4D archaeology

Abstract

IntroductionBy far the commonest absolute date estimates come from radiocarbon ages converted to dates by comparing them with the ages of tree rings of known date. There are still many problems with the technique. The quoted errors attached to most of the dates obtained between 1950 and around 1982 have to be increased by factors between 1.4 and 4 (Baillie 1990; Ashmore et al. 2000). There are plateaux in the calibration curve which mean that some ages correspond to an unacceptably wide range of calendar dates. Many archaeological sites contain pieces of charcoal much older than the main period of activity on them. Many charcoal dates obtained before about 1999 were from bulk samples and some demonstrably reflect mixing of charcoal of very different age, providing a meaningless date somewhere in between (Ashmore 1999a). There is now fairly abundant evidence that dates from poorly preserved bone, whether buried or cremated, can be centuries out. The marine effect, which has been assumed to make all Scottish shell dates 405 years too old, may fluctuate (Harkness 1983; Cook & Dugmore pers. comm.). The bones of people who ate food from marine sources show the marine effect and calculation of the required change to an age measured by a laboratory depends on a measurement of the strength of the marine effect at the time the person lived (Barrett et al. 2000). Some dates from residues on pots seem to represent accurately the time they formed; others for unknown reasons do not.