The Ownership of Time: Approved [sup.14]C Calibration or Freedom of Choice?

Article excerpt

Radiocarbon analysis does not yield an age but a dimensionless ratio of two carbon isotopes, [.sup.12]C and [sup.14]C (Mook & van der Plicht 1999) which must be converted to an age in years by calibration. The simplest [sup.14]C-age calibration rests on the assumption that [sup.14]C has always been generated at a constant rate, thus making [sup.14]C years equal to calendar years. This assumption is not valid, however, because [sup.14]C years may deviate from calendar years by up to several thousand years, especially in the latest Pleistocene, and corrections are essential.

In 1996 the Godwin Institute of Quaternary Research of Cambridge University initiated a major interdisciplinary study called the (Oxygen Isotope) Stage 3 Project (van Andel 2003a). Based on existing data and new high-resolution palaeoclimate modelling, the project has placed Middle and Upper Palaeolithic human activity (60-20 ka BP) in a glacial climatic context. The detailed record of climatic change was derived from annual varves in Greenland ice-cores while archaeological sites provided proxies for human responses to climate changes that were largely [sup.14]C-dated. Calibration of [sup.14]C dates was thus critical to the project, but in the mid-1990s [sup.14]C calibration by the INTCAL group (Stuiver et al. 1998) had not advanced much beyond ~15 ka BP while the dataset to be calibrated extended back to c. 45 ka BE An interim calibration method was thus required to allow the project to proceed.

A serviceable approach for this purpose was proposed by Laurent Labeyrie (pets. comm. 1996). It rested on the time-variable intensity of the earth's magnetic field that controls the [sup.14]C production rate in the upper atmosphere (Mazaud et al. 1991). Using cores of marine and lake sediments and stacks of lava flows, Laj et al. (1996) calculated the changes in [sup.14]C production between ~20 and ~45 ka BP and derived a correction for [sup.14]C dates. For a while this method served the Stage 3 Project (van Andel 1998) well until in 1998 Joris and Weninger (1998, 2000a, 2000b) provided a superior high-resolution calibration method named 'CalPal'.

Today two principal paths exist for the conversion of [sup.14]C years into calendar years. One is the venerable method of direct comparison of a suite of [sup.14]C dates with a calendric chronology based on tree-rings, the paired-dates calibration mode (Stuiver et al. 1998). The Holocene [sup.14]C calibration is the principal example of this method; its extension into Pleistocene uses paired [sup.14]C and uranium/thorium dates (Taylor et al. 1996) and is now known as 'INTCAL' (Stuiver et al. 1998).

By the late 1990s the only attempt to calibrate Pleistocene [sup.14]C dates, the INTCAL project, using paired [sup.14]C and U/Th dates from corals, had not advanced beyond about 20 ka cal b.p. Also, its resolution was poor due to wide sample spacing, and progress was slow (Bard 2001; Bard et al. 1990, 1993, 1998; Stuiver et al. 1998). The latest 'official' calibration curve (INTCAL04), having reached 26 cal ka BE was recently ratified (Bard et al. 2004) and we are now eagerly awaiting INTCAL05. Had the Stage 3 group waited for completion of the INTCAL calibration project as instructed (van der Plicht 1999), many years would still have to pass before the Project, completed in 2003, could have proceeded.

The other mode is the stratigraphic path to calibration (Bard et al. 2004) of which CalPal is currently the principal version. Cores from the Greenland ice-cap contain a late Pleistocene history of rapid cold-warm climate changes dated by annual varves and known as the Dansgaard/Oeschger (D/O) oscillations (Dansgaard et al. 1993). Early CalPal experiments used the D/O record as a calibrant (an event series dated in calendar years and suitable for calibrating radiocarbon dates) for [sup.14]C-dated events in North Atlantic pelagic sediments (e.g. Volker et al. 1998), and the large Stage 3 Project databases containing the human and mammalian [sup. …