Nigel A. Spooner

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.

The anomalous fading of infrared-stimulated luminescence from feldspars

Abstract Measurements of anomalous fading of infrared-stimulated luminescence (IRSL) are presented for a suite of identified feldspars stored for periods of up to 15 months at temperatures of ∼10°C and 100°C. Examples of feldspars representative of each major compositional regime of the ternary system are seen to fade, and similarly some are stable. Measured luminescence characteristics (emission spectra, relative luminescence intensities, glow curve shapes) and storage durations at these temperatures do not permit reliable separation of stable signals. In a consequent study to determine if preferential removal of the anomalously fading component by bleaching was possible, the optical bleaching response spectra of selected samples exhibiting representative fading and non-fading behaviour were measured. No preferential bleaching of the anomalous fading component was found, precluding an “optical wash” approach for feldspar. These same measurements suggested the IRSL signal originated from one defect type only in all feldspars studied. The ground-to-excited state transition energy was found to be 1.44±0.01 eV (corresponding to ∼861±6 nm) regardless of mineral composition or signal stability. However, the observation of stable IRSL [and optically stimulated luminescence (OSL)] from a variety of feldspar species indicates that anomalous fading is not an inherent property of this defect. It was deduced that the anomalous fading mechanism, at least at and above ambient temperatures, is dominated by a quantum-mechanical tunnelling mechanism. As such anomalous fading cannot be circumvented by any current laboratory procedures, monitoring is essential; discovery of significant anomalous fading is a sample rejection criterion.

The human colonisation of Australia: optical dates of 53,000 and 60,000 years bracket human arrival at Deaf Adder Gorge, Northern Territory

The date at which people entered Australia has important implications for the debate on modern human origins. Thermoluminescence dates of 50–60 ka, reported for initial occupation of the Malakunanja II site in northern Australia, have been used as a means of calibrating the rate of mitochondrial and nuclear DNA evolution in modern humans. Optical dating of unburnt quartzose sediments from a rock shelter site (Nauwalabila I, Lindner Site) in Deaf Adder Gorge, 70 km south of Malakunanja II, provides new evidence for the timing of the colonisation of the continent. Optical dates were determined for several stratigraphic levels within a 3 m deep excavation, in which flaked stone artefacts and ground pigments were found in primary depositional setting. The lowest human occupation levels are bracketed by dates of 53.4 ± 5.4 ka and 60.3 ± 6.7 ka, while the upper levels show good agreement between optical and calibrated 14C age estimates. High-quality haematite with ground facets and striations is associated directly with the 53 ka level and indicates the use of pigments by these early Australians. The optical dates independently confirm evidence for the colonisation of northern Australia shortly after 60 ka and should be seen in the context of this region as having been a likely entry route for the first human movements into Sahul.

On the optical dating signal from quartz

Abstract A series of experiments were conducted on the optical dating signal from quartz. Results are presented showing the quantitative equivalence of the light sums of the optically stimulated luminescence (OSL) emitted by quartz under exposure to 514.5 nm light and that of the quartz “325°C“ thermoluminescence (TL) peak. Discrepancies observed between the OSL and TL light sums under “typical” measurement conditions (i.e. when OSL is measured at ∼293 K and the 325°C TL peak is measured at ∼600 K, in both cases observing ultraviolet emissions) are attributed to thermal quenching of luminescence at the recombination centres (non-radiative recombination). The bleach response spectrum (using bleaching wavebands ranging from 400.6 Δ 10.1 nm to 899.1 Δ 13.2 nm) of OSL from quartz (514.5 nm stimulation) was measured and found, within experimental errors, to be indistinguishable from that previously measured for the 325°C TL peak of quartz. Also measured was the thermal dependence of the OSL signal under stimulation by various wavebands ranging from 480.9 Δ 6.8 nm to 861.1 Δ 12.5 nm, in the temperature range from ∼ 100 to ∼ 500 K. Strong thermal assistance was found for all wavebands tested, with the magnitude of thermal assistance increasing as stimulation wavelength increased— behaviour not inconsistent with photoionization. Supplementary work showed that OSL production involves a single-photon-absorption untrapping step. It is concluded that the OSL emitted by quartz under exposure to visible light, and the 325°C TL peak of quartz, originate from one and the same trapped electron population. Furthermore, optical untrapping of this trapped charge population, resulting in both the bleaching of the 325°C TL peak and the prompt emission of OSL, proceeds by the same mechanism: single-photon-absorption photoionization direct to the conduction band.

Continuous 150 k.y. monsoon record from Lake Eyre, Australia: Insolation-forcing implications and unexpected Holocene failure

Our reconstructed history of Lake Eyre provides the first continuous continental proxy record of Australian monsoon intensity over the past 150 k.y. This continental records broad correspondence to the marine isotope record demonstrates that this very large catchment, with its hydrology dependent on a planetary-scale climate element, responds to Milankovitch-scale climate forcing. Abrupt transitions from dry phases to wet phases (ca. 125 and 12 ka) coincide with Northern Hemisphere winter insolation minima rather than Southern Hemisphere summer insolation maxima, indicating that Northern Hemi- sphere insolation exerts a dominant control over the intensity of the Australian monsoon. Stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but, within the uncertainties, are consistent with Northern Hemisphere forcing. Regardless of the hemispheric forcing, the low intensity of the early Holocene Australian monsoon—by comparison with the last interglacial and particularly the last high-level lacustrine event at 65-60 ka when all forcing elements were modest— is an enigma that can be explained by a change in boundary conditions within Australia.

Optical dating: Preliminary results on the anomalous fading of luminescence from feldspars

Abstract Luminescence emitted from various beta dosed alkali feldspars and plagioclase feldspars has been measured under optical stimulation. The luminescence sensitivities were compared as the initial stage of an on-going study of short-term anomalous fading of the optically stimulated luminescence (OSL) signal. Anomalous fading was observed in the luminescence stimulated by both green light (514.5 nm) and IR (880 Δ 80 nm) from samples representing each of the major compositional regimes of the feldspar series. Means of alleviating the influence of this fading in luminescence dating applications are under investigation, particularly the effect of isothermal storage at 100°C. Initial findings show that anomalous fading in these samples does not lead to a stable remanent level within the storage times used. A practical consequence is that use of a fading monitoring procedure is strongly recommended in optical dating.

Tunnel afterglow, fading and infrared emission in thermoluminescence of feldspars

Abstract Anomalous fading of TL, OSL and IRSL has been observed in many samples of feldspars and attributed to the tunnel effect. Investigations do show expected tunnel afterglow except for samples with no fading. Its intensitym quite noticeable at LNT, is in proportion with reported rate of fading. The emission is entirely in the red and infrared part of the spectrum. An important thermal quenching is observed. Low temperature storage results in fading of TL. Cathodoluminescence emission spectra, monitored from 200 to 900 nm, do confirm TL observations. They show two well-separated ranges: one, “blue”, from UV to yellow (the only one observed in usual TL dating), displays various emission bands; the other, “IR”, red and infrared, shows a well-characterized narrow Gaussian emission band, with a maximum around 720 nm. Models are proposed, relating fading with disorder in crystals. Tunnel afterglow appears as a good criterion of fading in feldspars.

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 (

Kinetics of red, blue and UV thermoluminescence and optically-stimulated luminescence from quartz

The duration over which charge is retained at trapping sites is of fundamental importance for trapped electron dating. Here, we report measurements of the kinetic parameters of the prominent thermoluminescence (TL) glow peaks of quartz, and of the optically-stimulated luminescence (OSL) signal from quartz utilised for optical dating. The similarity in trapping lifetimes of the 325°C TL peak and the dominant component of the OSL signal are taken as further support for the hypothesis of their common origin in the same trapped electron population.

Age and origin of Terra Rossa soils in the Coonawarra area of South Australia

Abstract The famous Terra Rossa soil in the Coonawarra area, South Australia, is dominated by locally derived aeolian detritus, which probably accumulated over the last 120–130 ka. Four soil profiles and associated limestone and lunette deposits were investigated using the following methods: mass balance geochemistry of bulk soil samples (major and trace elements), quantitative X-ray diffraction (XRD) mineralogy, strontium isotopes (87/86), as well as grain-size analysis and cation exchange capacity. These data show that the Terra Rossa soil from the Coonawarra has a thick, clayey B-horizon which is geochemically homogeneous and dominated by smectite and kaolinite. Mass-balance calculations show unrealistic weathering scenarios when plotted using silicate residuum from the underlying limestone as parent. Realistic weathering scenarios are produced with fine-grained silicate material from local lunette deposits as parent. Strontium isotopes of silicate residuum from Gambier Limestone (0.78) contrast strongly with the clayey B-horizon (0.726). Strontium isotope ratios of silicate material from a local lunette (0.725) are similar to the B-horizon soil values. Strontium isotope ratios from regional geological units indicate that the strontium signature in the lunette and soil B-horizon is dominated by weathering products from the Palaeozoic Kanmantoo shales, extensively exposed upwind to the west on Kangaroo Island and the Fleurieu Peninsula. Optical (optically stimulated luminescence, OSL) dating of 61 individual quartz grains (single aliquot) from three samples in the Coonawarra soil profile (one from the A-horizon and two from the B-horizon) shows that most of the quartz sand grains have been buried for only a few thousand years. Many of the grains, however, have been buried for tens of thousands of years with three grains having exposure ages of between 105 and 109 ka. The large population of young exposure dates represents quartz sands recently exposed in the A-horizon and which have been translocated down to the B-horizon. The older exposure dates are interpreted as representing grains that were buried during or soon after the accumulation of wind-blown silt and clay. Our current model concerning the timing and conditions of aeolian deposition of the Coonawarra soil is that much of it accumulated during the relatively wet, last interglacial period around 120–130 ka. During that time span, it is thought that the playa–lunette systems in the low-lying areas to the west were particularly active and generated a significant local dust flux.