Introduction to Radioisotope Geochronology/Part Eight - Examples of Application

left|350px 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. Age of the Earth 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.Patterson C. Claire Patterson Age of meteorites and the earth Geochimica et Cosmochimica Acta 1956 10 230-237 10.1016/0016-7037(56)90036-9 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%).1997 Age of the Earth http://pubs.usgs.gov/gip/geotime/age.html U.S. Geological Survey 2006-01-10 http://web.archive.org/web/20051223072700/http://pubs.usgs.gov/gip/geotime/age.html 23 December 2005 noDalrymple G. Brent The age of the Earth in the twentieth century: a problem (mostly) solved Special Publications, Geological Society of London 2001 190 1 205–221 10.1144/GSL.SP.2001.190.01.14 2001GSLSP.190..205D Tectonic Applications Stratigraphic Applications thumb|right|400px|Badlands near Drumheller, Alberta, where erosion has exposed the K–T boundary 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.Fortey R Life: A Natural History of the First Four Billion Years of Life on Earth Vintage 1999 238–260 9780375702617 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.Gradstein F Ogg J, Smith A A Geologic Time Scale 2004 http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=0521781426 thumb|400px|Two geochronologists and a Paleontologist collecting an ash bed at the Cretaceous-Paleogene Boundary, Wyoming Thermochronology Quaternary Geochronology See Stirling and Mortensen (2009)StirlingAndersen2009 for a recent review of the U-Th chronometer applied to fossil coral reefs. References StirlingAndersen2009 Uranium-series dating of fossil coral reef: Extending the sea-level record beyond the last glacial cycle. Earth and Planetary Science Letters, 284 (2009) 3269-283 Amelin2002 Yuri Amelin, et al. Science 297, 1678 (2002)