living organisms absorb radiocarbon, an unstable form of carbon that has a half-life of about 5,730 years. During its lifetime, an organism continually replenishes its supply of radiocarbon by breathing and eating. After the organism dies and becomes a fossil, C-14 continues to decay without being replaced. To measure the amount of radiocarbon left in a fossil, scientists burn a small piece to convert it into carbon dioxide gas. Radiation counters are used to detect the electrons given off by decaying C-14 as it turns into nitrogen. The amount of C-14 is compared to the amount of C-12, the stable form of carbon, to determine how much radiocarbon has decayed and to date the fossil. Radioactive decay may take different routes. Thus, if the isotope decays by alpha emission, it loses the two protons and two neutrons that make up an alpha particle; the atomic number (number of protons) is reduced by two and the atomic mass (number of nuclear particles, or nucleons) by four. In beta decay, or electron loss, a radioactive nucleus can gain or lose one unit of electric charge without changing the number of nucleons. More radioactive substances are beta-ray emitters than alpha-ray emitters. A third important mode of decay involves electron capture; the nucleus of an atom absorbs an electron, which unites with a proton of the nucleus to form a neutron. Thus, the atomic number is reduced by one, but the mass of the nucleus remains unchanged. The fourth mode of decay, gamma radiation, consists of the emission of waves of electromagnetic energy.  Scientists describe the radioactivity of an element in terms of half-life, the time the element takes to lose 50 percent of its activity by decay. This covers an extraordinary range of time, from billions of years to a few microseconds. At the end of the period constituting one half-life, half of the original quantity of radioactive element has decayed; after another half-life, half of what was left is halved again, leaving one-fourth of the original quantity, and so on. Every radioactive element has its own half-life; for example, that of carbon-14 is 5730 years and that of uranium-238 is 4.5 billion years. Radiometric dating techniques are based on radio-decay series with constant rates of isotope decay. Once a quantity of a radioactive element becomes part of a growing mineral crystal, that quantity will begin to decay at a steady rate, with a definite percentage of daughter products in each time interval. These "clocks in rocks" are the geologists' timekeepers. Carbon-14 Method Radiocarbon dating techniques, first developed by the American chemist Willard F. Libby and his associates at the University of Chicago in 1947, are frequently useful in deciphering time-related problems in archaeology, anthropology, oceanography, pedology, climatology, and recent geology. Through metabolic activity, the level of carbon-14 in a living organism remains in constant balance with the level in the atmosphere or some other portion of the earth's dynamic reservoir, such as the ocean. Upon the organism's death, carbon-14 begins to disintegrate at a known rate, and no further replacement of carbon from atmospheric carbon dioxide can take place. The rapid disintegration of carbon-14 generally limits the dating period to approximately 50,000 years, although the method is sometimes extended to 70,000 years. Uncertainty in measurement increases with the age of the sample. Although the method is suited to a variety of organic materials, accuracy depends on the half-life to be used, variations in levels of atmospheric carbon-14, and contamination. (The half-life of radiocarbon was redefined from 5570 ± 30 years to 5730 ± 40 years in 1962, so some dates determined earlier required adjustment; and due to radioactivity more recently introduced into the atmosphere, radiocarbon dates are calculated from AD 1950.) The radiocarbon time scale contains other uncertainties, as well, and errors as great as 2000 to 5000 years may occur. Postdepositional contamination, which is the most serious problem, may be caused by percolating groundwater, incorporation of older or younger carbon, and contamination in the field or laboratory. Potassium-Argon Method The decay of radioactive potassium isotopes to argon is widely used for dating rocks. (The decay of potassium-40 to calcium-40 that also takes place is not useful.) Geologists are able to date entire rock samples in this way, because potassium-40 is abundant in micas, feldspars, and hornblendes. Leakage of argon is a problem if the rock has been exposed to temperatures above 125° C (257° F), because the age of the rock will then reflect the last episode of heating rather than the time of original rock formation. Rubidium-Strontium Method Used to date ancient igneous and metamorphic terrestrial rocks as well as lunar samples, this method is based on disintegration by beta decay of rubidium-87 to strontium-87. The method is frequently used to check potassium-argon dates, because the strontium daughter element is not diffused by mild heating, as is argon. Methods Involving Thorium-230 Thorium ratio methods are used to date older oceanic sediments beyond the range of radiocarbon techniques. Uranium in seawater eventually decays to the thorium isotope, thorium-230 (also called ionium), which is precipitated into ocean-floor sediments. Because it has been undergoing decay longer, scientists can detect a decrease in quantity in higher levels, and a time scale can be developed in this way. Thorium-230, part of the uranium-238 decay series, has a half-life of 80,000 years. Protactinium-231, derived from uranium-235, has a half-life of 34,300 years. Both parent elements are precipitated in the same proportions but at different rates. The ratio of the two changes regularly with time, showing greater differences in the quantity of undecayed parent isotopes in older sediments. The ionium-thorium age method, applied to deep-sea sediments formed during the last 300,000 years, is based on the assumption that the initial ionium content of accumulating sediments has remained constant for the total section under study and is not derived from uranium decay; the age of the sample depends on this ionium excess, which decreases with time. In the ionium-deficiency method, the age of fossil shell or coral from 10,000 to 250,000 years old is based on the growth of ionium toward equilibrium with uranium-238 and uranium-224, which entered the carbonate shortly after its formation or burial. Similar disequilibrium relationships can be used to assess ages of carbonates in soils; this method is a complement to carbon-14 methodology. Methods Involving Lead Lead-alpha age is estimated by spectrographically determining the total lead content and alpha-particle activity (uranium-thorium content) of zircon, monazite, or xenotime concentrates. The lead-alpha, or Larsen, method is applied to rocks younger than Precambrian. In the uranium-lead method, age in years is calculated for geologic material based on the known radioactive decay rate of uranium-238 to lead-206 and of uranium-235 to lead-207. Coupled with decay rates for thorium-232 to lead-208, three independent ages may be obtained for the same sample. The determined lead-206 and lead-207 ratios can be converted into a so-called lead-lead age. The method is most applicable to materials Precambrian in age. Additionally, a uranium-uranium age, derived from the ratio of uranium-235 to uranium-238, can be calculated as a by-product of uranium-thorium-lead dating. Fission-Track Dating The fission-track method, also known as spontaneous fission-track dating, involves the paths, or tracks, of radiation damage made by nuclear particles in a mineral or glass by the spontaneous fission of uranium-238 impurities. Age in years is calculated by determining the ratio of spontaneous fission-track density to that of induced fission tracks. The method works best for micas, tektites, and meteorites. It has been used to help date the period from about 40,000 to 1 million years ago, an interval not covered by carbon-14 or potassium-argon methods. Rocks subjected to high temperatures or exposed to cosmic-ray bombardment at the earth's surface, however, may yield erroneous ages. Recent Advances New techniques date various rock deposits by determining the concentrations of rhenium and osmium isotopes in them.