Free Essay: Carbon Dating: an Invaluable Yardstick in the Chronology of Humans Archeologists use many methods to analyze data from.
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Creationist Objections to Radiometric Dating. Independent Checks on Radiometric Dating. Theory of radiometric dating. What is radiometric dating? Simply stated, radiometric dating is a way of determining the age of a sample of material using the decay rates of radio-active nuclides to provide a 'clock.
The rules are the same in all cases; the assumptions are different for each method. To explain those rules, I'll need to talk about some basic atomic physics. There are 90 naturally occurring chemical elements. Elements are identified by their atomic number , the number of protons in the atom's nucleus. All atoms except the simplest, hydrogen- 1, have nuclei made up of protons and neutrons.
Hydrogen-1's nucleus consists of only a single proton. Protons and neutrons together are called nucleons , meaning particles that can appear in the atomic nucleus.
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A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons. Since all atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain.
For example, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-1's nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutron. Uranium contains 92 protons and neutrons, while uranium contains 92 protons and neutrons. Many nuclides are stable -- they will always remain as they are unless some external force changes them. Some, however, are unstable -- given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element.
There are two common kinds of radioactive decay, alpha decay and beta decay. In alpha decay, the radioactive atom emits an alpha particle. An alpha particle contains two protons and two neutrons.
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After emission, it quickly picks up two electrons to balance the two protons, and becomes an electrically neutral helium-4 He4 atom. When a nuclide emits an alpha particle, its atomic number drops by 2, and its mass number number of nucleons drops by 4. Thus, an atom of U uranium, atomic number 92 emits an alpha particle and becomes an atom of Th thorium, atomic number A beta particle is an electron. When an atom emits a beta particle, a neutron inside the nucleus is transformed to a proton.
The mass number doesn't change, but the atomic number goes up by 1. Thus, an atom of carbon C14 , atomic number 6, emits a beta particle and becomes an atom of nitrogen N14 , atomic number 7. A third, very rare type of radioactive decay is called electron absorption.
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In electron absorption, a proton absorbs an electron to become a neutron. In other words, electron absorption is the exact reverse of beta decay. So an atom of potassium K40 , atomic number 19 can absorb an electron to become an atom of argon Ar40 , atomic number The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay.
A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the nuclide. Some nuclides have very long half-lives, measured in billions or even trillions of years. Others have extremely short half-lives, measured in tenths or hundredths of a second. The decay rate and therefore the half-life are fixed characteristics of a nuclide.
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Different nuclides of the same element can have substantially different half-lives. The half-life is a purely statistical measurement. A sample of U ten thousand years old will have precisely the same half-life as one ten billion years old. Obviously, the major question here is "how much of the nuclide was originally present in our sample? Such cases are useless for radiometric dating. We must know the original quantity of the parent nuclide in order to date our sample radiometrically. Fortunately, there are cases where we can do that.
This is the second axiom of radiometric dating. The third and final axiom is that when an atom undergoes radioactive decay, its internal structure and also its chemical behavior change. Losing or gaining atomic number puts the atom in a different row of the periodic table, and elements in different rows behave in different ways. It may not form the same kinds of compounds. When the number of electrons change, the shell structure changes too. So when an atom decays and changes into an atom of a different element, its shell structure changes and it behaves in a different way chemically.
How do these axioms translate into useful science? This section describes several common methods of radiometric dating. To start, let's look at the one which almost everyone has heard of: The element carbon occurs naturally in three nuclides: C12, C13, and C The vast majority of carbon atoms, about About one atom in billion is C The remainder are C Of the three, C12 and C13 are stable. C14 is radioactive, with a half-life of years. C14 is also formed continuously from N14 nitrogen in the upper reaches of the atmosphere.
And since carbon is an essential element in living organisms, C14 appears in all terrestrial landbound living organisms in the same proportions it appears in the atmosphere. Plants and protists get C14 from the environment. Animals and fungi get C14 from the plant or animal tissue they eat for food. When an organism dies, it stops taking in C If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed.
When we know how much has decayed, we know how old the sample is. Many archaeological sites have been dated by applying radiocarbon dating to samples of bone, wood, or cloth found there. Radiocarbon dating depends on several assumptions. One is that the thing being dated is organic in origin. Radiocarbon dating does not work on anything inorganic, like rocks or fossils. Only things that once were alive and now are dead: The second assumption is that the organism in question got its carbon from the atmosphere. A third is that the thing has remained closed to C14 since the organism from which it was created died.
The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died. That last one is more important than it sounds. When Professor William Libby developed the C14 dating system in , he assumed that the amount of C14 in the atmosphere was a constant. A long series of studies of C14 content produced an equally long series of corrective factors that must be taken into account when using C14 dating.
So the dates derived from C14 decay had to be revised. One reference on radiometric dating lists an entire array of corrective factors for the change in atmospheric C14 over time. C14 dating serves as both an illustration of how useful radiometric dating can be, and of the pitfalls that can be found in untested assumptions. U and U are both nuclides of the element uranium. U is well known as the major fissionable nuclide of uranium.
It has a half-life of roughly million years. U is more stable, with a half-life of 4.
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Th is the most common nuclide of the element thorium, and has a half-life of All three of these nuclides are the starting points for what are called radioactive series. A radioactive series is a sequence of nuclides that form one from another by radioactive decay. The series for U looks like this: A indicates alpha decay; B indicates beta decay.
We can calculate the half-lives of all of these elements. All the intermediate nuclides between U and Pb are highly unstable, with short half-lives. Then any excess of Pb must be the result of the decay of U When we know how much excess Pb there is, and we know the current quantity of U, we can calculate how long the U in our sample has been decaying, and therefore how long ago the rock formed. Th and U also give rise to radioactive series -- different series from that of U, containing different nuclides and ending in different nuclides of lead.
Chemists can apply similar techniques to all three, resulting in three different dates for the same rock sample. Uranium and thorium have similar chemical behavior, so all three of these nuclides frequently occur in the same ores. If all three dates agree within the margin of error, the date can be accepted as confirmed beyond a reasonable doubt. Since all three of these nuclides have substantially different half-lives, for all three to agree indicates the technique being used is sound. But even so, radioactive-series dating could be open to question.
The rock being dated must remain a closed system with respect to uranium, thorium, and their daughter nuclides for the method to work properly. Both the uranium and thorium series include nuclides of radon, an inert gas that can migrate through rock fairly easily even in the few days it lasts. To have a radiometric dating method that is unquestionably accurate, we need a radioactive nuclide for which we can get absolutely reliable measurements of the original quantity and the current quantity. Is there any such nuclide to be found in nature?
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The answer is yes. Which brings us to the third method of radiometric dating. The element potassium has three nuclides, K39, K40, and K Only K40 is radioactive; the other two are stable. K40 is unusual among radioactive nuclides in that it can break down two different ways. It can emit a beta particle to become Ca40 calcium , or it can absorb an electron to become Ar40 argon Argon is a very special element. Argon is a gas at Earth-normal temperatures, and in any state it exists only as single atoms.
By contrast, potassium and calcium are two of the most active elements in nature. They both form compounds readily and hold onto other atoms tenaciously. What does this mean? It means that before a mineral crystallizes, argon can escape from it easily. It also means that when an atom of argon forms from an atom of potassium inside the mineral, the argon is trapped in the mineral. So any Ar40 we find deep inside a rock sample must be there as a result of K40 decay. That and some simple calculations produce a figure for how long the K40 has been decaying in our rock sample. What happens if our mineral sample has not remained a closed system?
What if argon has escaped from the mineral? What if argon has found its way into the mineral from some other source? If some of the radiogenic argon has escaped, then more K40 must have decayed than we think -- enough to produce what we did find plus what escaped.
In other words, a mineral that has lost argon will be older than the result we get says it is. In the other direction, if excess argon has gotten into the mineral, it will be younger than the result we get says it is. An isochron dating method isochron dating is described in the next section can also be applied to potassium-argon dating under certain very specific circumstances.
When isochron dating can be used, the result is a much more accurate date. Yet a fourth method, rubidium-strontium dating, is even better than potassium-argon dating for old rocks. The nuclide rubidium Rb87 decays to strontium Sr87 with a half-life of 47 billion years. Strontium occurs naturally as a mixture of several nuclides. If three minerals form at the same time in different regions of a magma chamber, they will have identical ratios of the different strontium nuclides. The total amount of strontium might be different in the different minerals, but the ratios will be the same.
Now, suppose that one mineral has a lot of Rb87, another has very little, and the third has an in-between amount. That means that when the minerals crystallize there is a fixed ratio of Rb As time goes on, atoms of Rb87 decay to Sr, resulting in a change in the Rb Sr87 ratio, and also in a change in the ratio of Sr87 to other nuclides of strontium. The decrease in the Rb Sr87 ratio is exactly matched by the gain of Sr87 in the strontium-nuclide ratio. The radiocarbon age of a certain sample of unknown age can be determined by measuring its carbon 14 content and comparing the result to the carbon 14 activity in modern and background samples.
The principal modern standard used by radiocarbon dating labs was the Oxalic Acid I obtained from the National Institute of Standards and Technology in Maryland. This oxalic acid came from sugar beets in When the stocks of Oxalic Acid I were almost fully consumed, another standard was made from a crop of French beet molasses.
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Over the years, other secondary radiocarbon standards have been made. Radiocarbon activity of materials in the background is also determined to remove its contribution from results obtained during a sample analysis. Background samples analyzed are usually geological in origin of infinite age such as coal, lignite, and limestone. A radiocarbon measurement is termed a conventional radiocarbon age CRA.
The CRA conventions include a usage of the Libby half-life, b usage of Oxalic Acid I or II or any appropriate secondary standard as the modern radiocarbon standard, c correction for sample isotopic fractionation to a normalized or base value of These values have been derived through statistical means. American physical chemist Willard Libby led a team of scientists in the post World War II era to develop a method that measures radiocarbon activity. He is credited to be the first scientist to suggest that the unstable carbon isotope called radiocarbon or carbon 14 might exist in living matter.
Libby and his team of scientists were able to publish a paper summarizing the first detection of radiocarbon in an organic sample. It was also Mr. Libby was awarded the Nobel Prize in Chemistry in recognition of his efforts to develop radiocarbon dating. Discovery of Radiocarbon Dating accessed October 31, Sheridan Bowman, Radiocarbon Dating: Interpreting the Past , University of California Press.
Accelerator Mass Spectrometry AMS dating involves accelerating ions to extraordinarily high kinetic energies followed by mass analysis. The application of radiocarbon dating to groundwater analysis can offer a technique to predict the over-pumping of the aquifer before it becomes contaminated or overexploited. Beta Analytic does not accept pharmaceutical samples with "tracer Carbon" or any other material containing artificial Carbon to eliminate the risk of cross-contamination. Radiocarbon Dating Groundwater The application of radiocarbon dating to groundwater analysis can offer a technique to predict the over-pumping of the aquifer before it becomes contaminated or overexploited.
Tracer-Free AMS Dating Lab Beta Analytic does not accept pharmaceutical samples with "tracer Carbon" or any other material containing artificial Carbon to eliminate the risk of cross-contamination.