| Fexybeaxary | Date: Δευτέρα, 2012-08-27, 2:21 PM | Message # 1 |
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What does one find in the calibration of carbon-14 against actual ages? If one predicts a carbon-14 age assuming that the ratio of carbon-14 to carbon-12 in the air has stayed constant, there is a slight error because this ratio has changed slightly. Figure 9 shows that the carbon-14 fraction in the air has decreased over the last 40,000 years by about a factor of two. This is attributed to a strengthening of the Earth's magnetic field during this time. A stronger magnetic field shields the upper atmosphere better from charged cosmic rays, resulting in less carbon-14 production now than in the past. (Changes in the Earth's magnetic field are well documented. Complete reversals of the north and south magnetic poles have occurred many times over geologic history.) A small amount of data beyond 40,000 years (not shown in Fig. 9) suggests that this trend reversed between 40,000 and 50,000 years, with lower carbon-14 to carbon-12 ratios farther back in time, but these data need to be confirmed.
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Electron spin resonance (ESR). Also called electron paramagnetic resonance, ESR dating also relies on the changes in electron orbits and spins caused by radioactivity over time. However, ESR dating can be used over longer time periods, up to two million years, and works best on carbonates, such as in coral reefs and cave deposits. It has also seen extensive use in dating tooth enamel.
The Samarium-Neodymium, Lutetium-Hafnium, and Rhenium-Osmium Methods. All of these methods work very similarly to the rubidium-strontium method. They all use three-isotope diagrams similar to Figure 4 to determine the age. The samarium-neodymium method is the most-often used of these three. It uses the decay of samarium-147 to neodymium-143, which has a half-life of 105 billion years. The ratio of the daughter isotope, neodymium-143, to another neodymium isotope, neodymium-144, is plotted against the ratio of the parent, samarium-147, to neodymium-144. If different minerals from the same rock plot along a line, the slope is determined, and the age is given by the same equation as above. The samarium-neodymium method may be preferred for rocks that have very little potassium and rubidium, for which the potassium-argon, argon-argon, and rubidium-strontium methods might be difficult. The samarium-neodymium method has also been shown to be more resistant to being disturbed or re-set by metamorphic heating events, so for some metamorphosed rocks the samarium-neodymium method is preferred. For a rock of the same age, the slope on the neodymium-samarium plots will be less than on a rubidium-strontium plot because the half-life is longer. However, these isotope ratios are usually measured to extreme accuracy--several parts in ten thousand--so accurate dates can be obtained even for ages less than one fiftieth of a half-life, and with correspondingly small slopes.
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