in which the proportion of carbon isotopes is counted directly (as contrasted with the indirect Geiger counter method) using an accelerator mass spectrometer.The method drastically reduces the quantity of datable material required.. Libby showed the essential uniformity of carbon-14 in living material and went on to measure the radiocarbon level in organic samples dated historically -- materials as old as 5,000 years from sources such as Egyptian tombs. The sediments they contain have a high percentage of content made up of the shells of small marine organisms and these sediments build up very slowly, from 10-50 mm per 1000 years, but their sequence is uninterrupted.One of the assumptions of is that Carbon 12, Carbon 13, and Carbon 14 are passed around the carbon cycle at similar rates.The three isotopes are chemically very similar, but slight differences between them may cause them to be taken up at different rates by some plants and animals.The technique is also used to analyze mollusk shells in an attempt to reconstruct the original aquatic environment.
Variations in the isotopic ratios for the raw materials can lead to a classification of types and even, in some cases, the suggestion of a source for the raw materials.It can be predicted theoretically and demonstrated experimentally, however, that subtle chemical effects do result from the difference in mass of isotopes.Isotopes of an element may have slightly different equilibrium constants for a particular chemical reaction, so that fractionation of the isotopes results from that reaction.Radiocarbon, potassium-argon, fission track, and thermoluminescence dating all rely on this phenomenon in different ways.CATEGORY: technique DEFINITION: A technique based on the relative abundance of lead isotopes, which differ according to the origin of the lead, allowing scientists to pinpoint the source of a piece of lead once the ratios of the isotopes have been determined.Thus dates emerge for glaciations and interglacial periods, which can assist in the age determination of archaeological material found in association with these glacial phases.Problems with the technique are the difficulty of correlating oceanic temperature changes with continental glacial and interglacial phases, and the disturbance by animals living on the ocean bottom.The technique based on the natural, spontaneous nuclear fission of Uranium 238 and its byproduct, linear atomic displacements/tracks.The basis for this technique is that a uranium isotope, U 238, as well as decaying to a stable lead isotope, also undergoes spontaneous fission. Fission is accompanied by an energy release which sends the resulting two nuclei into the surrounding material, the tracks causing damage to the crystal lattice.SYNONYMS OR RELATED TERMS: oxygen isotope examination CATEGORY: technique DEFINITION: Isotope analysis looking at the O18/O16 ratio in materials.The method can be used to classify glass types and to analyze mollusk shells in order to try and reconstruct their original environment and thus the source. The basis for this technique is the fact that the ratio of two of the stable isotopes of oxygen varies according to the material in which it is found.