By Aimee Komugabe
Uranium series (U-series) dating is based on the uranium and thorium radioactive decay chains. These decay chains involve a series of different elements and may be as long as 35 steps, before reaching the stable end product – lead. These elements are usually present in constant (equilibrium) amounts in a sample, as long as the system remains closed. A closed system is one in which there is no free exchange of elements in a material with the outside environment. Because these elements (and their isotopes) all have different half-lives, elements with longer half-lives will be present in greater quantities than those with shorter half-lives. Therefore, when equilibrium is disrupted (for example when a mineral breaks down leading to gain or loss of uranium and/or thorium), we can calculate the degree to which equilibrium has been restored between parent and daughter isotopes. This will give us the precise measure of time that has elapsed since the disruption.
The basic principle of uranium – thorium series dating in corals is that uranium in oxygenated environments is about ten thousand times more soluble in seawater than thorium is. As corals grow, they incorporate significant seawater uranium and virtually no thorium into their skeletons. This causes a disruption in the equilibrium of the 234U decay chain because the parent 234U isotope is incorporated in the skeleton preferentially, without the daughter 230Th isotope. Over the years, 234U in the skeleton decays into 230Th. The age of the coral is thus determined by calculating how long it took for the measured amount of 230Th to accumulate from the decay of 234U.