The researchers then came across an unusual resource: unused film footage taken by National Geographic photographers for a never-aired documentary on the original expedition. By matching features such as hills from the film to landmarks in the field, together with a hand drawn map from the late Paul Abell and photographs by Karl Butzer, both members of the 1967 expedition, the researchers were able to precisely locate the original fossil outcrops.
The scientists collected hundreds of new mammal fossils that helped differentiate layers of sediment and link the currently dry region to its moister climates of the past. But, precisely dating the outcrops required volcanic minerals with elements that undergo radioactive decay.
Brown and McDougall collected rocks containing volcanic debris, which McDougall dated to approximately 195,000 years old.
Forty years ago, radiometric dating methods were less accurate and suggested an age of roughly 130,000 years. No one was able to securely tie both sets of fossils to the same time period.
"Fortunately, small pumice fragments from two sediment layers were perfect for precise isotopic dating," says McDougall. “The pumice clasts are produced by explosive volcanic eruptions, and the rocks were probably carried to the Lake Turkana basin soon after, providing a time stamp for nearby sediments,?he adds. The results revealed how long ago the eruption occurred: roughly 196,000 years.
"This provided an unambiguous maximum age for the fossils,?says McDougall.
Further supporting the dates, the researchers linked sediments deposited by the Omo River to periodic, well-dated sapropels ?layers of organic-rich sediment
Source:National Science Foundation