Number of dates relating to each century
[This analysis is intended to demonstrate the potential of the database's calibrated data fields, it should not be read as a definitive interpretation of the information held in the database.]
There is a general view that radiocarbon dates are a necessary tool for those studying periods predating the widespread adoption of writing, but are too imprecise to be of value to those studying more recent times (see commentary in Haselgrove 1986 and Gerrard 2007).
A review of the use of radiocarbon dates in the national period-specific journals suggests this view is indeed widely held:
Proceedings of the Prehistoric Society: 570 references to dates from the study area, in 50 articles
Britannia: 47 references in 22 articles
Medieval Archaeology: 134 references in 36 articles
Post-medieval Archaeology: 5 references in 3 articles
But, as the chart below shows, a wealth of radiocarbon dates exist for recent times - far more than are available to scholars of early periods.
Number of determinations potentially relating to each century (dates, n = 5084; each appears more than once)
The IntCal04 calibration curve calibrates radiocarbon dates back to 12,400 BP based on tree-ring date, then from 12,400 - 26,000 BP it relies on marine records, and after this calibrations are performed by comparison with other sources of data (Bronk Ramsey et al 2006). In consequence it is possible to produce a calibrated date, for all radiocarbon dates, albeit with decreasing levels of accuracy the older the sample.
The chart shows the number of calibrated dates (y-axis) which may have an overlap with a specific century (x-axis). It includes all environmental and archaeological dates and as such is a measure of the availability of datable organic material, and of the intensity of research interest in specific periods.
A more accurate measure of the likely relevance of this radiocarbon dateset to a specific century would have been obtained by summingthe probability curves of all the calibrated dates. Unfortunately, my computer, powerful as it is, could not cope with the challenge.
40000 - 17000 BC There are less than 25 determinations potentially relevant to each century between these dates. This presumably reflects the 'new broom' effect caused by the last glaciation sweeping away earlier organic material from much of Wales and northern Britain.
17000 BC There are no dates potentially relevant to this time. This episode broadly correlates with the fullest extent of the ice sheets in the years preceeding the late glacial maximum (about 16000 BC) (Richards 2005, 19), at which time the amount of organic activity in the study area was probably at its minimum.
17000 - 12200 BC The number of potentially relevant dates rises for these centuries, as the ice sheets melt and the quantity of organic material in the study area increases. The rise is very rapid after 13000 BC reflecting the start of the Alleröd interstadial.
12200 - 9700 BC There is a slight trough in the number of dates in this period, possibly reflecting the resurgence of glaciers during the Younger Dryas, and a consequent short-lived downturn in the organic reservoir.
9700 - 5000 BC The number of dates potentially relevant to the first four millennia of the Holocene is much larger than for the Pleistocene. The climatic amelioration, consequent increase in organic activity, and the lack of a subsequent Ice Age to sweep this activity away, can all be seen as responsible.
More surprising is the trough in the curve between 7600 and 5800 BC. Although the calibration curve is quite flat for a few centuries after 7000 BC, this alone does not seem an adequate explanation for this phenomenon.
5000 - 2600 BC With Wales already fully forested and biologically active by this time, much of the dramatic rise in the number of potentially relevant dates during these centuries seems largely to be a result of human activity. The introduction of agriculture was accompanied by the happy habit of digging pits and building monuments - both sources of datable material. Indeed a peak in the number of dates from 3700 - 3400 BC seems likely to be a consequence of burial in tombs - a readily datable activity.
2600 - 900 BC Likewise during this period, the large peak in number of potential relevant dates around 1800 BC is probably a consequence of a second phase of 'organic-caching', via the building of cairns and barrows during the Early Bronze Age. The subsequent trough correlates with the decline in deliberate burial during the later Bronze Age.
900 BC - AD 43 The number of potentially relevant dates peaks around 400 BC during the Iron Age. This probably correlates with the widespread construction of hillforts, defended enclosures, and smaller settlements. It is worth noting that this was a period for which Haselgrove (1986) believed researchers did not naturally utilise radiocarbon dates. This suggests that there is a sizeable body of chronological data waiting to be exploited.
AD 43 - 400 The impression has been gained during this study that radiocarbon analysis has been applied to very few Romano-British sites, and yet the number of potentially relevant dates within the database is still extremely high. It seems likely that many of these dates have been generated by archaeologists who were expecting other results. This, in turn, raises the possibility that some types of sites, not traditionally studied by Romanists, might contain evidence for activity during this period.
AD 400 - 1100 This period, referred to in outdated textbooks as the Dark Ages, also has a large number of potentially relevant dates. As with the Romano-British period it is likely that many of these were obtained by chance, but again there is the strong possibility that radiocarbon dates are a seam of evidence which has yet to be mined.
Post AD 1100 The number of potentially relevant dates declines steadily in the centuries after AD 1100, declining to 'Pleistocence-levels' by AD 2000.
Bronk Ramsey, C., Buck, C. E., Manning, S. W., Reimer, P. J. and Van der Plicht, H. (2006) 'Developments in radiocarbon calibration for archaeology'. Antiquity 80, 783-98.
Gerrard, J. (2007) 'The temple of Sulis Minerva at Bath and the end of Roman Britain'. Antiquaries Journal 87, 148-64.
Haselgrove, C. (1986) 'An Iron Age community and its hillfort: the excavations at Danebury, Hampshire, 1969 - 79. A review'. Archaeological Journal 143, 363-9.
Richards, A. E. (2005) 'Stratigraphy'. In C. A. Lewis and A. E. Richards (eds), 'The glaciations of Wales and adjacent areas'. Almeley: Logaston, 27-40.