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Thursday, February 18, 2010

Fine tuning carbon dating

Radiocarbon dating is an extremely useful tool for archeologists or for paleontologists who work on the not too distant past.

All living things on earth contain carbon. Atmospheric carbon in the form of carbon dioxide is incorporated into plants during photosynthesis, and then consumed by animals. Most of that carbon is in the stable form carbon-12 (12C). However, there is a trace amount of radioactive carbon-14 (14C) in the atmosphere as well. While the organism is alive, both isotopes are being incorporated into the tissue at a ratio that matches that found in the atmosphere. Once the organism dies, no more 14C can be absorbed and the 14C that was there decays into nitrogen (14N). The half-life of 14C is just under 6000 years. In other words, in 6000 years, half the 14C that was in the specimen is gone, in 12,000 years, 3/4 is gone, etc. Once you get past about 50,000 years, the amount of 14C left is so small that no more meaningful data can be obtained.

This means that once you find a fossil, you can use the ratio of 12C to 14C in that fossil to determine how long ago the organism lived. There’s just one problem. You have to know the starting ratio of 12C to 14C in the atmosphere at the time the organism lived in order to assess what percentage of the radioactive carbon has decayed. Unfortunately, the amount of 14C in the atmosphere is not constant over time, but varies with fluctuations in both the Earth’s magnetic field and in the amount of solar activity. Therefore, the raw data must be corrected against a calibration curve that accounts for these fluctuations.

This is where an international group called INTCAL comes in. INTCAL has spent the past thirty years calibrating and perfecting the radiocarbon calibration curve. In 2004, by using data from tree rings and corals, the team reached consensus on a curve reaching back to 26,000 years ago. This curve was dubbed INTCAL04. More recently, the researchers have been able to extend that curve all the way to 50,000 years ago with their new INTCAL09 curve, which is published in the journal Radiocarbon.

Although carbon dating is useless for dating dinosaur fossils or other ancient life forms, it is an invaluable tool in dating early human cultures. For example, we now have a more accurate picture of when humans began painting caves, or when we migrated into Europe.

Lead author Paula Reimer of Queen’s University Belfast in Northern Ireland says they will continue to perfect the curve, and hope to have a new edition in 2011.