D. J. Huntley

Optical dating studies of quartz and feldspar sediment extracts

Abstract Continuing developmental work into optical dating has led to some clarification of the optical behaviour of quartz and feldspar sediment extracts. We have verified the key assumption intrinsic to the method — that the optical signal is zeroed completely upon exposure to daylight. We also show that the rate of zeroing in optical dating is much more rapid than it is in TL dating. Studies concerned with the extraction of the thermally stable signal from laboratory-irradiated samples showed that problems may arise from medium-to-high-temperature preheating of young samples. Optical alternatives to thermal stabilization were tested using dye and krypton lasers. Using a krypton laser we have succeeded in obtaining an equivalent dose of 0.0 ± 0.7 Gy for the quartz from a modern intertidal sand, and an average dose of 120 ± 22 Gy for the feldspar extract of a ∼70 ka sand unit. Both were obtained using six wavelengths, from the infrared to the violet, and no dependence on wavelength was found. We found that the photon energies of infrared and deep red light can stimulate luminescence in unirradiated quartz and feldspar extracts of sediments. This was not expected, and suggests that the process of trap emptying is more complex than we had envisioned, but implies a far greater than expected degree of technical flexibility for the optical dating method.

Thermoluminescence dating of sediments

Abstract The potential for using the thermoluminescence behaviour of sediments for dating them was first recognized by Soviet scientists G.V. Morozov and V.N. Shelkoplyas, and for over a decade their TL dates, obtained from a variety of sediments, have appeared in the Soviet literature. Since 1977 TL sediment dates have been published by six additional groups using a variety of methods. In this review we describe the principles of TL dating, the various methods used, and contrast TL dating of sediments with the now well-accepted TL dating of pottery. We conclude that while TL dating has the potential to solve many sedimentary problems, more fundamental research needs to be carried out before such dates should be accepted. A set of criteria for acceptable dates is proposed.

The stranded beach-dune sequence of south-east South Australia: A test of thermoluminescence dating, 0–800 ka

The remarkable sequence of stranded beach dunes in south-east South Australia deposited during the past 800 ka has provided us with a rare set of samples for testing thermoluminescence sediment-dating methods. Here we report thermoluminescence ages obtained from quartz separated from the dunes, and compare them with accepted ages for the high sea-levels responsible for the dunes. The independent ages of the dunes were obtained from earlier work by Schwebel which has been extended and refined by modelling the dune formation using sea-level variations derived from the oceanic δ18O record. The work is unusual in extending thermoluminescence dating to an 800 ka sequence with modest geological control of the age. The agreement between thermoluminescence and geological ages is satisfactory for eight dunes with ages in the range 120–800 ka. Quartz is not usually thought to be useful for thermoluminescence dating in this time span; two factors contribute to our success. The first is that the dose rates are low, about 0.5 Gy ka−1, or one quarter of that typical of sediments. The second is that the thermoluminescence vs. dose response continues to rise monotonically above the saturation region at doses above 300 Gy.

An explanation of the power-law decay of luminescence

Luminescence decay with time often shows a power-law dependence of the form intensity , where t is time and k is usually in the range 1–1.5. It is shown here that this power law can result from the tunnelling of trapped electrons to recombination centres that are randomly distributed, and that the range of exponents matches that of the observations. The explanation accounts for the most extreme case of an observed t−1.06 dependence extending over nine decades of time.

Optical dating using inclusions within quartz grains

A sequence of stranded beach dunes in the southeast of South Australia which resulted from the advance and retreat of the sea over a tectonically rising land surface during the past 800 ka has been securely dated by geological means and successfully used to test thermoluminescence dating based on quartz. We have now obtained a series of promising results from the same sequence with a test of optical dating on inclusions within the quartz grains. The innovative aspect of the measurements is the use of infrared irradiation to stimulate emission from the inclusions rather than conventional stimulation of the quartz itself by light of shorter wavelength. Satisfactory ages were found for seven dunes covering the time span 0-400 ka.

Light-induced emission spectra from some quartz and feldspars

Abstract Some luminescence spectra in the range 300–600 nm relevant to optical dating are reported. Quartz grains separated from the south-east South Australian dune sequence gave a single emission band at 365 nm with a width of 60 nm, quite different from the ≈350°C thermoluminescence (TL) spectra of the same material. Feldspars generally gave emission bands at 330, 400 and 570 nm, the 400 nm band being particularly strong for K feldspars and 570 nm band being strongest for Na-rich plagioclase feldspars, similar to their ≈300°C TL spectra.

Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating

Abstract A new thermoluminescence spectrometer is described. With it the whole spectrum from 300–750 nm is recorded while the sample is being heated. The spectrometer is sufficiently sensitive to permit the measurement of spectra from naturally occurring minerals found in sediments and to detect the presence of zircon in a quartz seperate. Spectra are for 12 feldspars; these show a wide range of behaviour although there are some systematic features. The main feldspar emission bands relevant to thermoluminescence dating are at 390, 450 and 570 nm; there is also emission below 350 nm and in the infrared. The 570 nm emission band is found only in the plagioclase feldspars and is thought to arise from Mn2+ substituting for Ca2+. It is shown that the blue emission in both single feldspar minerals and polymineral samples is often the sum of the 390 and 450 nm bands which overlap and have different temperature dependences. Several natural quartz samples were studied. These all showed similar spectra, with emission bands centered at 470 and 630 nm, the relative intensities varying from sample to sample. It was found that the predose effect in the 375°C thermoluminescence peak occurs in the 470 nm band and not the 630 nm band.

Further thermoluminescence dates from the dune sequence in the southeast of South Australia

Abstract Our thermoluminescence dating technique has been applied to several new samples from the remarkable sequence of stranded beach dunes in southeast South Australia that were formed during the past 800 ka. We show that Robe III is probably 100 ka as suggested by Schwebel and not 80 ka as we suggested earlier. A sample from the West Dairy dune is shown to belong to δ 18 O Stage 7 in accordance with Schwebels original suggestion and not Stage 5 as he suggested later. A new West Naracoorte sample yielded acceptable data and an age consistent with formation just after the Brunhes-Matuyama magnetic reversal at 780 ka as required by palaeomagnetic measurements. Two more samples from the Woakwine dune show that our reproducibility is consistent with our derived errors.

Optical dating of dune sand for the study of sea-level change

A new, widely applicable method to obtain well-constrained relative sea-level records, presented here, relies on optical dating of inorganic coastal landforms and sediments. Eighteen optical ages of basal dune sand from a barrier in Massachusetts, United States, indicate that local relative sea level rose 8 m during the past 5.5 k.y. A sea-level curve drawn from these optical ages shows good agreement with independent evidence from salt-marsh peat sampled behind the barrier, demonstrating the reliability of the method. Optical dating will be particularly useful in analyzing coastal records that contain too few organic indicators to allow high-resolution, 14 C-based analyses.

Thermoluminescence responses to alpha and beta irradiations, and age determination when the high dose response is non-linear

Abstract In thermoluminescence dating, in which the TL of naturally occurring minerals is used to obtain past radiation doses, the responses of these minerals to both alpha and beta irradiations must be measured. In this paper a new parameter, the-b-value, is introduced to relate these two responses in the low-dose linear response regions. It is then shown that in simple models for the high-dose non-linear responses, the b-value concept can still be applied and that it can be determined in a straightforward manner using equivalent beta dose and equivalent alpha track length measurements. The b-value equivalent dose so determined may be used in the standard age equation for archaeological and geological materials without errors due to non-linearity. An example supporting the model is given.