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Unformatted text preview: LECTURE 14 - DATING QUATERNARY DEPOSITS TYPES OF GEOCHRONOMETRIC DATING Radiometric that which provides a measure of the age of a sample, such as 14 C dating. Depends on natural rates of decay of radioactive isotopes Floating scale that which provides a "floating" chronology which may be tagged by radiometric dates, for tree ring or amino acid diagenisis. Depends mostly on natural processes . THE RADIOCARBON METHOD Principles: Method can be applied to wood, peat seeds, charcoal, bones, teeth, shells corals, speleothems, soil humus, leather, etc.. Most widely used method of radiometric age determination in Quaternary research. Based on production of 14 C from nitrogen (N) in the stratosphere as result of cosmic radiation . 14 N 7 + n -> 14 C 6 + p Oxidizes to CO 2 and mixes with normal CO 2 in the atmosphere which contains the stable isotope 12 C (~99%) and 13 C (~1%). 14 C occurs at an abundance of 10-12 . Atmospheric carbon is dissolved in fresh and salt water and absorbed by plants and animals. Through exchange with atmospheric carbon, the 14 C content of living plants and animals is at balance with the atmospheric composition. After death , however, the exchange ceases and the radioactive carbon within the body decays at a half life of 5730 30 years . By measuring ratio of 14 C to stable carbon in sample it is possible to determine when organism died. Analyses can be done using a gas-counting method (>5 grams of sample) that measures beta emissions ( 14 C -> 14 N + ), or with accelerator mass spectrometry (<1mg of sample) that measures the mass-ratio between 14 C and 12 C. Ages range between 300 to 70,000 years before present (1950) can be obtained (practical limit 8 half-lives) and dates given are in radiocarbon years BP or cal BP. Normally one or two sigma counting error () is included with the date (20 to 50 years highest precision). Problems with method: Assumes that amount of 14 C in the atmosphere is stable , and hence in living organic matter has been constant during last 70,000 years. Initial 14 C/ 12 C ratios have varied ( DeVries effect ) due to solar modulation of the cosmic ray flux and variation in strength of earths magnetic field. Ocean carbon exchange rate of about 2,000 yrs is also a factor. On account of the above plateaux of radiocarbon ages exist , e.g. 12,700 and 10,000 BP. Contamination affects age of sample, especially in the case of old samples. Also, dead carbon has been introduced into atmosphere due to burning of fossil fuels ( Suess effect ). Nuclear testing in atmosphere has also resulted in introduction of 14 C ( bomb effect )....
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- Spring '08