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Anatomy => Isotopic Tracer => Dating Methods
Dating Methods
INTRODUCTION
Dating Methods, in earth science, methods used to date the age of rocks and minerals. By applying this information, geologists are able to decipher the 4.6-billion-year history of the earth. The events of the geologic past-uplift of mountain ranges, opening and closing of seas, flooding of continental interiors, changes in climate-are all recorded in the strata of the earth's crust.
DEVELOPMENT OF RELATIVE AND ABSOLUTE METHODS
With the methods then available, 19th-century geologists could only construct a relative time scale. Thus, the actual age of the earth and the duration, in millions of years, of the units of the time scale remained unknown until the dawn of the 20th century. After radioactivity was discovered, radiometric dating methods were quickly developed. With these new methods geologists could calibrate the relative scale of geologic time, thereby creating an absolute one.
The relative scale was devised mainly by application of the principles of stratigraphy. An example of these is the law of superposition, which simply states that in an undisturbed succession of strata, the youngest beds are on top and the oldest on bottom (or, the higher beds are younger than the lower).
Based on the fossils they contain, strata in one area were correlated with those in other areas. As more and more such correlations were made, geologists began to make broad groupings of strata, which became the basis for dividing geologic time into vast blocks. Thus, the history of the earth was divided into four broad eras-Pre-cambrian, Paleozoic, Mesozoic, and Cenozoic; the eras were in turn divided into a number of periods.
ABSOLUTE DATING METHODS
Although development of radiometric methods led to the first and principal breakthroughs in establishing an absolute time scale, other absolute methods were devised that have limited applications. Chief among these are dendrochronology, varve analysis, hydration dating, and TL dating.
° Dendrochronology
This method of dating events and conditions of the recent past is based on the number, width, and density of annual growth rings of long-lived trees. In the southwestern United States, for example, a master tree-ring index has been constructed from the Douglas fir and bristlecone pine. This index enables dendrochronologists to date accurately events and climatic conditions of the past 3000 to 4000 years.
° Varve Analysis
One of the oldest methods employed for absolute age determination, varve analysis, was developed by Swedish scientists in the early 20th century. A varve is a sedimentary bed, or sequence of beds, deposited in a body of still water within a year's time. Counting and correlation of varves have been used to measure the ages of Pleistocene glacial deposits. By dividing the rate of sedimentation in terms of units per year by the number of units deposited following a geologic event, geologists can establish the age of the event in years.
° Obsidian Hydration Dating
Also referred to as hydration rind dating or obsidian dating, this method is used to calculate ages in years by determining the thickness of rims (hydration rinds) produced by water vapor slowly diffusing into freshly chipped surfaces on artifacts made of obsidian, or recent volcanic, glass. The method is applicable to glasses 200 to 200,000 years old.
° Thermoluminescence (TL) Dating
This method is based on the phenomenon of natural ionizing radiation inducing free electrons in a mineral that can be trapped in defects of the mineral's crystal lattice structure. These trapped electrons escape as TL when heated to a temperature below incandescence, so that by recording the TL of a mineral such as quartz and assuming a constant natural radiation level, the last drainage of the trapped electrons can be dated back to several hundred thousand years. In TL dating of pottery, for example, the specimen is heated until it glows with energy stored since it was fired.
° Radiometric Dating
Radiometric techniques were developed after the discovery of radioactivity in 1896. The regular rates of decay for unstable, radioactive elements were found to constitute virtual "clocks" within the earth's rocks.
A: Basic Theory
Radioactive elements such as uranium (U) and thorium (Th) decay naturally to form different elements or isotopes of the same element. (Isotopes are atoms of any elements that differ in mass from that element, but possess the same general chemical and optical properties.) This decay is accompanied by the emission of radiation or particles (alpha, beta, or gamma rays) from the nucleus, by nuclear capture, or by ejection of orbital electrons . A number of isotopes decay to a stable product, a so-called daughter isotope, in a single step (for example, carbon-14), whereas other series involve many steps before a stable isotope is formed. Multistep radioactive decay series include, for example, the uranium-235, uranium-238, and thorium-232 families. If a daughter isotope is stable, it accumulates until the parent isotope has completely decayed. If a daughter isotope is also radioactive, however, equilibrium is reached when the daughter decays as fast as it is formed.
Radioactive decay may take different routes. Thus, if the isotope decays by alpha emission, it loses the two protons and two neutrons that make up an alpha particle; the atomic number (number of protons) is reduced by two and the atomic mass (number of nuclear particles, or nucleons) by four. In beta decay, or electron loss, a radioactive nucleus can gain or lose one unit of electric charge without changing the number of nucleons. More radioactive substances are beta-ray emitters than alpha-ray emitters. A third important mode of decay involves electron capture; the nucleus of an atom absorbs an electron, which unites with a proton of the nucleus to form a neutron. Thus, the atomic number is reduced by one, but the mass of the nucleus remains unchanged. The fourth mode of decay, gamma radiation, consists of the emission of waves of electromagnetic energy.
Scientists describe the radioactivity of an element in terms of half-life, the time the element takes to lose 50 percent of its activity by decay. This covers an extraordinary range of time, from billions of years to a few microseconds. At the end of the period constituting one half-life, half of the original quantity of radioactive element has decayed; after another half-life, half of what was left is halved again, leaving one-fourth of the original quantity, and so on. Every radioactive element has its own half-life; for example, that of carbon-14 is 5730 years and that of uranium-238 is 4.5 billion years.
Radiometric dating techniques are based on radio-decay series with constant rates of isotope decay. Once a quantity of a radioactive element becomes part of a growing mineral crystal, that quantity will begin to decay at a steady rate, with a definite percentage of daughter products in each time interval. These "clocks in rocks" are the geologists' timekeepers.
B: Carbon-14 Method
Radiocarbon dating techniques, first developed by the American chemist Willard F. Libby and his associates at the University of Chicago in 1947, are frequently useful in deciphering time-related problems in archaeology, anthropology, oceanography, pedology, climatology, and recent geology. Through metabolic activity, the level of carbon-14 in a living organism remains in constant balance with the level in the atmosphere or some other portion of the earth's dynamic reservoir, such as the ocean. Upon the organism's death, carbon-14 begins to disintegrate at a known rate, and no further replacement of carbon from atmospheric carbon dioxide can take place. The rapid disintegration of carbon-14 generally limits the dating period to approximately 50,000 years, although the method is sometimes extended to 70,000 years. Uncertainty in measurement increases with the age of the sample.
Although the method is suited to a variety of organic materials, accuracy depends on the half-life to be used, variations in levels of atmospheric carbon-14, and contamination. (The half-life of radiocarbon was redefined from 5570 ± 30 years to 5730 ± 40 years in 1962, so some dates determined earlier required adjustment; and due to radioactivity more recently introduced into the atmosphere, radiocarbon dates are calculated from AD 1950.) The radiocarbon time scale contains other uncertainties, as well, and errors as great as 2000 to 5000 years may occur. Postdepositional contamination, which is the most serious problem, may be caused by percolating groundwater, incorporation of older or younger carbon, and contamination in the field or laboratory.
B: Potassium-Argon Method
The decay of radioactive potassium isotopes to argon is widely used for dating rocks. (The decay of potassium-40 to calcium-40 that also takes place is not useful.) Geologists are able to date entire rock samples in this way, because potassium-40 is abundant in micas, feldspars, and hornblendes. Leakage of argon is a problem if the rock has been exposed to temperatures above 125° C (257° F), because the age of the rock will then reflect the last episode of heating rather than the time of original rock formation.
C: Rubidium-Strontium Method
Used to date ancient igneous and metamorphic terrestrial rocks as well as lunar samples, this method is based on disintegration by beta decay of rubidium-87 to strontium-87. The method is frequently used to check potassium-argon dates, because the strontium daughter element is not diffused by mild heating, as is argon.
D: Methods Involving Thorium-230
Thorium ratio methods are used to date older oceanic sediments beyond the range of radiocarbon techniques. Uranium in seawater eventually decays to the thorium isotope, thorium-230 (also called ionium), which is precipitated into ocean-floor sediments. Because it has been undergoing decay longer, scientists can detect a decrease in quantity in higher levels, and a time scale can be developed in this way.
Thorium-230, part of the uranium-238 decay series, has a half-life of 80,000 years. Protactinium-231, derived from uranium-235, has a half-life of 34,300 years. Both parent elements are precipitated in the same proportions but at different rates. The ratio of the two changes regularly with time, showing greater differences in the quantity of undecayed parent isotopes in older sediments.
The ionium-thorium age method, applied to deep-sea sediments formed during the last 300,000 years, is based on the assumption that the initial ionium content of accumulating sediments has remained constant for the total section under study and is not derived from uranium decay; the age of the sample depends on this ionium excess, which decreases with time. In the ionium-deficiency method, the age of fossil shell or coral from 10,000 to 250,000 years old is based on the growth of ionium toward equilibrium with uranium-238 and uranium-224, which entered the carbonate shortly after its formation or burial. Similar disequilibrium relationships can be used to assess ages of carbonates in soils; this method is a complement to carbon-14 methodology.
E: Methods Involving Lead
Lead-alpha age is estimated by spectrographically determining the total lead content and alpha-particle activity (uranium-thorium content) of zircon, monazite, or xenotime concentrates. The lead-alpha, or Larsen, method is applied to rocks younger than Precambrian. In the uranium-lead method, age in years is calculated for geologic material based on the known radioactive decay rate of uranium-238 to lead-206 and of uranium-235 to lead-207. Coupled with decay rates for thorium-232 to lead-208, three independent ages may be obtained for the same sample. The determined lead-206 and lead-207 ratios can be converted into a so-called lead-lead age. The method is most applicable to materials Precambrian in age. Additionally, a uranium-uranium age, derived from the ratio of uranium-235 to uranium-238, can be calculated as a by-product of uranium-thorium-lead dating.
F: Fission-Track Dating
The fission-track method, also known as spontaneous fission-track dating, involves the paths, or tracks, of radiation damage made by nuclear particles in a mineral or glass by the spontaneous fission of uranium-238 impurities. Age in years is calculated by determining the ratio of spontaneous fission-track density to that of induced fission tracks. The method works best for micas, tektites, and meteorites. It has been used to help date the period from about 40,000 to 1 million years ago, an interval not covered by carbon-14 or potassium-argon methods. Rocks subjected to high temperatures or exposed to cosmic-ray bombardment at the earth's surface, however, may yield erroneous ages.
G: Recent Advances
New techniques date various rock deposits by determining the concentrations of rhenium and osmium isotopes in them.
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