Archeologists use many methods to analyze data from the past. One scientific tool they use is to analyze the radioactive decay of chemical elements found in plant and animal remains, pottery, and even in rocks. Radiocarbon dating, also known as carbon-14 dating, has been one of the most important radioisotope dating methods used. This scientific tool, which was first developed by Willard F.
Libby in the late 1940s, has significantly improved the accuracy of assigning dates to past events and artifacts as far back as 70,000 years. It is helping archaeologists, geologists, and anthropologists reconstruct the world’s history by filling in some of the many blank dates in the chronology of the history of our human world and by substantiating and revising other dates. In 1941, the radioactive atom, carbon-14, was isolated and discovered.
Utilizing this discovery of the unstable radioactive isotope of carbon, Libby formulated an idea for using the decay rate of this radioactive form of carbon to date the remains of once-living plants and animals such as charcoal, wood, bone, shells, and fossils. In 1948, while at the University of Chicago, he and his colleagues started experimenting with carbon-14 as a means for dating the past. The scientists proved that carbon-14, which is present in our atmosphere as carbon dioxide, is absorbed by plants, animals, and human beings at a constant rate, and that the amount of carbon-14 is stabilized at a specific amount.
A living organism can only intake a finite amount of carbon-14. Then, at the moment the living organism dies, it stops taking in any carbon-14, and the carbon-14 remaining in the organism starts to disintegrate at the half-life rate of 5,568 years (Poole 1961:27). (Today, based on refined calculations/techniques the half-life rate of carbon-14 is generally considered to be 5,730 years) (Wheatley 2004:98; DeYoung 2005:46). From the experimental results, Libby devised an apparatus to measure the amount of carbon-14 that had been lost and the amount that still remained in the substance. He planned to calculate the age of an object from the amount of carbon-14 left inside it after death.
To test the validity of his carbon-14 counting device and subsequent calculations, Libby tested many items that archaeologists had previously dated. Some of the items he tested included: acacia wood from the first stepped pyramid tomb of Egyptian ruler Zoser (established rate: 2700 B.C.; Libby date: 3979±350 years), cypress wood from the tomb of Sneferu in Egypt (established date: 4,575 B.P.; Libby date: 4802±210 years), cedar wood from the Egyptian pharaoh Sesostris’s funeral boat (established date: 3750 B.P.; Libby 3621±180 years), wood from a mummy coffin from the Ptolemaic period in Egypt (established date: 2280;
Libby 2190±450 years), wheat and barley seeds (established date: 5000 years old; Libby date 5256±230 years), and lastly, Libby dated charcoal from Iraq at 6596± 360 years which coincided with the known approximate date (Poole 1961:28-32, Libby 1952:70). Except for the Zoser sample date, which dated too far back in history, his experimental dates were accurate within an acceptable margin of error. These sample tests, along with many others, confirmed that his carbon-14 test dating method was scientifically dependable within an acceptable margin of error for objects already dated. Libby then continued his work on dating objects for which no dates had been established.
Scientists and scholars began to send him samples from all over the world to radiocarbon date. This included dating artifacts from the Dead Sea Scrolls, Pompeii, Stonehenge, and New Mexico. One of his most significant results occurred when his colleagues dated glacial debris near Two Creeks, Wisconsin. His scientific work provided strong evidence that the last Ice Age in North America had covered the land as recently as 11,000 BCE years ago, not 25,000 years ago as most geologists previously believed (Libby 1952:105). All of these accomplishments and the inroads Libby made in dating the past employing carbon-14 dating earned him the Nobel Prize in 1960.
Although radiocarbon dating methods have improved tremendously, Libby’s overall methodology is still utilized and accepted as a dating technique. It is based upon the fact that carbon-14 is a radioactive, unstable isotope of carbon-12 since its molecular composition has two more neutrons than protons. Carbon-14 is made when cosmic rays enter the earth’s atmosphere and collide with nitrogen atoms. The unattached carbon neutrons then combine with nitrogen-14 atoms to become carbon-14 (Nitrogen-14 + neutron → Carbon-14 + proton). As the resulting carbon-14 atoms drift down towards the earth’s surface they combine with oxygen to make carbon dioxide.
This carbon dioxide includes the stable, common isotope carbon-12 and also tiny amounts of radioactive carbon-14. Both kinds of carbon, C-14 and C-12, are absorbed and used by plants and trees during the photosynthesis process and become part of their cellulose structure. Animals then eat these plants containing carbon-14 and carbon-12 and thus forms of carbon enter their tissues. When plants and animals die, they no longer absorb carbon from the atmosphere and the trace amount of carbon-14 in them starts to slowly decay back to nitrogen (Carbon-14 → Nitrogen-14 + Beta (β)).
Beta particles are single electrons that are free from atoms and carry a negative charge (DeYoung 2005:25). At the point of death, all organisms contain one atom of carbon-14 for every trillion atoms of carbon-12 (Poole 1961:25). Carbon dating requires determining the amount of carbon-14 that has disintegrated in the sample and the amount that remains. Generally this is measured as the ratio of isotopes C-14/ C-12. This value is then compared to the initial carbon-14 content in the sample to determine its age taking into account carbon-14’s half-life and other calibration factors (DeYoung 2005:46-48).
Since the amount of carbon-14 in each sample is miniscule, it is necessary to have several “clean” samples of the same artifact being dated. It is imperative to avoid contamination of the artifacts as any carbon-14 found in the non-sample material, such as roots or other decaying remains which might be from a different time period, could significantly distort the results since the percentage of carbon-14 in a sample is minute (Hedman 2007:58). Therefore, scientists or archeologists generally collect large samples because when they cleanse the sample, which includes purification and distillation processes, small amounts of matter tend to be removed.
The trowels must be meticulously cleaned and the artifacts are packed in chemically neutral materials to ensure that the sample’s ratio of C-14 to C-12 remains the same. The stratigraphy of where the sample was taken must also be examined to ensure that the carbon sample location was not contaminated. In addition, scientists extract several samples in order to perform similar test on the artifact to confirm the accuracy of the dates they calculate. In order to determine a samples age, scientists perform experimental trials to identify and count the number of carbon-14 atoms in the sample based on carbon-14’s unique physical properties of larger mass and radioactivity (Hedman 2007:58).
Today, scientists manipulate two formulas to determine the age of an artifact. One is the formula t = (1/λ) ln (I0/I), where “λ” symbolizes the decay constant for carbon-14, “I” stands for the amount of carbon-14 in the dead matter, “t” represents time in years, and “I0” is the carbon-14 in living matter (L’Annunziata 2007:526). This formula calculates the time that has elapsed from the time of death of an organism. The second is an exponential decay formula which is A = A0* e^(-λk) (L’Annunziata 2007:523).
In this equation, “A” stands for the amount of carbon-14 atoms remaining after a given time “t”, “A0” denotes the number of carbon-14 atoms at the time of observation, and “λ” remains the decay constant. This formula allows scientists to know the ratio of the present amount of the radioactive isotope that remains versus the original amount in relation to time. This formula also determines the amount of carbon-14 that would diminish over a given time period. An important fact when using these formulas is that archaeologists and scientists are assuming that the production of radiocarbon in the atmosphere and the carbon-12 and carbon-14 ratio has remained constant over time.
When Libby and his colleagues developed this dating system in the 1940’s, they relied on the radioactivity of carbon-14’s unstable nuclei. They, as well as subsequent scientists, have used Geiger counters to detect if radioactive carbon is present in a sample. This device is able to detect the beta particles emitted by atoms of carbon-14 as they decay. When these rays hit the Geiger counter, the device clicks indicating that the substance is radioactive. After, scientists start their process of determining the samples historical age.
For example, if scientists wants to know the age of a piece of wood destroyed by fire, they take the piece of wood and soak it in chemicals to confirm any contamination particles that remain on the artifact are gone prior to it being tested. The next step is to separate the carbon-14 atoms from everything else in the wood. To do this, scientists place the wood in a heavy glass tube and then burn it. When this reaction occurs, the burning of the wood leaves very little ash and emits carbon dioxide, which is collected in a purification vacuum system. Once the gas containing carbon-14 is completely purified, the gas enters a glass jar, which then enters the carbon-14 counter.
This device is comprised of a ring of Geiger counters inside a casing, and all of this is surrounded by lead or iron bricks to filter out even more radiations from the atmosphere. The sample is then tested and an electronic panel counts and records the time elapsed as each carbon-14 atom disintegrates. From this, scientists can calculate the approximate age of the sample by manipulating the formulas mentioned previously and calculate the number of years that have elapsed since the sample stopped the intake of carbon-14 and began its half-life decay (Poole 1961:40-46). A second method of radiocarbon dating used is the Liquid Scintillation counting method. This was particularly popular during the 1960’s.
The Liquid Scintillation counting method converts a sample to carbon dioxide either through combustion or acid hydrolysis. The gas is purified until it is ready to be “reacted with molten lithium to form lithium carbide, before being catalytically trimerised to benzene” (Higham 2002: paragraph 2). Once this process finishes, the benzene is driven off and collected under a vacuum to be counted for carbon-14 content utilizing a Liquid Scintillation spectrometer. This apparatus counts the pulses of light emitted by the benzene compound when it is bombarded with photons and has a high precision in dating. Advancement in technology has led to the third method of radiocarbon dating, which is more precise than the other two methods. This process is accelerator mass spectrometry or AMS.
This technique uses multiple stages of acceleration and ionization, as well as several magnets to separate the carbon-14 isotopes from all other atoms and molecules in the sample. A major advantage of AMS is that all carbon-14 atoms in a sample are counted, not just the ones that happen to decay. This makes this method very sensitive and can give accurate dating even if only one milligram of carbon is provided (Hedman 2007:60). The limit of carbon-14 detection using AMS is about one carbon-14 atom for every 1016 stable carbon-12 atoms. This sensitivity is one part per ten thousand trillion or the equivalent of being able to detect a unique grain of sand along a 100 mile-long seashore (DeYoung 2005:50)!
This sensitivity means that artifacts can be analyzed without causing damage to them. It also enables objects with small amounts of carbon-14 (like steel tools) to be analyzed and dated. The only drawback is that AMS machines take up entire buildings, are only found in about thirty specialized places throughout the world, and it is very expensive to test samples utilizing this method (Hedman 2007:60). Various scientists use radiocarbon dating as a device to measure ages of artifacts; however, it is categorized under the Culture History theoretical school of thought in archaeology.
Culture History archaeologists focus their work on cultural processes and work to determine human behavior. In the field their focus is keened toward the distribution of artifacts that can define traits, which leads to defining cultures and changes over time. Carbon-14 dating falls under this theoretical school of thought because its purpose is to date artifacts, and when the dates of an artifact are known, they can be related to one another and aid in defining a culture or changes in culture over time (shortman). One of the areas where carbon dating has been particularly useful is in trying to date the evidence of human activity in the Americas.
Prior to carbon-14 dating, most scientists believed the last Ice Age ended about 25,000 years ago (Poole 1961:51). In 1950, ancient logs from spruce trees were found under glacial debris near Two Creeks, Wisconsin. Scientists were certain that these trees were crushed by the fourth Ice Age. Pieces of this wood were sent to Libby for dating. Based on his tests, the last Ice Age had spread across the land as recently as 11,000 years ago! This meant scientists had to restudy and revise previous dates of other natural events (Poole 1961:52-53). Another example of how carbon-14 dating helped was dating the evidence of humans coming to the New World.
This evidence revolves around the Clovis points, a group of artifacts, found in New Mexico. The Clovis points are large spearheads with a “flute” at their base and are made from rocks like flint or volcanic glasses like obsidian that can be chipped away to form sharp edges. Clovis points have been found throughout the United States, and at several sites, these artifacts were found with the remains of mammoths. Carbon-14 dating of these Clovis points places them at 11,000 BCE or 13,000 years ago after calibration (Hedman 2007:86). Using this data, the Clovis first-model was developed that suggested that there should not be any people in the Americas much before 13,000 to 14,000 years ago in view of the Ice Age that made travel difficult, even if it did provide a corridor into this land.
However, over the years archaeologists have claimed to discover sites that predate the Clovis points. As an example, carbon-14 dating of charcoal samples from the Meadowcroft rockshelter in Pennsylvania suggests that the site is over 15,000 years old. This leads to the belief that people landed in America well before the ice-free corridor opened. In addition, in 1997 independent archaeologists confirmed the accuracy of the date of wooden poles and posts from a site in Monte Verde, Chile. Carbon-14 testing
suggested the artifacts were 14,000 to 15,000 years old.
This site also appears to be older than Clovis, which is surprising given the fact that it is 10,000 miles south of Alaska (Hedman 2007:88-91). There are controversies surrounding the carbon dating of these sites. As to Meadowcroft, which is near Canada, remains of oak and hickory trees were found in the oldest layers. It seems unlikely deciduous trees could survive the ice age yet the excavators counter this argument saying the area was sheltered which made the climate milder. Meadowcroft skeptics also suggested that the samples might be contaminated since the site is in the heart of coal country.
If any of the surrounding material (which no longer had carbon-14 in it since it had already decayed) was mixed with charcoal from the fires, it would dilute the C-14 fraction and the dates would be too old. The archaeologists countered this by saying the dates in each layer of excavation were in order of age. As to Monte Verde, skeptic said the artifacts found were near a stream so the evidence was not reliable. They might be a mixture of material from a range of different times (Hedman 2007:89-91).
What is interesting about these examples is it points out some of the shortcomings of radiocarbon dating on the one hand, and on the other hand, it shows that it is a valuable tool for approximating the chronology of history when used with other dating techniques. Even though carbon-14 is a great tool in establishing the age of artifacts, it has some drawbacks. First, it can only date objects up to about 70,000 years ago since its half-life is 5,730 years (Wheatley 2004:98).
In addition, its accuracy is debated. Some scientists question the validity of using 5,730 years as a half-life since this assumes nuclear half-lives have always been stable. Moreover, radiocarbon dating assumes that the carbon-14 content of the atmosphere has remained constant over the years and that living things have a constant ratio of C-14 to C-12 in them when alive based on the C-14 content in the atmosphere. However, it is known that since the 1950s, the amount of carbon-14 in the atmosphere has increased because of nuclear bombs and reactors. Artifacts from this era would be dated younger than they really are since they have a higher C-14 to C-12 ratio.
By the same token, the ratio between carbon-14 and carbon-12 during the industrial era would be lower due to the burning of fossil fuels and the release of large amounts of carbon dioxide. Because of the increased levels of carbon dioxide, it would cause things to appear older than their true age. Fortunately, scientists have been able to adjust their radiocarbon dating results to account for changes in the carbon-14 levels in the atmosphere by taking into account information obtained from tree-ring dating. The carbon-14 data is calibrated to the tree ring dates.
This has greatly improved the accuracy of this dating technique (Wheatley 2004:98). The earth’s magnetic field can also impact carbon-14 dating results. Cosmic rays are charged particles, so they can be deflected by magnetic fields. Shifts in the magnetic field will influence the quantity of cosmic rays that enter the earth’s atmosphere. If the magnetic field is strong, the number of cosmic rays entering the atmosphere will be reduced. This in turn affects the amount of C-14 in the atmosphere. There have been variations in the earth’s magnetic field. For example, based on data from volcanic rock, the earth’s magnetic field was stronger around 2000 BCE than it is today.
At about the same time, the carbon-14 content of the atmosphere dropped. Today, the radiocarbon dating process considers these variations in magnetic fields when dating artifacts (Hedman 2007:75-76). Lastly, another key assumption in radiocarbon dating is the fraction of C-14 to C-12 was the same for organisms living in the past as it is for organisms living today. It is known that some living organisms can accumulate more carbon-14 in their bodies than others.
This is known as mass fractionation. For example, corn’s photosynthesis process causes it to have 2-3% higher carbon-14 fraction than sugar beets or tree leaves growing at the same time. If scientists did not take this into account, carbon-14 would underestimate the date of materials derived from corn (Hedman 2007:68-69). Despite these limitations, in my opinion, carbon-14 dating is an invaluable tool in helping date artifacts. Even though it can only date artifacts that are 70,000 years old or younger, it has enabled archeologists, geologists, and anthropologists to have a better understanding of how and where people lived over time in a large part to the chronological information it provides.
Specifically, it helped revise the human timetable when it dated the Ice Age to being as recent as 13,000 years ago, not 25,000 years. Radiocarbon dating has also aided in confirming previously established dates. Some may discredit this dating technique because of some of its assumptions, however, refinements to this carbon dating process and collaboration with other dating techniques, such as dendrochronology, continue to confirm that carbon-14 is still an important yardstick in measuring time and has significantly helped the field of archeology.
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