Carbon-Fourteen Dating Using the AMS Method
Author: Ian Taylor
The Source of C-14
The earth’s atmosphere is bombarded with cosmic rays thought to originate from our sun. The earth’s magnetic field serves to protect us from most of these cosmic rays. When the nucleus of a nitrogen atom (atomic mass 14) in the upper atmoshere is hit by a cosmic ray, a neutron is released and the atom becomes subject to the earth’s magnetic field and are seen as the Northern Lights in the northern hemisphere; ionized atoms drift to the north magnetic pole and discharge.
Other nitrogen atoms in the upper atmosphere capture those free neutrons and become unstable C-14 atoms, i.e. they are radioactive C-14, chemically identical to normal C-12 except that with time these atoms decay by emitting electrons (beta particles) and revert back to nitrogen-14 (N-14) atoms. Cosmic radiation converts about 16 atoms per gram per minute or 21 pounds of nitrogen per year into C-14 so that the proportion of C-14 to C-12 in the atmoshere is extremely small. The C-14 to C-12 ratio is given today as 1.2 X 10-12. This works out to approximately one atom of C-14 to every trillion atoms of C-12. Each C-14 atom combines with two oxygen atoms to form carbon dioxide and takes its place in the carbon chain together with the normal carbon dioxide produced from C-12. The ocean is a huge reservoir of carbon dioxide and, although mixing is much less rapid than in the atmosphere, the ocean does tend to keep the global system quite stable. The carbon dioxide and thus the C-14 content of the atmosphere is believed to have been in equilibrium for many thousands of years; thus, the ratio of C-12 to C-14 is believed to have been constant at least throughout human history. Plants and trees absorb the carbon dioxide from the atmosphere and thus as part of the food chain, C-14 finds its way into every living thing, including man. When living things die they cease to take in C-14. It is at this point that the proportion of C-14 in the dead body begins to decrease from what it had been when living. When living, the proportion of C-14 to C-12 was the same as that in the atmoshere. Knowing the rate at which the C-14 decays back to N-14, and knowing the proportion found in the sample, the age can be determined. The smaller the proportion of C-14, the older is the sample. The calculation depends upon comparing the ratio of C-14 to C-12 in the specimen with that believed to have been in the atmosphere at the time of death.
Materials Commonly Tested
- Wood. Starting with about 200 mg. of wood, peat, pine needles etc the cellulose is chemically extracted and 4 to 5 mg. taken for the test.
- Bone. Starting with 3 to 6 grams of material, the collagen (protein) is chemically extracted and the few mg. resulting used.
- Wooden statues. Rather than destructively use any of the wood, it is preferred to scrape away some of the lacquer or varnish; the wood may be much older.
- Cast iron contains < 3% carbon as graphite or Fe3C. A rod core is drilled from the sample and dissolved in acid. The carbon remains as a black powder, use 1 or 2 mg.
- Ivory/ sea shells. The carbon is present as carbonate and extracted chemically.
The sample is enclosed in an ampule together with copper oxide (CuO) and a small piece of silver wire to act as the “getter”: all traces of air are evacuated. The sealed ampule is heated to 800-900 degrees C. during which time all the organics are oxidized and the carbon converted to carbon dioxide gas. The silver scrubs out the sulphides and halides. The ampule is broken under vacuum and the carbon dioxide gas yield is measured. This CO2 gas is now converted to acetylene by reacting with metallic lithium to form lithium carbide; this carbide is then reacted with water to form acetylene gas.
The acetylene gas is dissociated to graphite powder by an electric discharge. Two electrodes are used with an AC spark and both electrodes become coated with graphite. This graphite powder is compressed to a pellet weighing about 250 micrograms. The pellet is made the target and bombarded with cesium atoms; about 10% of all carbon atoms are ionized to become a negative ion. The cesium atom is one of the largest and does not interfere with the ionized carbon atoms. The object here is to separate all the carbon atoms (C-12, C-13 and C-14) from the nitrogen atoms. Nitrogen 14 atoms have virtually the same weight and properties as C-14 but nitrogen does not form negative ions whereas carbon does. There are a few other elements that may be present and these are separated in the accelerator.
The mixed carbon ions are accelerated to 2 million volts and passed through argon gas. Here collisions take place with the heavier argon atoms and electrons are knocked off the carbon ions. The carbon ions are then passed through another gas chamber at a higher voltage to separate out the last traces of foreign elements.
The accelerator is really a molecular disintegrator because all the carbon products (C-12, C-13 and C-14) are disintegrated and lose electrons from their outer shells. The mass spectrometer sends the C-12, C-13 and C-14 into three separate beams that are then analyzed quantitatively and in sequential order: C-12 first, C-13 next then C-14. Carbon 14 decays with a half life of 5730 years meaning that the number of C-14 atoms left in the once-living material decrease with age: each 5730 years back into the past the number of C-14 atoms would have been double. Carbon 14 is determined by “counts per hour” in which the actual number of atoms are counted. A modern sample will typically have say, 40,000 counts; a sample of one half life or 5730 years old will have half this number of counts. A sample of two half-lives or 11,460 years old will have 10,000 counts and so on until at 14 half-lives the count is only six and the age would be (5730 X 14)= 80,220 years. The count becomes very uncertain at these low numbers and in practice the age limit for this method is about 70,000 years.
The number of C-12 atoms present, on the other hand, will be extremely great (at least 1012) and an atomic “count” would be impractical. The C-12 and C-13 are thus measured together as the current in milliamperes. A correction is made in the calculations to deduct the small fraction of C-13 that is occluded with the C-14.
Early tests (circa 1948) by Willard Libby used historically dated wood from caskets. When Egyptologist, I.E.S.Edwards, supplied dated wood he warned that the Egyptian chronology was actually younger than textbook ages. A range of wood samples, each about four ounces, was used to produce a calibration curve showing age versus C-14 count. Today, instead of using wood from dated caskets a standard solution of oxalic acid prepared by the US Standards Laboratory is used. However, this only provides one point on the calibration curve.
Reliability of the Method
Work in Leon, France, showed that over the years the results from 40% of the wood samples were rejected by archaeologists while the rejection rate of results from bone samples was only 10%. Clearly, there are problems with wood samples. In general, archaeologists have not met the C-14 tests with the enthusiasm shown by the geologist for their radiometric tests.
Calibration by Dendrochronology
In an attempt to provide some independent check on the C-14 test results, tree-ring analysis has been brought to a fine art using oak trees, bristlecone pine trees, etc. These trees live a long time and, as hardwoods, remain as wood a long time after being harvested. An especially useful set of calibration samples was found on the sea floor between Ireland and England. Here there were remains of oak trees standing in growth positions: those furthest from the shore were the oldest. The Irish oak calibration was carried out in the early 1980’s. There are also pine trees in Germany that have provided a more recent calibration curve that extends back to 11,000 BP (Before the Present).
Difficulties in Preparing Calibration Tree Samples
Under similar climatic conditions individual species of trees may form no rings or one or two or even more rings in a single year. Even among the same species, the number and pattern of rings that grow each year can vary from one valley to the next. Nevertheless, by analyzing several rings for C-14 rather than individual rings, a more uniform calibration curve is said to have been obtained. In practice, the tree section is turned on a lathe so that the sample for analysis consists of about twenty rings. The matching of trees earlier in history with those living later to provide a continuous record has had problems requiring extensive comparison of the patterns of the density of rings. Rings cluster in low growth years or become more separated in high growth years. The work in Germany (“Floating German pine chronology”) was carried out in 1993 and the difficulty of pattern matching overcome by averaging rings and ranking the results by C-14 analysis; this “circular” approach has resulted in a smooth calibration curve but the researchers are reluctant to report the details of their work.
Problems with C-12 to C-14 Equilibrium in the Atmoshere
Knowing the decay rate of C-14, it would take about 30,000 years for the production of C-14 in the upper atmosphere to build up to exactly match the rate of decay, that is, for the system to be in equlibrium. It is unquestionably assumed that the earth’s atmoshere is a lot older than 30,000 years and thus equibrium conditions have prevailed throughout the time that can be measured by the C-14 test method, say, 60,000 years. When the C-14 test was developed in 1948, the half life of C-14 was given as 5,500 years, today it is 5,730 years and, in recognition of the changing conditions, the date 1950 is taken as a base line so that whatever year the tests are carried out, they are all corrected back to this reference point. It is assumed that until the 17th century the ratio of C-12 to C-14 in the atmosphere was in equibrium but a number of factors are known to have influenced it:
- The Zeus Effect recognizes that since the Industrial Revolution a great deal of fossil fuel having virtually no C-14 has been burned adding more C-12 to the atmosphere thus the proportion of C-14 becomes effectively less making specimens living since the 18th century appear to be older.
- The Bomb Peak Effect is the result of atomic bomb testing that added a great deal of C-14 to the atmoshere thus tending to make specimens look younger. The “Bomb Peak” produced a maximum C-14 in 1965 and has been decreasing since; currently, the proportion of C-14 in the atmoshere is 13% above normal. However, the Bomb Peak Effect will only influence those living organisms that were alive from about 1965; it will have no effect on older specimens.
- The explosion of galactic nebulae is now known to increase the rate of decay of all radioactive elements. In the case of C-14 this means that specimens will appear older (having less C-14) than they really are.
- The geomagnetic field of the earth is known to have an effect on the rate of cosmogenic C-14 formation; less C-14 would be produced when the magnetic field was greater. The published data since 1830 shows that the average value of the magnetic field was consistently greater in the past; the Magsat program showed that the half-life of the earth’s magnetic field was an astounding 930 years. Thus, older specimens would contain less C-14 in the initial stage and appear older than their historical date today.
The Radiocarbon test begins by assuming the earth is old and the C-12 to C-14 ratio in the atmosphere has always been about what it is today — about one in a trillion. In fact, this is highly unlikely because today C-14 is actually forming 28 to 37% faster than it is decaying indicating non-equilibrium. It is commonly known among archeologists that specimens beyond about 3,000 years consistently date in excess of their historical age; the departure is greater the older the specimen. A reasonable explanation is as follows:
Sir James George Frazer recorded similar accounts of a world-wide flood in very early human history from 138 separate cultures from around the world. These were not the result of misionary teaching but were nevertheless very similar to the Genesis account of the Flood. If that account is true then at one point in earth’s history there was no vegetation to absorb the earth’s natural output of carbon dioxide. The bulk of the carbon dioxide production prior to the Industrial Revolution was from volcanoes.
Assuming the rate of C-14 production was lower in the past due to the earth’s greater magnetic field, production would nevertheless have continued but in the year of the flood the proportion of C-14 relative to the greater quantity of C-12 would have been less. Following the great flood, it would have taken some centuries for the C-14 to C-12 ratio to come into equilibrium and anything that lived during this period would have had a lower C-14 content and give the appearance of being older than is actually the case.