Exploring IRSL50 fading variability in bedrock feldspars and implications for OSL thermochronometry

Optically Stimulated Luminescence (OSL) is a well-established Quaternary dating method, which has recently been adapted to application in low-temperature thermochronometry. The Infra-Red Stimulated Luminescence (IRSL) of feldspar, which so far is the most promising target signal in thermochronometry, is unfortunately prone to anomalous fading. The fading of feldspar IRSL is at times not only challenging to measure, but also laborious to incorporate within luminescence growth models. Quantification of IRSL fading is therefore a crucial step in OSL thermochronometry, raising questions regarding (i) reproducibility and reliability of laboratory measurements of fading, as well as (ii) the applicability of existing fading models to quantitatively predict the level of IRSL field saturation in nature. Here we investigate the natural luminescence signal and anomalous fading of IRSL measured at 50 °C (IRSL₅₀) in 32 bedrock samples collected from a variety of lithologies and exhumation settings (Alaska and Norway). We report a large span of IRSL₅₀ fading rates between samples (g_2days ranging from ∼0.5 to ∼45%/decade), which further demonstrates (i) a good reproducibility between two common fading measurement protocols, and (ii) the ability of tunnelling models to predict the level of feldspar IRSL₅₀ field saturation in nature. We observe higher IRSL₅₀ fading in feldspar with increasing Ca content, although other factors cannot be dismissed at present. Finally, our dataset confirms that the applicability of feldspar IRSL₅₀ in OSL thermochronometry is limited to rapidly-exhuming settings or warm subsurface environments.     

Comparison of paired quartz OSL and feldspar post-IR IRSL dose distributions in poorly bleached fluvial sediments from South Africa

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 (D_e) 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 D_e values within a sample showed less variation in precision than the quartz D_e 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.     

MET-post-IR IRSL luminescence dating of cobbles buried in fluvial terraces in the Northern Chinese Tian Shan

Cobbles can be used as an alternative to the conventionally employed sand-sized mineral luminescence dating. In piedmont environments, cobbles are much more abundant than sand-sized material. The IRSL₅₀ signal has been widely used in previous studies due to its greater sensitivity to exposure events. However, it is well known that the low temperature IRSL signal is more prone to fade than elevated temperature post-IR IRSL signal. In this study, to test the reliability and applicability of cobble sub-surface elevated temperature IRSL luminescence dating, six light-color granite cobbles and two sand-sized samples from silty sand lens were collected from a high terrace of Manas River on the northern piedmont of Chinese Tian Shan. A modified multi-elevated-temperature post-infrared infrared stimulated luminescence (MET-post-IR IRSL) protocol was applied. The age-temperature (A-T) plateau of MET-post-IR IRSL measurement was combined with the conventional age-depth (A-D) plateau in luminescence-depth profile to evaluate the resetting and fading of MET-post-IR IRSL signals. Uncertainties of grain-sizes of K-feldspar within solidified slices were also explored by μ-XRF mapping of potassium content. The A-T plateau was identified between MET-post-IR IRSL₁₇₀ and MET-post-IR IRSL₂₂₅ signals of one cobble, which suggested completeness of bleaching before burial and negligible anomalous fading during burial. This cobble yielded MET-post-IR IRSL₂₂₅ ages of 15.8 ± 2.6 ka and 19.0 ± 3.2 ka for top and bottom side, respectively. These MET-post-IR IRSL₂₂₅ ages were consistent with independent coarse-grained quartz MAM OSL ages (15.7 ± 3.6 ka and 14.8 ± 2.6 ka) of two sand-sized samples. The MET-post-IR IRSL₂₂₅ age of 16.0 ± 1.2 ka for the bottom side of another cobble was also consistent with the independent age, even without the A-T plateau. It was inferred to be caused by anomalous fading of MET-post-IR IRSL signals other than that stimulated at 225 °C by refering to the A-D plateau observed. Our results show that MET-post-IR IRSL measurement can be employed to determine the burial ages of cobbles. The A-T plateau, complemented with the A-D plateau, could be used to assess the reliability of burial ages of cobble luminescence dating from the view of bleaching and fading.     

Luminescence dating of glaciolacustrine silt in overdeepened basin fills beyond the last interglacial

OSL and IRSL dating are applied to samples from a 152 m-long drill core to constrain the timing of three glaciolacustrine depositional periods within the infill of an overdeepened bedrock trough in the Lower Glatt valley, N Switzerland. The characterisation of the dose-response suggests that the polymineral IRSL₅₀ and pIRIR_180/225 signals are close to saturation, while quartz OSL ages are within the range of reliable dating. The demarcation of the upper quartz OSL dating limit, however, remains challenging. Dose-recovery tests performed with long storage periods were used to investigate the reliability of the high region of the dose-response curve. They suggest an upper limit for reliable dating of ∼400 Gy for these samples, which was considerably lower than the commonly used 2D₀ criterion. Lifetimes were calculated for the quartz OSL and the thermal stability of the signal is not considered as problematic for the determined ages. Allowing for a contribution from inherited dose due to partial bleaching, places the infill of the overdeepened valley within the penultimate glacial cycle (MIS6).     

Luminescence dating of Middle Pleistocene (MIS 7) marine shoreline deposits along the eastern coast of Tunisia: A comparison of K-feldspar and Na-feldspar IRSL ages

In the present study, we applied the IRSL_50°C dating method to both K- and Na-feldspar coarse grains from interglacial coastal deposits in north-eastern Tunisia. We used the yellow IRSL_50°C signal of Na-feldspars and the blue IRSL_50°C signal of K-feldspars. The key-sites for this study are at El Hajeb (Sahel area) and Dar Oufa (Cap Bon Peninsula). These deposits belong to the “Douira Unit” which has previously been assigned to marine isotope stage (MIS) 7 on the basis of amino acid ratios and chronostratigraphic evidence.In order to assess the reliability of the IRSL_50°C ages of the “Douira Unit”, we extended the IRSL dating technique to K- and Na-feldspars from two MIS 5 samples, with independent age control, collected within shallow-marine sands in the Sahel area and on the Jerba island (southern Tunisia).Two protocols of age correction for the observed fading in K- and Na-feldspars have been applied: (1) the Huntley and Lamothe (2001) fading correction and (2) the dose rate correction developed by Lamothe et al. (2003). The fading corrected IRSL ages of the “Douira Unit” and both MIS 5 control samples, measured on K- and Na-feldspars, are in good agreement with their expected ages.     

Small aliquot and single grain IRSL and post-IR IRSL dating of fluvial and alluvial sediments from the Pativilca valley,Peru

Infrared stimulated luminescence (IRSL) and post-IR IRSL are applied to small aliquots and single grains to determine the equivalent dose (D_e) of eleven alluvial and fluvial sediment samples collected in the Pativilca valley, Central Peru at ca. 10°S latitude. Small aliquot D_e distributions are rather symmetric and display over-dispersion values between 15 and 46%. Small aliquot g-values range between 4 and 8% per decade for the IRSL and 1 and 2% per decade for the post-IR IRSL signal. The single grain D_e distributions are highly over-dispersed with some of them skewed to higher doses, implying partial bleaching; this is especially true for the post-IR IRSL. Measurements of a modern analog reveal that residuals due to partial bleaching are present in both the IRSL as well as the post-IR IRSL signal. The g-values of individual grains exhibit a wide range with high individual uncertainties and might contribute significantly to the spread of the single grain D_e values, at least for the IRSL data. Electron Microprobe Analysis performed on single grains reveal that a varying K-content can be excluded as the origin of over-dispersion. Final ages for the different approaches are calculated using the Central Age Model and the Minimum Age Model (MAM). The samples are grouped into well-beached, potentially well-bleached and partially bleached according to the evaluation of the single grain distributions and the agreement of age estimates between methods. The application of the MAM to the single grain data resulted in consistent age estimates for both the fading corrected IRSL and the post-IR IRSL ages, and suggests that both approaches are suitable for dating these samples.     

Luminescence dating of marine sediments from the Sea of Japan using quartz OSL and polymineral pIRIR signals of fine grains

Luminescence dating has long been used for chronological constraints on marine sediments due to the ubiquitous dating materials (quartz and feldspar grains) and its applicability over a relatively long time range. However, one of the main difficulties in luminescence dating on marine sediments is partial bleaching, which causes age overestimations. Especially, partial bleaching is typically difficult to be detected in the fine grain fraction (FG) of marine sediments. The recently developed feldspar post-IR IRSL (pIRIR) protocol can detect non-fading signals and thus avoid feldspar signal instability. In the current study, fine grains were extracted from a gravity core in the northern Sea of Japan, and the aim is to test the feasibility of using different luminescence signals with various bleaching rates to explore the bleaching conditions of fine grain fraction in marine sediments. The results suggest that the quartz OSL signal and polymineral pIRIR signals at stimulation temperatures of 150 °C and 225 °C (pIRIR₁₅₀ and pIRIR₂₂₅) of FG were well bleached prior to deposition. The OSL ages were used to establish a chronology for this sedimentary core and the resulting age-depth relationship is self-consistent and comparable with radiocarbon dates. We conclude that different luminescence signals with various bleaching rates can be used to test the bleaching conditions of fine grain fraction in marine sediments; and the luminescence dating can be applied to marine sediments with great potential.     

Infrared stimulated luminescence and radiofluorescence dating of aeolian sediments from Hungary

Anomalous fading of the infrared stimulated luminescence (IRSL) signal from the polymineral fine-grain and K-feldspar fractions of aeolian sediments from Hungary has been studied. The samples in this study have previously been dated using the multiple aliquot additive dose (MAAD) protocol to measure the IRSL signal. The IRSL measurements using MAAD were conducted ～4 weeks after the irradiation, making it difficult to assess to what extent these age estimates were affected by anomalous fading. In this study, equivalent doses were obtained using the single aliquot regenerative dose (SAR) protocol. The fading rate for each sample was calculated using the different IRSL components and different parts of the decay curve. For each sample, the middle part of the decay curve always showed a lower fading rate than the initial part of the decay curve. The difference between the fading rates for different parts of the decay curve was greater for the K-feldspars than for the polymineral fine grains. Fading corrected ages were calculated by integrating both the initial and the middle part of the decay curve. These ages were compared with optically stimulated luminescence (OSL) ages from quartz, infrared radiofluorescence (IR-RF) ages obtained from K-feldspars and also with independent ages, provided by radiocarbon dating of shells and charcoal, and uranium-series dating of travertine.     

Validating post IR-IRSL dating on K-feldspars through comparison with quartz OSL ages

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.     

OSL and IRSL dating of raised beach sand deposits along the southeastern coast of Norway

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 (D_e) determination, sand-size quartz and feldspar extracts were used, applying a single aliquot regenerative (SAR) protocol. Both, OSL and IRSL D_e 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.     

OSL quartz age underestimation due to unstable signal components

When measuring samples from southern Peru, we discovered an age offset between feldspar infrared-stimulated luminescence (IRSL) and quartz optically stimulated luminescence (OSL) ages. This age offset was not only found in these particular samples but was also observed in samples from a range of geographical localities. Extensive analysis revealed that it is actually the quartz age that underestimates the true depositional age, and this is due to a very weak fast component combined with other thermally unstable OSL signal components. We discuss properties of these poorly behaving samples and how to detect such samples prior to the OSL single-aliquot regenerative-dose (SAR) measurement. Finally, existing approaches to improve equivalent dose (D_e) determination are discussed for samples from southern Peru.     

Timing of Holocene lake highstands around Dawa Co in inner Tibetan Plateau: Comparison of quartz and feldspar luminescence dating with radiocarbon age

Lakes over the inner Tibetan Plateau (TP) are very sensitive to the regional environmental transformations and climate changes. Well-preserved lake sediments around these lakes provide critical geomorphological and sedimentary evidence that can be used to infer the past hydroclimate changes. In this study, a lacustrine section from a sandy shoreline (∼74 m above the modern lake) situated to the northwest of modern Dawa Co in the inner TP was investigated using both luminescence and radiocarbon dating methods. Our results demonstrated: (1) the quartz optically simulated luminescence (OSL) dating yielded much younger ages (∼4 ka) than that of the post-infrared IRSL (pIRIR) dating of the K-feldspar fraction; (2) fading test showed g-values ranging between 1.34 and 4.46%/decade for quartz OSL signals, which is considered to be responsible for the underestimation of the corresponding ages; (3) the AMS ¹⁴C age of the charcoal sample from the section is in line with the K-feldspar pIRIR₂₂₅ ages, confirming the reliability of the pIRIR₂₂₅ dates and the underestimation of the quartz OSL ages. The anomalous fading of quartz OSL signals and the consequent age underestimation have been reported in several other lakes on the TP, we presented here for the first time firm evidence of the phenomenon with the help of a robust independent control of AMS ¹⁴C age of the charcoal. Based on the pIRIR₂₂₅ and AMS ¹⁴C ages, we conclude that Dawa Co underwent a prominent highstand during the early Holocene (∼9–7 ka), which was probably controlled by the large amounts of glacial meltwater input and the increased monsoonal precipitation.     

Luminescence dating of a megaflood event on a terrace of the Jinsha River,China

The numerical dating of megaflood sediments is a worldwide challenge, a fact that has impeded a full understanding of Late Quaternary dam-outburst flood processes that occurred along the river courses of the Tibetan Plateau. Optically stimulated luminescence (OSL) dating has been widely used on such sediments. Due to their short transportation distances prior to deposition, the OSL signals of megaflood sediments are often partially bleached, resulting in age overestimations. Here, we report on a comparison of OSL ages obtained using both quartz (4–11 μm FG; 90–125, 180–250 μm CG) and K-feldspar (180–250 μm CG) extracted from sediments taken from the Binghong-Bingnong Neolithic-Bronze Age site on the second Jinshan River terrace (T2), in Yunnan Province, southwestern China. Contrary to previous experience suggesting that CG fractions are usually better bleached than FG fractions prior to deposition, our results showed that the OSL ages for the FG quartz fraction were generally younger than those for the CG fraction. This would suggest that the two fractions may have come from different sources, and may have been subject to different geomorphological processes prior to deposition. FG quartz fractions may be suitable to define the maximum age of sediments located in alpine gorge regions. CG quartz fractions can be used by applying the minimum age model (MAM) to select relatively well-bleached grains yielded ages close to the ‘true’ burial ages of the sediments. The results showed that the post-IR IRSL₂₂₅ ages of single grain K-feldspar were overestimated by > 3 ka, suggesting K-feldspar may not be suitable dating material for megaflood sediments <30 ka. The OSL dates suggest that the T2 terrace was formed ∼8.4 ka, that aeolian sediments were deposited during ∼2.6–1.5 ka, and that the megaflood event occurred after ∼1.5 ka.     

Testing the application of quartz and feldspar luminescence dating to MIS 5 Japanese marine deposits

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.     

Luminescence chronology of fossiliferous fluvial sediments along the middle Atbara River,Sudan

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.     

Testing the potential of elevated temperature post-IR IRSL signals for dating Romanian loess

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.     

Isolating quartz-dominated OSL signal of rock slice by using pulsed stimulation: Implications for dating burial age of cobbles

Although the OSL signal sourced from quartz is expected to be more stable and bleached more rapidly than the IRSL signal sourced from feldspar in a general sense, the former is much less investigated than the latter for rock surface related luminescence dating. It is mainly due to the difficulty in isolating quartz-dominated OSL signals from rock slices, and the low sensitivity and non-fast component dominated OSL signals for quartz in most of rocks. In merit of the sub-conduction band transition of trapped electrons of feldspar under both green light stimulation and IR stimulation at elevated temperature (Jain and Ankjӕrgaard, 2011), it is expected that the contribution of feldspars to the green light stimulated luminescence could be substantially reduced by a prior IR stimulation at elevated temperature. Meanwhile, more fast-component dominated quartz OSL signal (if there is any) could be obtained by green light stimulation (Bailey et al., 2011). Therefore, in this study, we investigated the feasibility of using the post-IR pulsed green stimulation, which is performed at 25 °C (PGLSL₂₅) following two IR stimulations (IR₅₀IR₂₂₅), to isolate the quartz-dominated OSL signals from rock slices of granite cobbles for burial dating. The decay characteristics of stimulation curve, characteristic saturation dose and thermal stability of the pIR₅₀IR₂₂₅ PGLSL₂₅ signal suggest that this signal is quartz-dominated, but still not fast-component dominated. We tentatively validated the equivalent dose (D_e) measurement protocol by dose recovery experiment. The luminescence-depth profiles show that the bleaching depths of pIR₅₀IR₂₂₅ PGLSL₂₅ signals are slightly smaller than or close to that of the IR₅₀ signals, while they are much shallower for the pIR₅₀IR₂₂₅ signals. The pIR₅₀IR₂₂₅ PGLSL₂₅ procedure enables multiple D_e values determined from luminescence signals, with different bleachabilities and stabilities, from both feldspars and quartz in one measurement, which is of potential for buried age dating of cobbles.     

Rock and sediment luminescence dating of an ancient circular stone-walled enclosure at Sønnebøe,northern Scania,Sweden

Dating agricultural artefacts such as field walls and clearance cairns using radiocarbon can be challenging, especially since the association with datable material may be poor. Rock surface burial dating using luminescence offers an alternative. Here we report on the luminescence dating of a medieval circular stone-walled enclosure at Sønnebøe, northern Scania, Sweden, using both buried rocks and sediments. Luminescence burial profiles from IRSL signals measured at 50 °C (IR₅₀) indicated significant prior light exposure in 7 of the 8 samples tested (5 granite, 2 felsic gneiss), in some cases multiple exposure burial cycles were indicated. These rock surfaces had apparently been exposed for sufficient time to allow accurate IRSL ages for the most recent burial event. In contrasts, no useful post-IR IRSL profiles were obtained indicating that this signal was not sufficiently reset to allow accurate determination of the burial dose on any of these rocks. IR₅₀ fading corrections (typically ∼50%) were derived by comparing field saturation with that induced in the laboratory. Quartz extracted from sediments surrounding the rocks gave an average measured to given dose ratio of 1.03 ± 0.01 (n = 90), and these sediment samples were then dated using multigrain aliquots; the corresponding feldspar dose recovery ratio obtained using rock samples was 0.98 ± 0.05 (n = 28). A total of 15 ages were derived; 8 quartz OSL ages from the disturbed coarse grained sediments surrounding the structure, and 7 fading corrected IR₅₀ ages from the surfaces of rocks (2–3 mm chips, ∼1 mm thick) used in the construction of the structure itself. The exposure events preserved by the ring enclosure stones unequivocally show wall building taking place at the site between 800 and 300 years ago.     

A luminescence dating study of loess deposits from the Yili River basin in western China

The loess deposits surrounding the high mountainous regions of Central Asia play an important role in understanding environmental changes in Eurasia on orbital and sub-orbital timescales. However, problems with dating Central Asian loess have limited the interpretation of climatic and environmental data, especially on sub-orbital timescales. We selected a controversial loess section, Zeketai (ZKT, with a thickness of 23 m), in the Yili basin in Xinjiang Province in western China, to establish a detailed and systematic Optically Stimulated Luminescence (OSL) chronology. Quartz grains of 38–63 μm were isolated from 15 samples and the single-aliquot regenerative-dose (SAR) protocol was employed for D_e determination. OSL ages are in stratigraphic order and range from 13.8 ± 1 to 72 ± 6 ka, suggesting continuous loess accumulation during the last glaciation. We compared these dating results with that of the previously published fine-grain sized quartz (4–11 μm) using simplified multiple aliquot regenerative-dose (SMAR) protocol, and with the previous published radiocarbon dating (¹⁴C) ages on snail shells. With the exception of three young samples from the upper 6 m of the section, the SMAR dating results are basically consistent with the results using the SAR protocol. Both the SMAR and SAR OSL ages are consistently older than the ¹⁴C ages, and the radiocarbon date results should be used with caution since they appear to have been underestimated.     

Violet stimulated luminescence as an alternative for dating complex colluvial sediments in the Atacama Desert

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.