Making a date with light Essay

To measure the age of a layer of sediment, geologists most often rely on radiocarbon dating of the organic matter mixed in with the earth. But carbon-14 is not always abundant in sediment samples, and ages greater than 70,000 years are beyond the range of this technique. Now David Huntley, Dorothy Godfrey-Smith and Michael Thewalt at Simon Fraser University in Burnaby. British Columbia, have sidestepped these problems with a new method that dates the mineral grains of sediment directly.

Their technique is based on the ability of radiation, produced in the decay of naturally occurring isotopes, to ionize electrons from atoms in the mineral crystal. These electrons then fall into traps or defects in the crystal. They can escape the traps if the grains are exposed to light. But once the sediment is buried there is no escape and electrons pile up in the traps. The number of captured electrons is, therefore, a measure of the time elapsed since the last exposure to light, and presumably the deposition age of the sediment.

As discussed in the Jan. 10 NATURE, the researchers used an argon-ion laser to excite the electrons from traps in sediment samples. The amount of luminescence given off when these electrons return to crystal atoms can be related to the amount of ionizing radiation the grains received over time. Combining this with the radiation dose rate, measured by other means, results in an age for the grains.

The main drawback of the new method is that samples must be taken in the dark. Nonetheless, the researchers have shown that electron luminescence increases with sample age. The also report an age of 62 [plus-or-minus] 8 thousand years for a silt sample dated at 58.8 [plus-or-minus] 0.3 thousand years with the carbon-14 method.

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