Department of Plant Biology
Department of Geological Sciences
Michigan State University
Prior to the development of techniques for radiogenic
dating, the age of rock units was determined by the fossils, which were
present. Fossils used for dating/correlation of geological strata are called
guide or index fossils. The ideal index fossil is marine
(like this early Paleozoic trilobite), since marine organisms tend to be
widely distributed, with a restricted age distribution. Geologists attempted
to estimate the age of the various time units using criteria such as rates
of deposition of various kinds of sediments and the accumulation of salt
in the oceans. While prone to major errors, such attempts did indicate
that the earth was much older than the 6000 or so years suggested by biblical
chronologies.
C-14 is being constantly formed in the upper atmosphere by cosmic ray interactions with nitrogen. The balance of C-14 formation and decay results in a constant pool of C-14 in the atmosphere. Since living systems are constantly cycling carbon, all living organisms maintain an equilibrium level of C-14 in their tissues as long as the organism is alive.
Once an organism dies, the relative amount of C-14 declines by 50% for every 5730 years - the half-life of C-14. By measuring the relative amount of C-14 in a fossil, compared with modern material, the age can be calculated.
Complications emerged because our modern atmosphere has
enhanced levels of C-14 as a result of open-air testing of atomic weapons
between 1945 and the late 1960's, so actual standardization is done with
pre-atomic testing samples, normalized to 1950. The major limitation of
the technique is that after about 40,000 years, too little C-14 remains
for accurate measurement. Dating of older rocks is accomplished using other
isotopes (see above) which have half-lives long enough to date the age
of the solar system.
Ralph E. Taggart (taggart@msu.edu)