In last month’s ‘Science Notes’ we took you on a tour of the Oxford Radiocarbon Accelerator Unit, discussing the intricacies of radiocarbon dating. There is the risk of portraying the process as fixed and static, but it is always being updated with new treatments and techniques – to make the method even more precise and reliable. One of the most exciting techniques currently being refined is compound specific dating, also known as single-compound AMS dating.
Instead of dating bulk material, which may contain many different molecules, the compound specific method isolates and dates a single biomarker. This provides a high level of confidence that the carbon being dated is actually from the specimen and not a contaminant. At the ORAU, the team is using such an approach to extract the amino acid hydroxyproline from bone collagen. This amino acid has specifically been chosen as it is one of the main contributors to the total amount of carbon in bone from mammals.
Compound specific dating is not new. It has existed – at least in theory – since the 1980s, but until recently had never been totally successful. Similar techniques had been tried, but none were hugely effective. While still in its early days, the current method to isolate and date specific compounds is proving reliable and accurate.
In order to isolate a specific biomarker, the sample has to undergo more pre-treatment steps than normal – and, as a rule, the more steps added to a technique the more room there is for error and the more diligent laboratory technicians need to be. It also means that the procedure is costly and time-consuming: only five or six samples can be tested a week. As such, ORAU is primarily using the technique on samples that are not amenable to standard dating methods, such as those that have a high degree of contamination either from the burial environment or from museum conservation work; Palaeolithic bone samples are usually the most affected by such problems.
Recently, the ORAU team used this technique on a series of bone samples from Vindija Cave in Croatia. Neanderthal remains discovered in the cave were originally thought to be of significant archaeological importance, as radiocarbon dates from the bones suggested that Neanderthals may have inhabited the region as recently as 28,000-29,000 BP. This date would have meant that they co-existed with anatomically modern humans, and might have actually produced some of the Upper Palaeolithic artefacts normally attributed to Homo sapiens. Subsequent redating of the Vindija samples in the mid-2000s produced a slightly older date of 33,000-32,000 BP – which still kept the possibility of an overlap with modern humans. The dates found using the compound specific technique, however, revealed that the Neanderthal remains were actually even older, from before 40,000 BP, and so probably pre-dated the arrival of modern humans in the region.
A similar difference was seen in a sample from the Kostenki site in Russia. Originally, it was thought that a complete skeleton discovered at the site (Kostenki 14) dated to 33,000-32,000 BP. But, due to its unusual preservation, others argued that it could not possibly be that old, and direct dates suggested an alternative interpretation between 3,700-13,600 BP. In this instance, though, the compound specific dating technique proved that the original estimation was closer to the truth, yielding a result of 33,250 ± 500 BP.
While compound specific dating is currently being used almost exclusively for internal ORAU research projects, such as the ERC-funded PalaeoChron project led by Professor Tom Higham (www.palaeochron.org), the team is working on making this method commercially available to other researchers. Papers by the ORAU team that provide further discussions on the process are available for free at http://onlinelibrary.wiley.com/doi/10.1002/rcm.8047/epdf, www.ncbi.nlm.nih.gov/pmc/articles/PMC3344984/, and www.pnas.org/content/114/40/10606.full.
This article appeared in CA 336.