In-situ cosmogenic (super 14) C; production and examples of its unique applications in studies of terrestrial and extraterrestrial processes.

Abstract

Nuclear interactions of cosmic rays produce a number of stable and radioactive isotopes on the earth (Lal and peters 1967). Two of these, (super 14) C and (super 10) Be, find applications as tracers in a wide variety of earth science problems by virtue of their special combination of attributes: 1) their source functions, 2) their half-lives, and 3) their chemical properties. The radioisotope, (super 14) C (half-life = 5730 yr) produced in the earth's atmosphere was the first to be discovered (Anderson et al. 1947; Libby 1952). The next longer-lived isotope, also produced in the earth's atmosphere, (super 10) Be (half-life = 1.5 myr) was discovered independently by two groups within a decade (Arnold 1956; Goel et al. 1957; Lal 1991a). Both the isotopes are produced efficiently in the earth's atmosphere, and also in solids on the earth's surface. Independently and jointly they serve as useful tracers for characterizing the evolutionary history of a wide range of materials and artifacts. Here, we specifically focus on the production of (super 14) C in terrestrial solids, designated as in-situ-produced (super 14) C (to differentiate it from atmospheric (super 14) C, initially produced in the atmosphere). We also illustrate the application to several earth science problems. This is a relatively new area of investigations, using (super 14) C as a tracer, which was made possible by the development of accelerator mass spectrometry (AMS). The availability of the in-situ (super 14) C variety has enormously enhanced the overall scope of (super 14) C as a tracer (singly or together with in-situ-produced (super 10) Be), which eminently qualifies it as a unique tracer for studying earth sciences.