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Comparison of single-aliquot and single-grain MET-pIRIR De results for potassium feldspar samples from the Nihewan Basin, northen China
Quaternary Geochronology 56 (2020) 101040
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Research paper
Comparison of single-aliquot and single-grain MET-pIRIR De results for potassium feldspar samples from the Nihewan Basin, northen China Yujie Guo a, *, Bo Li b, c, Hailong Zhao a a
Institute of Nihewan Archaeology, Hebei Normal University, 050024 Shijiazhuang, Hebei Province, China Centre for Archaeological Science, School of Earth and Environmental Science, University of Wollongong, Wollongong, 2522 NSW, Australia c Australian Research Council (ARC) Centre of Excellence for Australian Biodiversity and Heritage, University of Wollongong, Wollongong, NSW 2522, Australia b
A R T I C L E I N F O
A B S T R A C T
Keywords: K-feldspar Single grain Single aliquot Fading rate Brightness
In this study we compared De results obtained from single-aliquot (SA) and single-grain (SG) measurements on potassium rich feldspars (K-feldspars) from the Nihewan Basin, north China, based on the multiple-elevatedtemperature post-infrared infrared (MET-pIRIR) stimulated luminescence. Discrepancy was observed between the SA and SG De results. Potential reasons for such discrepancy, including residual doses, sensitivity changes, fading rates, internal dose rates, and/or the combination of these possibilities, were investigated. Our results indicate that the disagreement between the SA and SG De results are mainly caused by the different fading rates and, to a small extent, by the internal K contents variations of different grains. Our study highlights the importance to investigate the relationship of single-grain De values and the luminescence behavior of individual grains, such as brightness. For partial-bleached or post-depositional mixed K-feldspar samples, statistical analysis should be conducted only on the grains with the same fading rate.
1. Introduction Infrared (IR) stimulated luminescence (IRSL) from potassium-rich feldspars (K-feldspar) has been successfully applied to determine the ages of Quaternary sediments worldwide (e.g., Thiel et al., 2011; Gong et al., 2014; Li et al., 2013a, Li et al., 2014a; Chen et al., 2015; Guo et al., 2016; Li et al., 2018; Jacobs et al., 2019), since the development of post-IR IRSL (pIRIR) dating procedures that are able to minimise the anomalous fading effect (Thomsen et al., 2008; Thiel et al., 2011; Li and Li, 2011, 2012). Most previous studies on K-feldspar IRSL dating focused on measurements of multiple grains using the single-aliquot regener ative-dose procedures (Murray and Wintle, 2000). Compared to multiple-grain techniques, single-grain techniques have the advantage of dealing with insufficiently bleached samples, identifying post-depositional mixing between sedimentary units of different ages and assessing stratigraphic integrity (Arnold et al., 2009; Bateman et al., 2007; David et al., 1997; Jacobs et al., 2006; Tribolo et al., 2010). Although there has been a large number of single-grain studies on quartz OSL (see Roberts et al. (2015) for a review), single-grain K-feldspar dating became applicable very recently (Trauerstein et al., 2012, 2014; Neudorf et al., 2012; Smedley et al., 2012, 2015, 2019; Nian et al., 2012; Smedley and Pearce, 2016; Gliganic
et al., 2017; Reimann et al., 2017; Buylaert et al., 2018; Brill et al., 2018; Schaarschmidt et al., 2019). Previous studies have shown that the equivalent dose (De) of single K-feldspar grains are much more complicated due to their varying luminescence properties from grain to grain. For example, Lamothe and Auclair (2000) demonstrated that different K-feldspar grains have significantly different anomalous fading rates in low temperature IRSL signals, which allowed them to develop an ‘isochron’ method to overcome the fading issue. Reimann et al. (2012) applied the pIRIR procedure to single-grain K-feldspars and observed that the K-feldspar De depends on the sensitivity of individual grains (i. e., brighter grains yielded higher De values), which they attributed to the correlation between blue emission and potassium content of individual grains. A similar pattern of correlation between sensitivity and De values was observed from some K-feldspar samples from Siberia (Derevianko et al., 2018; Jacobs et al., 2019), which shows that only the brightest portions of grains can give reliable results. Most of previous studies focused on either single aliquot or single grain, and there is a lack of systematic comparison between singlealiquot and single-grain of K-feldspar results on the same samples. In this study, we investigated three K-feldspar samples from the Nihewan Basin in northen China. Both multi-grained aliquots and individual grains were measured and compared. We investigated the reason for the
* Corresponding author. E-mail address: [email protected] (Y. Guo). https://doi.org/10.1016/j.quageo.2019.101040 Received 31 July 2019; Received in revised form 15 November 2019; Accepted 15 November 2019 Available online 25 November 2019 1871-1014/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Satellite images (courtesy of Google Earth) of the Nihewan Basin and the Xiabuzhuang Palaeolithic site. The red rectangle in (b) represents the Nihewan Basin and the red star represents the Beijing city. (c,d) Photos showing the location of the Xiabuzhuang site on the east scarp of the Qian River. (e) Sedimentary profile of the north face of the excavation pit, showing the locations of the OSL (XBZ-OSL-1 to 13) and radiocarbon (17XBZDT2:2209 and 17XBZDT2:3282) samples. ① disturbed layer, ② cultural layer and ③ layer with no remains.
discrepancy between the multi-grain and single-grain results.
Table 1 SA measurement procedure for the low-preheat-temperature MET-pIRIR mea surements (after Fu and Li, 2013). Step
Treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Give regenerative dose, Di Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s IRSL measurement at 170 � C for 100 s Give test dose, 13.1 Gy Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s IRSL measurement at 170 � C for 100 s IR bleaching at 280 � C for 100 s Return to step 1
2. Samples, experimental procedure and analytical facilities
Observed
The Xiabuzhuang site is located at the east bank of Qian River, in the south edge of the Nihewan Basin, northern China (Fig. 1). The geology of the river catchment area is mainly volcanic breccia, andesite, tuff, sili ceous limestone and quartz sandstone (Du, 2003; Xie et al., 2006; Yuan et al., 2011). The deposits are mainly alluvial-diluvial deposits, con sisting of silty sand interbedded with poorly rounded gravels. The cul tural remains were excavated from the top silty sand layer (Fig. 1c). A total of 15 OSL samples were collected from this site, but ten of them were measured (Fig. 1e). Apart from OSL samples, three radiocarbon samples, 17XBZDT2:2209 (animal bone, including two sub-samples 17XBZDT2:2209a and17XBZDT2:2209b) and 17XBZDT2:3282 (char coal) from the layer near samples XBZ-OSL-8 and -12 (Fig. 1e) were collected for comparison. The calibrated ages are 16,443‒16,026, 16, 310‒15,970, and 18,454‒18,115 yr cal BP (95% confidence interval, Reimer et al., 2013) for samples 17XBZDT2:2209a, 17XBZDT2:2209b and 17XBZDT2:3282, respectively. Three samples (XBZ-OSL-6, 8 and 12) were investigated in detail
Lx(50) Lx(80) Lx(110) Lx(140) Lx(170) Tx(50) Tx(80) Tx(110) Tx(140) Tx(170)
Note: For the first cycle natural signals, i ¼ 0 Gy and D0 ¼ 0 Gy. The entire sequence is repeated for several regenerative doses, including a zero dose and a repeat dose. 2
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Fig. 2. (a)–(c) De distributions for multi-grained single aliquots of samples XBZ-OSL-6, -8 and -12, respectively. The grey bands are centred on the CAM values. (d)– (f) De distributions for all the accepted grains of samples XBZ-OSL-6, -8 and -12, respectively; the grey bands are centred on the CAM values based on all the accepted grains (both the hollow circles and the black triangles), and the green bands centred on the CAM values based on the grains with brightness larger than 100, 140 and 80 ct/Gy (the black triangles) for samples XBZ-OSL-6, -8 and -12, respectively. The values of “n” in (a)–(c) represent the total number of the measured aliquots and the values in (d)–(f) represent the number of the accepted grains with brightness larger than 100, 140 and 80 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively.
here for comparing single aliquot and single-grain results. The other seven samples were used to establish standardised growth curves (SGC) and the dating results for these samples will be presented elsewhere. Kfeldspar grains from each sample were extracted following standard procedures (Aitken, 1998). Carbonates and organic matter were elimi nated using HCl acid and H2O2 solution, respectively. K-feldspar grains of 150–250 μm for samples XBZ-OSL-6 and 8, and 150–180 μm for
sample XBZ-OSL-12 were obtained by dry sieving and density separation using a heavy liquid solution (2.58 g/cm3). The grains were then etched in 10% HF acid for 40 min to remove the alpha-irradiated outer layer of each grain. The etched grains were sieved again to remove the portions that were reduced to less than 150 μm or broke into smaller pieces (Porat et al., 2015), and an overall reduction of 10 μm by HF etching was assumed. 3
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Table 2 SG measurement procedure for the low preheat temperature MET-pIRIR measurements. Step
Treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Give regenerative dose, Di Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s SG IR laser measurement at 170 � C for 3 s Give test dose, 12.5 Gy Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s SG IR laser measurement at 170 � C for 3 s IR bleaching at 280 � C for 100 s Return to step 1
Observed
Lx(50) Lx(80) Lx(110) Lx(140) Lx(170) Tx(50) Tx(80) Tx(110) Tx(140) Tx(170)
Note: For the first cycle natural signals, i ¼ 0 Gy and D0 ¼ 0 Gy. The entire sequence is repeated for several regenerative doses, including a zero dose and a repeat dose.
For multiple-grain analysis, K-feldspar grains were mounted onto the central ~3 mm diameter of stainless steel discs, corresponding to about several hundred grains on each aliquot. For single-grain measurements, the standard single-grain discs drilled with 100 holes, each 300 μm wide and 300 μm deep (Bøtter-Jensen et al., 2000), were used. Single-grain discs were checked under the microscope to verify that each hole con tained only one grain. The IRSL measurements were performed on a Risø TL/OSL-DA-20 reader equipped with a 90Sr beta source and IR diodes (870 nm) and IR laser (830 nm) for stimulation. IRSL signals were detected by an Electron Tubes Ltd 9235QB15 photomultiplier tube fitted with a blue filter pack comprising of Schott BG-39 and Corning 7–59 filters, which transmits blue-violet (320–480 nm) light. The internal dose rates for the three samples in this study were determined by assuming the K contents of 12 � 1%, based on energy dispersive spectroscopy analysis of single-grain K-feldspar samples from the same region (Rui et al., 2019a). A minor contribution from Rb was calculated by assuming a concentration of 400 � 100 ppm (Huntley and Lamothe, 2001). 3. De measurement procedures Given the relatively young age of our samples, a low preheat tem perature (200 � C for 60 s) was used in the SAR MET-pIRIR procedure (Fu and Li, 2013) to measure the De values for multi-grained aliquots and individual grains. The advantage of the low preheat MET-pIRIR pro cedure is that the residual doses are very small (see reviews in Li et al., 2014b), which is crucial for young samples. In the SAR procedure, dose response curve (DRC) is constructed using the sensitivity-corrected pIRIR signals (Lx/Tx) induced from a series of regenerative doses asso ciation with fixed test doses, including a zero and a repeated regenera tive dose to monitor the extent of recuperation and the recycling ratio, respectively. The De value of each aliquot/grain was determined by interpolating the sensitivity-corrected natural signal (Ln/Tn) on to its corresponding DRC. All the DRCs in this study were fitted using a single e D∕D0 Þ þ c, saturating exponential function of the form I ¼ I0 ð1 where I is the sensitivity-corrected IRSL intensity (Lx/Tx), D is the regenerative dose, D0 is the characteristic saturation dose, and I0 þ c define the maximum intensity of the DRC. The instrumental reproduc ibility errors of 1% and 1.5% were used for the SA and SG De mea surements respectively (based on the reproducibility test on our reader), following the procedure described in Li et al. (2018). The uncertainty on SG De was estimated using a Monte Carlo method with repeat fits value of 500. A systematic error of 2% was also added in quadrature to the measurement errors for possible bias in the calibration of the laboratory
Fig. 3. Fading uncorrected (a) and corrected (b) De results and ages (c) plotted against depth for samples XBZ-OSL-6, -8 and -12, respectively. The horizontal bars for each point indicate the standard errors. The blue diamonds represent SG results for all grains, and the green triangles in (c) represent SG ages results for grains after rejecting the dim ones with N-L1 lower than 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively.
beta source for both SA and SG De measurements. 3.1. Single-aliquot measurement The single-aliquot (SA) measurements were conducted using the MET-pIRIR procedure in Table 1. The IR stimulation was made consecutively at a range of temperatures, 50, 80, 110, 140 and 170 � C, for 100 s, respectively. The aliquots were held for 10, 10, 20, 20 and 30 s before IR stimulations at each of the temperatures, respectively, to monitor and minimise interference from isothermal decay signals (Fu et al., 2012). Typical natural IRSL (50 � C) and MET-pIRIR (80–170 � C) decay curves are shown in Fig. S1 for one aliquot of sample XBZ-OSL-12. The De value for each aliquot was calculated from the IRSL counts in tegrated over the first 15 s of IRSL decay, minus the counts from the final 15 s of stimulation as background. We noted that changing the 4
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Fig. 4. (a)–(c) Ranked individual De distributions according to their intrinsic brightness for all the accepted grains of samples XBZ-OSL-6, -8 and -12, respectively. The orange lines are running means with a fixed subset size of 5 data points. (d)–(f) The CAM De value (blue rhombus) distributions for the grains with brightness above different N-L1 thresholds. The points at the 0 threshold are offset laterally for clarity. The red squares represent the SA CAM De values for each sample, plotting at a ‘fictive’ N-L1 threshold of 320 ct/Gy for clarity. The dashed red lines in (d)–(f) are the ‘plateau’ values of 58.0, 61.3 and 59.1 Gy, calculated from the weighted mean of the De estimates with N-L1 thresholds above 100, 140 and 80 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively. The veticle bars for each point indicate the standard errors.
integration periods of the initial IRSL decay from 2 to 30 s have negli gible effect on the results. To choose a suitable preheat temperature, the De of the sample XBZOSL-12 was measured with preheat temperatures varying from 200 to 280 � C in increments of 20 � C. The obtained results were plotted in Fig. S2a, which indicated that the De results measured at 140 and 170 � C are independent of preheat temperature. For the other measuring tem peratures, 110, 80 and 50 � C, the best results are obtained for preheat temperatures of 200–220 � C. A dose recovery test (Galbraith et al., 1999; Wallinga et al., 2000) using the procedure in Table 1 at the preheat temperature of 200 � C was carried out on sample XBZ-OSL-12 to validate the applicability of the SAR procedure for our samples. Aliquots containing natural samples were first bleached for one month in November by sunlight at Shi jiazhuan, northern China. They were then given a dose of 59.0 Gy, which was then measured as an ‘unknown’ dose. The dose recovery ratios (or measured to given dose ratio) were plotted against IR stimulation tem perature in Fig. S2b. It shows that the given dose can be successfully recovered within 10%, suggesting that the SAR MET-pIRIR procedure can reliably determine the De values for samples from this site under
controlled laboratory conditions. Since the 170 � C signal suffers less anomalous fading than the 140 � C signal, we have used the 170 � C signal at the preheat of 200 � C to estimate final De results. For multi-grain aliquot De measurement, we applied the stand ardised growth curve (SGC) method (Li et al., 2015a,b) for our samples. Two aliquots from each of the 10 samples were measured using a full SAR procedure to construct their DRCs. Their DRCs were ‘re-normalised’ (Li et al., 2015a,b) using the signals measured at the regenerative-dose of 52.4 Gy to establish SGC (Fig. S3a). De values were then obtained by projecting the re-normalised natural signal onto the corresponding SGCs. We found that the De values obtained from individual DRCs are consistent with those obtained from the SGCs within 1σ (Fig. S3b), implying that the SGCs established are reliable. Based on this observa tion, we measured more aliquots by measuring only their natural signal, one regenerative-dose signal, and their corresponding test dose signals, and then projecting the re-normalised natural Ln/Tn signals on to the SGC to estimate their De values. The distributions of the SA De results for the three samples are plotted in the radial plots in Fig. 2a–c. The De values are distributed around a central value for each of the three samples and their over-dispersion (OD) values, calculated using the 5
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Fig. 5. (a)–(c) Ranked individual residual dose distributions according to their intrinsic brightness for all the accepted grain of samples XBZ-OSL-6, -8 and -12, respectively. (d)–(f) The arithmetic mean residual dose values distributions for the grains with brightness above different N-Tn thresholds. The red squares represent the arithmetic means of SA residual dose values for each sample calculated using the sum of the single-grained IRSL signals, plotting at ‘fictive’ N-L1 thresholds of 250 and 320 ct/Gy for clarity. The points at the 0 threshold are offset laterally. The veticle bars for each point indicate the standard errors.
Central Age Model (CAM) (Galbraith et al., 1999; Galbraith and Roberts, 2012), are zero for all the three samples. The CAM SA De values are 53.4 � 1.2, 54.5 � 1.2 and 55.6 � 1.2 Gy for samples XBZ-OSL-6, 8 and 12, respectively, corresponding to ages of 16.7 � 1.1, 16.6 � 1.0 and 17.4 � 0.7 ka, respectively.
saturation level of the DRC. A total of 600 grains mere measured for each sample, and 349, 397 and 369 were accepted for samples XBZ-OSL-6, 8 and 12, respectively, after applying the rejection criteria (Table S1). The CAM De values for the accepted grains were 45.49 � 1.17, 45.44 � 1.16 and 51.72 � 1.25 Gy, with OD values of 26.4 � 1.3, 28.0 � 1.2 and 22.7 � 1.1% for samples XBZ-OSL-6, 8 and 12, respectively (Fig. 2d–f), cor responding to ages of 14.19 � 0.91, 13.85 � 0.87 and 16.16 � 0.65 ka.
3.2. Single-grain measurement
3.3. Comparison of the SA and SG results
The single-grain (SG) measurement procedure was same as the SA measurement procedure but the last SA IRSL measurement at 170 � C was replaced with an IR laser stimulation at the same temperature (Table 2). Typical natural pIRSL signal at 170 � C decay curves for grains with different brightness are shown in Fig. S4 for sample XBZ-OSL-12. The net IRSL signals used for SG De estimation were calculated as the sum of counts in the first 0.15 s of IRSL decay minus a ‘late light’ background estimated from the mean count rate over the final 0.15 s in a total of 3 s stimulation time. Grains with unsuitable IRSL properties were rejected using the well-established criteria (e.g., Jacobs et al., 2006; Blegen et al., 2015; Guo et al., 2016; Rui et al., 2019b): (1) the net Tn signal is less than 3 times the standard deviation of its corresponding background signal, or the relative error on the test dose signal is greater than 25%; (2) the recycling ratio differs from unity by more than 2σ; (3) the extent of recuperation is greater than 10%; and (4) the Ln/Tn value exceeds the
The CAM SA and SG De results were plotted together in Fig. 3a, which shows that the SG De results are systematically lower than the corre sponding SA De results by ~15%, ~17% and ~7% for samples XBZ-OSL6, -8 and -12, respectively. There are several possible reasons for SA results being higher than SG results. The first one is that the samples were insufficiently bleached, and, as a result, the multi-grain measure ments were overestimated. However, the tight single-grain De distribu tions (Fig. 2d–f) and their relatively small OD values (Table S2) rule out this possibility. This is further supported by the radiocarbon dating re sults from the same layers of XBZ-OSL-8 and -12. The radiocarbon ages are statistically consistent with the SA ages, further supporting that the samples were sufficiently bleached prior to burial. We, therefore, infer that the disagreement between the SA and SG CAM De results might be 6
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CAM De values and OD values above different N-L1 thresholds were summarized in Table S2. Similar patterns of the De dependency on the grain brightness have also been observed in other studies (Reimann et al., 2012; Jacobs et al., 2019). The dependency of single grain De values on their inherent brightness might be related to: 1) the residual or “non-bleachable” (Li et al., 2013b) doses being lower for the dim grains than for the bright grains; 2) the sensitivity of dim grains changing during the natural pIRIR signal (Ln) measurement (Guo et al., 2015; Qin et al., 2018), which was not detected by the subsequent Tx and Lx signals measurements, i.e. the single-aliquot regenerative-dose (SAR) protocol might not be suitable for the dim K-feldspar grains for these samples; 3) the fading rates of the pIRIR signals are higher for the dim grains than the bright grains; 4) internal dosimetry variations caused by grain-to-grain variations in the internal potassium (K) content, i.e., there are lower K contents in the dim grains than in the bright grains (e.g., Huntley and Baril, 1997; Barr� e and Lamothe, 2010; Smedley et al., 2012; Reimann et al., 2012); or 5) a combination of these possibilities. Here we investigated these possibil ities based on a series of tests. 3.3.1. Residual doses In order to investigate whether the dimmer grains have different residual doses than brighter grains that can explain the difference in their De values, we measured residual doses on bleached samples. To do this, 600 grains of each of the three samples were bleached for one month in November by sunlight. The residual doses of the bleached grains were measured using the same procedure in Table 2, and a total of 364, 391 and 329 grains passed the rejection criteria for each of the samples (Table S1). The accepted grains were ranked according to their N-L1 signals, and the mean dose values of the ranked grains are plotted against N-L1 threshold in Fig. 5. It can be seen that there is a slight in crease trend above 40 ct/Gy threshold for each sample which might due to the higher internal K contents in the bright grains. However, all the mean residual values at each N-L1 thresholds are (nearly) consistent with zero. Hence, we conclude that the residual dose is not the major reason for the underestimation of the De values for the dim grains. 3.3.2. Dose recovery test The second possibility about initial sensitivity change can be tested using a dose recovery test (Galbraith et al., 1999, Wallinga et al., 2000, Guo et al., 2016; Qin et al., 2018). The same pattern of Fig. 4 should be seen if the sensitivity change in the natural SAR cycle for dimmer grains is the reason. To do this, 500, 600 and 400 grains of each of the samples XBZ-OSL-6, -8 and -12 were bleached for one month, the same as those used for residual dose measurements. After that, a beta dose of ~56 Gy was given to the bleached grains as the surrogated ‘natural dose’, which was then measured using the procedure in Table 2. A total of 311, 346 and 252 grains were accepted after applying above the rejection criteria (Tables S1 and S4). The dose recovery ratios were calculated applying the CAM on the measured doses (Galbraith et al., 1999), divided by the given dose (Fig. 6). It can be seen that all the ratio values are consistent with unity and there is no systematic increase of dose recovery ratio values with N-L1 threshold, suggesting that the initial sensitivity change is not an issue for dimmer grains, and the SAR protocol is suitable for all the accepted K-feldspar grains from these samples.
Fig. 6. Dose recovery ratios for accepted grains for different N-Tn thesdolds for samples XBZ-OSL-6, -8 and -12, respectively. The red squares represent the dose recovery ratios determined by the sum of the single-grained IRSL signals of each measured disc, plotting at a ‘fictive’ N-L1 threshold of 320 ct/Gy for clarity. The vertical bars for each point indicate the standard errors.
caused by the variable pIRIR behaviors between individual K-feldspar grains. We first investigated if there is any dependence of De on the brightness. The accepted SG grains results were ranked according to their inherent brightness. We used the first regenerative-dose of 24.1 Gy signal intensity (normalised to the calibrated beta source dose rate for each hole, marked as N-L1 with unit of cts/Gy) to indicate the brightness of the grain. A positive correlation between De values and N-L1 signal is observed. We then calculated the CAM De results by applying a series of N-L1 thresholds (Fig. 4d–f). It can be seen that the De values increase with N-L1 threshold before reaching a plateau after a certain threshold, which is ~100, ~140 and ~80 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively (Fig. 4d–f). We also noted that the OD values also decrease for larger N-L1 thresholds (Table S2). The number of the accepted grains,
3.3.3. Fading test The third possibility can be tested using a fading test (Huntley and Lamothe, 2001; Auclair et al., 2003; Guo et al., 2015, 2016). To do this, we conducted fading tests for both single aliquot and single grains. The aliquots or individual grains which had been used for De measurement were first bleached at 280 � C using IR light for 200 s to empty any residual/thermally transferred signals in the grains. Fading measure ments were then performed using a SAR procedure (Auclair et al., 2003). Repeated regenerative dose cycles were measured after different delay periods (up to 6 days) between preheat and pIRIR measurements. The 7
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Fig. 7. (a)–(c) The arithmetic means of the fading rates for grains with brightness above different N-L1 signal thresholds for samples XBZ-OSL-6, -8 and -12, respectively. (d)–(f) The CAM De values corrected using the fading rates in (a)–(c) for same grains with brightness above different N-L1 thresholds. The points at the 0 threshold are offset laterally. The dashed green lines in each panel of (a)–(c) denote ‘plateau’ of fading rate at about 0.34, 0.83 and 1.22%/decade for samples XBZOSL-6, -8 and -12, respectively. The dashed red lines in (d)–(f) are the ‘plateau’ values of 58.9, 64.5 and 65.3 Gy, calculated from the weighted mean of the De estimates with N-L1 thresholds above 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively. The red squares in each pannel represent the SA fading rates (in Fig. S3), plotting at a ‘fictive’ N-L1 threshold of 320 ct/Gy for clarity, and the corresponded fading-corrected CAM De values for samples XBZ-OSL-6, -8 and -12, respectively. The veticle bars for each point indicate the standard errors. Table 3 The dose rates, final SG and SA De values and ages for samples XBZ-OSL-6, -8 and -12. Samples
Grain size, μm
Water, %
U, ppm
Th, ppm
K,%
External dose rate,Gy/ka
Internal dose rate, Gy/kaa
Total environmental dose rate, Gy/ka
SG CAM De, Gy
SA CAM De, Gy
SG age, ka
SA age, ka
XBZOSL-6
150–250
10 � 3
1.62 � 0.07
7.09 � 0.23
1.68 � 0.06
2.444 � 0.075
0.762 � 0.173
3.206 � 0.188
58.4 � 2.5
150–250
1.82 � 0.08
7.50 � 0.24
1.70 � 0.06
2.517 � 0.076
XBZOSL-12
150–180
1.07 � 0.06
6.00 � 0.20
2.03 � 0.06
2.559 � 0.077
15.8 � 1.1b 17.7 � 1.3c 17.1 � 1.1b 18.9 � 1.3c 18.8 � 0.8b 19.5 � 0.9c
18.2 � 1.3
XBZOSL-8
50.5 � 2.0b 57.6 � 2.1c 56.2 � 1.7b 63.1 � 2.1c 60.3 � 1.7b 63.2 � 2.1c
10 � 3
10 � 3
0.817 � 0.186c 0.762 � 0.173 0.817 � 0.186c 0.641 � 0.067 0.687 � 0.070c
3.261 � 0.200
c
3.280 � 0.189 3.335 � 0.201c 3.201 � 0.102 3.246 � 0.104
c
60.7 � 1.7 59.5 � 1.9
18.5 � 1.2 18.6 � 0.8
a The internal dose rates was estimated by assuming K of 12 � 1% (Rui et al., 2019a) for the SA and SG (based on all the accepted grains) ages determination, and of 13 � 1% for the SG ages determination only using the bright grains. b The SG CAM De and age values were obtained based on all the accepted grains. The errors are propagated in quadrature. c The italics represent the internal and the total environmental dose rates, the SG CAM De and age values were obtained based on the grains whose brightness larger than 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively.
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fading results for four multi-grained aliquots per sample are shown in Fig. S5, which shows that there is a fading rate of ~1%/decade for the 170 � C signal. For the single grains of each sample, fading tests were conducted on the same 600 grains for each sample which had been used for De mea surement. The arithmetic means of the fading rates for the same accepted grains for previous De measurement above different N-L1 thresholds are displayed in Fig. 7a–c and summarized in Table S2. It can be seen that the fading rates decreased as the N-L1 thresholds increased, reaching a plateau at ~0.34%, ~0.83% and 1.22%/decade for samples XBZ-OSL-6, -8 and -12, respectively. The results from Fig. 7a–c appears to indicate that the dimmer grains are associated with larger fading rate, which could have resulted in lower De values and result in the pattern seen in Fig. 4. In order to further test this, we applied fading correction to the CAM De values of different N-L1 thresholds by using the fading rates obtained from the same N-L1 thresholds (Table S2). It can be seen that, the fading-corrected CAM De values reached a ‘plateau’ at an earlier threshold of 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively (Fig. 7d–f), compared to the fading un-corrected CAM De values in Fig. 4d–f. Furthermore, the fading corrected SG CAM De and SA CAM De are consistent with each other within 1σ for sample XBZ-OSL-12 (Fig. 3b), and the discrepancy between the SA and SG De results decreased to ~14% and ~7% for samples XBZ-OSL-6 and -8 respectively. The final ages calculated using the fading corrected De values are listed in Table 3. The SG (based on all the accepted grains) and SA ages are consistent with each other within 1σ for each sample, and consistent with the radiocarbon ages. Thus, we conclude that the higher fading rates for the dim grains, at least for samples XBZ-OSL-8 and -12, is the major reason here for the underestimation of the De values for the dim grains and the discrepancy between the SA and SG results.
routine check of the dependency of SG K-feldspar De estimates against their brightness. The weight of grains with lower K contents and higher fading fates in the single-grain De estimate is larger than in the singlealiquot De estimate. Thus, for well-bleached and homogeneous K-feld spar samples, the multi-grained single-aliquot procedure might be more appropriate for De determination because the IRSL signals are domi nated by those from the brightest grains associated with higher K con tent and lower fading rate. However, for single-grain measurements, which is usually conducted for partial bleached or post-depositionally mixed samples, it is suggested that the De values of the grains associ ated with the same fading rate should be grouped for statistical analyses, e.g., the brightest grains with the least fading are preferable for De estimation. Acknowledgment This research was supported by the Doctor Foundation of Hebei Normal University (grant No. L2018B30) and the National Natural Sci ence Foundation of China (grant No. 41702192) to Dr. Yujie Guo, and an Australian Research Council Future Fellowship to Bo Li (FT140100384). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.quageo.2019.101040. References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Arnold, L.J., Roberts, R.G., Galbraith, R.F., DeLong, S.B., 2009. A revised burial dose estimation procedure for optical dating of young and modern-age sediments. Quat. Geochronol. 4, 306–325. Auclair, M., Lamothe, M., Huot, S., 2003. Measurement of anomalous fading for feldspar IRSL using SAR. Radiat. Meas. 37, 487–492. Barr� e, M., Lamothe, M., 2010. Luminescence dating of archaeosediments: a comparison of k-feldspar and plagioclase IRSL ages. Quat. Geochronol. 5, 324–328. Bateman, M.D., Boulter, C.H., Carr, A.S., Frederick, C.D., Peter, D., Wilder, M., 2007. Detecting post-depositional sediment disturbance in sandy deposits using optical luminescence. Quat. Geochronol. 2, 57–64. Blegen, N., Tryon, C.A., Faith, J.T., Peppe, D.J., Beverly, E.J., Li, B., Jacobs, Z., 2015. Distal tephras of the eastern Lake Victoria basin, equatorial East Africa: correlations, chronology and a context for early modern humans. Quat. Sci. Rev. 122, 89–111. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523–528. Brill, D., Reimann, T., Wallinga, J., May, S.M., Engel, M., Riedesel, S., Brückner, H., 2018. Testing the accuracy of feldspar single grains to date late Holocene cyclone and tsunami deposits. Quat. Geochronol. 48, 91–103. Buylaert, J.P., Újv� ari, G., Murray, A.S., Smedley, R.K., Kook, M., 2018. On the relationship between K concentration, grain size and dose in feldspar. Radiat. Meas. 181–187. Chen, Y.W., Li, S.H., Li, B., Hao, Q.Z., Sun, J.M., 2015. Maximum age limitation in luminescence dating of Chinese loess using the multiple-aliquot MET-pIRIR signals from K-feldspar. Quat. Geochronol. 30, 207–212. David, B., Roberts, R., Tuniz, C., Jones, R., Head, J., 1997. New optical and radiocarbon dates from Ngarrabullgan Cave, a Pleistocene archaeological site in Australia: implications for the comparability of time clocks and for the human colonization of Australia. Antiquity 71, 183–188. Derevianko, A.P., Markin, S.V., Rudaya, N.A., Viola, B., Zykin, V.S., Zykina, V.S., Chabay, V.P., Kolobova, K.A., Vasiliev, S.K., Roberts, R.G., Li, B., Jacobs, Z., 2018. Interdisciplinary Studies of Chagyrskaya Cave – Middle Paleolithic Site of Altai. Institute of Archaeology and Ethnography, Siberian Branch of the Russian Academy of Sciences, Novosibirsk. Du, S.S., 2003. A preliminary study on raw material exploitation in Middle–Upper Palaeolithic sites in Nihewan Basin (in Chinese with English abstract). Acta Anthropol. Sin. 22, 121–130. Fu, X., Li, S.H., 2013. A modified multi-elevated-temperature post-IR IRSL protocol for dating Holocene sediments using k-feldspar. Quat. Geochronol. 17, 44–54. Fu, X., Li, B., Li, S.H., 2012. Testing a multi-step post-IR IRSL dating method using polymineral fine grains from Chinese loess. Quat. Geochronol. 10, 8–15. Galbraith, R.F., Roberts, R.G., 2012. Statistical aspects of equivalent dose and error calculation and display in OSL dating: an overview and some recommendations. Quat. Geochronol. 11, 1–27. 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.
4. Discussions and conclusions Previous studies have demonstrated that the SG K-feldspar De values may depend on their brightness (Reimann et al., 2012; Jacobs et al., 2019). Reimann et al. (2012) attributed this phenomenon as a result of dimmer grains being associated with lower K contents than brighter grains. Jacobs et al. (2019), however, found that this pattern seen in their samples may be related to sensitivity changes in the measurements of natural cycles (Ln and Tn), based on the observation that there is a similar pattern seen in their dose recovery test. They also demonstrated that most of their K-feldspar grains accepted for De determination have a high K content from 10 to 14%, which ruled out the internal K content being the major reason for the dependence of De on brightness. In our study, however, such a pattern was not seen in the dose recovery test (Fig. 6), indicating that the sensitivity correction for the natural signal is successful for both dimmer and brighter grains. We also demonstrated that the lower De for dimmer grains can be largely explained by their relatively higher fading rates (Fig. 7). However, we observed that there is still a systematic increase in the fading-corrected De values with N-L1 thresholds (Fig. 7d–f), implying that there may still be a slightly higher internal K content for brighter grains than for dimmer grains especially for sample XBZ-OSL-6. If we calculate the ages only using the fading corrected bright grains with brightness larger than 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12 respectively, the internal K was assumed as 13 � 1% (Rui et al., 2019a), the obtained ages are still consistent with the SA and SG (based on all the accepted grains) ages results (Table 3 and Fig. 3c). In conclusion, it is very important to investigate the dependency of SG De values with the luminescence behaviors of individual grains, such as brightness. The underestimation of SG De values for our samples is mainly caused by the relative higher fading rates in dim grains. How ever, other effects such as variable internal K contents and variable sensitivity changes in the measurement of natural signals may also contribute to the variable SG K-feldspar results. We recommend a 9
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Quaternary Geochronology 56 (2020) 101040 Porat, N., Faerstein, G., Medialdea, A., Murray, A.S., 2015. Re-examination of common extraction and purification methods of quartz and feldspar for luminescence dating. Anc. TL. 33, 22–30. Qin, J.T., Chen, J., Li, Y.T., Zhou, L.P., 2018. Initial sensitivity change of k-feldspar pirir signals due to uncompensated decrease in electron trapping probability: evidence from radiofluorescence measurements. Radiat. Meas. 120, 131–136. Reimann, T., Thomsen, K.J., Jain, M., Murray, A.S., Frechen, M., 2012. Single-grain dating of young sediments using the pIRIR signal from feldspar. Quat. Geochronol. 11, 28–41. Reimann, T., Rom� an-S� anchez, A., Vanwalleghem, T., Wallinga, J., 2017. Getting a grip on soil reworking - single-grain feldspar luminescence as a novel tool to quantify soil reworking rates. Quat. Geochronol. 42, 1–14. Reimer, P.J., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C. E., Cheng, H., Edwards, R.L., Friedrich, M., Grootes, P.M., 2013. IntCal13 and Marine13 radiocarbon age calibration curves 0‒50,000 years cal BP. Radiocarbon 55, 1869‒1887. Roberts, R.G., Jacobs, Z., Li, B., Jankowski, N.R., Cunningham, A.C., Rosenfeld, A.B., 2015. Optical dating in archaeology: thirty years in retrospect and grand challenges for the future. J. Archaeol. Sci. 56, 41–60. Rui, X., Guo, Y.J., Zhang, J.F., Hu, Y., Mei, H.J., Wang, Y.P., Xie, F., Li, B., 2019. Luminescence chronology of the Palaeolithic–Neolithic transition in the Yujiagou site at the Nihewan Basin, northern China. J. Quat. Sci. 34, 125–137. Rui, X., Li, B., Guo, Y.J., Zhang, J.F., Yuan, B.Y., Xie, F., 2019. Variability in the thermal stability of OSL signal of single-grain quartz from the Nihewan Basin, north China. Quat. Geochronol. 49, 25–30. Schaarschmidt, M., Fu, X., Li, B., Marwick, B., Khaing, K., Douka, K., Roberts, R.G., 2019. pIRIR and IR-RF dating of archaeological deposits at Badahlin and Gu Myaung Caves – first luminescence ages for Myanmar. Quat. Geochronol. 49, 262–270. Smedley, R.K., Pearce, N.J.G., 2016. Internal U, Th and Rb concentrations of alkalifeldspar grains: implications for luminescence dating. Quat. Geochronol. 35, 16–25. Smedley, R.K., Duller, G.A.T., Pearce, N.J.G., Roberts, H.M., 2012. Determining the kcontent of single-grains of feldspar for luminescence dating. Radiat. Meas. 47, 790–796. Smedley, R.K., Duller, G.A.T., Roberts, H.M., 2015. Bleaching of the post-IR IRSL signal from individual grains of k-feldspar: implications for single-grain dating. Radiat. Meas. 79, 33–42. Smedley, R.K., Buylaert, J.P., Újv� ari, G., 2019. Comparing the accuracy and precision of luminescence ages for partially-bleached sediments using single grains of K-feldspar and quartz. Quat. Geochronol. 53, 101007. Thiel, C., Buylaert, J.P., Murray, A., Terhorst, B., Hofer, I., Tsukamoto, S., Frechen, M., 2011. Luminescence dating of the Stratzing loess profile (Austria) - testing the potential of an elevated temperature post-IR IRSL protocol. Quat. Int. 234, 23–31. Thomsen, K.J., Murray, A.S., Jain, M., Botter-Jensen, L., 2008. Laboratory fading rates of various luminescence signals from feldspar-rich sediment extracts. Radiat. Meas. 43, 1474–1486. Trauerstein, M., Lowick, S., Preusser, F., Rufer, D., Schlunegger, F., 2012. Exploring fading in single grain feldspar IRSL measurements. Quat. Geochronol. 10, 327–333. Trauerstein, M., Lowick, S.E., Preusser, F., Schlunegger, F., 2014. Small aliquot and single grain IRSL and post-IR IRSL dating of fluvial and alluvial sediments from the Pativilca valley, Peru. Quat. Geochronol. 22, 163–174. Tribolo, C., Mercier, N., Rasse, M., Soriano, S., Huysecom, E., 2010. Kobo 1 and L’Abri aux Vaches (Mali, West Africa): two case studies for the optical dating of bioturbated sediments. Quat. Geochronol. 5, 317–323. Wallinga, J., Murray, A., Wintle, A., 2000. The single-aliquot regenerative-dose (SAR) protocol applied to coarse-grain feldspar. Radiat. Meas. 32, 529–533. Xie, F., Li, J., Liu, L.Q., 2006. Paleolithic Archeology in the Nihewan Basin [in Chinese]. Huashan Literature and Arts Press, Shijiazhuang. Yuan, B.Y., Xia, Z.K., Niu, P.S. (Eds.), 2011. Nihewan Rift and Early Man. Geology Press, Beijing (in Chinese).
Gliganic, L.A., Cohen, T.J., Meyer, M., Molenaar, A., 2017. Variations in luminescence properties of quartz and feldspar from modern fluvial sediments in three rivers. Quat. Geochronol. 41, 70–82. Gong, Z.J., Li, S.H., Li, B., 2014. The evolution of a terrace sequence along the Manas River in the northern foreland basin of Tian Shan, China, as inferred from optical dating. Geomorphology 213, 201–212. Guo, Y.J., Li, B., Zhang, J.F., Roberts, R.G., 2015. Luminescence-based chronologies for Palaeolithic sites in the Nihewan Basin, northern China: first tests using newly developed optical dating procedures for potassium feldspar grains. J. Archaeol. Sci.: Reports 3, 31–40. Guo, Y.J., Bo, L., Zhang, J.F., Yuan, B.Y., Fei, X., Graham, R.R., 2016. Luminescence ages for three ‘Middle Palaeolithic’ sites in the Nihewan basin, northern China, and their archaeological and palaeoenvironmental implications. Quat. Res. 85, 456–470. Huntley, D.J., Baril, M.R., 1997. The K content of the K-feldspars being measured in optical dating or in thermoluminescence dating. Anc. TL. 15, 11–13. Huntley, D.J., Lamothe, M., 2001. Ubiquity of anomalous fading in K-feldspars and the measurement and correction for it in optical dating. Can. J. Earth Sci. 38, 1093–1106. Jacobs, Z., Duller, G.A.T., Wintle, A.G., 2006. Interpretation of single grain De distributions and calculation of De. Radiat. Meas. 41, 264–277. Jacobs, Z., Li, B., Shunkov, M.V., Kozlikin, M.B., Bolikhovskaya, N.S., Agadjanian, A.K., Uliyanov, V.A., Vasiliev, S.K., O’Gorman, K., Derevianko, A.P., Roberts, R.G., 2019. Timing of archaic hominin occupation 1 of Denisova Cave in southern Siberia. Nature 565, 594–599. Lamothe, M., Auclair, M., 2000. The fadia method: a new approach in luminescence dating using the analysis of single feldspar grains. Radiat. Meas. 32, 433–438. Li, B., Li, S.H., 2011. Luminescence dating of K-feldspar from sediments: a protocol without anomalous fading correction. Quat. Geochronol. 6, 468–479. Li, B., Li, S.H., 2012. Luminescence dating of Chinese loess beyond 130 ka using the nonfading signal from K-feldspar. Quat. Geochronol. 10, 24–31. Li, B., Jacobs, Z., Roberts, R.G., Li, S.H., 2013. Extending the age limit of luminescence dating using the dose-dependent sensitivity of MET-pIRIR signals from K-feldspar. Quat. Geochronol. 17, 55–67. Li, B., Jacobs, Z., Roberts, R.G., Li, S.H., 2014. Review and assessment of the potential of post-IR IRSL dating methods to circumvent the problem of anomalous fading in feldspar luminescence. Geochronometria 41, 178–201. Li, B., Roberts, R.G., Jacobs, Z., 2013. On the dose dependency of the bleachable and non-bleachable components of irsl from k-feldspar: improved procedures for luminescence dating of quaternary sediments. Quat. Geochronol. 17, 1–13. Li, B., Roberts, R.G., Jacobs, Z., Li, S.H., 2014. A single-aliquot luminescence dating procedure for K-feldspar based on the dose-dependent MET-pIRIR signal sensitivity. Quat. Geochronol. 20, 51–64. Li, B., Roberts, R.G., Jacobs, Z., Li, S.H., 2015. Potential of establishing a ‘global standardised growth curve’ (gSGC) for optical dating of quartz from sediments. Quat. Geochronol. 27, 94–104. Li, B., Roberts, R.G., Jacobs, Z., Li, S.H., Guo, Y.J., 2015. Construction of a ‘global standardised growth curve’ (gSGC) for infrared stimulated luminescence dating of Kfeldspar. Quat. Geochronol. 27, 119–130. Li, B., Jacobs, Z., Roberts, R.G., Li, S.H., 2018. Single-grain dating of potassium-rich feldspar grains: towards a global standardised growth curve for the post-IR IRSL signal. Quat. Geochronol. 45, 23–36. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Neudorf, C.M., Roberts, R.G., Jacobs, Z., 2012. Sources of overdispersion in a k-rich feldspar sample from north-central India: insights from de, k content and irsl age distributions for individual grains. Radiat. Meas. 47, 696–702. Nian, X.M., Bailey, R.M., Zhou, L.P., 2012. Investigations of the post-IR IRSL protocol applied to single k-feldspar grains from fluvial sediment samples. Radiat. Meas. 47, 703–709.
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Research paper
Comparison of single-aliquot and single-grain MET-pIRIR De results for potassium feldspar samples from the Nihewan Basin, northen China Yujie Guo a, *, Bo Li b, c, Hailong Zhao a a
Institute of Nihewan Archaeology, Hebei Normal University, 050024 Shijiazhuang, Hebei Province, China Centre for Archaeological Science, School of Earth and Environmental Science, University of Wollongong, Wollongong, 2522 NSW, Australia c Australian Research Council (ARC) Centre of Excellence for Australian Biodiversity and Heritage, University of Wollongong, Wollongong, NSW 2522, Australia b
A R T I C L E I N F O
A B S T R A C T
Keywords: K-feldspar Single grain Single aliquot Fading rate Brightness
In this study we compared De results obtained from single-aliquot (SA) and single-grain (SG) measurements on potassium rich feldspars (K-feldspars) from the Nihewan Basin, north China, based on the multiple-elevatedtemperature post-infrared infrared (MET-pIRIR) stimulated luminescence. Discrepancy was observed between the SA and SG De results. Potential reasons for such discrepancy, including residual doses, sensitivity changes, fading rates, internal dose rates, and/or the combination of these possibilities, were investigated. Our results indicate that the disagreement between the SA and SG De results are mainly caused by the different fading rates and, to a small extent, by the internal K contents variations of different grains. Our study highlights the importance to investigate the relationship of single-grain De values and the luminescence behavior of individual grains, such as brightness. For partial-bleached or post-depositional mixed K-feldspar samples, statistical analysis should be conducted only on the grains with the same fading rate.
1. Introduction Infrared (IR) stimulated luminescence (IRSL) from potassium-rich feldspars (K-feldspar) has been successfully applied to determine the ages of Quaternary sediments worldwide (e.g., Thiel et al., 2011; Gong et al., 2014; Li et al., 2013a, Li et al., 2014a; Chen et al., 2015; Guo et al., 2016; Li et al., 2018; Jacobs et al., 2019), since the development of post-IR IRSL (pIRIR) dating procedures that are able to minimise the anomalous fading effect (Thomsen et al., 2008; Thiel et al., 2011; Li and Li, 2011, 2012). Most previous studies on K-feldspar IRSL dating focused on measurements of multiple grains using the single-aliquot regener ative-dose procedures (Murray and Wintle, 2000). Compared to multiple-grain techniques, single-grain techniques have the advantage of dealing with insufficiently bleached samples, identifying post-depositional mixing between sedimentary units of different ages and assessing stratigraphic integrity (Arnold et al., 2009; Bateman et al., 2007; David et al., 1997; Jacobs et al., 2006; Tribolo et al., 2010). Although there has been a large number of single-grain studies on quartz OSL (see Roberts et al. (2015) for a review), single-grain K-feldspar dating became applicable very recently (Trauerstein et al., 2012, 2014; Neudorf et al., 2012; Smedley et al., 2012, 2015, 2019; Nian et al., 2012; Smedley and Pearce, 2016; Gliganic
et al., 2017; Reimann et al., 2017; Buylaert et al., 2018; Brill et al., 2018; Schaarschmidt et al., 2019). Previous studies have shown that the equivalent dose (De) of single K-feldspar grains are much more complicated due to their varying luminescence properties from grain to grain. For example, Lamothe and Auclair (2000) demonstrated that different K-feldspar grains have significantly different anomalous fading rates in low temperature IRSL signals, which allowed them to develop an ‘isochron’ method to overcome the fading issue. Reimann et al. (2012) applied the pIRIR procedure to single-grain K-feldspars and observed that the K-feldspar De depends on the sensitivity of individual grains (i. e., brighter grains yielded higher De values), which they attributed to the correlation between blue emission and potassium content of individual grains. A similar pattern of correlation between sensitivity and De values was observed from some K-feldspar samples from Siberia (Derevianko et al., 2018; Jacobs et al., 2019), which shows that only the brightest portions of grains can give reliable results. Most of previous studies focused on either single aliquot or single grain, and there is a lack of systematic comparison between singlealiquot and single-grain of K-feldspar results on the same samples. In this study, we investigated three K-feldspar samples from the Nihewan Basin in northen China. Both multi-grained aliquots and individual grains were measured and compared. We investigated the reason for the
* Corresponding author. E-mail address: [email protected] (Y. Guo). https://doi.org/10.1016/j.quageo.2019.101040 Received 31 July 2019; Received in revised form 15 November 2019; Accepted 15 November 2019 Available online 25 November 2019 1871-1014/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Satellite images (courtesy of Google Earth) of the Nihewan Basin and the Xiabuzhuang Palaeolithic site. The red rectangle in (b) represents the Nihewan Basin and the red star represents the Beijing city. (c,d) Photos showing the location of the Xiabuzhuang site on the east scarp of the Qian River. (e) Sedimentary profile of the north face of the excavation pit, showing the locations of the OSL (XBZ-OSL-1 to 13) and radiocarbon (17XBZDT2:2209 and 17XBZDT2:3282) samples. ① disturbed layer, ② cultural layer and ③ layer with no remains.
discrepancy between the multi-grain and single-grain results.
Table 1 SA measurement procedure for the low-preheat-temperature MET-pIRIR mea surements (after Fu and Li, 2013). Step
Treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Give regenerative dose, Di Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s IRSL measurement at 170 � C for 100 s Give test dose, 13.1 Gy Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s IRSL measurement at 170 � C for 100 s IR bleaching at 280 � C for 100 s Return to step 1
2. Samples, experimental procedure and analytical facilities
Observed
The Xiabuzhuang site is located at the east bank of Qian River, in the south edge of the Nihewan Basin, northern China (Fig. 1). The geology of the river catchment area is mainly volcanic breccia, andesite, tuff, sili ceous limestone and quartz sandstone (Du, 2003; Xie et al., 2006; Yuan et al., 2011). The deposits are mainly alluvial-diluvial deposits, con sisting of silty sand interbedded with poorly rounded gravels. The cul tural remains were excavated from the top silty sand layer (Fig. 1c). A total of 15 OSL samples were collected from this site, but ten of them were measured (Fig. 1e). Apart from OSL samples, three radiocarbon samples, 17XBZDT2:2209 (animal bone, including two sub-samples 17XBZDT2:2209a and17XBZDT2:2209b) and 17XBZDT2:3282 (char coal) from the layer near samples XBZ-OSL-8 and -12 (Fig. 1e) were collected for comparison. The calibrated ages are 16,443‒16,026, 16, 310‒15,970, and 18,454‒18,115 yr cal BP (95% confidence interval, Reimer et al., 2013) for samples 17XBZDT2:2209a, 17XBZDT2:2209b and 17XBZDT2:3282, respectively. Three samples (XBZ-OSL-6, 8 and 12) were investigated in detail
Lx(50) Lx(80) Lx(110) Lx(140) Lx(170) Tx(50) Tx(80) Tx(110) Tx(140) Tx(170)
Note: For the first cycle natural signals, i ¼ 0 Gy and D0 ¼ 0 Gy. The entire sequence is repeated for several regenerative doses, including a zero dose and a repeat dose. 2
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Fig. 2. (a)–(c) De distributions for multi-grained single aliquots of samples XBZ-OSL-6, -8 and -12, respectively. The grey bands are centred on the CAM values. (d)– (f) De distributions for all the accepted grains of samples XBZ-OSL-6, -8 and -12, respectively; the grey bands are centred on the CAM values based on all the accepted grains (both the hollow circles and the black triangles), and the green bands centred on the CAM values based on the grains with brightness larger than 100, 140 and 80 ct/Gy (the black triangles) for samples XBZ-OSL-6, -8 and -12, respectively. The values of “n” in (a)–(c) represent the total number of the measured aliquots and the values in (d)–(f) represent the number of the accepted grains with brightness larger than 100, 140 and 80 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively.
here for comparing single aliquot and single-grain results. The other seven samples were used to establish standardised growth curves (SGC) and the dating results for these samples will be presented elsewhere. Kfeldspar grains from each sample were extracted following standard procedures (Aitken, 1998). Carbonates and organic matter were elimi nated using HCl acid and H2O2 solution, respectively. K-feldspar grains of 150–250 μm for samples XBZ-OSL-6 and 8, and 150–180 μm for
sample XBZ-OSL-12 were obtained by dry sieving and density separation using a heavy liquid solution (2.58 g/cm3). The grains were then etched in 10% HF acid for 40 min to remove the alpha-irradiated outer layer of each grain. The etched grains were sieved again to remove the portions that were reduced to less than 150 μm or broke into smaller pieces (Porat et al., 2015), and an overall reduction of 10 μm by HF etching was assumed. 3
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Table 2 SG measurement procedure for the low preheat temperature MET-pIRIR measurements. Step
Treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Give regenerative dose, Di Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s SG IR laser measurement at 170 � C for 3 s Give test dose, 12.5 Gy Preheat at 200 � C for 60 s IRSL measurement at 50 � C for 100 s IRSL measurement at 80 � C for 100 s IRSL measurement at 110 � C for 100 s IRSL measurement at 140 � C for 100 s SG IR laser measurement at 170 � C for 3 s IR bleaching at 280 � C for 100 s Return to step 1
Observed
Lx(50) Lx(80) Lx(110) Lx(140) Lx(170) Tx(50) Tx(80) Tx(110) Tx(140) Tx(170)
Note: For the first cycle natural signals, i ¼ 0 Gy and D0 ¼ 0 Gy. The entire sequence is repeated for several regenerative doses, including a zero dose and a repeat dose.
For multiple-grain analysis, K-feldspar grains were mounted onto the central ~3 mm diameter of stainless steel discs, corresponding to about several hundred grains on each aliquot. For single-grain measurements, the standard single-grain discs drilled with 100 holes, each 300 μm wide and 300 μm deep (Bøtter-Jensen et al., 2000), were used. Single-grain discs were checked under the microscope to verify that each hole con tained only one grain. The IRSL measurements were performed on a Risø TL/OSL-DA-20 reader equipped with a 90Sr beta source and IR diodes (870 nm) and IR laser (830 nm) for stimulation. IRSL signals were detected by an Electron Tubes Ltd 9235QB15 photomultiplier tube fitted with a blue filter pack comprising of Schott BG-39 and Corning 7–59 filters, which transmits blue-violet (320–480 nm) light. The internal dose rates for the three samples in this study were determined by assuming the K contents of 12 � 1%, based on energy dispersive spectroscopy analysis of single-grain K-feldspar samples from the same region (Rui et al., 2019a). A minor contribution from Rb was calculated by assuming a concentration of 400 � 100 ppm (Huntley and Lamothe, 2001). 3. De measurement procedures Given the relatively young age of our samples, a low preheat tem perature (200 � C for 60 s) was used in the SAR MET-pIRIR procedure (Fu and Li, 2013) to measure the De values for multi-grained aliquots and individual grains. The advantage of the low preheat MET-pIRIR pro cedure is that the residual doses are very small (see reviews in Li et al., 2014b), which is crucial for young samples. In the SAR procedure, dose response curve (DRC) is constructed using the sensitivity-corrected pIRIR signals (Lx/Tx) induced from a series of regenerative doses asso ciation with fixed test doses, including a zero and a repeated regenera tive dose to monitor the extent of recuperation and the recycling ratio, respectively. The De value of each aliquot/grain was determined by interpolating the sensitivity-corrected natural signal (Ln/Tn) on to its corresponding DRC. All the DRCs in this study were fitted using a single e D∕D0 Þ þ c, saturating exponential function of the form I ¼ I0 ð1 where I is the sensitivity-corrected IRSL intensity (Lx/Tx), D is the regenerative dose, D0 is the characteristic saturation dose, and I0 þ c define the maximum intensity of the DRC. The instrumental reproduc ibility errors of 1% and 1.5% were used for the SA and SG De mea surements respectively (based on the reproducibility test on our reader), following the procedure described in Li et al. (2018). The uncertainty on SG De was estimated using a Monte Carlo method with repeat fits value of 500. A systematic error of 2% was also added in quadrature to the measurement errors for possible bias in the calibration of the laboratory
Fig. 3. Fading uncorrected (a) and corrected (b) De results and ages (c) plotted against depth for samples XBZ-OSL-6, -8 and -12, respectively. The horizontal bars for each point indicate the standard errors. The blue diamonds represent SG results for all grains, and the green triangles in (c) represent SG ages results for grains after rejecting the dim ones with N-L1 lower than 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively.
beta source for both SA and SG De measurements. 3.1. Single-aliquot measurement The single-aliquot (SA) measurements were conducted using the MET-pIRIR procedure in Table 1. The IR stimulation was made consecutively at a range of temperatures, 50, 80, 110, 140 and 170 � C, for 100 s, respectively. The aliquots were held for 10, 10, 20, 20 and 30 s before IR stimulations at each of the temperatures, respectively, to monitor and minimise interference from isothermal decay signals (Fu et al., 2012). Typical natural IRSL (50 � C) and MET-pIRIR (80–170 � C) decay curves are shown in Fig. S1 for one aliquot of sample XBZ-OSL-12. The De value for each aliquot was calculated from the IRSL counts in tegrated over the first 15 s of IRSL decay, minus the counts from the final 15 s of stimulation as background. We noted that changing the 4
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Fig. 4. (a)–(c) Ranked individual De distributions according to their intrinsic brightness for all the accepted grains of samples XBZ-OSL-6, -8 and -12, respectively. The orange lines are running means with a fixed subset size of 5 data points. (d)–(f) The CAM De value (blue rhombus) distributions for the grains with brightness above different N-L1 thresholds. The points at the 0 threshold are offset laterally for clarity. The red squares represent the SA CAM De values for each sample, plotting at a ‘fictive’ N-L1 threshold of 320 ct/Gy for clarity. The dashed red lines in (d)–(f) are the ‘plateau’ values of 58.0, 61.3 and 59.1 Gy, calculated from the weighted mean of the De estimates with N-L1 thresholds above 100, 140 and 80 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively. The veticle bars for each point indicate the standard errors.
integration periods of the initial IRSL decay from 2 to 30 s have negli gible effect on the results. To choose a suitable preheat temperature, the De of the sample XBZOSL-12 was measured with preheat temperatures varying from 200 to 280 � C in increments of 20 � C. The obtained results were plotted in Fig. S2a, which indicated that the De results measured at 140 and 170 � C are independent of preheat temperature. For the other measuring tem peratures, 110, 80 and 50 � C, the best results are obtained for preheat temperatures of 200–220 � C. A dose recovery test (Galbraith et al., 1999; Wallinga et al., 2000) using the procedure in Table 1 at the preheat temperature of 200 � C was carried out on sample XBZ-OSL-12 to validate the applicability of the SAR procedure for our samples. Aliquots containing natural samples were first bleached for one month in November by sunlight at Shi jiazhuan, northern China. They were then given a dose of 59.0 Gy, which was then measured as an ‘unknown’ dose. The dose recovery ratios (or measured to given dose ratio) were plotted against IR stimulation tem perature in Fig. S2b. It shows that the given dose can be successfully recovered within 10%, suggesting that the SAR MET-pIRIR procedure can reliably determine the De values for samples from this site under
controlled laboratory conditions. Since the 170 � C signal suffers less anomalous fading than the 140 � C signal, we have used the 170 � C signal at the preheat of 200 � C to estimate final De results. For multi-grain aliquot De measurement, we applied the stand ardised growth curve (SGC) method (Li et al., 2015a,b) for our samples. Two aliquots from each of the 10 samples were measured using a full SAR procedure to construct their DRCs. Their DRCs were ‘re-normalised’ (Li et al., 2015a,b) using the signals measured at the regenerative-dose of 52.4 Gy to establish SGC (Fig. S3a). De values were then obtained by projecting the re-normalised natural signal onto the corresponding SGCs. We found that the De values obtained from individual DRCs are consistent with those obtained from the SGCs within 1σ (Fig. S3b), implying that the SGCs established are reliable. Based on this observa tion, we measured more aliquots by measuring only their natural signal, one regenerative-dose signal, and their corresponding test dose signals, and then projecting the re-normalised natural Ln/Tn signals on to the SGC to estimate their De values. The distributions of the SA De results for the three samples are plotted in the radial plots in Fig. 2a–c. The De values are distributed around a central value for each of the three samples and their over-dispersion (OD) values, calculated using the 5
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Fig. 5. (a)–(c) Ranked individual residual dose distributions according to their intrinsic brightness for all the accepted grain of samples XBZ-OSL-6, -8 and -12, respectively. (d)–(f) The arithmetic mean residual dose values distributions for the grains with brightness above different N-Tn thresholds. The red squares represent the arithmetic means of SA residual dose values for each sample calculated using the sum of the single-grained IRSL signals, plotting at ‘fictive’ N-L1 thresholds of 250 and 320 ct/Gy for clarity. The points at the 0 threshold are offset laterally. The veticle bars for each point indicate the standard errors.
Central Age Model (CAM) (Galbraith et al., 1999; Galbraith and Roberts, 2012), are zero for all the three samples. The CAM SA De values are 53.4 � 1.2, 54.5 � 1.2 and 55.6 � 1.2 Gy for samples XBZ-OSL-6, 8 and 12, respectively, corresponding to ages of 16.7 � 1.1, 16.6 � 1.0 and 17.4 � 0.7 ka, respectively.
saturation level of the DRC. A total of 600 grains mere measured for each sample, and 349, 397 and 369 were accepted for samples XBZ-OSL-6, 8 and 12, respectively, after applying the rejection criteria (Table S1). The CAM De values for the accepted grains were 45.49 � 1.17, 45.44 � 1.16 and 51.72 � 1.25 Gy, with OD values of 26.4 � 1.3, 28.0 � 1.2 and 22.7 � 1.1% for samples XBZ-OSL-6, 8 and 12, respectively (Fig. 2d–f), cor responding to ages of 14.19 � 0.91, 13.85 � 0.87 and 16.16 � 0.65 ka.
3.2. Single-grain measurement
3.3. Comparison of the SA and SG results
The single-grain (SG) measurement procedure was same as the SA measurement procedure but the last SA IRSL measurement at 170 � C was replaced with an IR laser stimulation at the same temperature (Table 2). Typical natural pIRSL signal at 170 � C decay curves for grains with different brightness are shown in Fig. S4 for sample XBZ-OSL-12. The net IRSL signals used for SG De estimation were calculated as the sum of counts in the first 0.15 s of IRSL decay minus a ‘late light’ background estimated from the mean count rate over the final 0.15 s in a total of 3 s stimulation time. Grains with unsuitable IRSL properties were rejected using the well-established criteria (e.g., Jacobs et al., 2006; Blegen et al., 2015; Guo et al., 2016; Rui et al., 2019b): (1) the net Tn signal is less than 3 times the standard deviation of its corresponding background signal, or the relative error on the test dose signal is greater than 25%; (2) the recycling ratio differs from unity by more than 2σ; (3) the extent of recuperation is greater than 10%; and (4) the Ln/Tn value exceeds the
The CAM SA and SG De results were plotted together in Fig. 3a, which shows that the SG De results are systematically lower than the corre sponding SA De results by ~15%, ~17% and ~7% for samples XBZ-OSL6, -8 and -12, respectively. There are several possible reasons for SA results being higher than SG results. The first one is that the samples were insufficiently bleached, and, as a result, the multi-grain measure ments were overestimated. However, the tight single-grain De distribu tions (Fig. 2d–f) and their relatively small OD values (Table S2) rule out this possibility. This is further supported by the radiocarbon dating re sults from the same layers of XBZ-OSL-8 and -12. The radiocarbon ages are statistically consistent with the SA ages, further supporting that the samples were sufficiently bleached prior to burial. We, therefore, infer that the disagreement between the SA and SG CAM De results might be 6
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CAM De values and OD values above different N-L1 thresholds were summarized in Table S2. Similar patterns of the De dependency on the grain brightness have also been observed in other studies (Reimann et al., 2012; Jacobs et al., 2019). The dependency of single grain De values on their inherent brightness might be related to: 1) the residual or “non-bleachable” (Li et al., 2013b) doses being lower for the dim grains than for the bright grains; 2) the sensitivity of dim grains changing during the natural pIRIR signal (Ln) measurement (Guo et al., 2015; Qin et al., 2018), which was not detected by the subsequent Tx and Lx signals measurements, i.e. the single-aliquot regenerative-dose (SAR) protocol might not be suitable for the dim K-feldspar grains for these samples; 3) the fading rates of the pIRIR signals are higher for the dim grains than the bright grains; 4) internal dosimetry variations caused by grain-to-grain variations in the internal potassium (K) content, i.e., there are lower K contents in the dim grains than in the bright grains (e.g., Huntley and Baril, 1997; Barr� e and Lamothe, 2010; Smedley et al., 2012; Reimann et al., 2012); or 5) a combination of these possibilities. Here we investigated these possibil ities based on a series of tests. 3.3.1. Residual doses In order to investigate whether the dimmer grains have different residual doses than brighter grains that can explain the difference in their De values, we measured residual doses on bleached samples. To do this, 600 grains of each of the three samples were bleached for one month in November by sunlight. The residual doses of the bleached grains were measured using the same procedure in Table 2, and a total of 364, 391 and 329 grains passed the rejection criteria for each of the samples (Table S1). The accepted grains were ranked according to their N-L1 signals, and the mean dose values of the ranked grains are plotted against N-L1 threshold in Fig. 5. It can be seen that there is a slight in crease trend above 40 ct/Gy threshold for each sample which might due to the higher internal K contents in the bright grains. However, all the mean residual values at each N-L1 thresholds are (nearly) consistent with zero. Hence, we conclude that the residual dose is not the major reason for the underestimation of the De values for the dim grains. 3.3.2. Dose recovery test The second possibility about initial sensitivity change can be tested using a dose recovery test (Galbraith et al., 1999, Wallinga et al., 2000, Guo et al., 2016; Qin et al., 2018). The same pattern of Fig. 4 should be seen if the sensitivity change in the natural SAR cycle for dimmer grains is the reason. To do this, 500, 600 and 400 grains of each of the samples XBZ-OSL-6, -8 and -12 were bleached for one month, the same as those used for residual dose measurements. After that, a beta dose of ~56 Gy was given to the bleached grains as the surrogated ‘natural dose’, which was then measured using the procedure in Table 2. A total of 311, 346 and 252 grains were accepted after applying above the rejection criteria (Tables S1 and S4). The dose recovery ratios were calculated applying the CAM on the measured doses (Galbraith et al., 1999), divided by the given dose (Fig. 6). It can be seen that all the ratio values are consistent with unity and there is no systematic increase of dose recovery ratio values with N-L1 threshold, suggesting that the initial sensitivity change is not an issue for dimmer grains, and the SAR protocol is suitable for all the accepted K-feldspar grains from these samples.
Fig. 6. Dose recovery ratios for accepted grains for different N-Tn thesdolds for samples XBZ-OSL-6, -8 and -12, respectively. The red squares represent the dose recovery ratios determined by the sum of the single-grained IRSL signals of each measured disc, plotting at a ‘fictive’ N-L1 threshold of 320 ct/Gy for clarity. The vertical bars for each point indicate the standard errors.
caused by the variable pIRIR behaviors between individual K-feldspar grains. We first investigated if there is any dependence of De on the brightness. The accepted SG grains results were ranked according to their inherent brightness. We used the first regenerative-dose of 24.1 Gy signal intensity (normalised to the calibrated beta source dose rate for each hole, marked as N-L1 with unit of cts/Gy) to indicate the brightness of the grain. A positive correlation between De values and N-L1 signal is observed. We then calculated the CAM De results by applying a series of N-L1 thresholds (Fig. 4d–f). It can be seen that the De values increase with N-L1 threshold before reaching a plateau after a certain threshold, which is ~100, ~140 and ~80 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively (Fig. 4d–f). We also noted that the OD values also decrease for larger N-L1 thresholds (Table S2). The number of the accepted grains,
3.3.3. Fading test The third possibility can be tested using a fading test (Huntley and Lamothe, 2001; Auclair et al., 2003; Guo et al., 2015, 2016). To do this, we conducted fading tests for both single aliquot and single grains. The aliquots or individual grains which had been used for De measurement were first bleached at 280 � C using IR light for 200 s to empty any residual/thermally transferred signals in the grains. Fading measure ments were then performed using a SAR procedure (Auclair et al., 2003). Repeated regenerative dose cycles were measured after different delay periods (up to 6 days) between preheat and pIRIR measurements. The 7
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Fig. 7. (a)–(c) The arithmetic means of the fading rates for grains with brightness above different N-L1 signal thresholds for samples XBZ-OSL-6, -8 and -12, respectively. (d)–(f) The CAM De values corrected using the fading rates in (a)–(c) for same grains with brightness above different N-L1 thresholds. The points at the 0 threshold are offset laterally. The dashed green lines in each panel of (a)–(c) denote ‘plateau’ of fading rate at about 0.34, 0.83 and 1.22%/decade for samples XBZOSL-6, -8 and -12, respectively. The dashed red lines in (d)–(f) are the ‘plateau’ values of 58.9, 64.5 and 65.3 Gy, calculated from the weighted mean of the De estimates with N-L1 thresholds above 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively. The red squares in each pannel represent the SA fading rates (in Fig. S3), plotting at a ‘fictive’ N-L1 threshold of 320 ct/Gy for clarity, and the corresponded fading-corrected CAM De values for samples XBZ-OSL-6, -8 and -12, respectively. The veticle bars for each point indicate the standard errors. Table 3 The dose rates, final SG and SA De values and ages for samples XBZ-OSL-6, -8 and -12. Samples
Grain size, μm
Water, %
U, ppm
Th, ppm
K,%
External dose rate,Gy/ka
Internal dose rate, Gy/kaa
Total environmental dose rate, Gy/ka
SG CAM De, Gy
SA CAM De, Gy
SG age, ka
SA age, ka
XBZOSL-6
150–250
10 � 3
1.62 � 0.07
7.09 � 0.23
1.68 � 0.06
2.444 � 0.075
0.762 � 0.173
3.206 � 0.188
58.4 � 2.5
150–250
1.82 � 0.08
7.50 � 0.24
1.70 � 0.06
2.517 � 0.076
XBZOSL-12
150–180
1.07 � 0.06
6.00 � 0.20
2.03 � 0.06
2.559 � 0.077
15.8 � 1.1b 17.7 � 1.3c 17.1 � 1.1b 18.9 � 1.3c 18.8 � 0.8b 19.5 � 0.9c
18.2 � 1.3
XBZOSL-8
50.5 � 2.0b 57.6 � 2.1c 56.2 � 1.7b 63.1 � 2.1c 60.3 � 1.7b 63.2 � 2.1c
10 � 3
10 � 3
0.817 � 0.186c 0.762 � 0.173 0.817 � 0.186c 0.641 � 0.067 0.687 � 0.070c
3.261 � 0.200
c
3.280 � 0.189 3.335 � 0.201c 3.201 � 0.102 3.246 � 0.104
c
60.7 � 1.7 59.5 � 1.9
18.5 � 1.2 18.6 � 0.8
a The internal dose rates was estimated by assuming K of 12 � 1% (Rui et al., 2019a) for the SA and SG (based on all the accepted grains) ages determination, and of 13 � 1% for the SG ages determination only using the bright grains. b The SG CAM De and age values were obtained based on all the accepted grains. The errors are propagated in quadrature. c The italics represent the internal and the total environmental dose rates, the SG CAM De and age values were obtained based on the grains whose brightness larger than 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively.
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fading results for four multi-grained aliquots per sample are shown in Fig. S5, which shows that there is a fading rate of ~1%/decade for the 170 � C signal. For the single grains of each sample, fading tests were conducted on the same 600 grains for each sample which had been used for De mea surement. The arithmetic means of the fading rates for the same accepted grains for previous De measurement above different N-L1 thresholds are displayed in Fig. 7a–c and summarized in Table S2. It can be seen that the fading rates decreased as the N-L1 thresholds increased, reaching a plateau at ~0.34%, ~0.83% and 1.22%/decade for samples XBZ-OSL-6, -8 and -12, respectively. The results from Fig. 7a–c appears to indicate that the dimmer grains are associated with larger fading rate, which could have resulted in lower De values and result in the pattern seen in Fig. 4. In order to further test this, we applied fading correction to the CAM De values of different N-L1 thresholds by using the fading rates obtained from the same N-L1 thresholds (Table S2). It can be seen that, the fading-corrected CAM De values reached a ‘plateau’ at an earlier threshold of 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12, respectively (Fig. 7d–f), compared to the fading un-corrected CAM De values in Fig. 4d–f. Furthermore, the fading corrected SG CAM De and SA CAM De are consistent with each other within 1σ for sample XBZ-OSL-12 (Fig. 3b), and the discrepancy between the SA and SG De results decreased to ~14% and ~7% for samples XBZ-OSL-6 and -8 respectively. The final ages calculated using the fading corrected De values are listed in Table 3. The SG (based on all the accepted grains) and SA ages are consistent with each other within 1σ for each sample, and consistent with the radiocarbon ages. Thus, we conclude that the higher fading rates for the dim grains, at least for samples XBZ-OSL-8 and -12, is the major reason here for the underestimation of the De values for the dim grains and the discrepancy between the SA and SG results.
routine check of the dependency of SG K-feldspar De estimates against their brightness. The weight of grains with lower K contents and higher fading fates in the single-grain De estimate is larger than in the singlealiquot De estimate. Thus, for well-bleached and homogeneous K-feld spar samples, the multi-grained single-aliquot procedure might be more appropriate for De determination because the IRSL signals are domi nated by those from the brightest grains associated with higher K con tent and lower fading rate. However, for single-grain measurements, which is usually conducted for partial bleached or post-depositionally mixed samples, it is suggested that the De values of the grains associ ated with the same fading rate should be grouped for statistical analyses, e.g., the brightest grains with the least fading are preferable for De estimation. Acknowledgment This research was supported by the Doctor Foundation of Hebei Normal University (grant No. L2018B30) and the National Natural Sci ence Foundation of China (grant No. 41702192) to Dr. Yujie Guo, and an Australian Research Council Future Fellowship to Bo Li (FT140100384). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.quageo.2019.101040. References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Arnold, L.J., Roberts, R.G., Galbraith, R.F., DeLong, S.B., 2009. A revised burial dose estimation procedure for optical dating of young and modern-age sediments. Quat. Geochronol. 4, 306–325. Auclair, M., Lamothe, M., Huot, S., 2003. Measurement of anomalous fading for feldspar IRSL using SAR. Radiat. Meas. 37, 487–492. Barr� e, M., Lamothe, M., 2010. Luminescence dating of archaeosediments: a comparison of k-feldspar and plagioclase IRSL ages. Quat. Geochronol. 5, 324–328. Bateman, M.D., Boulter, C.H., Carr, A.S., Frederick, C.D., Peter, D., Wilder, M., 2007. Detecting post-depositional sediment disturbance in sandy deposits using optical luminescence. Quat. Geochronol. 2, 57–64. Blegen, N., Tryon, C.A., Faith, J.T., Peppe, D.J., Beverly, E.J., Li, B., Jacobs, Z., 2015. Distal tephras of the eastern Lake Victoria basin, equatorial East Africa: correlations, chronology and a context for early modern humans. Quat. Sci. Rev. 122, 89–111. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523–528. Brill, D., Reimann, T., Wallinga, J., May, S.M., Engel, M., Riedesel, S., Brückner, H., 2018. Testing the accuracy of feldspar single grains to date late Holocene cyclone and tsunami deposits. Quat. Geochronol. 48, 91–103. Buylaert, J.P., Újv� ari, G., Murray, A.S., Smedley, R.K., Kook, M., 2018. On the relationship between K concentration, grain size and dose in feldspar. Radiat. Meas. 181–187. Chen, Y.W., Li, S.H., Li, B., Hao, Q.Z., Sun, J.M., 2015. Maximum age limitation in luminescence dating of Chinese loess using the multiple-aliquot MET-pIRIR signals from K-feldspar. Quat. Geochronol. 30, 207–212. David, B., Roberts, R., Tuniz, C., Jones, R., Head, J., 1997. New optical and radiocarbon dates from Ngarrabullgan Cave, a Pleistocene archaeological site in Australia: implications for the comparability of time clocks and for the human colonization of Australia. Antiquity 71, 183–188. Derevianko, A.P., Markin, S.V., Rudaya, N.A., Viola, B., Zykin, V.S., Zykina, V.S., Chabay, V.P., Kolobova, K.A., Vasiliev, S.K., Roberts, R.G., Li, B., Jacobs, Z., 2018. Interdisciplinary Studies of Chagyrskaya Cave – Middle Paleolithic Site of Altai. Institute of Archaeology and Ethnography, Siberian Branch of the Russian Academy of Sciences, Novosibirsk. Du, S.S., 2003. A preliminary study on raw material exploitation in Middle–Upper Palaeolithic sites in Nihewan Basin (in Chinese with English abstract). Acta Anthropol. Sin. 22, 121–130. Fu, X., Li, S.H., 2013. A modified multi-elevated-temperature post-IR IRSL protocol for dating Holocene sediments using k-feldspar. Quat. Geochronol. 17, 44–54. Fu, X., Li, B., Li, S.H., 2012. Testing a multi-step post-IR IRSL dating method using polymineral fine grains from Chinese loess. Quat. Geochronol. 10, 8–15. Galbraith, R.F., Roberts, R.G., 2012. Statistical aspects of equivalent dose and error calculation and display in OSL dating: an overview and some recommendations. Quat. Geochronol. 11, 1–27. 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.
4. Discussions and conclusions Previous studies have demonstrated that the SG K-feldspar De values may depend on their brightness (Reimann et al., 2012; Jacobs et al., 2019). Reimann et al. (2012) attributed this phenomenon as a result of dimmer grains being associated with lower K contents than brighter grains. Jacobs et al. (2019), however, found that this pattern seen in their samples may be related to sensitivity changes in the measurements of natural cycles (Ln and Tn), based on the observation that there is a similar pattern seen in their dose recovery test. They also demonstrated that most of their K-feldspar grains accepted for De determination have a high K content from 10 to 14%, which ruled out the internal K content being the major reason for the dependence of De on brightness. In our study, however, such a pattern was not seen in the dose recovery test (Fig. 6), indicating that the sensitivity correction for the natural signal is successful for both dimmer and brighter grains. We also demonstrated that the lower De for dimmer grains can be largely explained by their relatively higher fading rates (Fig. 7). However, we observed that there is still a systematic increase in the fading-corrected De values with N-L1 thresholds (Fig. 7d–f), implying that there may still be a slightly higher internal K content for brighter grains than for dimmer grains especially for sample XBZ-OSL-6. If we calculate the ages only using the fading corrected bright grains with brightness larger than 80, 40 and 40 ct/Gy for samples XBZ-OSL-6, -8 and -12 respectively, the internal K was assumed as 13 � 1% (Rui et al., 2019a), the obtained ages are still consistent with the SA and SG (based on all the accepted grains) ages results (Table 3 and Fig. 3c). In conclusion, it is very important to investigate the dependency of SG De values with the luminescence behaviors of individual grains, such as brightness. The underestimation of SG De values for our samples is mainly caused by the relative higher fading rates in dim grains. How ever, other effects such as variable internal K contents and variable sensitivity changes in the measurement of natural signals may also contribute to the variable SG K-feldspar results. We recommend a 9
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