[RADIOCARBON, Vol. 40, No. 3, 1998, P. xi]
Radiocarbon's previous special Calibration Issues in 1986 and 1993 (Stuiver and Kra 1986; Stuiver, Long and Kra 1993) have been among our most popular and widely distributed issues. This is not surprising, as reliable calibration of radiocarbon dates is crucial to researchers in disciplines where chronological interpretation of data is fundamental - archaeology, paleoenvironmental studies, and Near Eastern history, to name a few that are often represented in the pages of this journal. With INTCAL98, the precision of the calibration curves has been increased and their backwards range extended.
The theory of 14C calibration is relatively straighforward: naturally occurring materials that exhibit annual growth phenomena (e.g., tree rings, lake and marine varves) are 14C-dated as precisely as possible over age ranges that can (ideally) be dated absolutely. The resulting calibration curve shows the relation between conventional 14C dates and calendar ages, its trends and "wiggles" reflecting the variation over time of 14C in the geosphere. Once generated, the calibration curves (or more accurately, their underlying data sets) enable the conversion of a date in radiocarbon years (BP) to a calendar age range or ranges (cal BC/AD). For many users of 14C dates, this is a simple matter of plugging a conventional age into one of the computer calibration programs, or even of accepting and reporting the calibrated dates calculated by the laboratory when it returns results on a sample.
However, it is crucial to remember that a calibrated 14C age is probabilistic, and must not be confused with an absolute calendar date. The papers in this issue report in great detail on the data sets used to construct the INTCAL98 curves; the methods used in choosing, treating and measuring samples; and the statistical assumptions made to arrive at calibrated dates and their associated margins of uncertainty. Understanding this background is important when using calibrated 14C dates as evidence for a chronological argument, particularly when conventional 14C dates intersect multiple ranges on the calibration curve or when claiming very narrow calendar ranges as probable dates of origin.
Introductory discussions of 14C calibration can be found in Aitken (1990: Chapter 4), Bowman (1990: Chapter 4), Bronk Ramsey (1998b) and Taylor (1987: 133-142). The manuals for the OxCal (Bronk Ramsey 1998a), CAL25 (van der Plicht 1998) and CALIB (Stuiver and Reimer 1998) calibration software also discuss the principles of calibration as used in those programs.
David R. Sewell
Aitken, M. J. 1990 Science-Based Dating in Archaeology. London, Longman: 274 p.
Bowman, S. 1990 Radiocarbon Dating. Berkeley and Los Angeles, University of California Press: 64 p.
Bronk Ramsey, C. n.d./1998a OxCal Program [WWW document]. URL http://info.ox.ac.uk/departments/rlaha/oxcal/oxcal_h.html
Bronk Ramsey, C. n.d./1998b Radiocarbon Calibration. Radiocarbon WEB-Info [WWW document]. URL http://units.ox.ac.uk/departments/rlaha/calib.html Stuiver, M. and Kra, R., eds. 1986 Calibration Issue. Radiocarbon 28(2B): 805-1030.
Stuiver, M., Long, A. and Kra, R. S., eds. 1993 Calibration 1993. Radiocarbon 35(1): 1-244.
Stuiver, M. and Reimer, P. 1998 CALIB Version 4.0 [Computer program] URL http://depts.washington.edu/qil/
Taylor, R. E. 1987 Radiocarbon Dating: An Archaeological Perspective. Orlando, Academic Press: 212 p.
van der Plicht, J. 1998 The Groningen Calibration Program Version CAL25 [Computer program] URL http://www.cio.phys.rug.nl/HTML-docs/carbon14/cal25.html