The best method of assessing rates of change in ecosystems is by long-term monitoring. However, such information is unavailable for most ecosystems, and other means must be employed. In sedimentary environments, chronological scales can be determined by the distribution of radioactive isotopes in the sediment. These timescales are developed by using a known property of radioactive material, the "half-life." The half-life of an isotope is the amount of time it takes for half a given number of radioactive atoms to decay to another element. The age of the sediment containing a radioactive isotope with a known half-life can be calculated by knowing the original concentration of the isotope and measuring the percentage of the remaining radioactive material.
The requirements for a radioisotope to be a candidate for "dating" are that: (1) the chemistry of the isotope (element) is known; (2) the half-life is known; (3) the initial amount of the isotope per unit substrate is known or accurately estimated; (4) the only change in concentration of the isotope is due to radioactive decay; and (5) in order to be useful, it must be relatively easy to measure. If all these conditions are met, the effective range for each isotope is about eight half-lives. Four radioisotopes (7Be, 14C, 137Cs, and 210Pb) satisfy these criteria, and are useful for measuring sedimentary dynamics over the last 100 to 150 years. The following summarizes the uses and potential uses of these four radioisotopes in dating recent sediment.