on radioactive decay of unstable isotopes. The duration of time leading to the 

 state where half the original concentration remains is known as the half-life. 

 In general, the useful dating range of individual isotopic methods is about 

 10 times their half-life. The radiometric isotopes Carbon-14, Potassium- 

 Argon-40, Caesium-137, Lead-210, and Thorium-230 are the most commonly 

 used in standard geologic investigations (Faure 1977; Friedlander, Kennedy, 

 and Miller 1955). 



Radiocarbon (Carbon-14 or 14 C) dating is perhaps the most widely used 

 technique for assessing the age of Holocene and late Pleistocene organic 

 materials. Once an organism or plant dies, its radiocarbon ( 14 C) content is no 

 longer replenished and begins to decrease exponentially, achieving a half-life 

 after some 5,730 years. Substances that are often examined with 14 C dating 

 include wood, charcoal, peat, shells, bones, aqueous carbonates, rope, and 

 soil organics. Recent developments using mass spectrometers allow detection 

 of absolute amounts of 14 C content in samples as small as 5 mg. To be 

 comparable, radiocarbon dates are adjusted to a zero age at AD 1950. 

 Analytical error factors are given as one or two standard deviations about the 

 mean. Other errors, associated with sample contamination, changes in atmo- 

 spheric or oceanic 14 C content, and fractionation, are more difficult to 

 estimate. Absolute dates of samples less than 150 years old or greater than 

 50,000 years old are currently considered to be ambiguous. 



Potassium-argon dating (Potassium-Argon-40 or K:Ar) can be applied to a 

 wide range of intrusive and extrusive igneous rocks that contain suitable 

 minerals. In addition to constraints on rock type, it is necessary for the 

 sample to be unaltered by weathering or other geological processes that may 

 allow diffusion of radiogenic argon from the sample. The occurrence of such 

 rocks along coasts is generally restricted to regions adjacent to plate bound- 

 aries and regions of active tectonics. Potassium-argon dating of Holocene 

 deposits is generally imprecise, with errors of + 15 to 30 percent. Only 

 certain minerals, particularly those with a high K and low atmospheric Ar 

 content, are suitable for extending the K:Ar dates into the late Pleistocene. 

 For these reasons, potassium-argon dating has limited applications in studies 

 of the geologic history of coasts. 



Fission-track dating was developed as a complementary technique to 

 potassium-argon (K:Ar) dating. Most applications to Quaternary deposits 

 have involved dating airfall volcanic ash or glass deposits, a field known as 

 tephrochronology. This material usually has wide distribution and geologi- 

 cally speaking has infinitely narrow depositional time duration. However, it is 

 often absent or quickly removed in many coastal settings. If present, the 

 rapid deposition and large aerial extent of ash makes it an excellent tool for 

 correlation of rock strata and can provide radiometric age dates. A listing of 

 some of the important volcanic ash layers in North America, which include 

 very recent to Pleistocene dates, can be found in Sarna-Wojcicki, Champion, 

 and Davis (1983). 



Chapter 4 Laboratory Techniques and Approaches 



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