Dating methods

Relative and quantitative techniques used to arrange events in time and to determine the numerical age of events in history, geology, paleontology, archeology, paleoanthropology, and astronomy. Relative techniques allow the order of events to be determined, whereas quantitative techniques allow numerical estimates of the ages of the events. Most numerical techniques are based on decay of naturally occurring radioactive nuclides, but a few are based on chemical changes through time, and others are based on variations in the Earth's orbit. Once calibrated, some relative techniques also allow numerical estimates of age. See also Archeology; Astronomy; Geology; Radioisotope.

Relative dating methods rely on understanding the way in which physical processes in nature leave a record that can be ordered. Once the record of events is ordered, each event is known to be older or younger than each other event. In most cases the record is contained within a geological context, such as a stratigraphic sequence; in other cases the record may be contained within a single fossil or in the arrangement of astronomical bodies in space and time. The most important relative dating methods are stratigraphic dating and paleontologic dating. Other relative dating methods include paleomagnetic dating, dendrochronology, and tephrostratigraphy. See also Dendrochronology; Paleomagnetism; Sequence stratigraphy; Stratigraphy.

Several chemical processes occur slowly, producing changes over times of geological interest; among these are the hydration of obsidian, and the conversion of L- to D-amino acids (racemization or epimerization). Determination of age requires measurement of a rate constant for the process, knowledge of the temperature history of the material under study, and (particularly for amino acid racemization) knowledge of the chemical environment of the materials. See also Amino acid dating; Obsidian; Racemization.

Unlike chemical methods, in which changes depend both on time and on environmental conditions, isotopic methods which are based on radioactive decay depend only on time. A parent nuclide may decay to one stable daughter in a single step by simple decay [for example, rubidium decays to strontium plus a beta particle (⁸⁷Rb → ⁸⁷Sr + β)]; to two daughters by branched decay through different processes [for example, potassium captures an electron to form argon, or loses a beta particle to form calcium (⁴⁰K + e⁻ → ⁴⁰Ar; ⁴⁰K → ⁴⁰Ca + β)]; to one stable daughter through a series of steps (chain decay); or into two unequal-sized fragments by fission. In all cases, the number of parent atoms decreases as the number of daughter atoms increases, so that for each method there is an age-sensitive isotopic ratio of daughter to parent that increases with time. Many different isotopes have been exploited for measuring the age of geological and archeological materials. For example, the table shows parent isotopes, their half-lives, and the resulting daughter products. See also Half-life.

Astronomers have estimated the age of the universe, and of the Milky Way Galaxy, by various methods. It is well known that the universe is expanding equally from all points, and that the velocity of recession of galaxies observed from Earth increases with distance. The rate of increase of recession velocity with distance is called the Hubble constant; and knowing the recession rate and distance of galaxies at some distance, it is simple to find how long it took them to get there. Initial estimates for the age of the universe were approximately 20 billion years; but as the rate of expansion decreases with time, revised estimates are nearer 13 billion years. By contrast, the Earth and other bodies in the solar system are only about 4.5 billion years old. Comparison of present-day osmium isotope ratios with theoretically estimated initial ratios yields estimates of 8.6–15.7 billion years for the age of the Galaxy. See also Cosmochemistry; Cosmology; Milky Way Galaxy; Universe.