for National Geographic News
Radioactive traces from nuclear tests in the 1950s and '60s are allowing forensic scientists today to date, and to help identify, human remains found after accidents and natural disasters.
The new technique has already been used to help identify victims of last year's tsunami in Southeast Asia. It may prove useful in identifying the victims of Hurricane Katrina on the U.S. Gulf Coast.
The method, developed by Swedish and U.S. researchers, measures radioactive carbon-14, a byproduct of nuclear testing, in human tooth enamel.
The process can determine the age of a tooth to within 18 months, far more precise than other methods of evaluating skeletal remains.
"Tooth enamel preserves a permanent record of the amount of radioactive carbon present in the atmosphere when the tooth was formed," said Jonas Frisén, a forensic scientist at the Karolinska Institute in Stockholm, Sweden. "By determining the amount of radioactive carbon-14 in the tooth, we can then calculate the age of the tooth, and its owner."
The research was reported last week in the journal Nature.
Radiocarbon, or carbon-14, is produced when cosmic radiation hits the Earth's atmosphere, and it exists at low levels everywhere in the world.
The amount of radiocarbon in the atmosphere remained fairly stable until 1955, when above-ground nuclear testing caused it to rise dramatically. Although nuclear tests were conducted in only a few places, the radiocarbon spread evenly in the atmosphere.
Radiocarbon reacts with oxygen to form carbon dioxide, which plants incorporate through photosynthesis. Humans then eat the plants and animals that eat them, which transfers the plants' concentration of radiocarbon to the human body.
The radiocarbon gets into human teeth, but it remains locked inside the tooth enamel from the time the enamel is formed during childhood.
"Dental enamel is a mineral, not a live tissue so it doesn't have constant turnover like proteins," said Bruce Buchholz, a scientist at the Lawrence Livermore National Laboratory in California. "Whatever carbon got into the enamel during the formation of the teeth is then locked in there."
SOURCES AND RELATED WEB SITES