Carbon dating practice questions
But the tree ring record goes no further, so scientists have sought other indicators of age against which carbon dates can be compared.
One such indicator is the uranium-thorium dating method used by the Lamont-Doherty group.
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. The group theorizes that large errors in carbon dating result from fluctuations in the amount of carbon 14 in the air.
Changes in the Earth's magnetic field would change the deflection of cosmic-ray particles streaming toward the Earth from the Sun.
The Lamont-Doherty scientists conducted their analyses on samples of coral drilled from a reef off the island of Barbados.
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.
The percentage of Carbon 14 becomes extremely small and difficult to measure.
Also any contamination of the sample can distort the calculation of the age when the percentage is small At 9 half lives only 1/512 of the Carbon 14 is left. Most fossils have no Carbon left in them and Carbon 14 and radioactive dating can not be used to date them.
Since 1947, scientists have reckoned the ages of many old objects by measuring the amounts of radioactive carbon they contain.
New research shows, however, that some estimates based on carbon may have erred by thousands of years.This is a digitized version of an article from The Times’s print archive, before the start of online publication in 1996.