Single-aliquot Protocol Research Articles

Establishing a Luminescence-Based Chronostratigraphy for the Last Glacial-Interglacial Cycle of the Loess-Palaeosol Sequence Achajur (Armenia)

The loess-palaeosol section Achajur is part of the Sevkar loess area in north-eastern Armenia and comprises three prominent pedocomplexes with intercalated loess units, probably spanning at least three glacial-interglacial cycles. With its central position between the Black Sea and the Caspian Sea, the Sevkar loess area serves as important bridge between loess sections in southeastern Europe and Central Asia. Establishing a numerical chronology of the Achajur loess section enables correlation with other loess-palaeosol sections and a palaeoenvironmental reconstruction of the region. In this paper, we focus on the chronology of the last glacial-interglacial cycle using luminescence dating on fine grain (4–11 µm) material. We compare two post-IR-IRSL protocols on polymineral fine grain using different stimulation temperatures, in order to evaluate the influence of fading and potential unbleachable residuals on our samples. For a backup of the younger ages (<50 ka), we also applied a single aliquot protocol on the quartz fine grain fraction. The results indicate that the upper loess unit was deposited during MIS3 and 4, while MIS2 ages are not present. The underlying pedocomplex and loess unit fall into MIS5 and MIS6, respectively. This confirms that the loess was deposited during dry phases and pedocomplexes formed during wetter phases, as is also shown by comparison with other palaeoenvironmental proxies from the area.

Study of dental zirconia reinforced lithium silicate for its use as a personal accidental dosimeter

A recently introduced material for dental restorations, Zirconia reinforced Lithium Silicate (ZLS), is examined for its properties, in order to be used as a personal accidental thermoluminescence dosimeter. For this purpose, its main thermoluminescence characteristics are studied, such as its sensitivity, signal reproducibility, fading, dose response and lower detectable dose limit. Furthermore, the material is characterized by means of microscopic, spectroscopic and crystallographic techniques. The results propose that ZLS is promising to be used as a personal accidental thermoluminescence dosimeter for post-irradiation detection, with single aliquot protocols.

A SAR protocol for heat-sensitive materials exhibiting sensitization (SARHS) for the estimation of the equivalent dose

Accurate estimation of the radiation released at incidents, such as nuclear accidents or radiological terrorist events is of major importance for the prediction of health effects following such an incident. The luminescence properties of several materials are studied in order to assess their potential use as dosimeters in such cases.However, the protocols followed for such purposes are mainly those initially developed for dating. Multiple-aliquot protocols are mainly based on the Thermoluminescence response of the dosimeter used and require the availability of abundant material, while homogeneity of the luminescent substance in the entire sample is also a prerequisite in order to prepare similar aliquots. To account for the above drawbacks, several single-aliquot protocols have been suggested based on Optically Stimulated Luminescence (OSL), which however involve steps of heating the sample up to 300 °C, and as a result they can only be used for heat-resistant materials.In the present work, a novel single aliquot regenerative-dose protocol suitable for heat-sensitive materials exhibiting sensitization (called SARHS) for the estimation of the equivalent dose with OSL is proposed. The protocol eliminates any sensitization effects without encompassing any heating of the sample. Validation of the SARHS protocol was accomplished with three different heat-sensitive materials which exhibit sensitivity changes with the repeated use of the same sample (sensitization), i.e., dragonfly wings, a commercial solid drug (Daktarin) and chicken bones, in which the natural equivalent dose could be successfully recovered with negligible error even when fading, after exposure to irradiation, of the natural OSL signal was taken into account.

Developing a single-aliquot protocol for measuring equivalent dose in biogenic carbonates

Abstract Exploiting biogenic carbonates as thermoluminescence dosimeters requires an understanding of trap kinetics and an appropriate sequence with which to measure equivalent dose. The trap kinetics of two high temperature peaks (peaks II and III) from calcitic snail opercula have been investigated resulting in the calculation of lifetimes of 7.4 × 10 7 and 1.4 × 10 11 years for the two peaks respectively. Two measurement sequences, based upon changes in the application and measurement of a test dose, have been applied to peaks II and III, and though both methods were equally successful in dose recovery and production of a dose response curve some differences were observed. Primarily, the use of method 1 lead to dose dependant sensitivity change implying competition effects occurring during irradiation; method 2 did not experience this phenomenon. As a consequence method 2 was chosen as the most appropriate protocol for single-aliquot dating of this material. When assessing the TL behaviour of the two peaks, peak II performed poorly in dose recovery experiments recovering a dose 60–100% larger than that applied. Disproportionate growth of peak II in response to a beta dose applied prior to measurement, compared to growth following regeneration doses indicated that peak II was not suitable for use in single-aliquot protocols. However, dose recovery results for peak III were all within errors of unity of the given dose, and peak III was therefore chosen as the most appropriate peak for TL dosimetry in these single-aliquot procedures. The lifetime of charge in peak III is sufficient to date over many millions of years, and furthermore using the chosen method 2 the dose response curve has a D 0 of 3,250 ± 163 Gy allowing dating to over 3 million years.

Dissolution and subsequent re-crystallization as zeroing mechanism, thermal properties and component resolved dose response of salt (NaCl) for retrospective dosimetry

In the present study we report dosimetric properties of iodized salt aiming at using it as an accidental luminescent dosimeter. It was found that the very good sensitivity of its main dosimetric peak is strongly affected by thermal treatments. This is also the case for OSL emission. The sensitivity loss due to heating implies that caution should be exercised while applying single aliquot protocols for dose evaluation. The sequence of dissolution and subsequent re-crystallization was established to be an extremely effective zeroing mechanism for the TL signal. The linearity in the dose response was also monitored in the case of dissolved and subsequently re-crystallized salt. In the case of naturally occurring salt, zeroing of the TL signal due to dissolution as well as the linearity of dose response up to doses as large as 100 Gy were found to be very promising features for dating applications.

Simulations of thermally transferred OSL signals in quartz: Accuracy and precision of the protocols for equivalent dose evaluation

Thermally-transferred optically stimulated luminescence (TT-OSL) signals in sedimentary quartz have been the subject of several recent studies, due to the potential shown by these signals to increase the range of luminescence dating by an order of magnitude. Based on these signals, a single aliquot protocol termed the ReSAR protocol has been developed and tested experimentally. This paper presents extensive numerical simulations of this ReSAR protocol. The purpose of the simulations is to investigate several aspects of the ReSAR protocol which are believed to cause difficulties during application of the protocol. Furthermore, several modified versions of the ReSAR protocol are simulated, and their relative accuracy and precision are compared. The simulations are carried out using a recently published kinetic model for quartz, consisting of 11 energy levels. One hundred random variants of the natural samples were generated by keeping the transition probabilities between energy levels fixed, while allowing simultaneous random variations of the concentrations of the 11 energy levels. The relative intrinsic accuracy and precision of the protocols are simulated by calculating the equivalent dose (ED) within the model, for a given natural burial dose of the sample. The complete sequence of steps undertaken in several versions of the dating protocols is simulated. The relative intrinsic precision of these techniques is estimated by fitting Gaussian probability functions to the resulting simulated distribution of ED values. New simulations are presented for commonly used OSL sensitivity tests, consisting of successive cycles of sample irradiation with the same dose, followed by measurements of the sensitivity corrected L/T signals. We investigate several experimental factors which may be affecting both the intrinsic precision and intrinsic accuracy of the ReSAR protocol. The results of the simulation show that the four different published versions of the ReSAR protocol can reproduce accurately the natural doses in the range 0–400 Gy with approximately the same intrinsic precision and accuracy of ∼1–5%. However, these protocols underestimate doses above 400 Gy; possible sources of this underestimation are investigated. Two possible explanations are suggested for the modeled underestimations, possible thermal instability of the TT-OSL traps, and the presence of thermally unstable medium OSL components in the model.

On the intrinsic accuracy and precision of luminescence dating techniques for fired ceramics

Luminescence dating techniques have been used extensively for archaeological and geological samples. Such techniques are based on thermally or optically stimulated signals. This paper presents simulations of several luminescence techniques for equivalent dose (ED) estimation for ceramic materials containing quartz. The simulations are carried out using a recently published comprehensive kinetic model for quartz, consisting of 11 electron and hole traps and centers. The complete sequence of the experimental protocols for several thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques are simulated using the same set of kinetic parameters. The specific simulated protocols are: additive dose TL protocol, predose technique (both additive and multiple activation versions), phototransfer protocol, single aliquot regenerative optically stimulated luminescence (SAR-OSL) protocol, and SAR thermoluminescence protocol (SAR-TL). One hundred random variants of the natural samples were generated by keeping the transition probabilities between energy levels fixed, while allowing simultaneous random variations of the concentrations of the 11 energy levels. The relative intrinsic accuracy and precision of the protocols are simulated by calculating the equivalent dose (ED) within the model, for a given natural burial dose of the sample. The intrinsic accuracy of these techniques is estimated by simulating natural irradiation of the samples with a known burial dose, followed by simulation of the luminescence method used to recover the estimated dose ED. The percent difference between the burial dose and the ED value represents the simulated accuracy of the luminescence technique. The relative intrinsic precision of these techniques is estimated by fitting Gaussian probability functions to the ED values obtained with the 100 model variants. It is found that the various techniques can reproduce natural paleodoses in the range 10 mGy–10 Gy with a typical intrinsic accuracy of +1 to 10%. Techniques based on single aliquot protocols were found in general to be more precise than techniques requiring the use of multiple aliquots. In addition, techniques based on interpolation of experimental data were found to be consistently both more precise and accurate than those based on extrapolation of experimental data.

Towards development of a broadly-applicable SAR TT-OSL dating protocol for quartz

Difficulties in the application to 40–60 μm Chinese loess samples of the SAR thermally transferred OSL (TT-OSL) protocol outlined in Wang et al. (Wang, X. L., Wintle, A. G., Lu, Y. C., 2007. Testing a single-aliquot protocol for recuperated OSL dating. Radiation measurements 42, 380–391) are reported. These include poor recycling, negative intercepts on the sensitivity corrected TT-OSL axis and non-linear relationships between regeneration dose TT-OSL and test dose OSL that do not pass through the origin. A modified protocol is developed that attempts to circumvent these complications. This protocol involves correction for sensitivity change through the use of the TT-OSL response to a test dose and requires no correction for either charge carry over or basic transfer. A high temperature blue light bleach (400 s at 280 °C) is used in the middle and at the end of each SAR cycle to remove any TT-OSL signal remaining after previous dosing. The protocol appears to be applicable to a number of samples, producing reproducible dose response curves that within errors pass through the origin and saturate at high doses; the TT-OSL response to a test dose appears to be a satisfactory monitor of sensitivity changes. Testing the protocol on a Chinese loess sample shows that there is still signal growth up to a dose of at least 12 kGy. Dose recovery tests are also successful on a variety of samples and can recover known doses up to between 0.56 and 1.2 kGy. Reproducible growth is also observed using a number of coarse grained samples from various depositional environments and locations. However, signal strength is a limiting factor and many samples do not show sufficient TT-OSL sensitivity for application with any TT-OSL protocol.

Colluvial and alluvial sediment archives temporally resolved by OSL dating: Implications for reconstructing soil erosion

Colluvial and alluvial sediments represent important geoarchives to reconstruct long-term soil erosion and to gain insight into the complex system of sediment cascades and sediment fluxes within a catchment. In this respect, the temporal information of sediment archive formation is essential and achievable through optical stimulated luminescence (OSL) dating. In this study, colluvial and alluvial sediments from a mesoscale catchment in northern Bavaria were investigated and dated by OSL. Insufficient sediment bleaching was detected for some of the samples. In these cases, the method proposed by Fuchs and Lang [Fuchs, M., Lang, A., 2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece. Quaternary Science Reviews 20, 783–787.] was applied for equivalent dose ( D e ) determination. The calculated OSL ages are in stratigraphic order and their accuracy is confirmed by 14C age control. Based on the chronostratigraphies for alluvial and colluvial archives, there is a dominant synchronous sedimentation history for the Medieval and Modern period, but initiation of Holocene sedimentation occurred at different times in the upper and lower parts of the catchment. The latter phenomenon might be explained by the decoupled sediment fluxes between both the slope–channel system and the upper and lower catchment.

On the applicability of the leading edge method to obtain equivalent doses in OSL dating and dosimetry

The applicability of an objective methodology for dose distribution analysis proposed by Lepper and McKeever [2002. An objective methodology for dose distribution analysis. Radiat. Prot. Dosimetry 101 (1–4), 349–352] to samples, where available material only allows the measurement of a small number of aliquots, is investigated. The basis for the study are artificially and naturally insufficiently bleached samples for which a large number ( > 90 ) of aliquots were measured. Smaller sets of aliquots are then randomly drawn from this pool and a Gaussian function fitted to the rising limb of the dose distribution. It is found that when the method is based on a three-parameter fit, it is no longer applicable when the number of aliquots is significantly reduced. The percentage of successful fits for data sets made of only 20 aliquots was on average below 20%. In addition for data sets composed of 40 to 60 aliquots, palaeodoses obtained from fitting spread over a large dose range and showed a strongly asymmetric distribution. A significant improvement could be achieved by fixing the center of the Gaussian function to the mode D E bin and only fitting a two-parameter function to the data sets. With this approach an 80% change of successful fitting could be achieved even when using dose distributions constructed from only 20 to 30 aliquots. The median of the palaeodose distributions was approximately independent of the number of aliquots used. The modified method is then applied to samples from a sedimentary record in the northern Oman mountain range and the results compared with the method proposed by Fuchs and Lang [2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peleponnese, Greece. Quat. Sci. Rev. 20, 783–787]. It is concluded that the leading edge method proposed by Lepper and McKeever can be useful to derive D E 's from insufficiently bleached sediments even for a small number of aliquots if the Gaussian function is fitted as a two-parameter function.

Testing a single-aliquot protocol for recuperated OSL dating

A single-aliquot dating procedure for measuring the equivalent dose in fine-grained quartz extracted from Chinese loess has been developed and tested. The optically stimulated luminescence (OSL) signal used for construction of the dose–response curve is termed the recuperated OSL (ReOSL) signal; it is the difference between the thermally transferred OSL signal measured after heating to 260 ∘ C for 10 s following optical stimulation with blue light for 270 s at 125 ∘ C and the subsequent basic-transferred OSL signal measured when the same procedures are preceded by heating at 300 ∘ C for 10 s. Dose recovery experiments are reported, and the apparent impact of the size of test dose used to monitor sensitivity change is investigated. Equivalent doses obtained using the protocol developed from these experiments are compared with those obtained with multiple aliquots, using both conventional OSL and the ReOSL signals.

Chronostratigraphy of a sediment record from the Hajar mountain range in north Oman: Implications for optical dating of insufficiently bleached sediments

In this study we report on optical stimulated luminescence (OSL) ages of quartz extracted from a sedimentary record in the N-Oman mountain range. Equivalent dose ( D E ) distributions derived from single aliquot measurements (SAR) of small aliquots (ca. 200 grains) were investigated to test whether the skewness and broadness of the dose distribution can be used as criteria for the identification of insufficient bleaching. Furthermore, the methods proposed by Lepper and McKeever [2002 An objective methodology for dose distribution analysis. Radiation Protection Dosimetry 101 (1–4), 349-352]. Singhvi [Juyal, N., Chamyal, L.S., Bhandari, S., Bushan, R., Singhvi, A.K., in press. Continental record of the southwest monsoon during the last 130 ka: evidence from the southern margin of the Thar Desert, India. Quaternary Science Review] and Fuchs and Lang [2001 Fuchs, M., Lang, A., 2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece. Quaternary Science Review 20, 783–787.] to derive D E 's from insufficiently bleached sediments were compared. At first, the investigations were carried out on artificially bleached, irradiated and mixed quartz material from the Oman study area to simulate insufficiently bleached sediments. Then, the various statistical methods for identifying insufficient bleaching and D E derivation were applied to the natural samples from the study area, where 18 samples were measured. For the identification of insufficient bleaching the preferential parameter is the broadness of a distribution. For D E calculation, both the Singhvi method and the Fuchs and Lang method produce similar results, which are consistent with the stratigraphic order. A drawback of both methods is their sensitivity to low outliers. The Lepper and McKeever method was not applied to the natural samples, due to limitations in its application to a small number of aliquots and due to the ambiguous identification of the rising limb of the dose distributions.

Equivalent dose (palaeodose) estimation in thermoluminescence dating using a single aliquot of polymineral fine grains

A single aliquot protocol for the estimation of the natural dose in thermoluminescence dating (SATL) is presented that makes use of polymineral fine grains extracted from ceramic materials. The protocol is demonstrated using aliquots made from two Neolithic sherds and is compared with results from an additive dose polymineral technique. The results of both techniques are in close agreement and highlight the potential advantages of the new procedure, especially when sample availability is restricted, e.g. dating of small pottery fragments, or in cases of authenticity testing.

The multiphase Eulerian-Lagrangian transport (MELT-3D) approach for modeling of multiphase mixing in fragmentation processes : Nourgaliev, R. R. et al. Progress in Nuclear Energy, 2003, 42, (2), 123–157

In this study we report on optical stimulated luminescence (OSL) ages of quartz extracted from a sedimentary record in the N-Oman mountain range. Equivalent dose (DE) distributions derived from single aliquot measurements (SAR) of small aliquots (ca. 200 grains) were investigated to test whether the skewness and broadness of the dose distribution can be used as criteria for the identification of insufficient bleaching. Furthermore, the methods proposed by Lepper and McKeever [2002 An objective methodology for dose distribution analysis. Radiation Protection Dosimetry 101 (1–4), 349-352]. Singhvi [Juyal, N., Chamyal, L.S., Bhandari, S., Bushan, R., Singhvi, A.K., in press. Continental record of the southwest monsoon during the last 130 ka: evidence from the southern margin of the Thar Desert, India. Quaternary Science Review] and Fuchs and Lang [2001 Fuchs, M., Lang, A., 2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece. Quaternary Science Review 20, 783–787.] to derive DE's from insufficiently bleached sediments were compared. At first, the investigations were carried out on artificially bleached, irradiated and mixed quartz material from the Oman study area to simulate insufficiently bleached sediments. Then, the various statistical methods for identifying insufficient bleaching and DE derivation were applied to the natural samples from the study area, where 18 samples were measured. For the identification of insufficient bleaching the preferential parameter is the broadness of a distribution. For DE calculation, both the Singhvi method and the Fuchs and Lang method produce similar results, which are consistent with the stratigraphic order. A drawback of both methods is their sensitivity to low outliers. The Lepper and McKeever method was not applied to the natural samples, due to limitations in its application to a small number of aliquots and due to the ambiguous identification of the rising limb of the dose distributions.

Retrospective dose assessment: the measurement of the dose in quartz in dating and accident dosimetry.

In retrospective dosimetry using luminescence it is never possible to obtain a sample of the unaltered phosphor as it was at somc time in the past and before radiation exposure. This prevents direct calibration using conventional personnel dosimetry techniques, and so measurement protocols must be devised which allow accurate estimation of the dose using only the exposed phosphor. This has led to the use of complex protocols, requiring many tens of notionally identical subsamples, to obtain one estimate of the unknown dose. More recently, considerable effort has gone into devising protocols which provide an estimate of the unknown dose from only one subsample. These avoid the need to assume homogeneity in the sample. These protocols also allow faster measurement, which for the first time makes it practical to obtain multiple estimates of the unknown dose and thus externally calculated random errors. This paper discusses the development of single-aliquot protocols, with particular reference to those devised for use with quartz, and finally reviews some of their applications in the retrospective assessment of dose.

OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece

Optical ages of quartz extracts in Holocene sediments from the north-eastern Peloponnese, Greece, are reported. The single-aliquot regenerative-dose protocol proposed by Murray and Wintle (2000, Radiation Measurements 32, 57–73) was used to estimate the equivalent dose in these samples. A large inter-aliquot scatter in D E values was observed for all samples; this is ascribed to heterogeneous bleaching of the samples at deposition. To estimate a D E for age calculation, a technique is suggested based on the analysis of the D E scatter obtained from nine different quartz samples which were artificially bleached and irradiated. This procedure is especially useful when the sample quantity is limited. The optical age estimates increase with sampling depth. An average aggradation rate of 1.5 mm/yr is derived using the OSL ages; this value is consistent with the known uplift rate of 1–2 mm/yr.

Residual ages of modern sediments in an hyperarid region, Israel

Modern sediments from a variety of depositional environments in the hyperarid southern Arava region, Israel, were analyzed for their residual equivalent dose ( D e). Colluvial and fluvial sediments from drainage systems of different sizes and parent materials were collected and alkali feldspars were measured using the single-aliquot additive-dose protocol. All samples showed a large scatter in the D es. For most types of the sediments, the average D e is between 2 and 11 Gy, equivalent to residual ages of 1–5 ka. The lowest D e is usually in the range of 0.1–4 Gy, confirming that the single-aliquot protocol can identify the best bleached grains. Newly weathered material from active channels gave the largest D e's of up to 30 Gy. Reworked fluvial sediments, derived from terraces and fans, gave progressively smaller D e's with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high D e's and the large scatter result from the mode of sediment transport. Brief rain episodes result in flash floods, where large volumes of sediment are transported over short distances and only a small part of the sediment is exposed at any time to the sun. Only repeated cycles of transportation can homogenize the sediment and fully expose all its particles to the sun. Single-grain or small-aliquot protocols can overcome the high and variable residual D e values and enable better dating of sediments in hyperarid environments.

Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol

Single aliquot protocols are now widely used as a means of measuring the equivalent dose ( D e) in quartz and feldspar optical stimulated luminescence (OSL) dating of both heated and sedimentary materials. The most recent of these is the single-aliquot regenerative-dose (SAR) protocol, first suggested by Murray and Roberts (Radiation Measurements 29, 503–515, 1998). In this approach, each natural or regenerated dose OSL measurement is corrected for changes in sensitivity using the OSL response to a subsequent test dose (10–20% of D e). If the sensitivity correction is adequate, then the corrected OSL response should be independent of prior dose and thermal/optical treatment, i.e. there should be no change in the sensitivity-corrected dose–response curve on remeasurement. Here we examine the interpretation of the sensitivity corrected growth curve as a function of dose, and the effect of changing measurement conditions (e.g. preheat temperature, size of test dose, stimulation temperature) on the estimation of D e. The dependence of the dose response on prior treatment is tested explicitly, and the significance of thermal transfer discussed. It is concluded that a robust SAR protocol is now available for quartz, and that it is applicable to a wide range of heated and unheated materials.

A single-aliquot OSL protocol using bracketing regenerative doses to accurately determine equivalent doses in quartz

In most cases, sediments show inherent heterogeneity in their luminescence behaviours and bleaching histories, and identical aliquots are not available: single-aliquot determination of the equivalent dose (ED) is then the approach of choice and the advantages of using regenerative protocols are outlined. Experiments on five laboratory bleached and dosed quartz samples, following the protocol described by Murray and Roberts (1998. Measurement of the equivalent dose in quartz using a regenerative-dose single aliquot protocol. Radiation Measurements 27, 171–184), showed the hazards of using a single regeneration dose: a 10% variation in the regenerative dose yielded some equivalent dose estimates that differed from the expected value by more than 5%. A protocol is proposed that allows the use of different regenerative doses to bracket the estimated equivalent dose. The measured ED is found to be in excellent agreement with the known value when the main regeneration dose is within 10% of the true equivalent dose.

Optically stimulated luminescence dates for Late Pleistocene and Holocene sediments from Nørre Lyngby, Northern Jutland, Denmark

Quartz from late Pleistocene marine and freshwater deposits of known 14C age, and from a historically dated aeolian deposit, has been studied using multiple-aliquot and single-aliquot protocols. Comparisons are presented between equivalent doses derived using a multiple-aliquot additive-dose thermoluminescence approach, and three OSL protocols—Single aliquot regeneration and added dose (SARA; Mejdahl and Bøtter-Jensen, 1994), single-aliquot additive-dose (Murray et al., 1997) and single-aliquot regenerative-dose (Murray and Roberts, 1998). Dose rates, determined by laboratory gamma spectrometry, are compared with both field gamma scintillation measurements, and laboratory beta thermoluminescence dosimetry measurements. The agreement is satisfactory. The resulting dates are discussed in terms of absolute chronology and bleaching environment.