Unlike relative dating methods,methods provide chronological estimates of the age of certain geological materials associated with fossils, and even direct age measurements of the fossil material itself. To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly usemethods, based on the naturalof certain elements such as potassium and carbon, as reliable clocks to date ancient events. Geologists also use other methods - such asand, which assess the effects ofon the accumulation of electrons in imperfections, or "traps," in the crystal structure of a mineral - to determine the age of the rocks or fossils.

All elements contain protons and neutrons, located in the atomic nucleus, and electrons that orbit around the nucleus (Figure 5a). In each element, the number of protons is constant while the number of neutrons and electrons can vary. Atoms of the same element but with different number of neutrons are called isotopes of that element. Each isotope is identified by its atomic mass, which is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon 12 (12C), carbon 13 (13C), and carbon 14 (14C) (Figure 5a).



Figure 5: Radioactive isotopes and how they decay through time. (a) Carbon has three isotopes with different numbers of neutrons: carbon 12 (C12, 6 protons + 6 neutrons), carbon 13 (C13, 6 protons + 7 neutrons), and carbon 14 (C14, 6 protons + 8 neutrons). C12 and C13 are stable. The atomic nucleus in C14 is unstable making the isotope radioactive. Because it is unstable, occasionally C14 undergoes radioactive decay to become stable nitrogen (N14). (b) The radioactive atoms (parent isotopes) in any mineral decay over time into stable daughter isotopes. The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope. © 2013 All rights reserved.

Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both. This change is called radioactive decay. For example, unstable 14C transforms to stable nitrogen (14N). The atomic nucleus that decays is called the parent isotope. The product of the decay is called the daughter isotope. In the example, 14C is the parent and 14N is the daughter.

Some minerals in rocks and organic matter (e.g., wood, bones, and shells) can contain radioactive isotopes. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1.

The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay (argon, a gas, escapes into the atmosphere while the lava is still molten). When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral.

The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope (Figure 5b). When the quantities of the parent and daughter isotopes are equal, one half-life has occurred. If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated.

For example, if the measured abundance of 14C and 14N in a bone are equal, one half-life has passed and the bone is 5,730 years old (an amount equal to the half-life of 14C). If there is three times less 14C than 14N in the bone, two half lives have passed and the sample is 11,460 years old. However, if the bone is 70,000 years or older the amount of 14C left in the bone will be too small to measure accurately. Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past. Luckily, there are methods, such as the commonly used potassium-argon (K-Ar) method, that allows dating of materials that are beyond the limit of radiocarbon dating (Table 1).

Name of Method Age Range of Application Material Dated Methodology Radiocarbon

1 - 70,000 years

Organic material such as bones, wood, charcoal, shells

Radioactive decay of 14C in organic matter after removal from bioshpere

K-Ar dating 1,000 - billion of years

Potassium-bearing minerals and glasses

Radioactive decay of 40K in rocks and minerals

Uranium-Lead

10,000 - billion of years

Uranium-bearing minerals

Radioactive decay of uranium to lead via two separate decay chains

Uranium series

1,000 - 500,000 years

Uranium-bearing minerals, corals, shells, teeth, CaCO 3

Radioactive decay of 234U to 230Th

Fission track

1,000 - billion of years

Uranium-bearing minerals and glasses

Measurement of damage tracks in glass and minerals from the radioactive decay of 238U

Luminescence (optically or thermally stimulated)

1,000 - 1,000,000 years

Quartz, feldspar, stone tools, pottery

Burial or heating age based on the accumulation of radiation-induced damage to electron sitting in mineral lattices

Electron Spin Resonance (ESR)

1,000 - 3,000,000 years

Uranium-bearing materials in which uranium has been absorbed from outside sources

Burial age based on abundance of radiation-induced paramagnetic centers in mineral lattices

Cosmogenic Nuclides

1,000 - 5,000,000 years

Typically quartz or olivine from volcanic or sedimentary rocks

Radioactive decay of cosmic-ray generated nuclides in surficial environments

Magnetostratigraphy 20,000 - billion of years

Sedimentary and volcanic rocks

Measurement of ancient polarity of the earth's magnetic field recorded in a stratigraphic succession

Tephrochronology

100 - billions of years

Volcanic ejecta

Uses chemistry and age of volcanic deposits to establish links between distant stratigraphic successions

Table 1. Comparison of commonly used dating methods.



Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material. Dating methods like thermoluminescence, optical stimulating luminescence and electron spin resonance, measure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material. If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material. These methods are applicable to materials that are up to about 100,000 years old. However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged.