Feldspar Infrared Stimulated Luminescence Research Articles

Are quick equivalent doses realistic? Testing range-finder luminescence dating for water-lain and postglacial flooding sediments in NE Poland

Raw sediment samples which are without prior lengthy laboratory preparation can be used to obtain a rough but rapid estimate of the luminescence equivalent dose, and thus of the relative age of the sediment. In this study, we tested this range-finder method on clastic sediments in NE Poland for the first time, with special focus on Pleistocene meltwater sediments generated by highly energetic glacial lake outburst floods, and the post-flooding sediments. Two datasets with known doses from standard measurements were compared to range-finder doses determined from quartz and feldspar in untreated sediments. We found statistically significant correlation between equivalent doses of (1) standard quartz optically stimulated luminescence (OSL) and range-finder feldspar infrared stimulated luminescence (IRSL) at 50 ºC, and (2) standard quartz OSL and range-finder quartz OSL in low-dose samples (<80 Gy). However, these correlations should only be considered as approximate whereby preparing more than three range-finder aliquots has the potential of yielding more accurate results. Correlation between the range-finder quartz OSL and range-finder feldspar IRSL is also significant. The rangefinder measurements can be used for approximate dose determination to preliminary assess the sediment age or as a selection tool to avoid incompletely bleached samples. The sedimentary environment and especially sediment reworking and transportation seem to influence these correlations. We consider the sediments studied here to have undergone between one and four reworking stages, and samples with repeated reworking usually represent well-bleached material for luminescence dating.

Improved separation of quartz optically stimulated luminescence from a mixed sample using pulsed green light

In the luminescence dating of samples containing both quartz and feldspar (e.g. fine silt, or unseparated rock), the infra-red stimulated luminescence (IRSL) from feldspar is most often used, even though the luminescence characteristics of the OSL from quartz are better understood, and quartz OSL is known to be more stable than feldspar IRSL and more readily reset when exposed to daylight. Unfortunately, in almost all unseparated samples giving both quartz and feldspar luminescence signals, the feldspar OSL (stimulated by blue or green wavelengths) intensity is similar to, or greater than, that from quartz. Separation of quartz and feldspar signals has been achieved before using prior IR and/or pulsed visible light stimulation. This study investigates whether using pulsed green stimulation after IR stimulation gives better discrimination in favour of quartz optically stimulated luminescence (OSL) than using post-IR pulsed blue stimulation, by using the luminescence response of single mineral rocks (quartzite, potassium feldspar) at various temperatures. The results presented here support the view that pulsed blue and pulsed green stimulation of quartz (quartzite or calibration quartz grains) both excite charge to the conduction band, resulting in only a minor dependence on the temperature of the blue or green stimulation. We also confirm that IR depletes the quartz OSL signal only if undertaken at elevated temperature (>125 °C). In contrast, all 3 light sources can result in luminescence from potassium-rich feldspar, but the post-infrared pulsed blue light stimulated luminescence (post-IR PBSL) signal is significantly less dependent on the prior IR stimulation temperature than the post-infrared pulsed green light stimulated luminescence (post-IR PGSL) signal. This is presumably because green stimulation excites mainly into the band-tails. As a consequence, it is shown that the contribution of feldspar OSL to pulsed-green light stimulated luminescence (PGSL) is reduced more by a prior IR stimulation at elevated temperature, than the corresponding pulsed-blue light stimulated luminescence (PBSL). These results are consistent with the band models used to describe the origins of quartz and feldspar luminescence proposed by previous studies. We conclude that the post-IR PGSL provide better discrimination in favour of quartz OSL than does the corresponding post-IR PBSL, and that this should be taken advantage of in future studies on samples with mixed sources of luminescence.

The bleaching limits of IRSL signals at various stimulation temperatures and their potential inference of the pre-burial light exposure duration

Infrared Stimulated Luminescence (IRSL) techniques are being increasingly used for dating sedimentary feldspars in the middle to late Quaternary. By employing several subsequent stimulations at increasing temperatures, a series of post-IR IRSL (pIRIR) signals with different characteristics (stability and bleachability) can be obtained for an individual sample. It has been experimentally demonstrated that higher-temperature pIRIR signals are more stable, but they tend to exhibit larger residual doses up to few tens of Gy, potentially causing severe age overestimation in young samples. In this study we conducted comprehensive bleaching experiments of IRSL and pIRIR signals using a loess sample from China, and demonstrated that non-bleachable components in the IR (and possibly pIRIR) signals do exist. The level of such non-bleachable signal shows clearly positive correlation with preheat/stimulation temperature, which further supports the notion that lower temperature pIRIR are advantageous to date young samples and sediments especially from difficult-to-bleach environments. These results display a potential in constrain the pre-burial light exposure history of sediment utilizing multiple feldspar post-IR IRSL (pIRIR) signals. For the studied loess sample, we infer that prior to its last burial, the sample has received an equivalent of >264 h exposure to the SOL2 simulator (more than 2,000 h of natural daylight).

Coarse-grained K-rich feldspar and fine-grained polymineral IRSL dating of loess-palaeosol from the Chinese Loess Plateau: A comparison

In this study, the characteristics of infrared stimulated luminescence (IRSL) signals from coarse-grained (63–100 μm, CG) K-feldspar and fine-grained (4–11 μm, FG) polymineral fractions was compared using the Chinese loess/palaeosol samples from the Luochuan profile. The pIRIR 50, 225 and pIRIR 200, 290 protocols (pIRIR: post-IR IRSL. The subscript shows stimulation temperatures for the IR and pIRIR signals) were used. Natural signal intensities of eleven samples were used to construct the natural dose-response curve (DRC) for each signal. The laboratory and simulated-natural DRCs for each signal from the CG K-feldspar and FG polymineral fractions were constructed and were compared with the natural DRC. The natural DRCs were broadly consistent for the two pIRIR and the IR 200 signals for both fractions. The characteristic saturation dose ( D 0 ) of the natural DRCs for the pIRIR signals from the CG K-feldspar tended to be smaller than those from the FG polymineral fraction. Conversely, the D 0 s of the laboratory DRCs for all the signals from the CG K-feldspar were larger than those from the FG polymineral fraction. The ratio of simulated-natural DRC over natural DRC (N sim /N) was able to successfully determine the fading over- or under-correction tendencies for different signals and dose regions.

First reliable chronology for the Early Khvalynian Caspian Sea transgression in the Lower Volga River valley

During the late Quaternary, dramatic changes in relative sea level (~170 m) are known to have occurred in the Caspian Sea. However, all previous attempts at resolving the uncertainty associated with the timing of these transgressive/regressive events, primarily using radiocarbon, have produced inconclusive or controversial results. Here we present the first reliable chronology for the largest known transgression (Early Khvalynian). This was derived using optically stimulated luminescence (OSL) analysis of sand‐sized quartz, with support from infrared stimulated luminescence (IRSL) from K‐rich feldspar grains, all extracted from 16 sediment samples collected along the Lower Volga River. These samples were taken from loessic sediments, marine clays (known as Chocolate Clays) and the overlying modern soils exposed at three sections (Srednyaya Akhtuba, Raygorod, Leninsk) ~200 km upstream of the present‐day estuary. The differential bleaching rates of the quartz OSL and feldspar IRSL signals were used to evaluate the degree of resetting of quartz (and feldspar) signals; it was possible to conclude that all signals, and particularly quartz OSL, were sufficiently reset at deposition to allow accurate age estimation. Our results show unambiguously that the Early Khvalynian marine Chocolate Clays present at all three sections were deposited post‐LGM, between ~17 and ~13.1 ka ago. These age estimates are further constrained by those from the overlying Kastanozem soils (9.6–0.7 ka) and underlying loess‐soil series (32–19 ka), confirming a young (17–13 ka) age of the transgressive stage of the Early Khvalynian. Relative sea level during this period must have been well above the sampling altitudes of 11.7 m a.s.l. (Srednyaya Akhtuba), 11.3 m a.s.l. (Raygorod) and 4.7 m a.s.l. (Leninsk) to explain the absence of significant alluvial sand and to allow the deposition of the fine Chocolate Clays marker horizon.

Photon energy (8–250 keV) response of optically stimulated luminescence: Implications for luminescence geochronology

Understanding the energy response of dosimeters and scintillators for measurements of ionising radiation is an important area in the field of solid state dosimetry. Such an understanding is particularly critical for source calibration and for quantifying effective dose (rates) in wide-energy mixed-radiation fields. While there have been several investigations on characterising the ionising- energy response of artificial dosimeters (e.g. LiF, Al2O3:C, etc.), there exist no studies on natural dosimeters such as quartz and feldspar which are extensively used in luminescence geochronology and retrospective dosimetry. Currently, luminescence geochronology assumes that all beta and gamma irradiations in nature (from the decay of K, the U and Th radioactive chains) are equivalent in terms of luminescence production per unit dose to each other, and to the laboratory 137Cs, 60Co (gamma) and 90Sr/90Y (beta) irradiations used for calibration and/or equivalent dose determination. In this study, we experimentally determine the quartz Optically Stimulated Luminescence (OSL) and feldspar Infra-Red Stimulated Luminescence (IRSL: IRSL at 50 °C and post-IR IRSL at 290 °C) response to irradiations with photons of in the energy range 8–250 keV. We show that microdosimetric effects lead to a steady, significant decrease in quartz OSL production efficiency for < 80 keV photons. Similar to quartz, the local saturation of the electron traps also leads to a systematic decrease in luminescence efficiency for < 120 keV photons in feldspar; however, this effect competes with an increase in recombination efficiency due an increased number density of hole centres in the vicinity of the trap, thus giving a peak-shaped IRSL efficiency response in the range of 250–8 keV. Our results suggest that dose-rate conversion factors, specific to radiation type and radioisotope, should be used for age calculation, and significant care should be taken for luminescence based calibration of the dose rate of the laboratory irradiation source.

Testing the reliability of fading correction methods for feldspar IRSL dating: A comparison between natural and simulated-natural dose response curves

The reliability of the fading correction methods for feldspar infrared stimulated luminescence (IRSL) dating in the non-linear part of the dose response curve proposed by Wallinga et al. (2007) (modified after Lamothe et al., 2003) and Kars et al. (2008) was evaluated using samples from the Chinese loess-palaeosol sequence in Luochuan with the reference ages. Two protocols, the post-infrared (IR) IRSL at 225 °C (pIRIR225) and the pulsed IR at 50 °C, were applied in this study. The natural dose response curve (DRC) for the IR50 (pre-pIRIR225), pIRIR225 and pulsed IR50 was first constructed. The simulated-natural DRC and the fading corrected ages were then determined. The characteristic saturation dose (D0) of the simulated-natural DRC was found to be larger than that of the natural DRC for the IR50 (pre-pIRIR225) and pIRIR225 signals, whilst the two D0 values agreed with each other for the pulsed IR50. The saturation ratio (n/N) of the simulated-natural DRC agreed with the natural one within 10% uncertainty for all three signals. The fading corrected ages using these two methods agreed with the reference ages, confirming the reliability of the two fading correction methods. However, our numerical simulation on the fading correction showed that the method of Wallinga et al. (2007) significantly overcorrect ages when the fading rates are larger than ca. 4–5%/decade.

Chronology of desert margin in western India using improved luminescence dating protocols

The present study provides improved chronology for the desert margin fluvial sediments of semi-arid region located in the Mahi river basin, western India. The sequence has preserved a near-continuous record of climate change since the Last Interglacial. An earlier attempt of dating based on feldspar IRSL chronology shows a combined effect of anomalous fading and unbleached components resulting in age inversions. The present work tries to explore the possibility of using blue light stimulated luminescence (BLSL) of quartz, infra-red stimulated luminescence (IRSL) of feldspar and the newly developed methodologies, like natural correction factor based single aliquot regeneration (NCF-SAR) protocol and decision making schemes based on distribution of doses and beta heterogeneity concept for luminescence dating of sediments. Observations suggest that quartz suffered from significant sensitivity changes during natural signal measurement and partial bleaching. A combination of NCF-SAR protocol and sample specific equivalent dose computation helped in arriving at better age estimate for present samples. The study also compares the criteria for the selection of different age models that are used at present. The age of the alluvial sequence is now bracketed between 10 ka (upper aeolian unit) and 75 ka (lowermost fluvial unit).

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 (g2days 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.

Luminescence dating of interglacial coastal depositional systems: Recent developments and future avenues of research

Luminescence dating offers new opportunities to explore the evolution of Quaternary marine coastal facies and landforms. This review highlights the main advances in luminescence geochronology of interglacial coastal sediments through the analysis of 547 luminescence ages, most of which were published during the last decade. The majority of these reported luminescence investigations have been carried out along passive margin coasts. Since the turn of the century, the discovery of a normalization procedure known as Single Aliquot Regeneration (SAR) has drastically reduced data scatter and improved precision, with the consequence that quartz SAR optically-stimulated luminescence OSL has become the dating protocol of choice for the Last Interglacial (LIG) period. A more complex technique, known as thermally-transferred OSL (TT-OSL), is presumably proposed for dating older interglacials of the Mid-Pleistocene and beyond. Feldspar luminescence is increasingly being applied to dating Pleistocene sea level high stands due to a much higher dose saturation level than quartz OSL. The use of feldspar IRSL (Infrared-stimulated luminescence) is limited by the occurrence of variable, but ubiquitous anomalous fading (AF). Following the advent of AF-correction methods, several Middle Pleistocene sites have been amenable to dating, albeit with significant related uncertainties. Recently, new protocols involving the measurement of post-IR IRSL at elevated temperatures have yielded relatively coherent ages for interglacial sediments up to ca. 300 ka. Quartz OSL/TT-OSL, AF-corrected IRSL, and post-IR IRSL ages are generally correlated with periods of sea level high stands. A few ages are reported from the early and middle part of the Middle Pleistocene, as MIS11, 9 and more commonly MIS7 high stands are documented in strongly uplifting active margin coasts. However, by far the most obvious age peak corresponds to the end of the LIG. The MIS5e shoreline is probably the most studied and luminescence-dated coastal feature of all, as the chronology of the sea level markers is crucial to assess global eustatic sea level variations through the course of the last interglacial. Nevertheless, the observed abundance of young (100–120 ka) luminescence ages for presumed MIS5e sediments may underline methodological issues, and/or reflect the higher preservation potential of late regressive sequences. On the other hand, the occurrence of geographically distant reports of MIS5a high stand might reflect a true eustatic origin for this event. Age analysis supports the impression of general reliability of luminescence for the timing of former sea level high stands. There is a clear need to address issues in dose rate variability, in the phenomenology of fading in feldspar, and in the behaviour of luminescence growth with dose, both in laboratory and natural conditions. These could be addressed and properly evaluated by dating different minerals, as multiple or single grains, with consequent variable internal dose rates. More robust practices in the application of luminescence dating techniques could eventually constrain the age uncertainties to no better than 2–3%. Therefore, the strength of luminescence as a dating tool is more in terms of its extended age range and the ubiquity of datable material.

Infrared stimulated luminescence dating of 19&lt;sup&gt;th&lt;/sup&gt; century fluvial deposits from the upper Rhine River

Knowledge of the age of fluvial deposits is an important aspect in the understanding of river dynamics, which is pre-requisite for sustainable river management and restoration back to more natural conditions and processes. Presented here is a case study on using feldspar Infrared Stimulated Luminescence (IRSL) to date low-energy fluvial sediments that formed after correction of the Upper Rhine River in the first half of the 19&lt;sup&gt;th&lt;/sup&gt; century. A rigorous testing programme is carried out to characterise the IRSL properties of the samples, including thermal transfer, dose recovery and fading. All samples reveal complex distributions of equivalent dose, implying the presence of differential bleach-ing in the samples. It is shown that multi-grain aliquots overestimate the known-age by up-to 200 years, i.e. apparent IRSL ages are twice as old as the true age of the sediment. The use of single grains results in ages that are in excellent agreement with the expected age, therefore the age overestimation in multi-grain aliquot measurements is likely explained by signal averaging effects. While the application of single grains appears mandatory for dating young low-energy fluvial deposits, the small absolute offset associated with the multi-grain approach might be acceptable when dating sediments of such type that are older than a few 1000 years.

Applying luminescence methodology to key sites of Alpine glaciations in Southern Germany

This paper investigates the properties of quartz sampled at different sites in the Riss, Iller, Mindel, and Isar Valley of the northern Alpine Foreland of Germany and ties in with a previous study of optically stimulated luminescence (OSL) dating of glaciofluvial sediments from the type regions of the Würmian, the Rissian, and the Mindelian Glaciations. The previous investigation showed severe underestimation of quartz ages compared to the existing tentative chronology based on morphostratigraphic evidence as well as fading uncorrected feldspar Infrared Stimulated Luminescence (IRSL) ages. In this paper we present laboratory experiments and data analyses such as pulse annealing and isothermal decay measurements, De(t) and NR(t) plots as well as curve fitting analyses to demonstrate that the quartz has unfavourable luminescence characteristics. We conclude that the quartz from the study area is not suitable for OSL dating using standard single-aliquot regenerative-dose techniques.

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.

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.

Geomorphological correlation of the tectonically displaced Tejo River terraces (Gavião–Chamusca area, central Portugal) supported by luminescence dating

A suite of fluvial terraces (T1–T6, from top to bottom of the staircase) occur along a ∼70 km stretch of the Tejo River in central Portugal, between the small towns of Gavião and Chamusca. Terrace correlation was based upon the following: (a) aerial photograph analysis, geomorphological mapping and field topographic survey; (b) sedimentology of the terrace deposits, namely the maximum particle size, clast composition and lithofacies identification; and (c) K-feldspar infrared stimulated luminescence (IRSL) dating of the three lower terraces levels, as quartz optically stimulated (OSL) signal of the samples from these terraces was too close to saturation for all but two samples. The two upper terraces (T1 and T2) lack suitable materials for luminescence dating (e.g. sands/silts), but also their probable ages are beyond the upper range of the dating method. Faults affecting terraces and older deposits have been reported. The luminescence dating results suggest that some assignments of local terrace remnants were incorrect because of fault-related vertical displacements. The luminescence dating procedure also included a correction for anomalous fading in order to obtain more reliable estimates of the burial ages. The fading rate was identical for all samples, so for correlation of the terraces anomalous fading of the feldspar IRSL signal is considered not to be a significant problem. The T5 terrace has corrected ages of ∼42–99 ka, the T4 from ∼107 to 222 ka and the T3 terrace has a minimal age of ∼300 ka. Fluvial incision appears to have been principally controlled by regional uplift but also by localised movements along fault structures. Using the corrected ages of the T4 surface a time-averaged incision rate can be quantified as follows: (1) ∼0.29 m/ka for reach III, (2) ∼0.53 m/ka in the Chamusca area (east side on reach IV) and (3) ∼0.13 m/ka in the Mato Miranda area (west side on reach IV, the less uplifted block according to the geomorphic framework).

A new approach towards anomalous fading correction for feldspar IRSL dating — tests on samples in field saturation

Anomalous fading of the feldspar infrared stimulated luminescence (IRSL) signal hampers possibilities of using feldspar IRSL to obtain burial ages for sediments beyond the dating range of quartz optically stimulated luminescence. Here, we propose a new approach to quantify anomalous fading of the feldspar IRSL signal over geological burial times based on laboratory fading experiments. The approach builds on the description of the quantum mechanical tunnelling process recently proposed by Huntley [2006. An explanation of the power-law decay of luminescence. J. Phys. Condensed Matter 18, 1359–1365]. We show that our methods allow the construction of un-faded and natural IRSL dose–response curves as well as anomalous fading rates in field saturation. The predicted level of field saturation closely approximates the measured saturation level for five samples from fluvial deposits (Lower Rhine) known to be older than 1 Ma. The modelled anomalous fading rate in field saturation (13.4% per decade) is close to the measured value of 11.2% per decade. These results indicate that the proposed method may allow anomalous fading corrected IRSL dating beyond the linear part of the IRSL dose–response curve.

A test case for anomalous fading correction in IRSL dating

Infrared-stimulated luminescence (IRSL) dating of feldspars has the potential to date deposits beyond the age range of quartz optical (OSL) dating. Successful application of feldspar IRSL dating is, however, often precluded due to anomalous-fading, the tunnelling of electrons from one defect site to another. In this paper we test procedures proposed for anomalous-fading correction by comparing feldspar IRSL and quartz OSL dating results on a suite of samples from continental deposits from the southeastern Netherlands. We find that even after anomalous-fading correction IRSL ages underestimate the burial age of the deposits and argue that this may be a consequence of a dependency of anomalous fading rate on the dose rate and on the absorbed dose.

Feldspar IRSL emission spectra at high and low temperatures

The physical basis of feldspar dating using the Infrared Stimulated Luminescence (IRSL) method is still unknown to a large extent. This study shows the IRSL behaviour of a sediment-extracted microline in a temperature range from −130 to +200°C. The low temperature region was used to calculate the activation energy to 0.15 eV. The behaviour above room temperature, as well as additional experiments on museum specimens, gives cause for the introduction of a long lived state (some tens of milliseconds) to the IRSL model, which is situated in the recombination path of the 560 nm emission. Some consequences on dating are discussed.