DIRECT DATING OF FOSSIL MEGAFAUNA USING COSMOGENIC CL-36

The age of fossilized skeletal remains is usually determined by association with radiocarbon dates, luminescence dating of the encasing sediments, or uranium-series dating of bracketing speleothems. However, in all of these cases the age of the bones can be very different, even if reworking is not a factor. Direct dating has proven difficult, because bones tend to exchange isotopically with the environment and therefore not act as closed systems. In this paper we describe a new approach to directly dating tooth enamel using the cosmogenic isotope ³⁶Cl. This radionuclide is produced from a nuclear reaction with argon in the atmosphere, where it is dissolved into rain, and delivered to the Earth’s surface. The isotope is incorporated into hydroxyapatite during tooth formation and provides a decay clock with a half-life of 301,000 years. For initial dating trials, we chose the exceptionally well-preserved and independently dated fossil remains at the Naracoorte World Heritage area. We defined the modern ³⁶Cl/Cl ratio in the environment using widespread, large, grazing kangaroos (Marcopus giganteus or M. fuliginosus). Because of the close proximity of the site to the ocean, the modern ³⁶Cl/Cl is very low due to dilution by sea salt. In the caves there is some background ³⁶Cl production due to neutron production resultant from the decay of uranium and thorium in the sediments, and a minor contribution from muons penetrating the roof. Using a novel stepped-dissolution method, we were able to remove post-depositional contamination, minimize sample size and greatly reduce isobaric interference from collagen sulfur. The background-corrected fossil ³⁶Cl/Cl ratio in Victoria Fossil Cave gives an age of ~270,000 years, consistent with existing U/Th ages on bracketing speleothems. In Cathedral Cave the ratio gives a background-corrected age of ~430,000 years, older than existing U/Th ages.