Other forms of carbon dating
The group theorizes that large errors in carbon dating result from fluctuations in the amount of carbon 14 in the air. Room for Debate asks whether shorefront homeowners should have to open their land to all comers. Sincescientists have reckoned the ages of many old objects by measuring the amounts of radioactive carbon they contain.
But the tree ring record goes no further, so scientists have sought other indicators of age against which carbon dates can be compared. One reason the group believes the uranium-thorium estimates to be more accurate than carbon dating is that they produce better matches between known changes in the Earth's orbit and changes in other forms of carbon dating glaciation.
Since the rate of depletion has been accurately determined half of any given amount of carbon 14 decays in 5, yearsscientists can calculate the time elapsed since something died from its residual carbon Uraniuma radioactive element present in the environment, slowly decays to form thorium The Lamont-Doherty scientists conducted their analyses on samples of other forms of carbon dating drilled from a reef off the island of Barbados.
Dating Subject to Error But scientists have long recognized that carbon dating is subject to error because of a variety of factors, including contamination by outside sources of carbon. Alan Zindler, a professor of geology at Columbia University who is a member of the Lamont-Doherty research group, said age estimates using the carbon dating and uranium-thorium dating differed only slightly for the period from 9, years ago to the present.
But scientists have long recognized that carbon dating is subject to error because of a variety of factors, including contamination by outside sources of carbon. Politics Education Texas N.
According to carbon dating of fossil animals and plants, the spreading and receding of great ice sheets lagged behind orbital changes by several thousand years, a delay that scientists found hard to explain. They arrived at this conclusion by comparing age estimates obtained using two different methods - analysis of radioactive carbon in a sample and determination of the ratio of uranium to thorium in the sample.
But even if the method is limited to marine organisms, it will be extremely useful for deciphering the history of Earth's climate, ice, oceans and rocks, Dr. Carbon dating is unreliable for objects older than about 30, years, but uranium-thorium dating may be possible for objects up to half a million years old, Dr.
Because it is radioactive, carbon 14 steadily decays into other substances. The method is less suitable, however, for land animals and plants than for marine organisms, because uranium is plentiful in sea water but less so in most soils.
But when a plant or animal dies, it can no longer accumulate fresh carbon 14, and the supply in the organism at the time of death is gradually depleted. Changes in the Earth's magnetic field would change the deflection of cosmic-ray particles streaming toward the Earth from the Sun. Accurate tree ring records of age are available for a period extending 9, years into the past. In some cases, the latter ratio appears to be a much more accurate gauge of age than the customary method of carbon dating, the scientists said.
Fairbanks, a member of the Lamont-Doherty group, said that if the dates of glaciation were determined using the uranium-thorium method, the delay - and the puzzle - disappeared.
The samples represented animals that lived at various times during the last 30, years.
The best gauge they have found is dendrochronology: Using a mass spectrometer, an instrument that accelerates streams of atoms and uses magnets to sort them out according to mass and electric charge, the group has learned to measure the ratio of uranium to thorium very precisely. In principle, any material of plant or animal origin, including textiles, wood, bones and leather, can be dated by its content of carbon 14, a radioactive form of carbon in the environment that is incorporated by all living things.
New research shows, however, that some estimates based on carbon may have erred by thousands of years.
One such indicator is the uranium-thorium dating method used by the Lamont-Doherty group. Sweep to a Win Over the Heat.
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