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All pregnant women look forward to knowing the due date of delivery of the baby. A pregnancy ultrasound is a great option to determine the baby’s due date.Ultrasound has become very helpful for obstetricians in predicting due dates and also diagnosing twins.One such indicator is the uranium-thorium dating method used by the Lamont-Doherty group.Uranium 234, a radioactive element present in the environment, slowly decays to form thorium 230.Therefore they have sought ways to calibrate and correct the carbon dating method.The best gauge they have found is dendrochronology: the measurement of age by tree rings.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.
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.
The samples represented animals that lived at various times during the last 30,000 years. 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 9,000 years ago to the present.
'' But at earlier times, the carbon dates were substantially younger than the dates we estimated by uranium-thorium analysis,'' he said.
'' The largest deviation, 3,500 years, was obtained for samples that are about 20,000 years old.'' 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 global glaciation. 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.
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.
Accurate tree ring records of age are available for a period extending 9,000 years into the past.