Multiple approaches to date Japanese marker tephras using optical and ESR methods

The present study aimed to test reliability of luminescence and electron spin resonance (ESR) methods to date tephra. We investigated on three Japanese marker tephras, Ikeda-ko (6.4 ka), Aira-Tn (30 ka) and Aira-Iwato (45–50 ka). A systematic studies were performed using different minerals (quartz and feldspar), different grain fractions (75–250 and 250–500 μm), different luminescence and ESR signals, like optically stimulated luminescence (OSL) of quartz, infrared stimulated luminescence (IRSL) of feldspar, including recently developed least faded post infrared IRSL (pIR-IRSL), and ESR signals from paramagnetic centers Al and Ti–Li of quartz. Ages obtained using pIR-IRSL signal of plagioclase with preheat of 320 °C, 60 s and stimulation at 300 °C are consistent with the reference ages. High dose detection range (up to ∼600 Gy) and accurate age estimation enable pIR-IRSL of feldspar a promising methodology to date quaternary tephra. ESR ages from quartz are grossly correlated with the reference ages but large deviation and large associated errors are observed, possibly due to either low signal to noise ratio or heterogenous dose response of different aliquot in multiple aliquot additive dose (MAAD) approach.     

Evaluating a SAR TT-OSL protocol for dating fine-grained quartz within Late Pleistocene loess deposits in the Missouri and Mississippi river valleys,United States

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.     

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.     

Thermally re-distributed IRSL (RD-IRSL): A new possibility of dating sediments near B/M boundary

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.     

Optical dating of Chinese loess using sand-sized quartz: Establishing a time frame for Late Pleistocene climate changes in the western part of the Chinese Loess Plateau

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 (D_e) determinations and the suitability of our measurement protocol is confirmed by dose recovery tests. The influence of an IRSL signal on the quartz D_e 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 D_e 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).     

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.     

A comparative study of sand- and silt-sized quartz fractions for MAR-VSL dating using loess-palaeosol deposits in southern Germany

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.     

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.     

Luminescence dating of K-feldspar from sediments: A protocol without anomalous fading correction

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 (D_e) measurements using the IRSL and MET-pIRIR signals. The D_e values increase progressively from 50 °C to 200 °C, but similar D_e 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.     

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.     

Luminescence dating of scoria fall and lahar deposits from Somma–Vesuvius,Italy

Luminescence dating has been applied to scoria and lahar deposits from Somma–Vesuvius, Italy. Samples include scoria from the AD472 and 512 (or 536) eruptions and lahar deposits. In order to find a stable luminescence signal which is less affected by anomalous fading, infrared stimulated luminescence (IRSL) signals at elevated temperatures after bleaching with IR at 50 °C (termed post-IR IRSL; pIRIR) were tested at different preheat and elevated stimulation temperatures. The fading rates of both IRSL and pIRIR signals reduced dramatically with increasing preheat and pIRIR stimulation temperatures. A pIRIR signal measured at 290 °C after a preheat at 320 °C (60 s) and an IR stimulation at 50 °C (100 s) was selected to calculate the equivalent dose (D_e). The gamma spectrometry results indicate that the U-series nuclides are not in equilibrium and there is a large ²²⁶Ra excess. The dose rates and ages were calculated by assuming a ²²⁶Ra excess (over its parent ²³⁰Th) at deposition, and that this unsupported excess then decayed to the present level. The resulting luminescence ages of the two scoria samples agreed with the expected ages, and the ages of the lahar deposits indicate that they are associated with the AD1631 eruption.     

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.     

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.     

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.     

Feldspar multi-elevated-temperature post-IR IRSL dating of the Wulanmulun Paleolithic site and its implication

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 D_e 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.     

Luminescence signals from modern sediments in a glaciated bay,NW Svalbard

Landforming processes are highly active in the Arctic, and luminescence dating can be used to establish a chronological framework for these processes. For example, luminescence ages of raised littoral and marine deposits provide the age control for many reconstructions of Pleistocene events in the Arctic. Due to the nature of the depositional environment (e.g. short transport distance, turbid water, long polar night) these types of sediment may not be completely zeroed at the time of deposition. To test the significance of incomplete bleaching in this type of environment, surface sediments were sampled along a transect from the margin of a glacier out into a nearby bay on NW Svalbard. The water in the bay is very turbid (secchi depth 0.1 m), but there is significant reworking by waves along the shores.Quartz optically stimulated luminescence (OSL) and feldspar infrared stimulated luminescence (IRSL) were measured using sand-sized grains. For quartz OSL and feldspar IRSL (50 °C) the ice-proximal sample showed relatively high doses (∼12 Gy) while nearby beach sand and shallow-marine deposits, as well as ice-distal sandur sediments, had much lower doses: most OSL doses were consistent with zero, while IRSL (50 °C) ranged from 0.5 to 6.5 Gy. Post-IR IRSL (290 °C) doses were overall much higher (∼20–55 Gy), which partly is due to a significant (∼12 Gy) unbleachable residual, and partly due to slower bleaching rates than for the IRSL (50 °C) signal.In this Arctic environment it appears that bleaching is limited in the first ∼100 m of meltwater transport from the glacier margin, but for material transported at least 3 km bleaching of OSL and IRSL (50 °C) signals is more or less complete. Given the very limited light penetration through the seawater in the bay, any bleaching must have occurred during fluvial/subaerial transport to the bay or by wave-reworking on the beach. Apart from the ice-proximal glacifluvial sediments, residual apparent doses recorded by quartz OSL and feldspar IRSL (50 °C) are negligible for the luminescence dating of Pleistocene-aged deposits of ice-distal, littoral and shallow-marine origin.     

Maximum age limitation in luminescence dating of Chinese loess using the multiple-aliquot MET-pIRIR signals from K-feldspar

Numerical dating of loess is important for Quaternary studies. Recent progress in post-infrared infrared-stimulated luminescence (pIRIR) signals from potassium-rich feldspar has allowed successful dating of Chinese loess beyond the conventional dating limit based on quartz optically stimulated luminescence (OSL) signals. In this study we tested the multiple-aliquot regenerative-dose (MAR) pre-dose multiple-elevated-temperature post-IR IRSL (pMET-pIRIR) procedure on samples from the palaeosol S5 (∼480 ka) and S8 (∼780 ka) layers from the Luochuan and Jingbian sections, respectively. The results show that (1) compared to sensitivity-corrected signal (L_x/T_x), a higher saturation dose is observed for the sensitivity-uncorrected MET-pIRIR signals (L_x), suggesting that MAR is advantageous for dating old samples; (2) negligible fading component can be achieved using the pMET-pIRIR procedure; (3) for the sample from the top of palaeosol S5, D_e values (1360 + 226/-167 Gy) broadly consistent with expected D_e (1550 ± 72 Gy) can be obtained using the sensitivity-uncorrected 300 °C MET-pIRIR signal. Our study suggests that a D_e value of about 1800 Gy may be the maximum dating limit of Chinese loess using the MAR pMET-pIRIR procedure.     

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.     

K-feldspar IRSL dating of a Pleistocene river terrace staircase sequence of the Lower Tejo River (Portugal,western Iberia)

We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected D_e values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation, giving a minimum corrected D_e of about 1000 Gy, and thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (～Marine Isotope Stage [MIS]11); T4 is ca. 340–150 ka (～MIS9-6); T5 is 136–75 ka (～MIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that blankets T6 and some of the older terraces is 30–≥12 ka. Collectively, the luminescence ages seem to indicate that regional river downcutting events may be coincident with periods of low sea level (associated, respectively, with the MIS10, MIS6, MIS4 and MIS2).     

Luminescence dating reveals a rapid response to climate change of fluvial terrace formation along the Ani River,northeastern Japan,during the last glacial period

Fluvial terraces along the middle reaches of many Japanese rivers were formed during the last glacial period as a result of changes in sediment discharge related to cooler temperatures and/or reduced water discharge because of lower precipitation. The influence of climate change on these fluvial terraces is not yet fully understood because most previous studies lacked detailed reconstructions of the chronology of terrace development. This study provides a detailed luminescence chronology of fluvial terrace deposits along the Ani River, northeastern Honshu, Japan, and compares that chronology to paleoclimatic records. Eight samples for luminescence dating were obtained from an outcrop of terrace deposits (∼10 m thick) in the Ani River valley. The fading-corrected infrared stimulated luminescence (IRSL) ages are consistent with the fading-corrected post-IR IRSL ages for some samples, which suggests that fading corrections were effective despite the higher fading rates of the IRSL signal. However, for the other samples, the post-IR IRSL ages are significantly older than the fading-corrected IRSL ages due to incomplete bleaching. The pulsed IRSL signals are close to field saturation for older samples, which might have resulted in a greater variation of the ages. Fading-corrected IRSL ages demonstrate periods of rapid aggradation during 105–90 ka and 75–60 ka. Comparison of terrace development with paleoclimatic records indicates that the two periods of fluvial deposition correspond to decreases in precipitation caused by weakened East Asian summer monsoon precipitation and possibly decreases in temperature. The results of this study show that the Ani River responded rapidly to climate change on a time scale of a few tens of thousands of years during the last glacial period.