Kinds of Archaeological Dating Methods
Mar 23, Amino acid racemi-zation, obsidian hydration, fluorine dating, and in situ cosmogenic isotope dating fall into this category. Chemical dating. Aug 17, attempted to count backwards through the genealogies of the Bible, establishing The most well known and oft used form of radiometric dating is . RadioCarbon: An International Journal of Cosmogenic Isotope Research. Ages are determined from the rate of decay of the daughter isotope or the build- up of a Sm-Nd and U-Th-Pb methods and those datable by cosmogenic nuclides. eds., Chronometric Dating in Archaeology (New York: Plenum Press, );.
Furthermore, although it is not a limitation on what carbon can date, it is significant that carbon dating involves measuring for extremely minute quantities of the carbon isotope dispersed throughout a sample. This instrument is highly sensitive and allows precise ages on as little as one milligram of carbon, where the older method might require as much as 25 grams for ancient material. 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. It only provides a count of the amount of isotopes, such as carbon, currently in a sample. This count is then used in an equation with other factors, some of which are assumed, in order to calcute the age of the object. As the quote above specifically states, formerly at least 25 grams of sample were required.
A milligram is equal to about one twenty-five thousandths of an ounce. Consequently, carbon dates are based upon looking for one in a trillion atoms in samples that are smaller than one half of one percent of an ounce. Having established the limitations on what carbon can date, we arrive at 2 problems with carbon dating that are uncontested.
In other words, as we will see, these problems are acknowledged by evolutionary scientists and geologists. The first problem with carbon that is acknowledged by evolutionary scientists is its susceptibility to contaminations. This is explained in the quotes below. 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.
E Radiometric Dating, E2 Carbon Method — 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. As seen indicated by the minute ratios and sample sizes described previously, these quotes also attest that the amount of contamination needed to affect a sample would be quite small.
However, since the amount of carbon in the carbon cycle functions as the assumed starting ratio in the dated item, this equation both assumes and requires that the ratio of carbon to carbon in the carbon cycle has not changed for the entire 50, years that carbon can date. Carbon calculations assume and require that the ratio of carbon to carbon has not changed but has remained 1 trillion to 1 for the last 50, years. Any deviation in the starting ratio of carbon to carbon will affect the carbon age.
For example, we might consider what would happen if the carbon to carbon ratio in the carbon cycle were 2 trillion to 1 instead of the 1 trillion to 1 ratio we measure presently. If we assumed that the ratio was constant and has always been 1 trillion to 1 throughout the past 50, years and then we measured a 4 trillion to 1 present ratio in a dead organism, it would appear that three-quarters of the carbon had already decayed. Therefore, we would conclude that 2 half-lives of carbon would have occurred and that the organism had been dead for 11, years, which is equal to 2 half-lives of 5, years each.
In reality, however, if the starting ratio was 2 trillion to 1 in the past instead of 1 trillion to 1, then only 1 half-life would have occurred and the organism would have died only 5, years ago, which is well within the Biblical timescale, rather than 11, years ago, which exceeds the Biblical timescale. At this point, we can compare this example to the stated reality concerning variations in the carbon ratio. Concerning the need for the carbon ratio to remain uniform throughout the carbon cycle, we arrive at 2 essential questions.
First, is carbon distributed uniformly in plants and animals today? And second, has the present level been uniform throughout the past? His success initiated a series of measurements designed to answer two questions: Is the concentration of carbon uniform throughout the plant and animal kingdoms? And, if so, has today's uniform level prevailed throughout the recent past?
If not uniform, then the initial starting amount is not known. If the initial starting amount is not known, then it is impossible to know how much carbon is missing and how much decay has occurred. In fact, the quantity of carbon varies from place to place. 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.
Creation Science Issues, Radiometric Dating - A Christian Perspective
This is extraordinarily important given the role of volcanic activity in the Judeo-Christian account of a global flood. With volcanic activity occurring on a worldwide scale as the crust of the earth is broken up into plates and with enormous geyser-like fountains spewing volcanic gases up into the atmosphere all around the earth, a vast inflow quantity of normal carbon would suddenly be thrust into the carbon cycle.
And, as stated in the quote below, in addition to the fact that carbon ratios are not uniform around the world even today, the ratio of carbon is also known to vary over time.
A 2 to 3 percent depression of the atmospheric radioactive-carbon level since was noted soon after Libby's pioneering work, almost certainly the result of the dumping of huge volumes of carbonfree carbon dioxide into the air through smokestacks.
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 BC deviates measurably from the contemporary level.
In the year BC 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 lake sediment, data see above ; and, in archaeological research, certain materials of historically established ages.
These figures indicate just how sensitive this ratio is to environmental fluctuations, which in turn have a significant impact on the apparent age of dated items.
This means that cosmic rays affect carbon levels and ratios constantly and to an extent more significant than industrial or nuclear technology.
On this note, another question arises concerning how we know that the carbon level was different at 1, or 6, BC? In addition, the quote lists tree-ring dating and varve or lake sedimentation as a physical means of indicating and correcting for such deviations in the carbon level. In a later section examining non-radiometric forms of absolute dating, we will demonstrate that neither of these phenomena are reliable forms of dating either.
Thus, not only is carbon dating left without a means to identify and calibrate changes in the ratio of carbon, but this constitutes yet another example of circular reasoning in evolutionary dating. Moreover, the most important issue here is the impact that fluctuations in the carbon ratio have on calculating ages by carbon Note that a factor of two difference in the atmospheric carbon ratio, as shown in the top panel of Figure 9, does not translate to a factor of two offset in the age.
Rather, the offset is equal to one half-life, or 5, years for carbon The initial portion of the calibration curve in Figure 9 has been widely available and well accepted for some time, so reported radiocarbon dates for ages up to 11, years generally give the calibrated ages unless otherwise stated. The calibration curve over the portions extending to 40, years is relatively recent, but should become widely adopted as well.
These methods may work on young samples, for example, if there is a relatively high concentration of the parent isotope in the sample. In that case, sufficient daughter isotope amounts are produced in a relatively short time. As an example, an article in Science magazine vol. There are other ways to date some geologically young samples. Besides the cosmogenic radionuclides discussed above, there is one other class of short-lived radionuclides on Earth. These are ones produced by decay of the long-lived radionuclides given in the upper part of Table 1.
As mentioned in the Uranium-Lead section, uranium does not decay immediately to a stable isotope, but decays through a number of shorter-lived radioisotopes until it ends up as lead. While the uranium-lead system can measure intervals in the millions of years generally without problems from the intermediate isotopes, those intermediate isotopes with the longest half-lives span long enough time intervals for dating events less than several hundred thousand years ago.
Note that these intervals are well under a tenth of a percent of the half-lives of the long-lived parent uranium and thorium isotopes discussed earlier. Two of the most frequently-used of these "uranium-series" systems are uranium and thorium These are listed as the last two entries in Table 1, and are illustrated in Figure A schematic representation of the uranium decay chain, showing the longest-lived nuclides.
Half-lives are given in each box. Solid arrows represent direct decay, while dashed arrows indicate that there are one or more intermediate decays, with the longest intervening half-life given below the arrow. Like carbon, the shorter-lived uranium-series isotopes are constantly being replenished, in this case, by decaying uranium supplied to the Earth during its original creation.
Following the example of carbon, you may guess that one way to use these isotopes for dating is to remove them from their source of replenishment. This starts the dating clock. In carbon this happens when a living thing like a tree dies and no longer takes in carbonladen CO2.
For the shorter-lived uranium-series radionuclides, there needs to be a physical removal from uranium. The chemistry of uranium and thorium are such that they are in fact easily removed from each other.
Uranium tends to stay dissolved in water, but thorium is insoluble in water. So a number of applications of the thorium method are based on this chemical partition between uranium and thorium.
Sediments at the bottom of the ocean have very little uranium relative to the thorium. Because of this, the uranium, and its contribution to the thorium abundance, can in many cases be ignored in sediments. Thorium then behaves similarly to the long-lived parent isotopes we discussed earlier.
It acts like a simple parent-daughter system, and it can be used to date sediments. On the other hand, calcium carbonates produced biologically such as in corals, shells, teeth, and bones take in small amounts of uranium, but essentially no thorium because of its much lower concentrations in the water.
This allows the dating of these materials by their lack of thorium. A brand-new coral reef will have essentially no thorium As it ages, some of its uranium decays to thorium While the thorium itself is radioactive, this can be corrected for. Comparison of uranium ages with ages obtained by counting annual growth bands of corals proves that the technique is page. The method has also been used to date stalactites and stalagmites from caves, already mentioned in connection with long-term calibration of the radiocarbon method.
In fact, tens of thousands of uranium-series dates have been performed on cave formations around the world.
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Previously, dating of anthropology sites had to rely on dating of geologic layers above and below the artifacts. But with improvements in this method, it is becoming possible to date the human and animal remains themselves. Work to date shows that dating of tooth enamel can be quite reliable.
However, dating of bones can be more problematic, as bones are more susceptible to contamination by the surrounding soils. As with all dating, the agreement of two or more methods is highly recommended for confirmation of a measurement. If the samples are beyond the range of radiocarbon e. Non-Radiometric Dating Methods for the PastYears We will digress briefly from radiometric dating to talk about other dating techniques.
It is important to understand that a very large number of accurate dates covering the pastyears has been obtained from many other methods besides radiometric dating. We have already mentioned dendrochronology tree ring dating above. Dendrochronology is only the tip of the iceberg in terms of non-radiometric dating methods.
Here we will look briefly at some other non-radiometric dating techniques. One of the best ways to measure farther back in time than tree rings is by using the seasonal variations in polar ice from Greenland and Antarctica. There are a number of differences between snow layers made in winter and those made in spring, summer, and fall. These seasonal layers can be counted just like tree rings. The seasonal differences consist of a visual differences caused by increased bubbles and larger crystal size from summer ice compared to winter ice, b dust layers deposited each summer, c nitric acid concentrations, measured by electrical conductivity of the ice, d chemistry of contaminants in the ice, and e seasonal variations in the relative amounts of heavy hydrogen deuterium and heavy oxygen oxygen in the ice.
These isotope ratios are sensitive to the temperature at the time they fell as snow from the clouds. The heavy isotope is lower in abundance during the colder winter snows than it is in snow falling in spring and summer. So the yearly layers of ice can be tracked by each of these five different indicators, similar to growth rings on trees. The different types of layers are summarized in Table III. Page 17 Ice cores are obtained by drilling very deep holes in the ice caps on Greenland and Antarctica with specialized drilling rigs.
As the rigs drill down, the drill bits cut around a portion of the ice, capturing a long undisturbed "core" in the process. These cores are carefully brought back to the surface in sections, where they are catalogued, and taken to research laboratories under refrigeration.
A very large amount of work has been done on several deep ice cores up to 9, feet in depth. Several hundred thousand measurements are sometimes made for a single technique on a single ice core. A continuous count of layers exists back as far asyears. In addition to yearly layering, individual strong events such as large-scale volcanic eruptions can be observed and correlated between ice cores.
A number of historical eruptions as far back as Vesuvius nearly 2, years ago serve as benchmarks with which to determine the accuracy of the yearly layers as far down as around meters. As one goes further down in the ice core, the ice becomes more compacted than near the surface, and individual yearly layers are slightly more difficult to observe. For this reason, there is some uncertainty as one goes back towardsyears.
Recently, absolute ages have been determined to 75, years for at least one location using cosmogenic radionuclides chlorine and beryllium G. These agree with the ice flow models and the yearly layer counts.
Note that there is no indication anywhere that these ice caps were ever covered by a large body of water, as some people with young-Earth views would expect. Polar ice core layers, counting back yearly layers, consist of the following: Visual Layers Summer ice has more bubbles and larger crystal sizes Observed to 60, years ago Dust Layers Measured by laser light scattering; most dust is deposited during spring and summer Observed toyears ago Layering of Elec-trical Conductivity Nitric acid from the stratosphere is deposited in the springtime, and causes a yearly layer in electrical conductivity measurement Observed through 60, years ago Contaminant Chemistry Layers Soot from summer forest fires, chemistry of dust, occasional volcanic ash Observed through 2, years; some older eruptions noted Hydrogen and Oxygen Isotope Layering Indicates temperature of precipitation.
Heavy isotopes oxygen and deuterium are depleted more in winter. Yearly layers observed through 1, years; Trends observed much farther back in time Varves. Another layering technique uses seasonal variations in sedimentary layers deposited underwater. The two requirements for varves to be useful in dating are 1 that sediments vary in character through the seasons to produce a visible yearly pattern, and 2 that the lake bottom not be disturbed after the layers are deposited. These conditions are most often met in small, relatively deep lakes at mid to high latitudes.
Shallower lakes typically experience an overturn in which the warmer water sinks to the bottom as winter approaches, but deeper lakes can have persistently thermally stratified temperature-layered water masses, leading to less turbulence, and better conditions for varve layers. Varves can be harvested by coring drills, somewhat similar to the harvesting of ice cores discussed above. Overall, many hundreds of lakes have been studied for their varve patterns.
Each yearly varve layer consists of a mineral matter brought in by swollen streams in the spring. Regular sequences of varves have been measured going back to about 35, years. The thicknesses of the layers and the types of material in them tells a lot about the climate of the time when the layers were deposited. For example, pollens entrained in the layers can tell what types of plants were growing nearby at a particular time.
Other annual layering methods. Besides tree rings, ice cores, and sediment varves, there are other processes that result in yearly layers that can be counted to determine an age. Annual layering in coral reefs can be used to date sections of coral. Coral generally grows at rates of around 1 cm per year, and these layers are easily visible.
As was mentioned in the uranium-series section, the counting of annual coral layers was used to verify the accuracy of the thorium method. There is a way of dating minerals and pottery that does not rely directly on half-lives. Thermoluminescence dating, or TL dating, uses the fact that radioactive decays cause some electrons in a material to end up stuck in higher-energy orbits. The number of electrons in higher-energy orbits accumulates as a material experiences more natural radioactivity over time.
If the material is heated, these electrons can fall back to their original orbits, emitting a very tiny amount of light. If the heating occurs in a laboratory furnace equipped with a very sensitive light detector, this light can be recorded. The term comes from putting together thermo, meaning heat, and luminescence, meaning to emit light. By comparison of the amount of light emitted with the natural radioactivity rate the sample experienced, the age of the sample can be determined.
TL dating can generally be used on samples less than half a million years old. TL dating and its related techniques have been cross calibrated with samples of known historical age and with radiocarbon and thorium dating. While TL dating does not usually pinpoint the age with as great an accuracy as these other conventional radiometric dating, it is most useful for applications such as pottery or fine-grained volcanic dust, where other dating methods do not work as well.
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.