Geochronological dating methods

Carbon dating and other cosmogenic methods The occurrence of natural radioactive carbon in the atmosphere provides a unique opportunity to date organic materials as old as roughly 60, years. Unlike most isotopic dating methods, the conventional carbon dating technique is not based on counting daughter isotopes. It relies instead on the progressive decay or disappearance of the radioactive parent with time.

Newly created carbon atoms were presumed to react with atmospheric oxygen to form carbon dioxide CO2 molecules. Radioactive carbon thus was visualized as gaining entrance wherever atmospheric carbon dioxide enters—into land plants by photosynthesis, into animals that feed on the plants, into marine and fresh waters as a dissolved component, and from there into aquatic plants and animals.

In short, all parts of the carbon cycle were seen to be invaded by the isotope carbon Invasion is probably not the proper word for a component that Libby calculated should be present only to the extent of about one atom in a trillion stable carbon atoms.

So low is such a carbon level that no one had detected natural carbon until Libby, guided by his own predictions, set out specifically to measure it. His success initiated a series of measurements designed to answer two questions: Is the concentration of carbon uniform throughout the plant and animal kingdoms? After showing the essential uniformity of carbon in living material, Libby sought to answer the second question by measuring the radiocarbon level in organic samples dated historically—materials as old as 5, years from sources such as Egyptian tombs.

With correction for radioactive decay during the intervening years, such old samples hopefully would show the same starting carbon level as exists today.

His conclusion was that over the past 5, years the carbon level in living materials has remained constant within the 5 percent precision of measurement. A dating method was thus available, subject only to confirmation by actual application to specific chronologic problems.

Expressed as a fraction of the contemporary level, they have been mathematically converted to ages through equation 5 above. Archaeology has been the chief beneficiary of radioactive-carbon dating, but late glacial and postglacial chronological studies in geology have also been aided greatly. The occasional exceptions all involve nonatmospheric contributions of carbondepleted carbon dioxide to organic synthesis. Specifically, volcanic carbon dioxide is known to depress the carbon level of nearby vegetation, and dissolved limestone carbonate occasionally has a similar effect on freshwater mollusks, as does upwelling of deep ocean water on marine mollusks.

In every case, the living material affected gives the appearance of built-in age. In addition to spatial variations of the carbon level, the question of temporal variation has received much study. Of more recent date was the overcompensating effect of man-made carbon injected into the atmosphere during nuclear bomb testing. The result was a rise in the atmospheric carbon level by more than 50 percent.

Fortunately, neither effect has been significant in the case of older samples submitted for carbon dating. The ultimate cause of carbon variations with time is generally attributed to temporal fluctuations in the cosmic rays that bombard the upper atmosphere and create terrestrial carbon Whenever the number of cosmic rays in the atmosphere is low, the rate of carbon production is correspondingly low, resulting in a decrease of the radioisotope in the carbon-exchange reservoir described above.

Studies have revealed that the atmospheric radiocarbon level prior to bce deviates measurably from the contemporary level. In the year bce it was about 8 percent above what it is today. In the context of carbon dating, this departure from the present-day level means that samples with a true age of 8, years would be dated by radiocarbon as 7, years old. The problems stemming from temporal variations can be overcome to a large degree by the use of calibration curves in which the carbon content of the sample being dated is plotted against that of objects of known age.

In this way, the deviations can be compensated for and the carbon age of the sample converted to a much more precise date. Calibration curves have been constructed using dendrochronological data tree-ring measurements of bristlecone pines as old as 8, years ; periglacial varve, or annual lake sediment, data see above ; and, in archaeological research, certain materials of historically established ages.

It is clear that carbon dates lack the accuracy that traditional historians would like to have. Until then, the inherent error from this uncertainty must be recognized. A final problem of importance in carbon dating is the matter of sample contamination. If a sample of buried wood is impregnated with modern rootlets or a piece of porous bone has recent calcium carbonate precipitated in its pores, failure to remove the contamination will result in a carbon age between that of the sample and that of its contaminant.

Consequently, numerous techniques for contaminant removal have been developed. Among them are the removal of humic acids from charcoal and the isolation of cellulose from wood and collagen from bone. Today contamination as a source of error in samples younger than 25, years is relatively rare. Beyond that age, however, the fraction of contaminant needed to have measurable effect is quite small, and, therefore, undetected or unremoved contamination may occasionally be of significance.

A major breakthrough in carbon dating occurred with the introduction of the accelerator mass spectrometer. This instrument is highly sensitive and allows precise ages on as little as 1 milligram 0. The increased sensitivity results from the fact that all of the carbon atoms of mass 14 can be counted in a mass spectrometer. By contrast, if carbon is to be measured by its radioactivity, only those few atoms decaying during the measurement period are recorded.

By using the accelerator mass spectrometer, possible interference from nitrogen is avoided, since it does not form negative ion beams, and interfering molecules are destroyed by stripping electrons away by operating at several million volts.

The development of the accelerator mass spectrometer has provided new opportunities to explore other rare isotopes produced by the bombardment of Earth and meteorites by high-energy cosmic rays. Many of these isotopes have short half-lives and hence can be used to date events that happened in the past few thousand to a few million years. In one case, the time of exposure, like the removal of rock by a landslide , can be dated by the presence of the rare beryllium 10Be isotope formed in the newly exposed surface of a terrestrial object or meteoroidal fragment by cosmic-ray bombardment.

Other applications include dating groundwater with chlorine 36Cl , dating marine sediments with beryllium 11Be and aluminum 26Al , and dating glacial ice with krypton 81Kr. In general, the application of such techniques is limited by the enormous cost of the equipment required. Uranium-series disequilibrium dating The isotopic dating methods discussed so far are all based on long-lived radioactive isotopes that have survived since the elements were created or on short-lived isotopes that were recently produced by cosmic-ray bombardment.

The long-lived isotopes are difficult to use on young rocks because the extremely small amounts of daughter isotopes present are difficult to measure. A third source of radioactive isotopes is provided by the uranium - and thorium -decay chains. Uranium—thorium series radioisotopes, like the cosmogenic isotopes, have short half-lives and are thus suitable for dating geologically young materials.

The decay of uranium to lead is not achieved by a single step but rather involves a whole series of different elements, each with its own unique set of chemical properties. In closed-system natural materials, all of these intermediate daughter elements exist in equilibrium amounts.

That is to say, the amount of each such element present is constant and the number that form per unit time is identical to the number that decay per unit time.

Accordingly, those with long half-lives are more abundant than those with short half-lives. Once a uranium-bearing mineral breaks down and dissolves, the elements present may behave differently and equilibrium is disrupted. For example, an isotope of thorium is normally in equilibrium with uranium but is found to be virtually absent in modern corals even though uranium is present. Over a long period of time, however, uranium decays to thorium , which results in a buildup of the latter in old corals and thereby provides a precise measure of time.

Most of the studies using the intermediate daughter elements were for years carried out by means of radioactive counting techniques; i. The introduction of highly sensitive mass spectrometers that allow the total number of atoms to be measured rather than the much smaller number that decay has resulted in a revolutionary change in the family of methods based on uranium and thorium disequilibrium.

Thorium dating The insoluble nature of thorium provides for an additional disequilibrium situation that allows sedimentation rates in the modern oceans to be determined. In this case, thorium in seawater, produced principally by the decay of uranium, is deposited preferentially in the sediment without the uranium parent.

This is defined as excess thorium because its abundance exceeds the equilibrium amount that should be present.

With time, the excess decays away and the age of any horizon in a core sample can be estimated from the observed thoriumto-thorium ratio in the seawater-derived component of the core.

Sedimentation rates between 1 and 20 mm 0. Lead dating The presence of radon gas as a member of the uranium-decay scheme provides a unique method for creating disequilibrium. The gas radon Rn escapes from the ground and decays rapidly in the atmosphere to lead Pb , which falls quickly to the surface where it is incorporated in glacial ice and sedimentary materials.

By assuming that the present deposition rate also prevailed in the past, the age of a given sample at depth can be estimated by the residual amount of lead Principal cosmogenic and uranium-thorium series radioisotopes The principal cosmogenic and uranium-thorium series radioisotopes are listed in the table.


A wide variety of geochronological tools or methods can be employed to estimate quantitative and qualitative dating of rocks and sediments. Scientific knowledge of Earth’s geologic history has advanced significantly since the development of radiometric dating, a method of age determination based on the principle that radioactive atoms in geologic materials decay at constant, known rates to daughter magami.gaetric dating has provided not only a means of numerically quantifying .

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