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Unit 1 Radiometric Dating
Numerous factors, however, can complicate the carbon-dating process and lead to
inaccuracies. One potential complication is that the ratio of carbon-14 to carbon-12 in the
atmosphere fluctuates over time. The ratio today, for example, is different than it was 14,000
years ago. Thus, to accurately determine the date of a particular fossil, scientists must first
take into account these fluctuations. Furthermore, contamination of a sample by living
microorganisms, which may be feeding on the sample itself, can skew the results of carbon
dating significantly. A final limitation is that, in geological terms, carbon-14 has a relatively
quick decay rate. After 50,000 years or so, the amounts of carbon-14 remaining in a sample
are so minute that reliable dating becomes impossible.
For samples exceeding this age limit, uranium-lead dating is the most common method.
Uranium-lead dating has a number of advantages. First, since uranium has a very slow decay
rate, this method can be used to date samples that are billions of years old. Second, uranium-
lead dating produces highly accurate results because it averages the decay rates of two
different uranium isotopes. The first of these is uranium-235, which decays into lead-207. The
second is uranium-238, which decays into lead-206. By measuring the decay rates of both
isotopes and averaging out the results, uranium-lead dating can achieve a higher degree of
accuracy than would be possible measuring either isotope in isolation.
Like carbon dating, however, uranium-lead dating also has its limitations. The most
problematic is that organic materials contain only trace amounts of uranium, which are
immeasurable. Furthermore, uranium is only found in large amounts in certain kinds of
minerals, most notably igneous rocks. Thus uranium-dating, while highly accurate, can only be
used in certain specialized circumstances. Uranium-lead dating suffers from other problems as
well. Uranium decay is a much more energetic process than carbon decay, and the energy
released by these atoms can actually propel surrounding atoms from their original locations,
causing microscopic damage to a sample which can cause in accurate results.
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Numerous factors, however, can complicate the carbon-dating process and lead to
inaccuracies. One potential complication is that the ratio of carbon-14 to carbon-12 in the
atmosphere fluctuates over time. The ratio today, for example, is different than it was 14,000
years ago. Thus, to accurately determine the date of a particular fossil, scientists must first
take into account these fluctuations. Furthermore, contamination of a sample by living
microorganisms, which may be feeding on the sample itself, can skew the results of carbon
dating significantly. A final limitation is that, in geological terms, carbon-14 has a relatively
quick decay rate. After 50,000 years or so, the amounts of carbon-14 remaining in a sample
are so minute that reliable dating becomes impossible.
For samples exceeding this age limit, uranium-lead dating is the most common method.
Uranium-lead dating has a number of advantages. First, since uranium has a very slow decay
rate, this method can be used to date samples that are billions of years old. Second, uranium-
lead dating produces highly accurate results because it averages the decay rates of two
different uranium isotopes. The first of these is uranium-235, which decays into lead-207. The
second is uranium-238, which decays into lead-206. By measuring the decay rates of both
isotopes and averaging out the results, uranium-lead dating can achieve a higher degree of
accuracy than would be possible measuring either isotope in isolation.
Like carbon dating, however, uranium-lead dating also has its limitations. The most
problematic is that organic materials contain only trace amounts of uranium, which are
immeasurable. Furthermore, uranium is only found in large amounts in certain kinds of
minerals, most notably igneous rocks. Thus uranium-dating, while highly accurate, can only be
used in certain specialized circumstances. Uranium-lead dating suffers from other problems as
well. Uranium decay is a much more energetic process than carbon decay, and the energy
released by these atoms can actually propel surrounding atoms from their original locations,
causing microscopic damage to a sample which can cause in accurate results.
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