G.A.T. Duller

Distinguishing quartz and feldspar in single grain luminescence measurements

Abstract Luminescence dating of single grains requires a method of distinguishing between quartz and feldspar based solely on their luminescence behaviour. Several methods are tested, including the shape of optically stimulated luminescence (OSL) decay curve, the effect of infrared stimulation upon the OSL signal, and the change in OSL decay curve shape and OSL intensity as a function of the temperature of the sample during stimulation. Of these, the last requires a large number of additional measurements, whilst the first is a poor discriminant. The most efficient means of distinguishing quartz and feldspar grains is by taking the ratio of the OSL signal when measured with and without prior infrared stimulation. This is termed the OSL IR depletion ratio.

Developments in radiation, stimulation and observation facilities in luminescence measurements

Abstract This paper summarises recent developments in the Riso TL/OSL reader. These new developments include an elevated temperature irradiation facility that has been expanded to measure variable temperature radioluminescence (RL) during beta irradiation. This RL facility allows the measurement of TL, OSL and RL in the same software-controlled automatic sequence. To provide an alternative to the 90 Sr beta source, the performance of a software-controlled mini X-ray generator has been investigated. This has the advantage of providing a range of dose rates spanning more than three orders of magnitude up to more than 2 Gy / s . A separate bleaching facility has also been added to the reader, new powerful infrared (IR) light-emitting diodes have replaced the IR laser diode in the general OSL unit, and an IR laser has been included in the single-grain OSL attachment. This paper describes these new facilities, and illustrates their applications with representative measurements.

Equivalent dose determination using single aliquots

Two methods for the determination of equivalent dose using single aliquots of potassium feldspar are described. The non-destructive nature of infrared stimulated luminescence (IRSL) has made it possible to make repeated luminescence measurements on a single aliquot without apparently changing the sensitivity of the material. Such an approach has been made practicable using the IRSL add-on to the Riso automated thermoluminescence (TL) reader (Botter-Jensen et al., Nucl. Tracks18, 257–263, 1991). The hardware and software that have made this possible are described. The method based upon the regeneration technique does appear to suffer from problems of changes in sensitivity when bleaching between doses. It is hypothesized that this is due to partial emptying of the trapped charge population. In the case of the additive dose technique, no such problem is seen. The precision of both techniques is far superior to normal TL and IRSL techniques. The accuracy appears good for the additive dose technique, but not for regeneration.

A new system for measuring optically stimulated luminescence from quartz samples

Abstract An optically stimulated luminescence (OSL) unit based on a halogen lamp light source has been developed for the stimulation of quartz samples. In addition to the halogen light source, the OSL unit incorporates an infra-red diode array for infra-red-stimulated luminescence (IRSL) measurements of feldspar samples and for screening the purity of quartz. The new OSL unit can be fitted directly onto the automated Riso TL dating apparatus. The stimulation spectrum filtered from the halogen lamp delivers about 16mW cm -2 at the sample. Preliminary OSL studies of quartz samples are described, and dating results are presented.

Optical dating of single sand-sized grains of quartz: sources of variability

Optically stimulated luminescence (OSL) measurements have been made of over 3000 sand-sized grains of quartz. Analysis at this scale highlights the variability in the luminescence sensitivity and the dose saturation characteristics of individual quartz grains. Using a new instrument capable of measuring single grains it is feasible to routinely measure the equivalent dose from many hundreds of grains from each sample. Analysis of such datasets requires assessment of the uncertainties on each equivalent dose since these may vary significantly. This paper assesses the significance of signal intensity, dose saturation characteristics and instrument uncertainty in equivalent dose calculation.

Underestimation of equivalent dose in single-aliquot optical dating of feldspars caused by preheating

The ability of single-aliquot techniques to recover a known laboratory dose in coarse-grain potassium-rich feldspar separates is tested. It is shown that, for the samples under study, single-aliquot methods underestimate the known dose. It is deduced that this phenomenon is caused by an increase in electron trapping probability as a consequence of heating. The same phenomenon would lead to an underestimation in the equivalent dose in a natural sample. Multiple-aliquot techniques for equivalent dose determination may not suffer from this problem, but are not suitable for samples that might not have been completely bleached at the time of deposition. It is argued that testing for changes in electron trapping probability should become routine practice in the single-aliquot dating of feldspar.

Age and dynamics of linear dunes in the Namib Desert

Bristow, C. S., Duller, G. A. T., Lancaster, N. (2007). Age and dynamics of linear dunes in the Namib Desert. Geology, 35(6), 555-558. Sponsorship: American Chemical Society

Testing optically stimulated luminescence dating of sand-sized quartz and feldspar from fluvial deposits

Abstract We apply single-aliquot optically stimulated luminescence (OSL) dating to quartz- and feldspar-rich extracts from fluvial channel deposits of the Rhine–Meuse system in The Netherlands. The time of deposition of these deposits is tightly constrained by radiocarbon dating or historical sources. This allows us to compare OSL ages obtained on quartz and infrared OSL (IR-OSL) ages obtained on potassium-rich feldspar with independent ages over the range of 0.3–13 ka. We show that the quartz OSL ages are in good agreement with the expected age. Using IR-OSL dating of feldspar, we find a slight age overestimate for the youngest sample, whereas for older samples the age is significantly underestimated. We also apply OSL dating to older fluvial and estuarine channel deposits with limited independent chronological constraints. Comparison of feldspar IR-OSL ages with the quartz OSL ages up to ∼200 ka shows a clear trend, where the former severely underestimates the latter. This trend is similar to that found for the samples with independent age control, indicating that the feldspar IR-OSL ages are erroneously young for the entire age range. In the youngest samples, incomplete resetting of the IR-OSL signal prior to deposition probably masks the age underestimation. We show that the IR-OSL age underestimation is partly caused by changes in trapping probability due to preheating. Correction for this phenomenon improves the IR-OSL ages slightly, but does not provide a complete solution to the discrepancy. We suggest that, in the light of the problems encountered in the IR-OSL dating of feldspar, quartz is the mineral of choice for OSL dating of these deposits. However, feldspar dating should continue to be investigated, because it has potential application to longer time scales.

Temperature dependence of OSL decay curves : Experimental and theoretical aspects

The factors which affect the shape of Optically Stimulated Luminescence (OSL) decay curves are examined, both experimentally and theoretically, in an effort to understand and describe the behavior of OSL as a function of temperature. The processes considered include: (1) retrapping by shallow traps; (2) thermally assisted optical stimulation; (3) thermal quenching; and (4) localized donor-acceptor type recombination. Experimental OSL data from natural quartz and feldspars, stimulated with both green and infra-red light, are examined in the light of the theoretical considerations.

Unprecedented last-glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska

Abstract A high-resolution chronology for Peoria (last glacial period) Loess from three sites in Nebraska, midcontinental North America, is determined by applying optically stimulated luminescence (OSL) dating to 35–50 μm quartz. At Bignell Hill, Nebraska, an OSL age of 25,000 yr near the contact of Peoria Loess with the underlying Gilman Canyon Formation shows that dust accumulation occurred early during the last glacial maximum (LGM), whereas at Devil’s Den and Eustis, Nebraska, basal OSL ages are significantly younger (18,000 and 21,000 yr, respectively). At all three localities, dust accumulation ended at some time after 14,000 yr ago. Mass accumulation rates (MARs) for western Nebraska, calculated using the OSL ages, are extremely high from 18,000 to 14,000 yr—much higher than those calculated for any other pre-Holocene location worldwide. These unprecedented MARs coincide with the timing of a mismatch between paleoenvironmental evidence from central North America, and the paleoclimate simulations from atmospheric global circulation models (AGCMs). We infer that the high atmospheric dust loading implied by these MARs may have played an important role, through radiative forcing, in maintaining a colder-than-present climate over central North America for several thousand years after summer insolation exceeded present-day values.