Thorium dating

Learn about a controversial 2017 study that used thorium/uranium dating of broken mastodon bones to argue that a prehistoric human species, such as Neanderthals, lived in North America some 130,000 years ago.The Earth can be assumed to be a very large sample containing lead evolving from primordial lead by radiogenic increments.This method does not require a constant rate of sedimentation of ionium but simply that the two isotopes are precipitated in a constant proportion.The validity of the ionium-thorium age method is based upon the following assumptions: (1) the ratio of thorium-230 (ionium) to thorium-232 in ocean water remains constant during the age span of the sample to be dated; (2) during precipitation there is no chemical fractionation of ionium and thorium, so that they are precipitated in a constant ratio; (3) the sediments do not contain any detrital material that has a significant amount of either nuclide; and (4) after deposition the thorium isotopes do not migrate within the sediments.By Aimee Komugabe Uranium series (U-series) dating is based on the uranium and thorium radioactive decay chains.These decay chains involve a series of different elements and may be as long as 35 steps, before reaching the stable end product – lead.

In this comment, we discuss three concepts introduced by these authors that could lead to a misunderstanding of the method and its application in the archaeological community and in the scientific debate about rock art chronology.

The application of the U/Th method for the dating of prehistoric rock art is still experimental.

Technical improvements (for less damageable sampling) and fundamental research on the causes of errors are needed.

The lead incorporated within the Earth has been evolving continuously from primordial lead and from the radioactive decay of uranium and thorium isotopes.

Thus, the lead isotopic composition of any mineral or rock depends upon its age and the environment from which it was formed; that is, it would depend upon the ratio of uranium plus thorium to lead in the parent material.

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