New USU Lab Sheds Light on Age of Strata
By Joe Bauman Deseret Morning News
An amazing new tool for dating ancient remains, a luminescence geochronology laboratory, has been installed at Utah State University.
Since the 1950s carbon dating has set a high standard among archaeologists and geologists for checking the age of remains. But the method only works with biological material, which may not be available — or, in the case of ancient graves, would not be desirable to use.
“In the last five or 10 years the technology has changed, and there are new scientific procedures,” said Joel L. Pederson, director of USU’s new Luminescence Laboratory. One of these methods is that used by the lab, optically stimulated luminescence.
On Jan. 26 university officials hosted an open house and opened the laboratory, located at the university’s Research Park in North Logan. The main instrument, a luminescence reader developed in Denmark, was purchased through a $170,000 grant from the Val A. Browning Foundation.
“It provides absolute age dates on sediments … up to about 300,000 years old,” said Pederson, who is an associate professor. Tammy Rittenour, also of USU, does much of the actual running of the lab, he said.
Besides dating archaeological strata and remains, the instrument can answer such questions as when earthquake faults ruptured, when formations eroded, and when sediments were laid down by lakes and streams.
Quartz, which makes up sand, may be the most common mineral on Earth’s surface, he said. Grains of sand imbedded within pottery, soil disturbed by grave-digging, bits of mineral moved during a landslide — all can be dated by the luminescence lab, as long as the samples are no more than 300,000 years old.
Quartz grains hold a luminescence signal. Sand in the ground, untouched by sunlight, slowly acquires a charge from the radioactive material that is everywhere. A bit of quartz is “filled” with the most charge it can absorb after 300,000 years and then it discharges and starts the process again.
But when sand grains are exposed to sunlight, the light is so strong that it causes the electron signal to fill completely and discharge, within about two seconds. The quartz is reset.
Sand grains that were exposed and covered by a landslide or earthquake or digging would have discharged when they were exposed to the sun. When buried shortly afterward, they began their slow recharging.
A group of electrons gives off a flash when discharged. The light is far too dim to see by eye, but photomultiplier devices can detect it.
“We bring it (a sample) into a laboratory and stimulate the grains to release their luminescence signal, and our reader reads it,” Pederson said.
The instrument bombards the sample with both controlled sources of light and controlled radiation, according to Pederson. “So we actually have a radioactive source that is part of the instrument.” The bombardment simulates the natural radiation in the environment, making the grains discharge.
If this signal turns out to be weak or small, the sediment was buried a relatively short time; if more substantial, a longer time. Formulae have been developed to calibrate the discharge with the age of the sample.
The sample must not be exposed to sunlight. “We can just take a metal tube from a hardware store and we hammer it into the outcrop,” Pederson said. To preserve its sensitivity, “we end up throwing away half the sample in the laboratory.”
The lab is working on projects concerning earthquake hazards. Besides its own work, it is “a little oversubscribed” with paying customers.
“We’re already involved in an archaeological project in the Grand Canyon where we are beginning to explore and date some of the sediments that you find archaeological remains in.”
Launching the instrument, the lab studied samples from sediment below the university. The material, exposed by a recent landslide, was laid down on the Provo level of Lake Bonneville before the lake dropped.
“The age we got from deep underneath the university was 19,300 years before present,” Pederson said, “plus or minus 1,200 years.”
E-mail: bau@desnews.com
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