The Carbon Method

For organic objects such as wood less than 40,000 years old, measurement is made of a certain form of carbon, Carbon-14. This radioactive form of carbon is the product of the action of cosmic rays in the upper air. It unites with oxygen to form radioactive carbon dioxide, which is taken up by living plants and then by animals that eat the plants. It is found in a constant proportion in all living tissues, decay of the radioactive carbon being balanced by constant replacement of cells. The intake ceases when the organism dies, but the decay of its radioactive carbon continues. After 5,565 ± 30 years (the sign ± means plus or minus that amount for experimental error) half of the carbon will have changed to the stable form. Measurement of Carbon 14, therefore, makes possible precise age determinations on some relatively recent fossils and carbon-containing materials, including carbonate rocks. Archaeologists carefully remove hitherto "worthless" charcoal or wood bits found along with pots and arrowheads for CH dating of their finds.

Carbon-14 dating of the wood of logs uprooted by the most recent continental glacier in Wisconsin established that the event happened 11,40 0 ± 200 years ago. Until this test set a more definite figure, it had been assumed that the glacier made its call 25,00 0 years ago.

Many measurements have been made by these methods and have been used to confirm and make more precise the findings of paleontologists all over the world. Perhaps as good an example as any, which seems more personal because it casts light on the genealogy of man, comes from the discoveries made by Dr. L. S. B. Leakey in Africa.

When Leakey discovered fossils of precursors of man in Olduvai Gorge in Tanzania, the age of the rocks in which they lay became a matter of major paleontological importance. Luckily, the fossils were preserved in volcanic ash, so that fresh igneous minerals were present right with the fossils.

Potassium-argon tests set the age of the volcanic ash at 1.7 5 million years. This determination was cross-checked by another method that made use of fragments of volcanic glass in the rocks. These were polished and examined under the microscope for tracks only a few atoms wide made in the glass by fission of uranium atoms. After the tracks had been counted, the samples were irradiated to cause all their contained uranium to produce tracks. These were counted, and comparison of the two sets of numbers gave a measure of the extent of uranium decay. From it the age of the rocks was estimated at 2 million ± 3 0 , 0 0 0 years. After allowing for innate sources of error in the two methods, the experimenters regarded their results as substantial confirmation of an age for the fossils of nearly 2 million years.

Some ingenious fellow has reduced the incredible dimensions of geologic time to more familiar terms by bringing it within the scale of a calendar year. If the history of the earth were compressed into twelve months, the first eight months would be represented by virtually blank pages in the calendar. These would be the Proterozoic and Archeozoic eras. September and October would see the development of algae and bacteria and the gradual appearance of the invertebrates as fossils. Mammals would not come on the scene until mid-December. Man himself would not have evolved until the last few minutes of the last hour of the last day of December in this hypothetical year, and what we know as written history would span little more than the last minute of the year.

Such is geologic time; such is the theater of paleontology.

0 0

Post a comment