Australia: The Land Where Time Began

A biography of the Australian continent 

Spring Creek and Mowbray Swamp – New Dates and XAD-2

At sites in Spring Creek, Victoria and Mowbray Swamp, Tasmania, bones of extinct Australian marsupials have been recovered that yielded radiocarbon ages that were scattered and anonymously young on collagen, gelatin and ultrafiltered gelatin. Material from Mt Cripps in Tasmania and the 2 sites mentioned above, that had previously been dated, was resampled by Gillespie et al., as well as a control sample from Zaglik, Russia, to compare chronologies based on XAD-2 purified total amino acid fractions and collagen purified by ultrafiltration (UF) or methods that were less rigorous. A similar infinite age was given by the control site bone, though 6 out of 7 other comparisons yielded radiocarbon ages on total amino acid fractions older than those that were previously measured on collagen or gelatin. A date of more than 53,500 years ago was found for the Palorchestes from Spring Creek.

Poor organic preservation and ages that are close to the limit of the radiocarbon method have hampered the dating of the bone collagen of extinct Australian marsupials (Gillespie et al., 2006). The discovery of well-preserved bones and teeth in limestone caves in Tasmania, that were subsequently dated to a range from 30,600-44,500 years BP, which have been reported for several extinct and extant species of marsupial changed significantly this situation (Cosgrove et al., 2010; Gillespie et al., 2012; Turney et al., 2008). However, the use of the Oxford method that has been described (Brock et al., 2010) on ultrafiltered gelatin from extinct marsupials recovered from the Mowbray Swamp, northwestern Tasmania, produced dark-coloured gelatin that failed to deliver the background ages that were expected (Gillespie et al., 2010).

It is common for megafauna assemblages to be preserved in swamps, and it has proved especially difficult to date sites on mainland Australia, with few if any credible radiocarbon results being obtained from the bone. Lancefield Swamp in central Victoria is an example that has given dates from 3,000-27,000 BP on various bone fractions, though no collagen was found and there is prevalent humic contamination (Gillespie et al., 1978). There is a similar situation with fossil bones from Black Creek Swamp, Kangaroo Island, with no collagen being found and radiocarbon ages on soil organics in the sediments that were fossiliferous are anomalously young when compared with other dates such as luminescence (Grün et al., 2006). At Spring Creek, southwestern Victoria, ages were obtained that ranged from 19,800 BP on plant remains (Flannery & Gott, 1984) to 36,500 BP on bone gelatin (White & Flannery, 1995) that have been claimed for the extinct fauna.

The removal of small organic molecules, that included soil humic and fulvic acids, and many others, which may be younger or older than the bone, was achieved by the development of the ultrafiltration of gelatin solutions. Humic acids are complex organic molecules that have a high content of aromatic carbon, though very small amounts of nitrogen, the C:N ratio often being utilised as an indicator of the quality of the gelatin – and when the C:N is markedly higher than 3.2, which is the typical value, it is likely that there is exogenous carbon-rich contaminants present (van Klinken, 1999).

Ultrafiltration does not work as expected for some bone samples; also, the ability of the method to achieve the appropriate separations has been questioned (Brock et al, 2013).

The use of polymeric XAD-2 resin specifically to remove humic/fulvic contamination is an alternative treatment (Stafford et al., 1988). The affinity of the resin for non-ionic organic compounds leads to adsorption using hydrophobic interactions and van der Waals bonding, and it has been suggested by comparisons that the XAD-2 method is superior to dating collagen or Longin gelatin (Stafford et al., 1991). In this study Gillespie et al. explore the situations where “correct” ages are not given by the ultrafiltration method, and they report new results for the use of the XAD-2 method on 1 extant and 5 extinct marsupial bones, that were dated previously, from 3 sites of Australian megafauna, and a woolly rhinoceros bone from Zaglik, Russia, as a background control (Storm et al., 2013).

Conclusions

XAD-2 purification methods were tested by Gillespie et al. for total amino acids prepared from several samples of bone that were expected to be near or beyond the limit of the radiocarbon method, comparing their results with ages that had previously been reported for the same bones that were prepared by the Oxford ultrafiltration method or the collagen and Longin gelatin that are less rigorous. The XAD-2 processed total amino acids ages reported here that all except 1 case are older than the other ages, and Gillespie et al. suggest this is the result of removal of fulvic acid contamination that is more effective. Work is not complete on the Mowbray Swamp megafauna, but the results obtained by Gillespie et al. are now close to the background ages that are suggested by the geology, and they say the simple XAD-2 method used in this study merits further testing.

Gillespie et al. say their results further demonstrate that in common with Lancefield Swamp, Black Creek Swamp and Spring Creek are not Australian megafauna sites that are late-surviving.

Sources & Further reading

1. Gillespie, R., R. Wood, S. Fallon, T. W. Stafford and J. Southon (2015). "New 14C dates for Spring Creek and Mowbray Swamp megafauna: XAD-2 processing." Archaeology in Oceania 50(1): 43-48.