European Middle and Upper Palaeolithic Radiocarbon Dates Are Often Older Than They Look: Problems with Previous Dates and Some Remedies
Higham, Thomas, Antiquity
The European Middle to Upper Palaeolithic is widely accepted as being the transitional period over which the final Neanderthals became extinct and were replaced by anatomically modern humans (Mellars 1989, 1999; Zilhao & D'Errico 1999; Zilhao 2006 and references therein). Questions of major importance arise. Did Neanderthals independently develop symbolic and adaptive behaviours before the ~36 500 [sup.14]C BP arrival of modern humans in Western Europe? Did they copy incoming modern human behaviour ('acculturation') whilst the two co-existed in the few millennia prior to Neanderthal extinction? Where, when and how did Neanderthals become extinct and how did their extinction relate to the spatial dispersai of modern human populations? Did Neanderthals and modern humans meet, were they contemporaries in some or all parts of Europe, and did they mate?
The resolution of many of these questions awaits an unambiguous and reliable chronology, bur for 50 years, this has been unattainable. Radiocarbon dates exhibit an asymptotic tendency as they approach the measurement limit, which is itself determined in individual laboratories by careful repeat radiocarbon measurements of material known to be beyond the reach of the technique, i.e. greater than about 60 000 or 70 000 years old (Chappell et al. 1996). Subsequent radiocarbon measurements can never be older than this, since if a radiocarbon date is within two standard deviations of the background value, a 'greater than' age is calculated.
Several scholars have attempted to explore the issues using databanks of radiocarbon dates obtained from archaeological sites (e.g. Bocquet-Appel & Demars 2000, bur see Pettitt & Pike 2001; Joris et al. 2003; van Andel et al. 2003; Dolukhanov & Shukurov 2004; Gamble et al. 2004, 2005; Kuzmin & Keates 2005) and received guidance on how best to view the many available dates (Pettitt et al. 2003). But research undertaken by the Oxford Radiocarbon Accelerator Unit (ORAU) over the last decade throws doubt on the validity of many of these published dates. The principal areas of error lie in the selection of inappropriate samples and the effects of inadequate pre-treatment to remove contamination. This paper addresses some of the problems and illustrates recent progress and a way forward.
It is clear that many samples dated in the past have failed to pass the basic test of bearing witness to the presence of humans. Types of useful samples include cut-marked or humanly-modified bones, human remains, organic artefacts or charcoal from clearly identified features, such as hearths. It is axiomatic that all of these samples must come from secure archaeological contexts, bur even if they do not, they have the distinct advantage over other types of nondescript bone of still being able to provide information regarding the age of human presence. Samples of bone without cut marks ought to be avoided because they may be deposited by animals such as hyena that regularly use the same types of cave and rockshelter sites frequented by humans. Such problems have been particularly acute in the British Paiaeolithic (Jacobi et al. 2006), but are almost certainly more widely applicable. In some contexts it is exceedingly difficult to identify cut marks, since surface etching, degradation, overprinting by animals and deposits of carbonate and sediment can obscure them. Without careful sample selection, no amount of improved pre-treatment chemistry will result in accurate ages.
Samples of Middle/Upper Palaeolithic age are particularly susceptible to errors from contamination. Whilst laboratory and chemical background is quantifiable, however, trace contamination derived from archaeological contexts rarely is. It is this material that may be responsible for producing finite dates for samples that are, in reality, older than the [sup.14]C limit. Historically, this has left us a situation in which we now have many radiocarbon ages that are artificially younger than they ought to be. …