Geochemical and geophysical implications of the radiocarbon calibration

摘要

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 ~(14)C dating method has been widely applied since the 1950s, it is recognized that the atmospheric ~(14)C/~(12)C 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 ~(14)C 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 ~(14)C ages being significantly younger than calendar ages during most of the last 45,000 years. Abrupt ~(14)C shifts, which occurred over centuries to millennia, are superimposed on this long-term trend of decreasing atmospheric ~(14)C/~(12)C ratio. To a certain extent, it is possible to outline the different causes of atmospheric ~(14)C variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes: (1) Most high-frequency changes in the atmospheric ~(14)C/~(12)C 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 ~(14)C/~(12)C events with ~(10)Be and ~(36)Cl concentration maxima in polar ice cores. (2) The long-term trend shift of ~(14)C 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 ~(10)Be and ~(36)Cl analysed in low and high latitude records. (3) A prominent and rapid atmospheric ~(14)C/~(12)C excursion occurred between 13,000 and 11,500 cal-yr-BP, thus corresponding to the Younger Dryas cold period. By contrast with the ~(14)C variability mentioned above, which is linked to ~(14)C production changes, this so-called ~(14)C 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.