Biology Reference
In-Depth Information
medical devices and foodstuffs, as well as in CT (computed tomography) or
the older CAT scans (computed axial tomography), gamma-knife surgery,
scanning foreign merchant ship containers before entering domestic ports,
and converting the outwardly mild-mannered but clearly unstable Bruce
Banner into the Incredible Hulk.
Radioactive isotopes have other characteristics that render them im-
mensely important in the geological and biological sciences. The decay pro-
cess is irreversible once it has begun, and the rate is constant. Moreover, as
the atomic structure of the isotope, for example uranium, is altered through
loss of neutrons, protons, electrons, and the emission of electromagnetic
rays, its physical properties are also altered, and the isotope is converted
to another element, in this case, lead. No philosopher's stone is involved,
and it is not an alchemic transformation. It results from the fact that the
physical properties of an element are a refl ection of its atomic makeup. If
this makeup changes, then the altered product takes on the characteristics
of some different element (i.e., uranium to lead). Other decay products
are argon from radioactive potassium, and strontium from radioactive ru-
bidium. The rate at which an isotope decays is its half-life, and this rate
differs among different elements. For example, the half-life of uranium 238
is 4.5 billion years, cobalt is 5.2 years, sodium is 15 hours, and bismuth is
1.9
1019 years. The familiar carbon
14
has a half-life of 5730 years.
Because rocks and fossils from the geologic column include elements
undergoing irreversible isotopic transformations at different but constant
rates, it becomes possible to assign an absolute age to rocks provided other
criteria are met. First, the origin of the rock containing, for example, ura-
nium must preclude the initial presence of the decay product, for example,
lead (in the case of igneous rocks any original lead is volatilized). Second,
the rock must contain both uranium and lead. Third, the transformation
must still be going on, for otherwise it would not be known how long ago
the decay process had stopped. Theoretically, at the most precise level of
application, particles one billionth of a gram can be dated. More commonly,
ages are assigned to samples down to about the size of minute teeth. Be-
cause of their half-life, lead-uranium ratios are mostly used to date rocks
billions to hundreds of millions of years old, potassium-argon for rocks hun-
dreds to a few millions of years, and many other radioactive elements fall
in intervening and younger ranges. In the later part of the geologic column
there, are marine sediments several thousand years old deposited and lying
undisturbed, as in the Cariaco Basin off the coast of Venezuela, that have
been dated with the resolution of a decade or less. Sediments that are varved
(annually layered), such as lake deposits, ice cores from Greenland, Ant-
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