The contribution of radioactive and chemical dating to the understanding of the environmental system

Radioactive and chemical dating methods are yielding most valuable information on the history of the earth and the planetary system. In this paper mainly methods using cosmic ray produced isotopes are discussed. During the recent past, fluctuations in radioisotopes produced by cosmic radiation in the earth's atmosphere have been observed, the most convincing example being the fluctuations of the 14C/C-ratio observed in tree-ring samples. Such fluctuations complicate the interpretation of radioactive ages in terms of absolute ages, and their interpretation asks for the development of models considering not only isotope production variations but also the geochemical behavior of the isotopes of the different elements. For this purpose, it is useful to distinguish between noble gas radioisotopes (e.g., 39Ar, 81Kr), radioisotopes which get incorporated in molecules of gases and vapors (14C, 3H), and radioisotopes of solids (10Be, 36Cl) which get attached to aerosol particles and are deposited with precipitation. In polar obtained by drilling through the ice caps therefore constitute a continuous set of ancient air samples. 39Ar/Ar and 81Kr/Kr measurements on these samples primarily reflect the production rates of these radioisotopes averaged over a few half-lives. It is expected that due to the short residence time of Be and Cl in the atmosphere, 10Be and 36Cl measurements on ice cores will directly reveal isotope production variations. Due to dilution in the CO2 exchanging system the atmospheric 14C/C-ratio shows a dampened response to 14C production rate variations. In contrast to the noble gas radioisotopes the size of the effective dilution reservoir - atmosphere plus parts of the ocean and biosphere - depends on the characteristic times of the production rate variations. In addition, 14C/C variations in atmospheric CO2 may be caused by variations in the CO2 exchange dynamics, as indicated by the observation of changes in the atmospheric CO2 concentration in ice cores. Finally a strategy for the study of the environmental system and its history is proposed. Dating methods provide the time scale for ancient system states, and fluctuations in the parameters used for dating point to important changes in system processes. Recent developments in field and analytical methods as well as model calculations promise accelerated progress regarding a quantitative understanding of processes determining our environment. This is badly needed in view of possible natural and/or anthropogenic changes with effects on society