Reservoir models and production rate variations of natural radiocarbon

Distribution and time constants of the exchangeable carbon on the earth's surface are discussed in terms of a two-box model and a three-box model. Emphasis is given to the question of the functional relationship of the 14C level in the atmospheric CO2 to the cosmic ray production rate Q. It is shown in a quantitative way that relatively slow changes (with a time constant of more than 30 years) of the atmospheric 14C content can be described adequately by a two-box model. More rapid changes require more complicated models. The most appropriate definition of 'residence time' of 14C (and CO2) in the atmosphere depends on the time scale of the phenomenon under consideration. The observed fluctuations of the 14C content of atmospheric CO2 during the past 7000 years can be explained by assuming production rate variations due to geomagnetic and heliomagnetic modulations of the cosmic ray flux, as long as these fluctuations occur on a time scale longer than ∼100 years. More rapid fluctuations with amplitudes in excess of 1%, if they exist, will require a different explanation.