Developments in Radiocarbon Calibration for Archaeology

Article excerpt

Introduction

Radiocarbon dating underpins most of the chronologies used in archaeology for the last 50 000 years. However, it is universally acknowledged that the radiocarbon 'ages' themselves (usually expressed in terms of 14C years BP--because they are measured relative to the standard which corresponds to AD 1950) are not an accurate reflection of the true age (in calendar years) of samples, because the proportion of radiocarbon in the atmosphere has fluctuated in the past and because the half-life used for the calculation of radiocarbon ages is not correct. For this reason, where possible, radiocarbon dates are calibrated against material of known age (giving ages expressed in terms of cal AD, cal BC or cal BP--which is absolute relative to AD 1950). For recent periods (in practice, the Holocene) this is now standard practice amongst archaeologists. However, as we seek to extend the timescale over which calibration is possible, it is important to be aware of the diverse nature of calibration datasets and the limits to their reliability. It is also worth considering some of the reasons behind the controversy over the term 'calibration' (van Andel 2005).

Data for radiocarbon calibration

Until recently the main data that have been employed to generate the estimates of the radiocarbon calibration curve have been measurements of the radiocarbon concentration of wood which has been dendro-chronologically dated to the nearest year. This is ideal from the point of view of archaeologists since the wood in trees is laid down with carbon taken from the atmosphere. The same can be said for most plant fragments, and, through the food chain, for terrestrial animals. So, for the vast majority of archaeological material, the carbon in the samples should have a radiocarbon concentration very close to that of the tree rings used to generate the calibration curve. Only when there are samples from marine or fluvial environments, or other unusual situations (for example depleted [sup.14]C[O.sub.2] from volcanic sources, or significant oceanic upwelling in some coastal situations) do we have to worry about reservoirs of carbon with radiocarbon concentrations that are substantially different to those in the atmosphere.

Over the last couple of decades the extent of tree ring records available has been greatly expanded. In 1986 the firmly dated sections of the calibration curve extended back to about 7300 cal BP (Stuiver 1986), although floating sections could be used to infer its form back over the full extent of the Holocene. When the IntCal04 calibration curve (Reimer et al. 2004) was constructed the tree ring data extended back to about 12 400 cal BP. This record is in most places duplicated many times over, both in terms of the dendro-chronology and with dates measured at a number of different high-precision laboratories. This lends great strength to our conviction that, within the uncertainty quoted on IntCal04, the tree ring section of the IntCal04 curve closely represents a true record for the atmosphere of the mid-latitude Northern Hemisphere (see Figures 1 & 2). The 2004 estimate of the calibration curve for the past 1000 years from the Southern Hemisphere, which has a slightly different radiocarbon concentration (this difference equates to no more than c. 100 [sup.14]C years in this time period), is also available in the form of the SHCal04 curve (McCormac et al. 2004) (see also Figure 2). Furthermore, more data are always being added to this corpus and floating sections of wood from Germany now extend well back into the late glacial. This will almost certainly allow us to extend the terrestrial calibration curve back further in time. Equally interesting is the fact that kauri trees from New Zealand are found with ages that extend right out beyond the range of radiocarbon and are currently being dated in the age range 25-55 000 BP (oral presentation by Chris Turney at the nineteenth International Radiocarbon Conference, Oxford). …