J.T. Hutton

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.

Ionic exchange between soil solution and bone: toward a predictive model

Accurate paleoecological inferences from the elemental composition of human and faunal bone recovered from archaeological and paleontological sites will depend on controls for postmortem diagenesis. In vivo chemical signatures in bone can be obscured by postmortem changes in the soil environment. Models addressing the fossilization process in bone must consider both ionic substitution and mineralization mechanisms of diagenesis. Current solubility profile techniques for the differentiation of biogenic and diagenetic chemical phases in fossil bone neglect ionic substitution. Ionic soil solution data from an arid-land South Australian archaeological site are presented to demonstrate the importance of such information to the development and testing of these predictive models. Elemental ratios in the soil solution are compared with those in rib bone from archaeological human skeletons and modern terrestrial mammal controls. The archaeological bone is enriched with Si, AI, Mn, Ba, Fe, S, Sr, K and Ti and depleted in Mg relative to the modern controls. A variety of processes including contamination with quartz and secondary carbonates and ionic substitution from the soil solution are responsible for these postmortem chemical alterations.

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.

The use of soil chemistry data to address post-mortem diagenesis in bone mineral

Abstract The soluble and exchangeable ions for a soil profile from the Roonka archaeological site. Lower Murray Basin, South Australia, are compared with the total elemental content of the soils. Since the chemical composition of post-mortem ionic substitution phases and secondary minerals in archaeological bone will depend on the availability of ions to the soil solution under field conditions, it is suggested that models addressing diagenesis in bone employ soluble and exchangeable ions rather than total elemental soil data.

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.

Role of Salts in Development of Granitic Tafoni, South Australia

Fresh Proterozoic and Paleozoic granitic rocks in South Australia possess approximately 0.06% soluble salts (chiefly Ca, Na, S, and Cl), possibly in fluid inclusions. Active tafoni developed in these rocks show small-scale flaking on their ceiling and walls. Fresh and flaking granite are similar in all respects (chemistry, mineralogy, degree of alteration, texture, and structures), except that soluble salts are 2-13x more abundant in the flakes, where they occur as gypsum and halite crystals. An external source for these salts is ruled out by their chemical character. They are most likely a concentration of soluble substances present in the fresh rock and leached to a near-surface location when available moisture is evaporated. Tafoni configuration, distribution, and mineralogy argue against either thermal changes or wetting-and-drying being major contributors to the flaking process. We believe that salts cause flaking by crystal growth. Residual strain energy in the granites could help explain how only 0.4% salts can create 10-20% fracture porosity in an otherwise fresh rock.

Some quartz thermoluminescence spectra relevant to thermoluminescence dating

Abstract A new high-sensitivity spectrometer was used to study the thermoluminescence spectra of several quartzes. It was found that natural quartz extracted from sediments, but otherwise untreated or given a gamma dose, yields two broad emission bands, one centred at 630 nm and the other extending from 350 nm to at least 550 nm. The dependence of the spectra on radiation dose were examined for both laboratory dose and natural dose, which differ by 9 orders of magnitude in dose rate, and no effect due to dose rate was observed. The dose responses of both emission bands were examined in the range 0–1600 Gy and were found to be highly non-linear and different. It is shown that the predose effect in the high temperature thermoluminescence peak is associated with the blue emission and not the red emission. For some sediment samples the presence of zircon in “quartz” extracts was readily identified by its spectrum; in some cases the zircon even dominated the thermoluminescence. Thus it is clearly necessary to remove the zircon if one wishes to study the thermoluminescence of the quartz. For a commercial quartz it was found that annealing at 700°C caused a dramatic spectral change and an increase in sensitivity, thus casting doubt on the applicability of thermoluminescence studies on annealed quartz to TL dating of natural quartz extracted from sediments.

Composition and genesis of silcretes and silcrete skins from the Beda Valley, southern Arcoona plateau, South Australia

Abstract Detailed petrographic and chemical studies of silcretes collected from Beda Valley near the southern extremity of Lake Torrens, South Australia, have shown that there are at least two distinct types. One type, often found attached to quartzite, has very angular grains of quartz in a matrix very high in titanium. Electron probe studies clearly show that the areas between the quartz grains are mostly titania containing very little silica. These skins, besides containing 2–10% Ti, contain .05 to .25% Zr with a very high correlation between ratio of the amount of these elements in the skin to that in the quartzite and it is suggested they have been produced by the loss of silica. The other type of silcrete has subrounded grains of quartz in a matrix of chalcedonic silica. It is also low in titanium (<1%) and low in zirconium (<.04%). It is suggested this type, which in this area is massive with columnar structure, is formed by the addition of silica.