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.
Since the rate of depletion has been accurately determined (half of any given amount of carbon 14 decays in 5,730 years), scientists can calculate the time elapsed since something died from its residual carbon 14.
Carbon-14 has a half-life of 5,730 ± 40 years— during the succeeding 5,730 years.
Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.
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.
Scientists at the Lamont-Doherty Geological Laboratory of Columbia University at Palisades, N.