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	Potassium-Argon Dating	Amino Acid Racemization
Field	Archaeology	Archaeology
Family	Regression model	Regression model
Year of origin	1999	1997
Originator	Developed from 1940s-1950s radiometric work; codified for the 40Ar/39Ar successor by McDougall and Harrison	Reviewed for archaeology by Beverly Johnson and Gifford Miller
Type	Radiometric dating clock based on radioactive decay of potassium-40 to argon-40	Chemical kinetic dating clock based on the racemization of amino acids in biogenic materials
Seminal source	McDougall, I., & Harrison, T. M. (1999). Geochronology and Thermochronology by the 40Ar/39Ar Method (2nd ed.). Oxford University Press. ISBN: 9780195109207	Johnson, B. J., & Miller, G. H. (1997). Archaeological Applications of Amino Acid Racemization. Archaeometry, 39(2), 265-287.
Aliases	K-Ar Dating, Potassium-Argon Geochronology, K-Ar Radiometric Dating, Potassium-Argon Method	AAR Dating, Amino Acid Geochronology, Amino Acid Epimerization, D/L Ratio Dating
Related	3	3
Summary	Potassium-argon (K-Ar) dating is a radiometric technique that determines the age of volcanic rocks and minerals from the slow radioactive decay of potassium-40 to argon-40. Potassium is abundant in many rock-forming minerals, and a fixed fraction of its naturally radioactive isotope decays to argon gas at a precisely known rate, so the amount of argon trapped inside a crystal is a clock that starts when the mineral cools below its argon-retention temperature. By measuring how much radiogenic argon has accumulated relative to the remaining potassium, the analyst inverts the decay equation to obtain the time elapsed since crystallization. Because potassium-40 has a half-life of about 1.25 billion years, the method reaches far beyond the radiocarbon range and became the workhorse for dating the volcanic deposits that bracket Plio-Pleistocene hominin fossils at sites such as Olduvai Gorge.	Amino acid racemization (AAR) dating estimates the age of biogenic materials such as mollusc shell, ostrich eggshell, bone, and teeth from the slow chemical conversion of amino acids from one mirror-image form to the other after an organism dies. Living tissue builds proteins almost entirely from left-handed (L) amino acids, but after death these gradually interconvert toward an equilibrium mixture of left- and right-handed (D) forms, so the measured ratio of D to L rises predictably with time. Because the reaction is a temperature-dependent chemical process rather than a radioactive decay, AAR is fundamentally a kinetic clock that must be calibrated against an independently dated reference and corrected for the sample's thermal history. Reviewed for archaeology by Johnson and Miller and covered as a standard chronometric tool in Renfrew and Bahn's textbook, it offers a rapid, inexpensive way to date or correlate deposits across the Quaternary, well beyond the radiocarbon range.