Q. How can scientists accurately date when stone tools were made, like those found at Lake Turkana in Kenya?
A. “An artifact like a tool made from flint is usually not dated directly but instead according to the age of the sediment layers it is found in,” said Dennis V. Kent, a research scientist at the Lamont-Doherty Earth Observatory of Columbia University.
Radiocarbon dating is widely used to date materials like charcoal from hearths and carbonate in snail shells, Dr. Kent said, but it is limited to about the last 50,000 years because of the short half-life of carbon 14. For older sediments, techniques include tephrochronology (involving potassium) and magnetostratigraphy (involving iron).
In tephrochronology, layers of volcanic ash, tephra, often contain potassium-bearing minerals whose crystallization age can be determined, even going back billions of years. But the infrequency of volcanic eruptions may make it hard to date intervening sediments.
These sediments, however, are likely to contain traces of iron-bearing minerals like magnetite, which act like compasses. Their magnetic orientation is preserved when they are encased, and there has been a well-dated series of reversals of the earth’s magnetic polarity.
The sediment around the Turkana tools was deposited soon after a reversal that occurred 1.778 million years ago, Dr. Kent said, thus helping establish an estimated age for the tools: 1.76 million years.
Ray, C. Claiborne. 2011. "Dating Stone Tools". The New York Times. Posted: September 12, 2011. Available online: http://www.nytimes.com/2011/09/13/science/13qna.html?_r=1