In-situ cosmogenic ¹⁴C: Production and examples of its unique applications in studies of terrestrial and extraterrestrial processes

Nuclear interactions of cosmic rays produce a number of stable and radioactive isotopes on the earth (Lal and Peters 1967). Two of these, ¹⁴C and ¹⁰Be, 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, ¹⁴C (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, ¹⁰Be (half-life = 1.5 myr) was discovered independently by two groups within a decade (Arnold 1956; Goel et al. 1957; Lal 199 la). 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 ¹⁴C in terrestrial solids, designated as in-situ-produced ¹⁴C (to differentiate it from atmospheric ¹⁴C, initially produced in the atmosphere). We also illustrate the application to several earth science problems. This is a relatively new area of investigations, using ¹⁴C as a tracer, which was made possible by the development of accelerator mass spectrometry (AMS). The availability of the in-situ ¹⁴C variety has enormously enhanced the overall scope of ¹⁴C as a tracer (singly or together with in-situ-produced ¹⁰Be), which eminently qualifies it as a unique tracer for studying earth sciences.