John Head

Beyond the radiocarbon limit in Australian archaeology and Quaternary research

Allen (1994) and Allen & Holdaway (1995), noticing the pattern in early radiocarbon dates from Australia, have advanced the notion their limit records the human settlement of the continent. A critical analysis of context and content in those carbon determinations leads to a different view. The results may be disconcerting for every region which builds its late Pleistocene chronologies on radiocarbon!

Terrestrial vegetation change inferred from n-alkane σ13C analysis in the marine environment

Abstract We report gas chromatography-isotope ratio monitoring-mass spectrometry (GC-IRM-MS) measurements of the δ13C values of individual biomarker compounds (n-alkanes) extracted from a 3 m marine sediment core taken near the mouth of the Johnstone River, North Queensland, Australia. The technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine-derived carbon. The results indicate that there has been a 2% increase in the δ13C values of terrestrially derived n-alkanes (C29−C33) since clearing of the forested Johnstone River drainage basin for sugarcane and pasture began in the late 19th century. A much slower ∼1% increase in δ13C values after 6,000 years BP and prior to European settlement may be related to a decrease in rainfall in the basin, or to an increase in the abundance of C4 plants as a result of increased aboriginal burning. The results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin, and demonstrate that the δ13C values of terrestrially derived n-alkanes in the marine environment can be used to assess basin-wide vegetation changes in adjacent river catchments on geological timescales.

Review of Radiocarbon Data from Atmospheric and Tree Ring Samples for the Period 1945-1997 Ad

A summary of 14C data from atmospheric sampling and measurements on wood from annual tree rings for the period 1945-1997 AD is presented and evaluated. Atmospheric records are characterized by different distributions of bomb-test 14C between the Northem and Southem Hemispheres, latitude dependence, and seasonal fluctuations. Radiocarbon data from tree rings are summarised and plotted against atmospheric records from similar latitudes. In some cases, discrepancies are found. Possible reasons for this include: 1) the use of stored carbohydrate from the previous year, 2) different 14C levels in the air around subcanopy trees due to respiration of CO2, 3) regional and local effects of anthropogenic CO2 and 14C sources, 4) sampling of wood material too close to ring boundaries, and 5) insufficient pretreatment of tree ring sampies for dating. But in cases where trees were carefully selected and the sampies adequately pretreated, radiocarbon data from tree rings show excellent agreement with direct atmospheric sampling records.

Reconnaissance thermoluminescence dating of northern Australian coastal dune systems

Abstract Quaternary lithostratigraphic units in coastal dunes have been dated at three locations in northern Australia, Cobourg Peninsula, Shelburne Bay, and Cape Flattery, by both radiocarbon dating of shell and organic carbon and thermoluminescence (TL) sediment dating. Both coarse fraction and fine fraction TL methods were used. Seventeen TL dates were measured. None of the TL dates contradict the ages given by radiocarbon. Where multiple TL dates were taken from a unit, they overlap within 2 standard deviation giving added confidence in the results. A phase of dune emplacement during the late Holocene (ca. 2700-1800 yr B.P.) was identified in two of the dune-fields, an early Holocene phase of dune emplacement (ca. 8600-7500 yr B.P.) in two of the dune-fields, and a late Pleistocene episode (ca. 24,000–17,000 yr B.P.) in both the Cape York dunefields. Three older units gave dates of about 29,000, 81,000, and 171,000 yr B.P., but these must be treated with caution.

New optical and radiocarbon dates from Ngarrabullgan Cave, a Pleistocene archaeological site in Australia : implications for the comparability of time clocks and for the human colonization of Australia

The human settlement of Australia falls into that period where dating is hard because it is near or beyond the reliable limit of radiocarbon study; instead a range of luminescence methods are being turned to (such as thermoluminescence at Jinmium: December 1996 ANTIQUITY). Ngarrabullgan Cave, a rock-shelter in Queensland, now offers a good suite of radiocarbon determinations which match well a pair of optically stimulated luminescence (OSL) dates - encouraging sign that OSL determinations can be relied on.

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.

The 14C “bomb spike” determines the age spread and age of CO2 in Law Dome firn and ice

We report a precise, model-independent determination of the age and age spread of CO2 in air trapped in ice. A large pulse of atmospheric radiocarbon, generated in the atmosphere by nuclear tests, peaked in the early-to-mid 1960s. We measure the profile of the radiocarbon “bomb spike” in firn air and ice bubbles from high snow-accumulation sites drilled in 1987 and 1993 on Law Dome, East Antarctica, by employing high precision AMS (Accelerator Mass Spectrometry). Large 14C atmospheric growth rates and a high signal-to-noise ratio lead to a direct and precise determination of the CO2 age and age-spread in the ice. A least-squares comparison with the atmospheric history gives a mean CO2 age of 8.9±0.5 years at the bottom of the firn (where vertical gas diffusion ceases) with an age spread in the ice (full width of a moving average smoothing window) of 12.5±1.5 years. These results confirm the possibility of examining decadal trace gas variations prior to direct instrumental measurements.

Multidating studies of Batadomba Cave, Sri Lanka

Abstract In order to explore the reliability of dating techniques and to determine which materials are most useful for the establishment of precise chronologies, 14 C, ESR, TL and OSL analyses were carried out on charcoal, bones. shells and sediments from the archaeological site of Batadomba Cave, Sri Lanka. The dating techniques were complemented by Fourier transform infra-red (FTIR) spectrometry, X-ray diffraction analysis as well as scanning electron microscopy (SEM). Compared with the new and existing charcoal 14 C chronology, radiocarbon dating of shells, luminescence dating of sediments as well as ESR dating of shells provided reasonable agreement. Outliers could be explained by diagenetic alteration of the samples. ESR dating of bones seems to he fraught with many complications, but also provided some reasonable age estimates.

Abandoned Orange-footed Scrubfowl (Megapodius reinwardt) nests and coastal rainforest boundary dynamics during the late Holocene in monsoonal Australia

Abstract The late Holocene history of monsoon rainforest retreat in northern Australia was explored by radiocarbon dating abandoned Orange-footed Scrubfowl ( Megapodius reinwardt ) nests. Scrubfowl can only build ‘nests’ (large heaps of soil and leaf litter) in coastal rainforests in northern Australia. A detailed study from a site with a known history of rainforest contraction demonstrated that it is impossible to accurately determine the date of rainforest boundary retreat by radiocarbon dating abandoned Scrubfowl nests. Nonetheless radiocarbon dates from abandoned nests at three sites on the coast of the Northern Territory suggested that the rainforests contracted sometime within the late Holocene. It is argued that the cause of such localised rainforest contraction is the combined effect of tropical cyclone damage and subsequent severe fires occurring in the storm debris, and not regional climatic change, a cause shown to be inconsistent with existing ecological and geomorphological data. The observed maintenance of rainforest boundaries under a regime of Aboriginal burning and the geographically patchy occurrence of abandoned Scrubfowl nests does not support the idea that Aborigines have been the primary cause of widespread rainforest retreat in the late Holocene.

14C and ESR dating of calcrete and gypcrete cores from the Amadeus Basin, Northern Territory, Australia

Abstract More than forty calcrete and gypcrete samples from bore holes in the Curtin Springs area of the Amadeus Basin, Central Australia, have been dated by means of the 14 C and ESR techniques. The results indicate that the application of multiple dating techniques to samples from calcrete and gypcrete deposits enables age limits to be expanded and the number of datable material types to be increased with respect to Quaternary studies. The ability of the 14 C and ESR techniques to determine the approximate age of calcrete and gypcrete deposits has been demonstrated.