Introduction to Radioisotope Geochronology/Part Eight - Examples of Application

Introduction
Radio-isotopic dating is the determination of ages of geologic materials through the comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.

Brief History of Determining the Age of the Earth
In regards to the age of the Earth, the first accurate estimate that is still broadly accepted today was proposed by Claire Patterson in 1953. Patterson's dissertation project was focused on using radiogenic lead (i.e. lead that was produced by the radioactive decay of uranium) to calculate the age of meteorites. It was assumed that meteorites and asteroids represented the left-over materials from the formation of the Solar System and had remained relatively undisturbed and therefore by measuring the lead isotopes of a meteorite on could assume the age of the Earth to be equivalent. Patterson found that the age of five meteorites from Canyon Diablo in Arizona, Nuevo Laredo in Mexico, and Henbury in Northern Australia was 4.55±0.07 billion years old. Amazingly, the current estimate for the age of the Earth is still within uncertainty of Patterson's first calculation at 4.54 ± 0.05 billion years (4.54 × 109 years ± 1%).

Stratigraphic Applications


The Cretaceous–Tertiary extinction event, which occurred approximately 65.5 (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period. The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy, the K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.



Quaternary Geochronology
See Stirling and Mortensen (2009) for a recent review of the U-Th chronometer applied to fossil coral reefs.