When did these Natural Fossil Reactors start operating?

Geological Time:
The age of the earth (4550 million years) is based on the age of the solar system as measured by the uranium to lead dating of meteorites. The 4550 year old geological record has been subdivided by geologists into 5 'eras'.

These eras are based largely on the evidence left behind by fossils. The sequence of fossils are thought to show the evolution of life forms since the beginning of the earth with the boundary between eras representing a major change in the types of life forms present on the earth. Changes in the major types of life forms have been associated with mass extinctions such as could have been caused by the collision of a large meteor with the earth.

Follow the diagram for more details.

An Oklo Time Line:
Using a number of radioactive clocks the Oklo fossil reactors have been radioactively dated to be about 2000 million years old. The uranium in these reactors is thought to have come from the tiny amounts of uranium orginally scattered throughout the earth's crustal rocks during its formation.

The evolution of plants and the resulting oxidizing rainwaters slowly dissolved and mobilized the uranium which was then slowly concentrated by other chemical conditions into the resulting U orebodies. This process took hundereds of millions of years

The reactors themselves only operated for a relatively short period of time (1 million years) but the time which radioactive wastes have had to escape from the reactor sites is about 2000 million years.

Radioactive Clocks:
Radioactive clocks play important roles in deciphering the Oklo phenomenon. To date the age of the U deposit, uranium-lead, rubidium-strontium and samarium-neodymium clocks have been used.
The fact that sufficient ^235>U relative to ^238>Uwas present in the reactors is related to the difference in the decay rates of these two isotopes.

During the reactors a vast array of radioactive clocks were inititated by the fission reactions. Virtually every fission product isotopes produced was a radioactive clock. The majority of these isotopes are far too short to be of any real benefit in studying the Oklo phenomenon but there are still several dozen with half lives of several years to millions of years or more which have been used to determine many reactor parameters including

• the time period over which each of the individual reactors operated
• how much ²³⁵U each the individual reactors "bred" from ²³⁸U
• how long it took for some of the more mobile fission products to move out from the reactor zones and
• how much of each of the fission products was retained inside the reactors.