J.R. Prescott

Cosmic ray contributions to dose rates for luminescence and ESR dating: Large depths and long-term time variations

Abstract In previous work, we have discussed the way in which the contribution of cosmic rays to the dose rates for TL/ESR dating depends on altitude, latitude and depth below ground level. The present paper extends the discussion to greater depths. Long-term changes in the geomagnetic field and the galactic cosmic ray intensity could affect the dose rate. The effects are shown to be small over the past 500 ka in most circumstances.

New ages for human occupation and climatic change at Lake Mungo Australia

Australias oldest human remains, found at Lake Mungo, include the worlds oldest ritual ochre burial (Mungo III) and the first recorded cremation (Mungo I). Until now, the importance of these finds has been constrained by limited chronologies and palaeoenvironmental information. Mungo III, the source of the worlds oldest human mitochondrial DNA, has been variously estimated at 30 thousand years (kyr) old, 42–45 kyr old and 62 ± 6 kyr old, while radiocarbon estimates placed the Mungo I cremation near 20–26 kyr ago. Here we report a new series of 25 optical ages showing that both burials occurred at 40 ± 2 kyr ago and that humans were present at Lake Mungo by 50–46 kyr ago, synchronously with, or soon after, initial occupation of northern and western Australia. Stratigraphic evidence indicates fluctuations between lake-full and drier conditions from 50 to 40 kyr ago, simultaneously with increased dust deposition, human arrival and continent-wide extinction of the megafauna. This was followed by sustained aridity between 40 and 30 kyr ago. This new chronology corrects previous estimates for human burials at this important site and provides a new picture of Homo sapiens adapting to deteriorating climate in the worlds driest inhabited continent.

Cosmic ray and gamma ray dosimetry for TL and ESR

The cosmic ray dose as a function of depth has been measured to a depth of 450 g cm-2 in limestone and an expression is given that extends this to any depth of likely interest. The use of CaSO4:Dy in copper capsules as a quartz-equivalent dosimeter and the application of an NaI(TI) scintillometer for very low level in situ dosimetry are discussed.

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.

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.

The mechanism of thermoluminescence in an Australian sedimentary quartz

Abstract The thermoluminescence and related properties of a specific Australian quartz are considered, using both original experiments and data from the literature on comparable systems. It is concluded that the rapidly-bleaching peak at about 305 °C in the glow curve and the peaks at 220,180 and 100 °C, which all emit at wavelengths below about 430 nm in this quartz, form a “family”, the electron traps of which all feed the same luminescence centre using the conduction band for electron transport. The slowly-bleaching peak at about 350 °C and smaller peaks at 260 °C and above about 450 °C, emitting at about 470 nm, are outside this system. These peak temperatures are for unirradiated (“natural”) material measured at a ramp rate of 3.1 K/s. While the paper deals in detail with only one specific quartz sample, the properties described have been seen in several others. Whether the conclusions apply more generally remains to be seen.

The effect of illumination wavelength on the bleaching of the thermoluminescence (TL) of quartz

Abstract A study was made of the relative efficiencies with which the component wavelengths of sunlight bleach the natural thermoluminescence (NTL) of quartz (100 μm grains) extracted from sedimentary deposits. The bleaching susceptibility of all NTL glow peaks was found to increase with decreasing wavelength. UV wavelengths (

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.

Three-dimensional thermoluminescence spectra of feldspars

The paper describes a systematic study of the thermoluminescence emission spectra of thirty feldspars covering the whole composition range from high potassium (orthoclase) through high sodium (albite) to high calcium (anorthite). The study was stimulated by the need to understand the properties of feldspars in connection with the application of thermoluminescence to the dating of sediments. The data were obtained with a high-sensitivity Fourier transform spectrometer, which allows measurements at the low light levels found in natural samples. Three-dimensional displays in which intensity is plotted as a function of photon energy and temperature assist identification of a wide range of spectral features. A number of common features are found: an emission at 3.1 eV is strong in alkali feldspars with more than 80 mole.% orthoclase and occurs with lower intensity in most other alkali feldspars and some plagioclases; a broad band with a flat maximum near 2.6 eV is found in all alkali feldspars at temperatures near 200 degrees C when artificially irradiated, and at higher temperatures in some natural samples. Emission at 2.2 and 4.4 eV is characteristic of plagioclases with more than 75 mole.% of albite. Plagioclases with more than 50 mole.% anorthite, and alkali feldspars with less than 50 mole.% orthoclase, emit mainly in the range 1.5-2 eV. These features can be identified with luminescence centres suggested by previous workers. The application to practical thermoluminescence dating is discussed.

Thermoluminescence dating of the Malan loess at Zhaitang, China

Although the widespread loess deposits of north China have been well described, there has been limited information on the absolute age of the various layers. Loess, a wind-carried deposit exposed to sunlight and subsequently buried, should be amenable to dating by thermoluminescence. Through use of the fine grain technique, ages from 18 to 84 ka have been obtained for a sequence in the Malan loess exposed in the Zhaitang section near Beijing. The dates are consistent with the evidence from geology and magnetostratigraphy and provide a chronology for the Zhaitang section.