Radiocarbon Calibration and Late Glacial Occupation in Northwest Europe
Blockley, S. P. E., Donahue, R. E., Pollard, A. M., Antiquity
Various methods of analysing the dating of the late Glacial suggest various interpretations. Here, in answer to a paper from 1997, radiocarbon dates are calibrated and used to reconsider the dating of this contentious period.
Key-words: northwest Europe, Late Glacial, radiocarbon calibration, population movement, climatic fluctuation
In a recent paper Housley et al. (1997) presented 127 AMS and 14 conventional radiocarbon estimates from Magdalenian, Hamburgian and Creswellian contexts, in eight areas of Europe (FIGURE 1a). The dates were predominantly on bone, most of which had been humanly-modified. Using these dates they attempted to model movements of people in Europe during the last deglaciation. They argued that much of northwest Europe was abandoned around the time of the Last Glacial Maximum, with populations dwelling in refugia. They proposed a re-colonization of northwestern Europe constrained by the retreat of the ice, and outlined a chronological sequence for this process. A `moving sum' was used to count the dates (Housley et al. 1997: 44). They assumed that this method implicitly accounted for the 1 sigma errors on the uncalibrated dates (since the `bin width' was chosen to be roughly the same as the average 1[Sigma] error), and therefore allowed the treatment of the data as point estimates. The method produced a series of histograms (FIGURE 1a) for the areas of Europe, which were interpreted as supporting a model of population movement. The earliest occupied `bin' on each histogram was taken as identifying the initiation of colonization (`pioneer phase'); the mode of the histogram was interpreted as a `residential phase', when populations were fully established.
[FIGURE 1a ILLUSTRATION OMITTED]
There are, however, some difficulties with this approach, centring around two key areas. Firstly, it may be argued that whilst the moving-sum method may take into account the 1[Sigma] errors on the uncalibrated dates, it does not account for errors at 2[Sigma] (required for 95% confidence). A second difficulty is that it is totally based on uncalibrated radiocarbon dates. Because the [.sup.14]C timescale is incorrect and nonlinear, the true chronological relationships between dates and between groups of dates are not known. It is therefore not possible to use uncalibrated [.sup.14]C age estimates as a linear relative chronology. In this case apparent chronological differences between groups of uncalibrated dates from European regions are used incorrectly to imply population movement (Housley et al. 1997: 43).Thirdly, the definition of the geopolitical regions used in the study is not necessarily relevant to Late Glacial geography. Finally, the dating programme was based around bone which was, where possible, humanly modified (Housley et al. 1997). The archaeological logic behind this is sound, but it leaves open the problem that some bone can be notoriously difficult to date (e.g. Pollard & Heron 1996: 288-90; Taylor et al. 1996).
Here we use a stepwise approach to the reanalysis of the [.sup.14]C dates in the original paper. Firstly, we look at the effects of applying the 2[Sigma] radiocarbon errors to the moving-sum method used by Housley et al. (1997). Secondly, we calibrate the dates using the curve published in 1993 (Bard et al.) which was available when the paper of Housley et al. was written, in order to demonstrate that the original conclusions could have been shown to be unsound at the time. Thirdly, we examine the effect, if any, of using the latest calibration curve published in Radiocarbon 40/3 (Stuiver & van der Plicht 1998). By calibrating, it becomes possible to compare the radiocarbon dates in this study with ice-core temperature curves (e.g. Alley et al. 1993), which are independent of the radiocarbon timescale. This allows us to test the relationship between climate and population movement against high-resolution climatic data. …