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1.
Recent developments have opened up the possibilities of using potassium feldspar for dating Pleistocene sediments; a stable (less-fading) part of the infrared stimulated luminescence (IRSL) signal can be selected by largely depleting the unstable part of the IR signal, using a combination of thermal and IR stimulation: post IR-IRSL dating (pIR-IRSL).This study aims to test the validity of pIR-IRSL dating on feldspars. We obtained pIR-IRSL ages on a large suite of samples from several locations in the Netherlands area, covering a wide range of depositional environments and ages. Age control was provided by quartz optically stimulated luminescence (OSL) ages on the same samples; these ages were shown to accord with geological age constraints. Comparison with IRSL ages enabled us to evaluate the improvement that pIR-IRSL dating provides over conventional IRSL methods.The majority of feldspar ages obtained with pIR-IRSL showed good agreement with both the quartz OSL ages and the geological age constraints. Our study demonstrates that pIR-IRSL dating is more robust than conventional IRSL and should be the method of choice in feldspar luminescence dating of Pleistocene sediments.  相似文献   

2.
We report here the results of a feasibility study of luminescence dating of polymineralic volcanic ash. Of the several possible protocols that used different emission bands and different IR stimulation regimes, the post infrared-infrared stimulated luminescence (pIR-IRSL) signal (detected using a violet-blue emission window with stimulation temperature in the region 260–320 °C) provided the most stable signal. This involved, i) identification of the most suitable temperature for pIR-IRSL read out, ii) determination of alpha efficiency and, iii) estimation of anomalous (athermal) fading rate. Anomalous fading rate (g-value in %/decade) of pIR-IRSL signal at 300 °C was 0.0–1.6%/decade and it ranged from 2.4 to 5.2%/decade for IRSL at 50 °C, both preheated to 320 °C. Thus, though more stable, pIR-IRSL signals may fade in nature, and even during laboratory extended irradiation. Of the models for fading correction by Huntley and Lamothe (2001) and Kars et al. (2008), the Kars et al. (2008) model performed better as the natural luminescence intensity was closer to the onset of saturation in the luminescence dose response curve. Our measurements suggest that alpha efficiencies of the pIR-IRSL signals are higher than that of IRSL. Fading corrected pIR-IRSL single aliquot regeneration (SAR) protocol based ages on three of the five volcanic ash beds are in agreement with the expected ages of ∼74 ka, based on geochemical association of the present samples to be the Youngest Toba Tuff (YTT). Other ash samples that gave ages of <24 ka and <37 ka, were inferred to have been in their secondary context, reworked from their original depositional sites. The onset of saturation dose of the pIR-IRSL signal (D0) was ∼330 Gy and this implied a maximum measurable equivalent dose of 660 Gy. The minimum detectable dose was ∼5 Gy. These dose limits correspond to a typical age range of 1–150 ka using the pIR-IRSL signal for volcanic ashes.  相似文献   

3.
A comparative study using quartz optically stimulated luminescence (OSL) and feldspar post-infrared infrared stimulated luminescence (post-IR IRSL) was undertaken on Quaternary fluvial sediments from an unnamed tributary of the Moopetsi River in South Africa. The aim is to assess whether the post-IR IRSL signal can be used to date incompletely bleached sediments. Several post-IR IRSL signals using varying stimulation and preheat temperatures were investigated; of these the post-IR IRSL225 signal was deemed most appropriate for dating because it bleached most rapidly. The feldspar post-IR IRSL225 equivalent dose (De) values from this site are consistently larger than those from quartz OSL, probably due to differences in the bleaching characteristics of the two signals. Additionally, the post-IR IRSL225 De values within a sample showed less variation in precision than the quartz De data, possibly due to greater averaging between grains in the feldspar small aliquots. The agreement between ages based on the OSL and post-IR IRSL225 signals was better for younger samples (<20 ka) than for older ones (>50 ka); the cause of this variation is unclear.  相似文献   

4.
A protocol for optical dating of potassium-rich feldspar (K-feldspar) is proposed. It utilizes the infrared stimulated luminescence (IRSL) signal measured by progressively increasing the stimulation temperature from 50 to 250 °C in step of 50 °C, so-called multi-elevated-temperature post-IR IRSL (MET-pIRIR) measurements. Negligible anomalous fading was observed for the MET-pIRIR signals obtained at 200 and 250 °C. This was supported by equivalent dose (De) measurements using the IRSL and MET-pIRIR signals. The De values increase progressively from 50 °C to 200 °C, but similar De values were obtained for the MET-pIRIR signal at 200 and 250 °C. Measurement of modern samples and bleached samples indicates that the MET-pIRIR signals have small residual doses less than 5 Gy equivalent to about 1–2 ka. We have tested the protocol using various sedimentary samples with different ages from different regions of China. The MET-pIR IRSL ages obtained at 200 and 250 °C are consistent with independent and/or quartz OSL ages.  相似文献   

5.
The Wulanmulun site found in 2010 is an important Paleolithic site in Ordos (China), from which lots of stone and bone artifacts and mammalian fossils have been recovered. It was previously dated by radiocarbon and optically stimulated luminescence (OSL) techniques on quartz. To further confirm the reliability of the chronology constructed based on OSL ages and test the applicability of the recently developed pIRIR procedure on sediments from northern China, twenty-four sediment samples (including eolian, lacustrine and fluvio-eolian sands) from the site were determined using the multi-elevated-temperature post-IR IRSL (MET-pIRIR or pIRIR) procedure on potassium feldspar. The results show that the studied samples have two MET-pIRIR De preheat plateaus (280–320 and 340–360 °C), and the bleaching rates of the luminescence signals are associated with sample ages and stimulation temperatures. All the pIRIR ages (7–155 ka) corrected for anomalous fading and residual dose obtained after solar bleaching for 15 h are larger than the corresponding quartz OSL ages (4–66 ka) previously determined, even for the young eolian samples (<10 ka). But the corrected IRSL(50 °C) ages (6–85 ka) are broadly consistent with the quartz ages. It appears that the IRSL(50 °C) ages are more reliable, although this contradicts the previously results obtained by other people. On the other hand, we also obtained an extended age plateau between the stimulation temperatures of 50 and 290 °C in the plot of age versus stimulation temperature (A-T plot) by subtracting different residual doses obtained after different bleaching times. The reliability of the plateau ages requires further investigation. For the sediment samples from this site, quartz should be more suitable for dating than K-feldspar, and the quartz OSL ages of 50–65 ka for its cultural layer should be reliable.  相似文献   

6.
A thick Middle and Late Pleistocene loess-palaeosol sequence is exposed at the Stari Slankamen section in the Vojvodina region situated in the south-eastern part of the Pannonian basin, Serbia. The profile exposes an about 45 m thick series of loess intercalated by at least eight pedocomplexes. Ten samples were dated by luminescence methods using a modified single aliquot regenerative dose (SAR) protocol for polymineral fine grains and for quartz extracts from the upper part of the Stari Slankamen loess sequence. The infrared stimulated luminescence (IRSL) and post-IR optically stimulated luminescence (OSL) signals from all polymineral samples showed anomalous fading, suggesting that the post-IR OSL signal is still dominated by feldspar OSL. The ages ranging from 4.6 to 193 ka were obtained after fading correction. These ages indicate that the loess unit V-L1L1, the weakly developed soil complex V-L1S1 and the loess unit V-L1L2 were deposited during marine isotope stage (MIS) 2, 3, and 4, respectively, and also indicate that the loess unit V-L2 is of the penultimate glacial age.  相似文献   

7.
When electron spin resonance (ESR) is applied to sedimentary quartz, dealing with the poor bleachability of the signals is particularly challenging. In this study, we used both the single-grain optically stimulated luminescence (OSL) and the single aliquot ESR dating of quartz from deep sand deposits preserving a Stone Age archaeological sequence to combine the advantages of the two methods: good bleaching behaviour and extended age range. Using the youngest samples at each sampling site we were able to calculate the mean ESR residual age from the difference between the OSL ages and the apparent ESR ages. Focusing mainly on the single aliquot regenerative dose (SAR) protocol here, we were able to calculate the mean ESR residual age for the Ti and Al centres, including the non-bleachable signal component for the latter. For the NP site, residual ages of 209 ± 13 ka and 695 ± 23 ka were calculated for the two centres, whereas for the ZS site 268 ± 39 ka and 742 ± 118 ka were determined. These residual ages are significant and cannot be neglected. Thus, the residual age was subtracted from the apparent ESR ages. The validity of the residual subtraction method was tested through a comparison of the oldest OSL age from each site with the residual subtracted ESR age. For both NP and ZS sites, the residual subtracted Ti and Al ages were consistent with the OSL age within 2-σ uncertainty, and therefore confirm the robustness of the subtraction method. Within the NP sequence, we were able to locate the end of the Early Stone Age at 590 ± 86 ka, and this provides a maximum age for the transition to the Middle Stone Age in this part of south-central Africa.  相似文献   

8.
Constraining the ages of fluvial terraces is essential to understanding fluvial responses to climate and sea-level changes and estimating uplift/incision. Luminescence dating of sand or silt grains from fluvial terrace deposits in Japan is difficult because sand layers are often absent from gravelly deposits, quartz grains are typically dominated by medium/slow components and/or contaminated by feldspars, and short transport distances and short residence times in riverbeds result in poor bleaching of luminescence signals. Luminescence dating of cobbles may overcome these difficulties, but few studies have applied this technique to fluvial terrace deposits. Here, we examine the utility of luminescence dating applied to three granodiorite cobbles from a late Pleistocene fluvial terrace deposit of the Ara River, Japan. We investigated variations of the infrared stimulated luminescence (IRSL) and post-IR IRSL signals with depth in each cobble. The IRSL and post-IR IRSL signals generally increase with depth, indicating that the cobbles were not completely bleached before deposition. Nonetheless, the IRSL ages of the cobble surfaces (19–17 ka) are consistent with the age of a tephra layer (16–15 ka) at the base of loess deposits overlying the terrace. In contrast, IRSL ages of sand-sized feldspar grains overestimate the depositional age because of incomplete bleaching, whereas silt-sized quartz grains greatly underestimate the depositional age, likely because of the thermal instability of the medium component. Our results demonstrate that luminescence dating of cobbles could provide a better understanding of fluvial systems in which luminescence dating of sand grains is difficult.  相似文献   

9.
The applicability of both quartz and feldspar luminescence dating was tested on twenty-five samples from a marine succession now forming a coastal cliff at Oga Peninsula, Honshu Island, Japan. The quartz optically stimulated luminescence (OSL) signal shows thermal instability and linear modulated (LM)-OSL analysis revealed the dominance of a slow component. When compared with independent age control provided by two marker tephras, the quartz OSL ages grossly underestimate the depositional age. In contrast, potassium (K)-rich feldspar is a suitable dosimeter when measured using post-IR infrared stimulated luminescence (IRSL) at 225 °C (pIRIR225). Scanning electron microscope (SEM) analyses on the feldspar extracts revealed that the grains are amorphous with small crystalline inclusions; using standard internal dose rate parameters, this would result in a too large dose rate. Dose rates were calculated using the observed grain size of 40 ± 20 μm with an assumed K concentration of 12.5 ± 0.5%. The fading corrected pIRIR225 ages agree well with independent age control, and the sediments of the Katanishi Formation were deposited between 82 ± 6 and 170 ± 16 ka. This study demonstrates that pIRIR dating of feldspar is a powerful chronological tool for the dating of sediments of volcanic origin.  相似文献   

10.
This study attempted to probe a geologically more stable IR stimulated luminescence signal (IRSL) that explored so far. IRSL, probes the proximal pairs and, pIRSL measurements at elevated temperatures consume more distant pairs. We surmised that the residual IRSL after pIR-IRSL should arise from most distant pairs and hence should be more stable. A thermal stimulation after pIR-IRSL leads to redistribution of charges including distant pairs and this can be probed by further IRSL and pIRSL (post IR IRSL) measurements. This post IRSL following a thermal treatment is termed as redistributed IRSLs (RD IRSL and RD pIRSL) and contributes about 10% of total IRSL counts from a pristine sample. As expected RD, IRSLs are poor to be solar bleachable. Burial age of around 800 ka(with 30–40 %) using RD-IRSL and RD-pIRSL were comparable with the palaeomagnetic dating and were a factor of three higher than conventional pIR-IRSL, TRL, IR-RL and VSL ages.  相似文献   

11.
An up to 15 m thick alluvial fan and aeolian sandsheet complex is exposed in the upper Senne area, on the southern slope of the Teutoburger Wald Mountains (NW Germany). The origin and age of these deposits have been controversially discussed for many years, ranging from Saalian glaciofluvial to periglacial Weichselian deposits. In order to provide a high-resolution chronological framework for the deposits, we conducted luminescence dating of 12 samples from two localities (Oerlinghausen and Augustdorf pits). Both coarse-grain potassium-rich feldspar and quartz minerals were used for luminescence dating. Feldspar was measured using an elevated temperature post-IR infrared stimulated luminescence (IRSL). Quartz was measured using optically stimulated luminescence (OSL) with a conventional single aliquot regenerative dose (SAR) protocol. Feldspar results tend to overestimate quartz ages for the lower part of the sections (alluvial plain and alluvial fan facies) but are consistent with quartz ages for the upper part of the sections (aeolian facies). Quartz ages from both central and minimum age models suggest deposition during the Late Pleistocene Pleniglacial to Late Glacial.  相似文献   

12.
Raised beach sand deposits along the southeastern coast of Norway were dated by optical (OSL) and infrared stimulated luminescence (IRSL) and the quartz and K-feldspar luminescence characteristics were described. Due to the poor quartz luminescence characteristics, only a limited number of samples were suitable for OSL dating. More promising are the K-feldspar extracts, with typical K-feldspar luminescence characteristics and no sign of fading. For equivalent dose (De) determination, sand-size quartz and feldspar extracts were used, applying a single aliquot regenerative (SAR) protocol. Both, OSL and IRSL De estimates show a wide distribution, unexpected for beach deposits. The calculated OSL and IRSL age estimates were generally in good agreement and the correctness of the ages was confirmed by independent age control. Because only a limited number of the quartz samples were suitable for OSL dating, IRSL dating of the K-feldspar represents an alternative to OSL quartz dating.  相似文献   

13.
Luminescence dating has become a key tool in studies of the Quaternary. The typically stable luminescence response of quartz grains and the absence of a significant internal dose, make quartz minerals the preferred dosimeter for monitoring the burial dose in sediments. Unfortunately, the reliability of conventional OSL (optically stimulated luminescence) dating, based on blue stimulation, can be compromised when the luminescence decay is not dominated by a rapidly decaying and stable part of the luminescence signal (i.e. the fast component). On the other hand, standard methods in luminescence dating are limited to ages of a few hundred kiloyears. In this study, violet stimulated luminescence (VSL) has been used as a means to overcome both problems, applied to a series of colluvial deposits in the Atacama Desert, Chile. Quartz from this region, characterized by poor blue-OSL response, showed a reproducible and stable VSL signal capable of recovering given doses up to ∼500 Gy and a saturation dose twice as high as conventional OSL. The VSL response from these samples has been studied in detail and the estimated ages have been compared with an already established chronology for the same site, based on IRSL of potassium feldspar single grains. Results agree for the dose range of the profile studied, ∼100–250 Gy, equivalent to ages of 29–79 ka confirming the suitability of VSL for dating sedimentary quartz with unreliable blue-OSL response and to extend the age range of conventional OSL dating.  相似文献   

14.
Fluvial sediments of the middle Atbara River Valley, eastern Sudan, contain abundant vertebrate fossils and stone tools. Previous work described two sedimentary units, the Butana Bridge Synthem (BBS) and the Khashm El Girba Synthem (KGS), with three divisions each (BBS1-3 and KGS1-3, from bottom to top, respectively). 230Th/U dating on bivalve shells suggested an age of ∼126 and ∼92 ka for the basal KGS2 and basal KGS3, respectively, and mammalian biochronology in combination with magnetostratigraphy suggested an age of late Early to early Middle Pleistocene for the underlying BBS. To establish a detailed chronology of this fluvial sedimentary sequence, we collected 17 luminescence samples from both sides of the Atbara River close to the Butana Bridge. Quartz OSL dating was applied to samples from the upper part of the profile (upper KGS2 and KGS3), but the signal reached saturation within the upper ∼10 m of the sequence. To select a suitable feldspar signal to date older samples beyond the limit of the quartz OSL, a comparison of the quartz OSL, feldspar post-IR IRSL at 225 and 290 °C, and pulsed IRSL signal at 50 °C was conducted for a sample from KGS3. The result showed that only the fading corrected pulsed IRSL yielded an age consistent with the quartz OSL, and the post-IR IRSL signals (both at 225 and 290 °C) overestimated the quartz age significantly. We therefore selected the pulsed IRSL signal to date the older deposits. The luminescence ages indicate that the entire BBS - KGS sequence was deposited between 224 ± 23 ka and <17 ± 1 ka, corresponding to marine isotope stages (MIS) 7–2, significantly revising previous conclusions.  相似文献   

15.
In this study we test the potential of the elevated temperature infrared stimulated luminescence (IRSL) signals for dating Romanian loess. The recently developed post-IR IRSL protocol is applied to Romanian loess using polymineral fine grains extracted from the loess-palaeosol sequence at Mircea Vodă (SE Romania). This approach is aimed at obtaining an additional age control to examine the age discrepancy obtained from previous optically stimulated luminescence (OSL) studies using different grain-sizes of quartz (4–11 μm and 63–90 μm).Two preheat post-IR IR stimulation temperature combinations were used, 250–225 °C and 325–300 °C, respectively. The signals obtained are documented in terms of dose response curve, laboratory tests and fading. Although both post-IR IRSL signals exhibit small fading rates, dose response characteristics indicate that these rates may be laboratory artefacts. The post-IR IRSL signal stimulated at 300 °C is observed to suffer from dose dependent initial sensitivity changes as both natural and regenerated signals are observed to lie above the saturation level of the dose response curve. Uncorrected age results obtained using both post-IR IRSL signals are in general agreement with previously reported silt-sized quartz OSL ages for samples collected from the uppermost loess unit L1. For older material, the post-IR IRSL signal stimulated at 225 °C is considered to provide reliable age results, in agreement with independent age control available for this sequence.  相似文献   

16.
Optical dating was applied to two loess-paleosol sections (Lujiaowan and Shuixigou) from the northern piedmont of Tianshan Mountain, Xinjiang province, China. The two sections are over 200 km apart and have a similar depositional sequence, which consists of two paleosol layers embedded by one loess layer. Two difficulties were met in optical dating. First, because the sections are located on the slope of the mountain, it was found that some cliff debris, with coarse grains (>200 μm), were mixed with the eolian sediments by rainfall, especially in the paleosol layers. Second, the optically stimulated luminescence (OSL) signals of quartz grains from the deposits were too dim to obtain a reliable equivalent dose (De). The 63–90 μm K-feldspar grains were separated to decrease the debris portion, and they yielded bright infrared stimulated luminescence (IRSL) signals. A multiple-elevated-temperature post-IR IRSL (MET-pIRIR) procedure was applied to determine De. Comparing the optical dating ages of the two sections, the Lujiaowan (LJW) and Shuixigou (SXG) sections recorded almost the same depositional process during the Holocene. The ages of the two loess layers (2.44–3.38 ka at LJW; 2.47–4.36 ka at SXG) suggested that one drought event happened widely in this westerly dominated area. The same drought event 2.5–3.5 ka ago also happened in the Chinese Loess Plateau (CLP), where the summer monsoon dominated. However, the paleosol development period (6.6–4 ka) in the study area was distinguished from the monsoon dominated area (8–4 ka), which suggests an arid early Holocene in the westerly area.  相似文献   

17.
《Quaternary Geochronology》2008,3(1-2):99-113
The Chinese Loess Plateau (CLP) is of major interest to Quaternary geologists because it represents an important terrestrial archive of palaeoclimatic fluctuations. Previous multiple-aliquot luminescence dating studies of Chinese loess mainly used thermoluminescence (TL) and infrared stimulated luminescence (IRSL) signals of polymineral fine-grains; these are known to be subject to anomalous fading and thus will tend to yield age underestimations. In this paper we investigate whether the blue-light stimulated luminescence (BLSL) signals from 63 to 90 μm quartz grains extracted from three western Chinese loess sites (Zhongjiacai, Le Du and Tuxiangdao) can be used to establish a reliable chronology. The single-aliquot regenerative-dose (SAR) procedure is used for the equivalent dose (De) determinations and the suitability of our measurement protocol is confirmed by dose recovery tests. The influence of an IRSL signal on the quartz De measurements derived from BLSL has been investigated. From these results we conclude that an IRSL contamination, expressed as an IRSL/BLSL ratio, of up to 10% can be accepted before the values of De are significantly affected. All three sites yield stratigraphically consistent and spatially highly reproducible optical ages up to about 50–70 ka. At the Tuxiangdao site a marked hiatus in the record is identified between ∼20 and ∼30 ka; this remained undetected in previous studies and clearly highlights the importance of high-resolution optical dating in Chinese loess research. The optical ages presented in this work provide more evidence for episodic loess deposition and varying loess accumulation rates in the western part of the CLP. Our study seems to confirm the potential of optically stimulated luminescence (OSL) dating using the SAR procedure applied to the very fine sandy quartz fraction in Chinese loess back to ∼40–50 ka (∼120–150 Gy).  相似文献   

18.
Multiple-aliquot regenerative-dose violet stimulated luminescence (MAR-VSL) dating studies of the Chinese loess-palaeosol sequence in Luochuan using sand- and silt-sized quartz have previously produced inconsistent results; the VSL ages were in agreement with their independent ages up to ∼900 ka for sand-sized quartz, whereas the silt-sized VSL ages underestimated the independent chronology beyond ∼100 ka. Here we therefore evaluate the VSL dose response pattern of sand- (63–100 μm) and silt-sized (4–11 μm) quartz grains from the loess-palaeosol sequence in southern Germany in high resolution but with a limited age range up to ∼160 ka. All the samples studied benefit from good age control provided by reliable quartz optically stimulated luminescence (OSL) ages and fading corrected feldspar post-infrared infrared stimulated luminescence at 225 °C (pIRIR225) ages, which can be used for assessing the validity of the estimated VSL ages. The comparison of the MAR standardised dose response curve (DRC) using regeneration doses up to ∼1000 Gy for both grain size fractions demonstrates that they are almost similar in shape with comparable characteristic saturation doses. The comparison of the natural and laboratory generated DRCs of each grain size reveals that they broadly overlap in the low dose range for both fractions, while in the high dose range the deviation between natural and laboratory DRCs is higher for the silt-sized quartz fraction. It is also shown that the magnitude of the characteristic saturation dose is dependent upon the size of the maximum given dose, especially for the silt-sized quartz. The constructed laboratory standardised DRCs to very high doses (up to ∼6000 Gy) showed continuous signal growth at high doses, particularly in the case of silt-sized quartz grains, thereby confirming our previous observation. The sand-sized quartz has a much less pronounced linear growth component and can therefore be considered more suitable for dating samples with equivalent doses falling on the high dose region of the DRC.  相似文献   

19.
Northeastern China is located in the East Asian monsoon region; it is sensitive to both high and low latitude global climate systems. Loess deposits in the region have considerable potential as sensitive archives of past climate changes. However, research into loess deposition and climate change in this region is restricted by the lack of independent age control. In this study, coarse-grained quartz SAR OSL and K-feldspar post-IR infrared (IR) stimulated luminescence (post-IR IRSL; pIRIR290) methods have been used to date the Sanbahuo loess site in northeastern China. The quartz OSL characteristics are satisfactory. The measured pIRIR290 De's do not vary significantly with IR stimulation temperatures between 50 °C and 260 °C; a first IR stimulation temperature of 200 °C was adopted. Dose recovery tests were performed by adding different laboratory doses to both laboratory bleached (300 h SOL2) samples and natural samples; the results are satisfactory up to ∼800 Gy. Resulting quartz OSL and feldspar pIRIR290 ages are in good agreement at least back to ∼44 ka; beyond this feldspar pIRIR290 ages are older. The feldspar ages are consistent with the expected age of the S1 palaeosol (MIS 5). There appears to have been a period of fast loess deposition at ∼62 ka, perhaps indicative of winter monsoon intensification with a very cold and dry climate that lead to a serious desertification of dunefields in northeastern China.  相似文献   

20.
The Mississippi and Missouri river valleys in the midcontinental United States contain extensive loess-paleosol sequences that are used to constrain the timing of expansion and retreat of the Laurentide Ice Sheet. Previous studies have been unsuccessful in producing finite ages for sediments older than ∼150 ka due to saturation of luminescence emissions. The thermally transferred optically stimulated luminescence (TT-OSL) dating technique is tested on the fine-grained (4–11 μm) quartz fraction of these loess deposits because the TT-OSL signal has been shown to grow with high (kGy) radiation doses. The TT-OSL signal continued to increase with radiation dose above 900 Gy. The optical and thermal stabilities of this TT-OSL signal are evaluated. Equivalent dose values are highly sensitive to preheat temperatures. Recycling ratios, zero-dose response values, and dose recovery tests all yield acceptable values for samples with burial doses >∼200 Gy. The apparent TT-OSL ages for the Roxana Silt (∼52–63 ka), Teneriffe Silt (∼66 ka), and Loveland Silt (∼133–192 ka) agree at 1σ level with previously published TL and IRSL ages derived from the same samples. For the oldest unit, the Crowley's Ridge Silt, TT-OSL ages (∼167–200 ka) are younger than IRSL or TL ages by ∼20%. This is interpreted as underestimation related to TT-OSL signal contamination, which can be avoided by isolating the fast component of the TT-OSL. Preliminary fast component TT-OSL ages for the Crowley's Ridge Silt (∼197–241 ka) favor deposition during marine oxygen-isotope stage (MIS) 7 or 8, contrary to a previous inference of a MIS 12 deposition.  相似文献   

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