Oxidizable carbon ratio dating is not an objective independent dating method and does not provide evidence for long ages “While the OCR procedure provides good age estimates for many archaeological samples, it cannot be applied to all situations. Specific environmental conditions must be met before meaningful age estimates are possible. The change in the oxidizable C ratio through time and the formulation of the OCR-date equation, were derived from samples obtained from moderately to well drained aerobic soils. Results from the analyses conducted on samples obtained from poorly drained anaerobic soils yielded spurious data, suggesting that OCR-date equation pertains to an O2 dependent system.
Cosmic rays bombard Earth’s atmosphere, creating the unstable isotope carbon-14. This isotope lets scientists learn the ages of once-living things. Image via /Simon Swordy/NASA. Radiocarbon dating is a technique used by scientists to learn the ages of biological specimens – for example, wooden archaeological artifacts or ancient human remains – from the distant past.
It can be used on objects as old as about 62,000 years. Here’s how it works. What is an isotope? To understand radiocarbon dating, you first have to understand the word isotope. An isotope is what scientists call two or more forms of the same element. If you could peer at the atoms of two different isotopes, you’d find equal numbers of protons but different numbers of neutrons in the atoms’ nucleus or core. So there’s a difference in the relative atomic masses of two isotopes.
But they still have the same chemical properties. A carbon atom is a carbon atom is a carbon atom … Although an element’s number of protons cannot change, the number of neutrons can vary slightly in each atom. Atoms of the same element that have different numbers of neutrons are called isotopes.
Here’s an example using the simplest atom, hydrogen. Radiocarbon dating uses isotopes of the element carbon. Image via Radiocarbon dating uses carbon isotopes.
Radiocarbon dating relies on the carbon isotopes carbon-14 and carbon-12. Scientists are looking for the ratio of those two isotopes in a sample. Most carbon on Earth exists as the very stable isotope carbon-12, with a very small amount as carbon-13. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12. Cosmic rays – high-energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14.
Carbon-14 is considered a radioactive isotope of carbon. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes. Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.
Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere. They have the same ratio of carbon-14 to carbon-12 as the atmosphere, and this same ratio is then carried up the food chain all the way to apex predators, like sharks.
But when gas exchange is stopped, be it in a particular part of the body like in deposits in bones and teeth, or when the entire organism dies, the ratio of carbon-14 to carbon-12 begins to decrease.
The unstable carbon-14 gradually decays to carbon-12 at a steady rate. And that’s the key to radiocarbon dating. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive. This plot shows the level of carbon-14 in the atmosphere as measured in New Zealand (red) and Austria (green), representing the Southern and Northern Hemispheres, respectively. Aboveground nuclear testing almost doubled the amount of carbon-14 in the atmosphere.
That’s why aboveground nuclear testing was banned. The black arrow shows when the Partial Test Ban Treaty was enacted that banned aboveground nuclear tests. Image via Hokanomono via Wikimedia Commons. A special kind of radiocarbon dating: Bomb radiocarbon dating. As we mentioned above, the carbon-14 to carbon-12 ratio in the atmosphere remains nearly constant. It’s not absolutely constant due to several variables that affect the levels of cosmic rays reaching the atmosphere, such as the fluctuating strength of the Earth’s magnetic field, solar cycles that influence the amount of cosmic rays entering the solar system, climatic changes and human activities.
Among the significant events that caused a temporary but significant spike in the atmospheric carbon-14 to carbon-12 ratio were above-ground nuclear test detonations in the two decades following World War II. Bomb radiocarbon dating is a term for radiocarbon dating based on timestamps left by above-ground nuclear explosions, and it is especially useful for putting an absolute age on organisms that lived through those events. In Ethan Siegel writes: The only major fluctuation [in carbon-14] we know of occurred when we began detonating nuclear weapons in the open air, back in the mid-20th century.
If you ever wondered why nuclear tests are now performed underground, this is why. Most radiocarbon dating today is done using an , an instrument that directly counts the numbers of carbon-14 and carbon-12 in a sample. A detailed description of radiocarbon dating is available at the .
Bottom line: Radiocarbon dating is a technique used by scientists to learn the ages of biological specimens from the distant past.
best carbon dating ratio is being measured in - How Accurate is Carbon Dating? We Bet You Didn't Know This
About the Archive This is a digitized version of an article from The Times’s print archive, before the start of online publication in 1996. To preserve these articles as they originally appeared, The Times does not alter, edit or update them. Occasionally the digitization process introduces transcription errors or other problems. Please send reports of such problems to May 31, 1990, Page 00021 The New York Times Archives Since 1947, scientists have reckoned the ages of many old objects by measuring the amounts of radioactive carbon they contain.
New research shows, however, that some estimates based on carbon may have erred by thousands of years. It is too soon to know whether the discovery will seriously upset the estimated dates of events like the arrival of human beings in the Western Hemisphere, scientists said. But it is already clear that the carbon method of dating will have to be recalibrated and corrected in some cases.
Scientists at the Lamont-Doherty Geological Laboratory of Columbia University at Palisades, N.Y., reported today in the British journal Nature that some estimates of age based on carbon analyses were wrong by as much as 3,500 years.
They arrived at this conclusion by comparing age estimates obtained using two different methods - analysis of radioactive carbon in a sample and determination of the ratio of uranium to thorium in the sample. In some cases, the latter ratio appears to be a much more accurate gauge of age than the customary method of carbon dating, the scientists said.
In principle, any material of plant or animal origin, including textiles, wood, bones and leather, can be dated by its content of carbon 14, a radioactive form of carbon in the environment that is incorporated by all living things. Because it is radioactive, carbon 14 steadily decays into other substances. But when a plant or animal dies, it can no longer accumulate fresh carbon 14, and the supply in the organism at the time of death is gradually depleted.
Dating Subject to Error But scientists have long recognized that carbon dating is subject to error because of a variety of factors, including contamination by outside sources of carbon. Therefore they have sought ways to calibrate and correct the carbon dating method. The best gauge they have found is dendrochronology: the measurement of age by tree rings. Accurate tree ring records of age are available for a period extending 9,000 years into the past. But the tree ring record goes no further, so scientists have sought other indicators of age against which carbon dates can be compared.
One such indicator is the uranium-thorium dating method used by the Lamont-Doherty group. Uranium 234, a radioactive element present in the environment, slowly decays to form thorium 230. Using a mass spectrometer, an instrument that accelerates streams of atoms and uses magnets to sort them out according to mass and electric charge, the group has learned to measure the ratio of uranium to thorium very precisely.
The Lamont-Doherty scientists conducted their analyses on samples of coral drilled from a reef off the island of Barbados. The samples represented animals that lived at various times during the last 30,000 years.
Uranium-Thorium Dating Dr. Alan Zindler, a professor of geology at Columbia University who is a member of the Lamont-Doherty research group, said age estimates using the carbon dating and uranium-thorium dating differed only slightly for the period from 9,000 years ago to the present.
''But at earlier times, the carbon dates were substantially younger than the dates we estimated by uranium-thorium analysis,'' he said. ''The largest deviation, 3,500 years, was obtained for samples that are about 20,000 years old.'' One reason the group believes the uranium-thorium estimates to be more accurate than carbon dating is that they produce better matches between known changes in the Earth's orbit and changes in global glaciation.
According to carbon dating of fossil animals and plants, the spreading and receding of great ice sheets lagged behind orbital changes by several thousand years, a delay that scientists found hard to explain.
But Dr. Richard G. Fairbanks, a member of the Lamont-Doherty group, said that if the dates of glaciation were determined using the uranium-thorium method, the delay - and the puzzle - disappeared. The group theorizes that large errors in carbon dating result from fluctuations in the amount of carbon 14 in the air. Changes in the Earth's magnetic field would change the deflection of cosmic-ray particles streaming toward the Earth from the Sun. Carbon 14 is thought to be mainly a product of bombardment of the atmosphere by cosmic rays, so cosmic ray intensity would affect the amount of carbon 14 in the environment at any given time.
#30,000-Year Limit The Lamont-Doherty group says uranium-thorium dating not only is more precise than carbon dating in some cases, but also can be used to date much older objects. Carbon dating is unreliable for objects older than about 30,000 years, but uranium-thorium dating may be possible for objects up to half a million years old, Dr. Zindler said. The method is less suitable, however, for land animals and plants than for marine organisms, because uranium is plentiful in sea water but less so in most soils.
But even if the method is limited to marine organisms, it will be extremely useful for deciphering the history of Earth's climate, ice, oceans and rocks, Dr. Fairbanks said.
Carbon dating, or radiocarbon dating, like any other laboratory testing technique, can be extremely reliable, so long as all of the variables involved are controlled and understood. Several factors affect radiocarbon test results, not all of which are easy to control objectively. For this reason, it’s preferable to date objects using multiple methods, rather than relying on one single test.
Carbon dating is reliable within certain parameters but certainly not infallible. When testing an object using radiocarbon dating, several factors have to be considered: First, carbon dating only works on matter that was once alive, and it only determines the approximate date of death for that sample. For example, a steel spearhead cannot be carbon dated, so archaeologists might perform testing on the wooden shaft it was attached to.
This provides good information, but it only indicates how long ago that piece of wood was cut from a living tree. Radiocarbon dating can’t tell the difference between wood that was cut and immediately used for the spear, and wood that was cut years before being re-used for that purpose. Nor can it tell if a much older spearhead was attached to a brand-new shaft.
Most archaeological items can’t be directly carbon dated, so their dating is based on testing done on nearby objects or materials.
This makes the results subject to the researchers’ assumptions about those objects. If the spear head is dated using animal bones nearby, the accuracy of the results is entirely dependent on the assumed link between the spear head and the animal.
This is perhaps the greatest point of potential error, as assumptions about dating can lead to circular reasoning, or choosing confirming results, rather than accepting a “wrong” date.
Second, radiocarbon dating becomes more difficult, and less accurate, as the sample gets older. The bodies of living things generally have concentrations of the isotope carbon-14, also known as radiocarbon, identical to concentrations in the atmosphere. When an organism dies, it stops taking in new carbon-14, and whatever is inside gradually decays into other elements.
Carbon-14 normally makes up about 1 trillionth (1/1,000,000,000,000) of the earth’s atmosphere. So even brand-new samples contain incredibly tiny quantities of radiocarbon. Eventually, the amount of carbon-14 remaining is so small that it’s all but undetectable.
Tiny variations within a particular sample become significant enough to skew results to the point of absurdity. Carbon dating therefore relies on enrichment and enhancement techniques to make smaller quantities easier to detect, but such enhancement can also skew the test results.
Normal errors in the test become magnified. As a result, carbon dating is only plausible for objects less than about 40,000 years old. The other major factor affecting the results of carbon dating is gauging the original proportion of carbon-14 itself. Carbon dating is based on the loss of carbon-14, so, even if the present amount in a specimen can be detected accurately, we must still know how much carbon-14 the organism started with.
Scientists must assume how much carbon-14 was in the organism when it died. Complicating matters is the fact that Earth’s carbon-14 concentrations change drastically based on various factors. As samples get older, errors are magnified, and assumptions can render carbon dating all but useless. For example, variations in greenhouse effects and solar radiation change how much carbon-14 a living organism is exposed to, which drastically changes the “starting point” from which a radiocarbon dating test is based.
Likewise, different living things absorb or reject carbon-14 at different rates. Two plants that died at the same moment, but which naturally contained different levels of radiocarbon, could be dated to drastically different times.
Modern effects such as fossil fuel burning and nuclear testing have also changed atmospheric carbon-14 levels and in turn change the “starting point” for a radiocarbon test.
All in all, setting the parameters of the carbon-14 test is more of an art than a science. Contamination and repeatability are also factors that have to be considered with carbon dating. A tiny amount of carbon contamination will greatly skew test results, so sample preparation is critical.
Even then, a large proportion of radiocarbon dating tests return inconsistent, or even incoherent, results, even for tests done on the same sample. The explanation given for these outliers is usually “contamination.” Inconsistent results are another reason why multiple samples, multiples tests, and various parallel methods are used to date objects.
Due to all these factors, it’s common for carbon dating results of a particular sample, or even a group of samples, to be rejected for the sole reason that they don’t align with the “expected” results. That’s not unusual in science, so far as it goes, but the relationship between assumptions and interpretations must be kept in mind. At best, it needs to be acknowledged. At worst, it can make carbon dating circular and self-confirming, though there are other means of dating that can reduce this risk.
In short, carbon dating is as useful as any other technique, so long as it’s done properly and the results are objectively interpreted. It is not, however, an inherently error-free or black-and-white method for dating objects. In order to explain the Carbon 14 dating process itself, were going to have to get a little science-cee. Atoms are the basic building blocks of matter. Atoms are made up of much smaller particles called protons, neutrons, and electrons. Protons and neutrons make up the center (nucleus) of the atom, and electrons form shells around the nucleus.
The number of protons in the nucleus of an atom determines the element. For example, all carbon atoms have 6 protons, all atoms of nitrogen have 7 protons, and all oxygen atoms have 8 protons. The number of neutrons in the nucleus can vary in any given type of atom.
So, a carbon atom might have six neutrons, or seven, or possibly eight—but it would always have six protons. An “isotope” is any of several different forms of an element, each having different numbers of neutrons. The illustration below shows the three isotopes of carbon. Carbon-14, is expressed as (14C) also referred to, as I stated earlier, as radiocarbon.
Biblical claims of a young earth (about 6,000 years) has been in question, since 14C dates of tens of thousands of years have become common. When a scientist’s interpretation of data does not match the clear meaning of the text in the Bible, we should never reinterpret the Bible.
God knows just what He meant to say, our science as far as God is concerned is laughable and menial and His understanding of our science is infallible, whereas ours is fallible. So we should never think it necessary to modify His Word. Genesis 1 defines the days of creation to be literal days (a number with the word “day” always means a normal day in the Old Testament, and the phrase “evening and morning” further defines the days as literal days). Since the Bible is the inspired Word of God, we should examine the validity of the standard interpretation of 14 C dating by asking several questions: Some isotopes of certain elements are unstable; they can spontaneously change into another kind of atom in a process called “radioactive decay.” Since this process presently happens at a known measured rate, scientists attempt to use it like a “clock” to tell how long ago a rock or fossil formed.
There are two main applications for radiometric dating. One is for potentially dating fossils (once-living things) using carbon-14 dating, and the other is for dating rocks and the age of the earth using uranium, potassium and other radioactive atoms.
Radiocarbon (14C) is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14C combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire 14C by eating the plants.
When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14C it contains begins to decrease as the 14C undergoes radioactive decay. Measuring the amount of 14C in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The older a sample is, the less 14C there is to be detected, and because the half-life of 14C (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.
In nature, carbon exists as two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C), and a radioactive isotope, carbon-14 (14C), also known as "radiocarbon". The half-life of 14C (the time it takes for half of a given amount of 14C to decay) is about 5,730 years, so its concentration in the atmosphere might be expected to reduce over thousands of years, but 14C is constantly being produced in the lower stratosphere and upper troposphere, primarily by galactic cosmic rays, and to a lesser degree by solar cosmic rays.
There are three naturally occurring isotopes of carbon in the environment: carbon-12, carbon-13, and carbon-14. They all have six protons since they are carbon, and therefore they are identical chemically. Biological and chemical processes cannot tell them apart, so they maintain approximately the same relative abundance in living tissue as they do in the atmosphere.
The chemical processes that bring carbon into the body and those which remove it from the body treat all carbon the same. Therefore, the percentages of the three isotopes will be the same in a living organism as it is in the organism’s environment. Carbon-14 is not stable against beta-minus decay with a half-life of 5,730±40 years, but is constantly replenished in the atmosphere by cosmic ray interaction with nitrogen-14.
The relative abundance of carbon-14 varies slightly with solar flares, magnetic field fluctuations, etc. At any given time, there will be a small fraction of all the carbon which is carbon-14.
The naturally occurring ratio of C-14/C-12 is about 10^-12 (0.000000000001). After a living organism dies, the carbon-14 in the organism will decay away, but it is no longer replenished by intake by breathing or eating.
Therefore the ratio of C-14/C-12 decreases. If any chemical or mechanical processes remove carbon from the dead organism, all carbon will be equally affected so the ratio is unaffected. Only the radioactive decay of the carbon-14 can affect the C-14/C-12 ratio.
By measuring how much the ratio has changed, the date of dead organisms can be calculated. After about ten half-lives there is so little C-14 left that dating is impossible. Therefore carbon dating can only be used on organisms that were alive less than ≈57,000 year ago.
Note that we don’t measure carbon-14 and compare it to how much carbon we assume was in the sample. We don’t need to know how much carbon was in the sample. We don’t care if some carbon was removed.
We measure the current ratio of C-14/C-12 and compare it to naturally occurring ratio. It does not depend on the amount of carbon in the environment or even the amount in the sample. It only depends on the ratio.
So how do we know what the naturally occurring ratio of C-14 to C-12 was in the past? The most straightforward method is with tree rings. By counting tree rings and by correlating them with older tree rings (by matching up sequences of drought, etc.) we can go back about 8000 years. We can do similar analysis with ice cores and varves. We can measure the C-14/C-12 ratio in the tree rings.
We can count the rings to see how old the ring is to a very high precision. Since we know the half-life of carbon-14 we can calculate what the C-14/C-12 ratio was at the time the ring was formed. Through this method we can produce a curve or table that shows exactly what the C-14/C-12 ratio was for thousands of years into the past.
So when we find a sample of organic material and we want to know how old it is, we measure the ratio of C-14/C-12 in the sample. This is generally not done by measuring the radioactivity of the carbon-14 atoms. Instead scientist use an accelerator mass spectrometer to measure the ratio of the carbon-14 atoms to the carbon-12 atoms.
A mass spec can do this easily with proper preparation. When a sample is dated, the ratio of C-14/C-12 is measured and compared to the curve or table I discussed in the previous paragraph. That gives how long ago the biological sample stopped exchanging carbon with the environment.
That is when it died.