Ewan Lawson

Optical and radiocarbon dating at Jinmium rock shelter in northern Australia

The Jinmium rock shelter is located in the Kimberley region of northern Australia. Claims for ancient rock art and an early human presence at this site were based on thermoluminescence ages of 50–75 thousand years (kyr) for quartz sands associated with buried circular engravings (pecked cupules) and on thermoluminescence ages of 116–176u2009kyr for the underlying artefact-bearing deposits. Here we report substantially younger optical ages for quartz sand, and ages based on measurements of radioactive carbon in charcoal fragments, from the occupation deposit. Using conventional (multiple-grain) optical dating methods, we estimate that the base of the deposit is 22u2009kyr. However, dating of individual grains shows that some have been buried more recently. The single-grain optical ages indicate that the Jinmium deposit is younger than 10u2009kyr. This result is in agreement with the late-Holocene ages obtained for the upper two-thirds of the deposit from radiocarbon measurements. We suggest that some grains have older optical ages because they receivedinsufficient exposure to sunlight before burial. The presence of such grains in a sample will cause age overestimates using multiple-grain methods, whether using thermoluminescence or optical dating.

Sea-level and environmental changes since the last interglacial in the Gulf of Carpentaria, Australia: an overview

Abstract The Gulf of Carpentaria is an epicontinental sea (maximum depth 70xa0m) between Australia and New Guinea, bordered to the east by Torres Strait (currently 12xa0m deep) and to the west by the Arafura Sill (53xa0m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf was separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, an isolation basin, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. A preliminary interpretation is presented of the palaeoenvironments recorded in six sediment cores collected by the IMAGES program in the Gulf of Carpentaria. The longest core (approx. 15xa0m) spans the past 130xa0ka and includes a record of sea-level/lake-level changes, with particular complexity between 80 and 40xa0ka when sea-level repeatedly breached and withdrew from Gulf/Lake Carpentaria. Evidence from biotic remains (foraminifers, ostracods, pollen), sedimentology and geochemistry clearly identifies a final marine transgression at about 9.7xa0ka (radiocarbon years). Before this transgression, Lake Carpentaria was surrounded by grassland, was near full, and may have had a surface area approaching 600xa0km×300xa0km and a depth of about 15xa0m. The earlier rise in sea-level which accompanied the Marine Isotopic Stage 6/5 transgression at about 130xa0ka is constrained by sedimentological and biotic evidence and dated by optical- and thermoluminescence and amino acid racemisation methods.

AMS at ANTARES – The first 10 years

Abstract The status and capabilities of the ANTARES AMS facility after 10 years are reviewed. The common AMS radioisotopes, 10 Be, 14 C, 26 Al, 36 Cl and 129 I, are routinely analysed. A capability for the detection of 236 U and other actinide isotopes has been developed. The measurement program includes support to Quaternary science projects at Australian universities and to ANSTO projects in global climate change and nuclear safeguards.

Bomb radiocarbon in annual tree rings from Thailand and Australia

We have examined the atmospheric 14C excess in the tropics and the southern hemisphere temperate region in the bomb pulse period, using two sets of cross-dated tree rings. One set was from a medium-sized three-leaf pine (Pinus kesiya) grown in northwestern Thailand and the other was from a Huon pine (Lagarostrobos franklinii) grown in northwestern Tasmania, Australia. A total of 48 annual tree rings (24 pairs) from 1952 to 1975 AD were pretreated to alpha-cellulose, combusted to CO2 and converted to graphite for 14C measurement in the tandem accelerator at ANSTO. Excellent agreement was found between our measured 14C data from tree rings and atmospheric 14C records at similar latitudes. A large depletion of atmospheric 14C for Thailand in 1953–1954 AD was observed. This might be due to a combination of the Suess effect and upwelling in the tropical Indian Ocean. The results also showed the rise and decay of bomb 14C peaks from north to south with a time delay of about 1.5 yr, and the effects of minor atmospheric nuclear tests in the late 1960s and early 1970s. A delay of at least one month for 14C in tree cellulose of Huon pine compared with that in the atmosphere was also found.

Measurements of the 14CO2 bomb pulse in firn and ice at Law Dome, Antarctica

Abstract 14CO2 produced in the atmosphere by nuclear weapons testing in the 1960s is now incorporated in the air bubbles of Antarctic ice. The high atmospheric radiocarbon growth rates through the period of tests and subsequent decline provide a unique and independent test for the smoothing of atmospheric CO2 signals due to firn diffusion and bubble close off. The level of smoothing quantifies the time resolution with which atmospheric trace gas histories can be reconstructed from ice cores. In this paper, the methodologies for the preparation and AMS measurements of ice core and firn 14CO2 from high accumulation sites at Law Dome are detailed. The results are compared with predictions of a numerical model incorporating firn air diffusion and bubble close-off. The sample sizes, precision of measurements and sources of contamination are discussed for both firn and ice samples.

New radiocarbon dates for the Roonka Flat Aboriginal burial ground, South Australia

The Roonka Flat site has produced the largest well-provenanced pre-European Aboriginal burial population in Australia. The site is located on an elevated sand flat on the west bank of the lower Murray River near Blanchetown, South Australia. The skeletal remains have been studied by a range of specialists in mortuary analysis, physical anthropology, dental anthropology, palaeopathology and palaeodiet (Pretty and Kricun 1989; Pate 1997a, 1998, in press).

14C AMS measurements of the bomb pulse in N- and S-hemisphere tropical trees

The 14C bomb-pulse signature has been measured by AMS on cross-dateable teak samples from N- and S-hemisphere locations in the tropics. Excellent agreement is found with the atmospheric 14C content in the period 1955 to 1980 for the respective hemispheres. These results demonstrate that 14C measurements can be used to facilitate growth rate determinations in tropical trees.

Research and measurement program at the ANTARES AMS facility

Abstract The ANTARES AMS facility is fully operational and supports a research program in environmental science, with emphasis on global climate change and nuclear safeguards. A measurement program for external projects involving Australian and overseas institutions is also carried out, mainly in Quaternary science studies.

HIGH-PRECISION RADIOCARBON MEASUREMENTS AT THE ANTARES AMS CENTRE

Abstract The ANTARES AMS Centre at Lucas Heights Research Laboratories is currently used for measurements of the long-lived radioisotopes 14C, 26Al, 36Cl and 129I, with measurement of 10Be and actinides under development. Improved precision for 14C (to 0.5%) has been achieved with the recent commissioning of a 59 sample ion source coupled with automated data acquisition. We have developed rigorous data evaluation methods which are invaluable in assessing reproducibility and aid in the identification of physical problems with the system. Low backgrounds, in the accelerator and in the sample preparation laboratories, have allowed us to measure very old samples (to 50 000 years BP) and are also vital for high-accuracy dating of small samples.

AMS radiocarbon dating of bone collagen: Establishing a chronology for the Swanport Aboriginal burial ground, South Australia

Abstract The Swanport Aboriginal skeletal population has played a significant role in physical anthropological research in Australia. This paper provides the first chronometric dates for this important burial population. AMS radiocarbon determinations on bone collagen from six individuals showed a calibrated 2a range from 1027 BC to 1521 AD. On the basis of this sample, the Swanport population appears to pre-date all European contact in Australia. These dates contradict previous assumptions that associated the Swanport burial population with a recent protohistoric period or a discrete period of time related to historic smallpox epidemics in the 19th century. The current chronometric range of approximately 2500 years for inhumations at Swanport indicates the use of the site as a burial ground over an extended period of time during the late Holocene.