Bear Lake Methods: Isotopes

Isotopes

Isotopes are atoms with the same number of protons in the nucleus but varying numbers of neutrons. Isotopes can be stable or unstable, depending on how many more neutrons than protons they have. Stable isotopes commonly used in environmental studies are carbon 12 and 13; nitrogen 12, 13 and 14; and oxygen 16 and 18. If the isotope's nucleus is too heavy, the atom will tend to eject particles until it stablizes. When particles are ejected from an atomic nucleus, we call it radiation and the isotopes that do it are radioactive. Common radioactive isotopes are carbon 14; lead 210; cesium 137; potassium 40; rubidium 87; uranium-234, 235, and 238; and thorium-232. Radioactive isotopes are commonly used for dating.

Stable Isotopes:

The relative amounts of one isotope to another (their ratio) is often dependent on biological and physical processes. Geologists have studied the way the ratios respond to changes in salinity, temperature, precipitation, photosynthesis and decomposition. By using modern examples, and assuming that the processes in the past work the same way they do now, geologists have calibrated the ratios. Using these calibrations, we can estimate how the environment around our research site has changed over time. Common stable isotopic ratios include ¹²C/¹³C and ¹⁸O/¹⁶O.

Visit these sites for more information on stable isotopes:

Jeffrey S. Amthor, Carbon and the Carbon Cycle
Measurements and Modeling of Atmospheric Methane Using Stable Carbon Isotopes
Marine Geology: use of stable carbon isotopes
Marine Geology: use of stable oxygen isotopes

Radioactive Isotopes:

The radiation activity (rate of emissions) is used to estimate the concentration of the radioactive element in a sample. This concentration is often compared to that of another isotope, usually a stable isotope, and their ratios are used to calculate age. In radiocarbon dating, the amount of ¹⁴C measured is compared to the amount of ¹³C in the sample. Factors that can affect the measurement of radioactivity include a change in the rate of isotope production, a change in the source of the radioactive or stable isotopes, and contamination by material with significantly different levels of either isotope.

To learn more about isotopes and radiation:

Basics
Fundamentals of Isotope Geochemistry
The Third Millennium Online:Chapter Five
Radiation Protection Program Publication
Caltech Radiation Safety Training and Reference Manual
Radioactive Dating

Return to Lead210 Page
Return to C14 Page
Return to Geochemistry Page

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