/locate/quageo

Research paper

Examining the potential of high sampling resolution OSL dating of Chinese loess Thomas Stevensa,, Simon J. Armitagea, Huayu Lub,c, David S.G. Thomasa a

Oxford University Centre for the Environment, South Parks Road, Oxford OX1 3QY, UK State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, PR China c The Department of Geography, Nanjing University, Nanjing 210093, PR China

b

Received 22 March 2006; accepted 22 March 2006 Available online 30 May 2006

Abstract Detailed analysis of the depositional characteristics of Chinese loess is required to determine the nature of the paleoclimate record preserved in these extensive sediments. High sampling resolution optically stimulated luminescence (OSL) dating has the potential to facilitate such an analysis. However, high-resolution dating is extremely time consuming and therefore of limited practical applicability. This study assesses the luminescence characteristics of loess from three sections on the Chinese Loess Plateau in an attempt to identify methods of increasing sample throughput without compromising data quality. Using the single-aliquot regenerative-dose technique, samples yield internally consistent results. However, dose recovery data indicate that care is required in selecting preheating regimes for different sections. The standardized growth curve approach was tested and found to be applicable within, but not between, sites. Nonetheless, the use of standardized growth curves offers increases in sample throughput that will allow more routine high-resolution dating of Chinese loess. High-resolution dose rates calculated using inductively coupled plasma (ICP) methods show relative homogeneity of radioisotope concentrations and are comparable to lower resolution field gamma-spectrometry measurements. Consequently, high-resolution OSL dating has great potential to elucidate the depositional characteristics of Chinese loess and facilitate more precise use of the paleoclimatic information it preserves. r 2006 Elsevier Ltd. All rights reserved. Keywords: Chinese loess plateau; OSL; SAR; Standardized growth curve

1. Introduction Loess is one of the most widely investigated terrestrial sediments with uninterrupted deposition assumed to occur on the Chinese Loess Plateau, potentially providing a continuous and detailed record of Quaternary East Asian Monsoon and paleoenvironmental change (e.g. Porter, 2001). However, the paucity of dateable material within Chinese loess has hampered study of the paleoclimatic record it contains and further assumptions have been made in order to date loess through correlation of proxies to marine oxygen-isotope stratigraphy (e.g. Zheng et al., 1995) and through modelling of accumulation rates based on grain-size (e.g. Nugteren et al., 2004). These age models Corresponding author. Tel.: +44 7968 967 609; fax: +44 1865 275885.

E-mail address: [email protected] (T. Stevens). 1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.03.004

have been used to support evidence for the existence of rapid climate change events (e.g. Porter and An, 1995). However, recent investigations (Kohfeld and Harrison, 2003; Lu et al., 2004) have highlighted discrepancies between these age models and those derived from absolute dating techniques, suggesting that some of the model assumptions may be in error. Consequently, a highresolution, systematic examination of loess sedimentation characteristics is required to test the existing age models and the underpinning assumptions. Optically stimulated luminescence (OSL) dating techniques are particularly suitable for application to Chinese loess due to its high content of aeolian sedimentary mineral grains. However, OSL dating is time consuming and labour intensive. In addition, the errors associated may be too large to be of use in developing detailed sedimentation histories. This study aims to test the feasibility of high sampling resolution OSL

ARTICLE IN PRESS 16

T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22

dating of Chinese loess by detailing the luminescence properties of samples from two sections and investigating possible strategies for increasing sample throughput.

(upper Malan loess, the Holocene S0 soil and an unusually distinct loess layer lying above S0, here termed L0). 3. Sample preparation and measurement

2. Sites Two sites located at the SE (Shiguanzhai) and NW (Beiguoyuan) ends of the Loess Plateau were selected in order to investigate the potential spatial variability of deposition associated with significantly different environmental, sedimentary and diagenetic contexts (Fig. 1). The modern annual precipitation at Shiguanzhai is over 150 mm higher than at Beiguoyuan, and the site lies further from hypothesized dust sources of the North and northeast (Lu and Sun, 2000). These differences cause significant facies variations in the respective Malan (Last Glacial) and Holocene loess/paleosol sequences. The Beiguoyuan site is comprised of two sections located near Huanxian, Gansu Province, on the northern Loess Plateau (Fig. 1). The section with the most complete Holocene record (361 370 21.300 N, 1071 170 12.200 E, 1523 m a.s.l.) lies several km NW of the main section (361 370 36.200 N, 1071 160 57.400 E, 1545 m a.s.l.) where a longer record of loess deposition is preserved. Samples from the entire Holocene section and from the upper 5 m of the main section (which contains the Holocene soil and upper Malan loess) were analyzed. At the Shiguanzhai site (341 100 22.200 N, 1091 110 45.500 E, 708 m a.s.l.), located near Lantian, Shaanxi Province on the southern fringe of the Loess Plateau, the upper 1.75 m of the section was sampled

Samples were collected at 10–20 cm intervals from freshly cleaned sections, either as 10  10  10 cm blocks or in metal tubes hammered into the section face. The sunlight-exposed outer surfaces of blocks and the ends of tube samples were removed in the laboratory under subdued red light. The unexposed loess was prepared for equivalent dose (De) determination while exposed material was retained for radioisotope content analysis. De determinations were carried out on 40–63 mm quartz, this size fraction being easy to extract and abundant in the samples analyzed (Fig. 2). Although it is more time consuming to prepare, quartz was preferred to polyminerals due to potential errors associated with equivalent doses measured from the latter (Roberts and Wintle, 2003; Wang et al., 2006). Carbonates and organic matter were removed from the bulk sample by treatment with 1 M HCl and 15% H2O2, respectively. Quartz was isolated from the resulting material by immersion in 35% H2SiF6 for up to 4 weeks, with a subsequent 0.1 M HCl wash to remove fluorite precipitates. Grains (40–63 mm) were obtained by wet sieving and mounted as a monolayer on aluminium discs using silkospray silicone oil. Equivalent doses were measured using the SAR procedure (Fig. 3) of Murray and Wintle (2000). All measurements were performed using a Risø TL-DA-15 TL/OSL reader fitted with a blue LED (l ¼ 470720 nm) stimulation source. The OSL signal was measured using a 9235QA photomultiplier tube filtered by 6 mm of Hoya U340 glass (Bøtter-Jensen et al., 2000). The OSL signal was integrated from the first 0.6 s of stimulation minus a background estimated from the last 6 s of stimulation. All growth curves were fitted using a saturating exponential plus linear function. The uncertainty on individual De values was estimated using Monte Carlo simulation. Typically, 12–14 aliquots were measured for each sample and the weighted mean De (with one standard error uncertainty) was calculated.

7 6 5 %

4 3 2 1 0 0

Fig. 1. Map of sites described in study, modern isohyets and potential dust transport directions (arrows).

1

2.2

4.3 8.5 18 Grain-Size (microns)

36

75

Fig. 2. Grain-size distribution of sample CH04/1/2.

150

ARTICLE IN PRESS T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22

Figs. 4a, c, and 5a show the distribution of equivalent doses down section at the main section of Beiguoyuan, the Holocene section of Beiguoyuan and Shiguanzhai, respectively. Equivalent doses show a general, although punctuated, increase down section and most errors are well under 5%. In order to determine whether the SAR

Preheat (PH1) 260˚C for10s

Firstly, the purity of each sample aliquot was tested using the IR depletion ratio test (Duller, 2003). In this test, a regeneration dose is repeated, except that the aliquot is exposed to IR stimulation prior to measurement of luminescence intensity (Lx). The ratio of sensitivity corrected luminescence intensities (RIR/RnoIR) should be unity within error, where there is no significant IRbleachable (feldspar) contribution to the luminescence signal. Aliquots yielding IR depletion ratios of o0.9 were rejected. The average IR-ratio of 0.9770.04 indicates that sample preparation was successful in isolation of the quartz fraction. Secondly, it is necessary to check for ‘recuperation’ of a residual signal in ‘zeroed’ samples after heating, dosing and optical stimulation of aliquots. To test this, a zero dose cycle is included in the SAR protocol. The ratio of normalized luminescence responses from the zero dose cycle (l0/t0) to the final regeneration point (l01/t01) as a percentage shows all samples have less than 2%

Test dose 20 Gy

Preheat (PH2) 220˚C for10s

OSL (Tx) Blue LEDs @ 125˚C for 60s

Blue wash Blue LEDs @ 280˚C for 60s

Yes

Finish

Fig. 3. Modified SAR protocol used in study.

(a)

De (Gy) 50 100

(b) Dose Rate (Gy ka-1) 2.5

3.0

3.5

(c)

4.0

0

0

0

100

50

200

100

300

Depth (cm)

Depth (cm)

0

4. Luminescence characteristics

4.1. Internal checks

OSL (Lx) Blue LEDs @ 125˚C for 60s

Growth curve complete?

technique is working and consequently whether these equivalent doses are accurate estimates of the paleodose, it is necessary to apply certain tests.

The SAR protocol allows the inclusion of several internal checks on De reliability. It is only appropriate to use OSL dating to establish age models for Chinese loess deposits if these internal checks demonstrate that SAR is appropriate for these samples.

β regeneration dose (Omitted for cycle n=0)

No

17

De (Gy) 25 50

(d) Dose Rate (Gy ka-1) 3.0

3.5

4.0

150

400

200

500

250

600

300

Fig. 4. (a), (c) Equivalent doses and (b), (d) dose rates for Beiguoyuan main and Holocene sections, respectively. Errors on equivalent doses are standard errors while errors on dose rates are calculated as described in Section 5. Dark diamonds and light squares show dose rates calculated using ICP and field g-spec techniques, respectively.

ARTICLE IN PRESS T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22

18

(a) 0

De (Gy) 50

100

(b) 3

Dose Rate (Gy ka-1) 4

5

0

20

40

60

Depth (cm)

80

100

120

140

160

180

SAR sensitivity correction can be examined further by plotting recycling ratio against sensitivity change (Tx/T1). Fig. 6 demonstrates that there is no correlation between recycling ratio and the amount of sensitivity change, further indicating that the SAR sensitivity correction procedure works as intended. The majority of Tx/T1 ratios are greater than unity (mean ¼ 1.3770.25) indicating that aliquot sensitivity increased during the SAR measurement procedure, although significant inter-aliquot variability was observed. Sensitivity change during the SAR measurement procedure can be monitored by plotting relative sensitivity (Tx/Tn) versus measurement cycle (Armitage et al., 2000). Fig. 7 shows relative sensitivity plotted against measurement cycle for various samples. All samples show increased sensitivity through the SAR sequence however rates and patterns of change vary. In some samples most of the sensitization occurs over the first two SAR cycles. There is then little sensitization until the zero dose cycle (cycle 7) where there is a sharp increase in sensitivity followed by a return to lower values. Other samples show a monotonic increase in sensitivity with SAR cycle, irrespective of regeneration dose. By plotting Tx/Tn change between cycles 7 and 8 (100 and 0 Gy, respectively) against De (Fig. 8), sensitivity change is shown to reduce with increases in De. Due to the relatively homogenous dose rates (Fig. 4b, d and 5b), this indicates that sensitivity 1.3

200

1.1 1 0.9 0.8 0.7 0.6 0.5

1

1.5

2

2.5

3

Tx/T1

Fig. 6. Recycling ratio plotted against sensitivity change (Tx/T1).

Relative Sensitivity (Tx/Tn)

recuperation with a mean value of 0.0270.18%. This low value is likely due to the high-temperature bleach between each SAR cycle (Murray and Wintle, 2003). Sensitivity changes (changes in the measured luminescence intensity per unit dose) are known to occur in quartz during single-aliquot measurement procedures. The SAR protocol uses the luminescence response to a small, constant test-dose administered after measurement of the natural or regenerated signal, to monitor sensitivity changes (Fig. 3). Sensitivity corrected luminescence intensity is then calculated by dividing the natural/regenerated Lx by the test dose luminescence intensity (Tx). The effectiveness of this sensitivity correction can be tested by repeating the first dose point in the growth curve at the end of the measurement sequence. The ratio of sensitivity corrected luminescence intensities (Rx/R1), hereafter termed ‘‘recycling ratio,’’ should be consistent with unity if the correction procedure is appropriate. Murray and Wintle (2000) suggested that aliquots should be rejected if recycling ratios are outside 10% of unity. In this study, most aliquots fell within 10% of unity with a mean recycling ratio of 0.9970.07, indicating that the SAR sensitivity correction is appropriate for these samples. The

(Lx/Tx)/(Ln/Tn)

1.2

Fig. 5. (a) Equivalent doses and (b) dose rates for Shiguanzhai section. Errors on equivalent doses are standard errors while errors on dose rates are calculated as described in Section 5.

3 2.5 2 1.5 1 1

2

3

4 5 6 Measurement Cycle

7

8

9

Fig. 7. Test dose response (Tx) normalized by the test dose response of the natural (Tn) plotted against SAR cycle for selected samples. All samples were analyzed using the same dose and test dose points (the natural, 20, 5, 10, 50, 100, 0, 20 and 20 Gy dose points and 20 Gy test dose).

ARTICLE IN PRESS T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22 0.6

1.2 y = -0.0091x + 0.614

Ratio Dosed:Recovered

(T0/Tn)-(T100/Tn)

0.5

R2 = 0.7718

0.4 0.3 0.2 0.1 0 -0.1

19

0

20

40 De (Gy)

60

80

1.1

1

0.9

0.8 -

change between these cycles is largely controlled by sample age, with larger changes occurring for younger samples. These results appear to confirm the suggestion by Wintle and Murray (1999) that significant sensitivity changes occur during burial. 4.2. Ability to recover a known dose Dose recovery tests check the ability of the SAR protocol to recover a known laboratory administered dose. Successful dose recovery indicates that the SAR procedure produces internally consistent results for any given sample, and hence, enhances the credibility of De measured from naturally dosed material (Wallinga et al., 2000). Fig. 9 shows ratios of laboratory administered to measured (recovered) dose for selected samples used in this study. Laboratory administered doses were chosen to approximate the paleodose for each sample (10 Gy increments from 20–70 Gy). All doses recovered are within 10% of unity and support the conclusion that the SAR technique is suitable for De determination in these samples. However, there seems to be increased De overestimation with increased given dose, suggesting that SAR ages from this suite of samples may become less accurate for older samples. 4.3. Varying the preheat temperature High-resolution dating of loess requires a rapid, standardized measurement procedure. In particular, the use of a wide range of preheating conditions for each sample is time consuming and impractical where a large number of ages must be produced. Dose recovery tests, using PH1 temperatures from 180–280 1C (for 10 s) and PH2 temperatures 40 1C lower were performed to identify suitable preheating conditions for these samples. The results for CH04/1/23, CH04/1/249 and CH04/4/7 are shown in Fig. 10. For sample CH04/1/249, preheats of less than 220 1C overestimate the given dose while above 280 1C, the given dose is underestimated. For sample CH04/4/7 the opposite trend is observed. For this sample all preheats except for 180 1C give equivalent doses within 10% of unity. For sample CH04/1/23, a true ‘plateau’ is shown with all values being near identical and close to

Dose

+

Fig. 9. Ratios of given to recovered dose for ten samples from all three studied sections. 1.15 Ratio Dosed/Recoevered

Fig. 8. Difference between 0 and 100 Gy dose Tx/Tn ratio plotted against De.

1.1 1.05 1 0.95 0.9 0.85 0.8

0

16

0

18

0

20

0 0 22 24 Preheat Temp(°C)

0

26

0

28

0

30

Fig. 10. Ratio of given dose to recovered dose for CH04/1/23 (circles), CH04/1/249 (dark diamonds) and CH04/4/7 (white triangles) as a function of preheat (and cutheat) temperature.

unity within limits. In all three samples, the 260 1C preheat and 220 1C used elsewhere in this study yields acceptable results. Only PH1 temperatures between 220 and 260 1C yield acceptable dose recovery results, in sharp contrast with the wide range of PH1 temperatures (140–300 1C) successfully employed by Roberts and Wintle (2001) on fine grained Chinese loess. The relatively restricted range of acceptable preheating conditions indicated by the present study demonstrates that preheat tests must be performed at every site prior to selection of standardized measurement conditions for high-resolution dating. 4.4. Is the SAR technique valid for quartz blue emission? The above results suggest that with careful sample preparation and determination of SAR measurement conditions, the UV OSL emission from 40–63 mm quartz grains is suitable for accurate and reproducible De determination on Chinese loess samples with equivalent doses 70 Gy and below. Sensitization dependant on time since burial occurs through the SAR procedure although the application of a test dose is correcting for any changes. 5. Obtaining dose rates The determination of accurate dose rates is as important in OSL dating as obtaining accurate equivalent doses.

ARTICLE IN PRESS 20

T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22

Hossain et al. (2002) demonstrated that NaI(Tl) field gamma-ray spectrometry calibrated using the doped U, Th, K and undoped ‘background’ blocks at the University of Oxford yielded reliable results. However, field gamma spectrometry can be time consuming especially when radioisotope concentrations as opposed to total dose rates are required. In relatively isotopically homogenous deposits, such as loess, ICP mass spectrometry (ICP-MS) and atomic emission spectrometry (ICP-AES) may be used to rapidly determine U, Th and K concentrations. The accuracy of the technique was assessed by comparison with field gamma-spectrometry data. Dose rates were calculated from U, Th and K concentrations using the conversion factors of Adamiec and Aitken (1998) and assuming a mean water content of 874% (see below) and an a efficiency of 0.0470.02 (Rees-Jones, 1995). Alpha and beta attenuation was calculated using Bell (1980) and Mejdahl (1979). Uncertainties were calculated by the propagation, in quadrature, of errors associated with individual errors for all measured quantities. In addition to uncertainties calculated from counting statistics, errors due to (1) b source calibration (3%, Armitage and Bailey, 2005), (2) radioisotope concentration (3%), (3) dose rate conversion factors (3%), and (4) attenuation factors (3%) have been included (Murray and Olley, 2002). The cosmic dose was calculated using present day burial depth (Prescott and Hutton, 1994). Figs. 4b, d and 5b show dose rates based on radioisotope concentrations determined by ICP-MS/AES at 10 cm intervals and field g-spec measurements made at larger intervals. The two techniques yield mostly indistinguishable results within errors demonstrating that ICP-MS/AES methods can be used as a rapid and accurate alternative to field gamma-spectrometry measurements. The ICP-MS/ AES dose rates calculated show remarkable homogeneity down section with most dose rates being indistinguishable within errors. At Shiguanzhai there is a reduction in dose rate at 120 cm. The dose rate between sites appears to vary such that it remains desirable to obtain high-resolution dose rates for different sections. However, sediment homogeneity suggests that field gamma-spectrometry measurements at less regular intervals than De determinations may be sufficient for many applications. Roberts and Wintle (2001) also found relatively homogenous dose rates at Duowa on the Chinese Loess Plateau. One significant potential error in dose rate determination is the estimation of mean water content during burial. The dose attenuation caused by water is substantial and unknown variations in water content during burial may cause large errors (Aitken, 1985). Present-day water content was assessed in the laboratory using two different methods. In the first method, light exposed parts of the sample collected for De determination are weighed, dried at 110 1C for 48 h and then weighed again. The second method was used where field gamma-spectrometry measurements were made. A 30 cm hole was augured into the section face in order place the field gamma spectrometer

into the section. Sediment at the back of the hole was scrapped into an airtight pot. These samples were then weighed, dried at 110 1C for 48 h and then weighed again. Water contents calculated using the former method are much lower than those using the latter (3.772.5% versus 7.773.5%). It is possible that the first method underestimates present-day water content due to surficial drying of the sediment face since it was cut, or water loss during transport. The water content values from the field gamma-spectrometry samples yielded water contents between 1% and 13% with the majority of values around 8%. Water contents may be slightly higher in paleosols but no trend is obvious between stratigraphic units or depth. The differences between sites are minimal (although moisture contents are slightly higher at Shiguanzhai), suggesting that the dominant control on moisture content may be porosity and compaction of sediment and not climate. Consequently, it is valid to assume that within (and probably between) sections a single water content value can be used in dose rate calculations. In the present study a mean burial water content of 874% was used throughout, based on the present-day values for samples from gamma-spectrometry auger holes. However, estimation of mean burial water content remains a significant limiting factor in the accuracy of dose rate determination. 6. Application of a standardized growth curve Roberts and Duller (2004) recently suggested that quartz measured using the SAR technique may exhibit a uniform response to laboratory dose, since the test-dose response normalizes all data for signal intensity. Consequently, growth curves for samples from the same site or region will be identical within errors. This approach enabled Roberts and Duller (2004) to construct standardized growth curves for coarse-grained quartz (180–212 mm) and polymineral fine-grains (4–11 mm). A standardized growth curve (SGC) is constructed by calculating the mean sensitivity-corrected luminescence intensity (Lx/Tx) for each dose point in the growth curve. In principle, equivalent doses can be calculated for aliquots where only the sensitivity-corrected natural luminescence intensity (Ln/Tn) has been measured, provided that the measurement conditions (principally preheats and test dose) are identical to those used to produce the SGC. The SGC approach potentially facilitates high-resolution dating of Chinese loess by dramatically increasing sample throughput. To test the validity of this approach, an SGC was constructed using dose response data (regeneration doses of 5, 10, 20, 50 and 100 Gy using a 20 Gy test dose) from nine samples (121 aliquots) from main section at Beiguoyuan. Monte Carlo simulation was then used to calculate the De for nine different samples from the Beiguoyuan main section, five samples from the Holocene section at Beiguoyuan and three samples from Shiguanzhai. The mean equivalent doses for individual samples

ARTICLE IN PRESS T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22

obtained using the SGC approach are plotted against those calculated using the standard SAR approach in Fig. 11. For Beiguoyuan main section the ratios SGC/SAR equivalent doses are within errors of unity, indicating that the SGC approach is valid. However, this is not the case with samples from the Holocene section at Beiguoyuan or Shiguanzhai. SGC values seem to be higher than standard SAR values although there is a possibility that this is a systematic offset for younger samples. These data indicate that the SGC approach is probably only valid where an SGC is constructed and applied to samples from the same section and not as Lai (2006) has suggested, for samples across the Loess Plateau. Indeed, it appears that samples from the two Beiguoyuan sections, located within a few kilometres of each other, do not share dose response characteristics. Despite the apparent limitations, the SGC approach is particularly suited to high-sampling resolution studies of loess deposition in which large numbers of samples from the same section are analyzed. Using the standard SAR technique, De determination for 30 prepared Late Pleistocene and Holocene samples takes 2 months. The construction of an SGC using luminescence intensity of ten samples that were run using the SAR protocol and subsequent use of the SGC for the next 20 samples halves measurement time to 1 month. Dose rate determination and sample preparation for De determination using the methods outlined in this paper can be done at a rate enabling a constant availability of samples for De measurement.

7. Conclusions

20

The SAR procedure allows precise and accurate depositional ages to be determined for samples of Chinese loess. However, the sampling resolution required to determine the depositional characteristics of Chinese loess, and therefore accurate analysis of the paleoclimate data it contains, makes detailed investigations extremely time consuming. In this study we have investigated the luminescence properties of Chinese loess in an attempt to increase sample throughput in the laboratory and therefore to potentially enhance the viability of high sampling resolution studies. Loess from all three sites studied yield acceptable recycling ratios irrespective of the magnitude of sensitivity change that occurs during the measurement sequence. Dose recovery tests indicate that appropriate preheating regimes may vary between sections. However all sites appear to yield acceptable results using a 260 1C PH1 and a 220 1C PH2. Using this preheat combination, doses between 10 and 70 Gy were successfully recovered. The standardized growth curve approach appears to be valid within but not between sections, potentially allowing rapid equivalent dose determination for single sections. Dose rates do not vary significantly down section. ICP-MS/AES radioisotope concentrations are consistent with those determined using field gamma-spectrometry, allowing rapid dose rate determination for large numbers of samples. These data indicate that there is considerable potential for streamlining OSL dating of Chinese loess. Consequently, high-resolution dating of Chinese loess is now feasible and should be applied to the wide range of sections containing hitherto poorly dated paleoclimate records. Previous age models have often emphasized the continuity of loess deposition (e.g. Porter and An, 1995). However, the distribution of equivalent dose values at our study sites demonstrates significant down-section variation, indicating the occurrence of rapid changes in sedimentation rate. This suggests that high sampling resolution OSL models will provide a more realistic and detailed indication of age and sedimentation than other methods. Nonindependent age models are unable to account for or highlight pedogenic overprinting, sediment disturbance, sub-millennial scale changes in sedimentation and unconformities. However, age models derived from high sampling resolution OSL ages do not suffer from these limitations. Further investigation using OSL ages will allow examination of the assumptions underpinning much of the paleoclimatic work on Chinese loess.

10

Acknowledgements

80

70

60 SGC De(Gy)

21

50

40

30

10

30

50 SAR De(Gy)

70

Fig. 11. Equivalent doses (Gy) determined using an SGC and the SAR method. The dark line is the 1:1 line. Dark diamonds are samples from Beiguoyuan main section, squares are samples from Beiguoyuan Holocene section and white triangles are samples from Shiguanzhai.

The authors thank Yi Shuangwen and Sun Xuefeng for their help in the field. We also acknowledge the support of NERC for provision of data for dose rate determination through the use of their ICP-MS and AES facilities (Award OSS/279/0205); and the Chinese Academy of Sciences

ARTICLE IN PRESS 22

T. Stevens et al. / Quaternary Geochronology 2 (2007) 15–22

(Grant 2003-06) and National Natural Sciences Foundation of China (Grants 40325007, 40121303) for supporting the fieldwork. TS thanks Jesus College, Oxford University, Oxford University Centre for the Environment and Dr. Steven Stokes for fieldwork and conference support. Thanks to Stephen Stokes and Dr. Richard Bailey for guidance and to Ailsa Allen for help on the map. Editorial handling by: R. Gru¨n References Adamiec, G., Aitken, M.J., 1998. Dose rate conversion factors: update. Ancient Thermoluminescence 16, 37–50. Aitken, M.J., 1985. Thermoluminescence Dating. Oxford University Press, Oxford 75pp. Armitage, S.J., Bailey, R.M., 2005. The measured dependence of laboratory beta dose rates on sample grain size. Radiation Measurements 39, 123–127. Armitage, S.J., Duller, G.A.T., Wintle, A.G., 2000. Quartz from Southern Africa: Sensitivity changes as a Result of Thermal Pretreatment, vol. 32. pp. 571–577. Bell, W.T., 1980. Alpha dose attenuation in quartz grains for thermoluminescence dating. Ancient Thermoluminescence 12, 4–8. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrumentation. Radiation Measurements 32, 523–528. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiation Measurements 37, 161–165. Hossain, S.M., De Corte, F., Vandenberghe, D., Van den haute, P., 2002. A comparison of methods for the annual radiation dose determination in the luminescence dating of loess sediment. Nuclear Instruments and Methods in Physics Research A 490, 598–613. Kohfeld, K.E., Harrison, S.P., 2003. Glacial–Interglacial changes in dust deposition on the Chinese Loess Plateau. Quaternary Science Reviews 22, 1859–1878. Lai, Z.P., 2006. Testing the use of an OSL standardised growth curve (SGC) for De determination on quartz from the Chinese Loess Plateau. Radiation Measurements 41, 9–16. Lu, H.Y., Sun, D.H., 2000. Pathways of dust input to the Chinese Loess Plateau during the Last Glacial And Interglacial periods. Catena 40, 251–261. Lu, H.Y., Zhang, F.Q., Liu, X.D., Duce, R., 2004. Periodicities of palaeoclimatic variations recorded by loess–paleosol sequence in China. Quaternary Science Reviews 23, 1891–1900.

Mejdahl, V., 1979. Thermoluminescence dating: beta-dose attenuation in quartz grains. Archaeometry 21, 61–72. Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1–16. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose procedure. Radiation Measurements 32, 57–73. Murray, A.S., Wintle, A.G., 2003. The single aliquot regenerative dose protocol: potential for improvements in reliability. Radiation Measurements 37, 377–381. Nugteren, G., Vandenberghe, J., van Huissteden, J.K., An, Z., 2004. A Quaternary climate record based on grain size analysis from the Luochuan loess section on the central loess plateau, China. Global and Planetary Change 41 (3-4), 167–183. Porter, S.C., 2001. Chinese loess record of monsoon climate during the last Glacial–Interglacial cycle. Earth-Science Reviews 54, 115–128. Porter, S.C., An, Z., 1995. Correlation between climate events in the North Atlantic and China during the last glaciation. Nature 375, 305–308. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long term variations. Radiation Measurements 23 (2/3), 497–500. Rees-Jones, J., 1995. Optical dating of young sediments using fine-grain quartz. Ancient Thermoluminescence 13, 9–13. Roberts, H.M., Duller, G.A.T., 2004. Standardised growth curves for optical dating of sediment using multiple-grain aliquots. Radiation Measurements 38 (2), 241–252. Roberts, H.M., Wintle, A.G., 2001. Equivalent dose determinations for polymineralic fine-grains using the SAR protocol: application to a Holocene sequence of the Chinese Loess Plateau. Quaternary Science Reviews 20, 859–863. Roberts, H.M., Wintle, A.G., 2003. Luminescence sensitivity changes of polymineral fine grains during IRSL and [post-IR] OSL measurements. Radiation Measurements 37, 661–671. Wallinga, J., Murray, A., Duller, G., 2000. Underestimation of equivalent dose in single-aliquot optical dating of feldspar caused by pre-heating. Radiation Measurements 32, 691–695. Wang, X., Lu, Y.C., Zhao, H., 2006. On the performances of the singlealiquot regenerative-dose (SAR) protocol for Chinese loess: fine quartz and polymineral grains. Radiation Measurements 41, 1–8. Wintle, A.G., Murray, A.S., 1999. Luminescence sensitivity changes in quartz. Radiation Measurements 30, 107–118. Zheng, H., Rolph, T., Shaw, J., An, Z., 1995. A detailed palaeomagnetic record for the Last Interglacial period. Earth and Planetary Science Letters 133, 339–351.