Don't be an isodope, learn about an isotope.

Any single atomic element has a fixed number of protons. However, nearly all elements are capable of posessing more than one fixed number of neutrons. For example, hydrogen is defined as an atom with only one proton. Hydrogen commonly has zero neutrons, giving this type of atom an atomic mass of 1. This means that 6.0225 × 1023 (one mole) of these common hydrogen atoms weighs 1 gram. However, every once in a while a hydrogen atom will also have a neutron, which has basically the same mass as a proton. These heavier hydrogen atoms are referred to as 2H, or deuterium, and have an atomic mass of 2. Even more rarely, a hydrogen atom will have two neutrons. These atoms have an atomic mass of 3 and are referred to at 3H, or tritium. 1H, 2H, and 3H are all isotopes of hydrogen.

Some isotopes are classified as stable isotopes and others are classified as unstable, or radioactive, isotopes. Stable isotopes maintain constant concentrations on Earth over time. Unstable isotopes are atoms that disintegrate at predictable and measureble rates to form other isotopes by emitting a nuclear electron or a helium nucleus and radiation. These isotopes continue to decay until they reach stability. As a rule, the heavier an isotope is than the most common isotope of a particular element, the more unstable it is and the faster it will decay. Because the rates of radioactive decay are measurable, unstable isotopes are useful tools in determining age. For example, nuclear bombs put large and detectable amounts of certain radioactive isotopes into the atmosphere. After the near-elimination of nuclear bomb testing due to the Limited Test Ban Treaty in 1963, the carbon-14 concentration in the atmosphere began decreasing immediately. Therefore, a 5 year old has a significantly lower concentration of carbon-14 in her bones than a person born in 1960. Anybody born after

1965 or so possesses a significantly higher concentration of carbon-14 than someone born before nuclear testing. Thus, we can tell how old many living organisms are based on the recent history of carbon-14 in the atmosphere. Because carbon-14 decays slowly (decreases to half of its original concentration every 5,370 years), we can also tell how long ago much older organisms died based upon what we know the pre-industrial carbon-14 concentration in the atmosphere was and how much radioactive decay of this isotope has occurred since the organism's death. This is a very useful tool in dating petrified wood, bones from ancient civilizations, and shells in ocean-floor sediments. For very old specimens such as dinosaur bones, more stable radioactive isotopes must be used because of their slower decay rates.

While radioactive isotopes are very cool, I am looking for a little more stability in my life and am therefore interested in stable isotopes for my current research. Because stable isotopes don't decay, they remain in the environment at a constant concentration. However, the distribution of stable isotopes throughout the environment constantly changes as a result of changing environmental preferences. For example, much like salinity, the concentrations of heavy hydrogen and oxygen isotopes in the ocean increases significantly during glacial periods because cold air is not as good at absorbing and holding onto heavy water molecules as warm air is. This means that water molecules made of light hydrogen and oxygen isotopes evaporate from the ocean more easily than heavy molecules. During glacial times, much more water is trapped on land as ice than during interglacial times, redistributing the light water molecules that preferentially evaporated from the ocean onto land. Trapping water on land decreases the volume of the ocean and therefore increases the concentration of heavy hydrogen and

oxygen isotopes in the ocean. Because the concentration of these heavy isotopes in the ocean is a function of temperature, the environment has been keeping a record of sea surface temperature for millions of years by the constant piling up of dead organic matter on the ocean floor (sedimentation). For example, foraminifera living at the ocean's surface make their calcium carbonate (CaCO3) shells out of the chemicals they pull out of the ocean. If the ocean is changing its isotopic composition over time, and if these shells are constantly falling to the ocean floor as new shells are created, then measuring concentrations of heavy oxygen isotopes in shells going down in depth from the ocean floor is like traveling back in time and measuring sea surface temperature over millions of years.

There are many other processes that influence the preferential distribution of other heavy and light isotopes as well. For example, the concentration of nitrogen-15 in an animal indicates whether or not that animal is starving. The combination of nitrogen-15 and carbon-13 in a human indicates how much meat that person eats, and comparing these measurements around the world can tell us much about dietary differences from country to country. Changes in carbon-13 concentration throughout the hoof of a herbivore, such as a deer, can indicate drought because of changes in chemistry of the plants that the dear eats, and changes in carbon-13 of bones from 10 million to 3 million years ago indicate a significant change from woody plants (C3) to grasses (C4) about 5 million years ago.