Isotopes are powerful tools for investigating a wide number of processes. However, in order to understand the extent to which these isotopes can be used it is necessary to understand their origin and distribution in the marine biosphere.
Atoms are made up of three subatomic particles: protons, neutrons, and electrons. The protons and neutrons are packed together in the nucleus at the center of the atom (see Figure). Electrons orbit the nucleus. It is the number of protons within the nucleus that determines what material (element) the atom is.
Figure from The Ohio State University web site
DefinitionsN = Neutrons, neutral charge
Z = Protons, positive charge
e = electrons, negative charge
A = Atomic Number = N + Z
As an example, the element Sodium written as 2311Na has an A = 23, Z= 11, and N = 12
When an element has atoms that differ only in the number of neutrons, these various atoms are called isotopes. Every chemical element has more than one isotope, although one isotope usually dominates in nature. All isotopes of one element have identical chemical properties. This means it is difficult to separate isotopes from each other by only chemical means. However, the physical properties of the isotopes, such as their masses, boiling points, and freezing points, do differ and enable both separation and identification.
Certain isotopes of elements are unstable, and release excess energy in the form of ionizing radiation, also known as radioactivity. In fact, radioactivity is defined as the release of energy resulting from a spontaneous change in the structure of the nucleus. Thus, radioactive decay is often accompanied by the ejection of a particle from the nucleus. Some lightweight isotopes are radioactive, and all elements that have an atomic number > 82 are radioactive.
All unstable isotopes are called radioisotopes. For example, carbon-14 (14C) is a radioisotope of carbon. In some cases, all of the isotopes of a given element are radioactive and differ only in the types of radiative energy emitted. Uranium is one such element.
Figure modified from Olympus Digital Microscope web site
Radiation has many forms, but is most easily described as energy in the form of high speed particles or electromagnetic waves. It can be ionizing or non-ionizing. Non-ionizing radiation lacks the energy to alter atoms (e.g., visible light and microwaves). Ionizing radiation has enough energy to change normal cellular functioning and even cause cells to die or transform into a cancerous cell. Ionizing radiation is generally categorized by strength or energy level into three main categories:
1) Alpha particles. Most densely ionizing, but weakest form of ionizing radiation. These particles travel a few inches through air, but can be stopped by a sheet of paper. This means that cells can be protected or shielded from damage by alpha particles by clothing. Even your skin will protect you from damage from alpha particles. However, if alpha particles are inhaled or ingested or get into a cut on the skin, they can cause damage to cells. As alpha particles decay inside the body, the surrounding cells absorb the radiation.
2) Beta particles. More energetic. These particles can travel several feet through air, but are stopped with denser materials such as wood, glass or aluminum foil.
3) Gamma rays. High-energy electromagnetic energy waves and the most penetrating type of radiation. They travel at the speed of light through the air. Cells must be shielded from gamma rays with concrete, lead or steel.
Figure from www.gcse.com web site
The reason why radioactive isotopes are so powerful is that they decay over a very specific time period called a half-life, t½. The half-life is the amount of time it takes for half of the starting material to decay away. For example, 14C has a half-life of 5730 years. If we started with 10 14C atoms, after 5730 years, or one t½, only five 14C atoms would remain. After another 5730 years, or 2 t½, only 2.5 atoms would remain and so on.
Radioisotopes can be thought of as built in clocks that enable researchers to investigate a number of processes over timescales ranging from less than a minute to thousands of years. Radioactive elements are everywhere, from the air we breathe to even our fire alarms.
Radioisotopes in Oceanography
Radioisotopes enter the oceans through atmospheric deposition (rain), rivers, groundwater, and sediment. They are produced both naturally and by man. By using differences in how the isotopes react chemically and how long their half-lives are it is possible to explore processes ranging from groundwater input to particle scavenging. Natural Radioactive Decay Series - isotopes produced by the decay of long-lived uranium (238U & 235U) and thorium (232Th) parent isotopes.
Elements in the Natural Radioactive Decay Series
Natural Radioactive Decay Series - isotopes produced by the decay of long-lived uranium (238U and 235U) and thorium (232Th) parent isotopes.
Uranium - a heavy metal present in almost all rocks and soils that may dissolve in water containing oxygen and carbonate or sulfate ions. Uranium is enriched in granitic rocks and organic-rich sediments. It accumulates only slightly in the body where it may affect the kidneys. The danger is due to its chemical effect rather than its radioactivity.
Thorium - a heavy metal present in almost all rocks and soils but is very difficult to dissolve. Because of its limited mobility it rarely poses a health threat.
Radium - an element that behaves like calcium and barium (member of the alkaline earth series). It accumulates in calcium-rich tissue such as bones and teeth. Its danger derives from its radioactivity (and that of its daughters) in close proximity to bone marrow.
Radon - a gaseous element that generally does not form chemical compounds and readily escapes from rocks and soil. Its danger derives from its radioactivity (and that of its daughters) in the lungs.
Polonium - a daughter of radon that decays with a very high energy alpha particle. It is produced in the lungs by radon decay. It may also enter the lungs by smoking tobacco.
Lead - a stable element formed at the end of the decay chains. The amount that enters the body due to radioactivity is negligible.