GEOCHEMICAL AND GEOPHYSICAL IMPLICATIONS OF THE RADIOCARBON CALIBRATION*

Abstract

A precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45,000 years. Although the 14C dating method has been widely applied since the 1950s, it is recognized that the atmospheric 14C/12C ratio has not been stable during the past. In order to calculate accurate ages, these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14C measurements with true calendar ages provided by independent dating methods.The calibration curve obtained so far is characterized by a long-term trend with raw 14C ages being significantly younger than calendar ages during most of the last 45,000 years. Abrupt 14C shifts, which occurred over centuries to millennia, are superimposed on this long-term trend of decreasing atmospheric 14C/12C ratio. To a certain extent, it is possible to outline the different causes of atmospheric 14C variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes: (1) Most high-frequency changes in the atmospheric 14C/12C ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the Sun are available. A similar conclusion is derived by comparing 14C/12C events with 10Be and 36Cl concentration maxima in polar ice cores. (2) The long-term trend shift of 14C ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10,000 and 40,000 years BP. This interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10Be and 36Cl analysed in low and high latitude records. (3) A prominent and rapid atmospheric 14C/12C excursion occurred between 13,000 and 11,500 cal-yr-bp, thus corresponding to the Younger Dryas cold period. By contrast with the 14C variability mentioned above, which is linked to 14C production changes, this so-called 14C age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle. This interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle.