Carbon 14 Dating

Categories: DatingRadioactivity

Archaeologists utilize a number of methods to examine data from the past. One clinical tool assists to analyze the radioactive decay of chemical aspects discovered in plant and animal stays, pottery, and even rocks. Radiocarbon dating, also referred to as carbon-14 dating, has become among the most essential radioisotope dating approaches archaeologists employ. This scientific tool, very first established by Willard F. Libby in the late 1940s, started with the discovery of the isotopic carbon-14 atom. Following this discovery, researchers started to contemplate methods to make use of carbon-14 to date previously living organisms.

Since researchers understood that living organisms absorb carbon-14 at a constant rate while alive, they started to create a process to measure C-14 to C-12 ratios in dead organisms. This procedure made it possible for researchers to age an organism utilizing this carbon ratio. To guarantee the formula was right, they began to perform experimental trials of radiocarbon dating to check its accuracy, and while testing, found several approaches of carbon 14 dating that yielded accurate results including the Geiger counting approach, liquid scintillation method, and AMS dating approach.

These three approaches have significantly improved the precision of assigning dates to previous occasions and artifacts, dating as far back as 70,000 years. Using and explaining the techniques, treatments, and applications, the anthropological school of idea for carbon-14 dating will be obtained. Although this method has drawbacks and critics, carbon-14 dating has shaped and influenced the historic classification artifacts to the point that archaeologists, geologists, and anthropologists now have the ability to construct the world’s history by completing some of the numerous blank dates in the chronology of the history of our human world and by validating and revising other dates.

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The structure of radiocarbon dating started with researcher Williams F. Libby and, throughout history, has progressed into a valid dating strategy showed through numerous experiments. Carbon-14 dating emerged in 1941, when scientists separated and found the radioactive atom, carbon-14.

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 using discovered charcoal, wood, bone, shells, and fossils from the past. 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; thus, organisms contain a constant amount of carbon-14 throughout their lifespan. A living organism can only intake a certain amount of carbon-14, and since the ratio of C-14 to C-12 in a living organism is approximately 1.35×10-12, scientists can assume the amount of carbon-14 an organism absorbs. Then, at the moment the living organism dies and stops respiring, 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 remained in the substance. He planned to calculate the age of an object from the amount of carbon-14 remaining 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 date: 2700-2600 B.C.; Libby date: 4650 B.P., which stands for Before Present acting as if present is 1950, therefore it can be translated to 2700 B.C.); cypress wood from the tomb of Sneferu in Egypt (established date: 4575 B.C.; Libby date: 4802 B.C); cedar wood from the Egyptian pharaoh Sesostris’s funeral boat (established date: 3750 B.C.; Libby 3621 B.C); wood from a mummy coffin from the Ptolemaic period in Egypt (established date: 2280 B.C; Libby 2190 B.C); wheat and barley seeds (established date: 5000 years old; Libby date 5256 years old); and, lastly, Libby dated charcoal from Iraq at 6596 years old, which coincided with the known approximate date (Poole 1961:28-32, Libby 1952:70).

His experimental dates were accurate within an acceptable margin of error, estimated to be approximately 300 years. These sample tests, along with many others, confirmed that his carbon-14 test dating method was scientifically dependable for objects already dated. Libby then continued his work on objects with unknown ages. Scientists and scholars began to send him samples from all over the world to radiocarbon date. These samples included artifacts from the Dead Sea Scrolls, Pompeii, Stonehenge, and New Mexico.

One of his most significant contributions came from dating 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 years ago. Previously, geologists believed that the last Ice Age in North America occurred 25,000 years ago; however, this new technique proved that this date was inaccurate. (Libby 1952:105). All of these accomplishments and the inroads Libby paved for archaeological and historical dating by using carbon-14 earned him the Nobel Prize in Chemistry in 1960.

As Libby worked tediously to create a method of chronological dating, other scientists experimented and tested his method to verify it, and they began to improve the method by creating formulas in order to calculate the approximate age of an artifact. While new methods and calculations formed, radiocarbon dating methods improved tremendously; however, Libby’s overall methodology is still commonly accepted as a dating technique. 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 in the atmosphere includes the stable, common isotope carbon-12 and tiny amounts of radioactive carbon-14. Subsequently, both isotopes of carbon are absorbed and used by plants and trees during the photosynthesis process and become part of their cellulose structure. Animals then eat these plants and, thus, the various 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 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 a ratio of one atom of carbon-14 for every trillion atoms of carbon-12 (Poole 1961:25). Carbon dating requires determining the amount of carbon-14 in a sample prior and after death, found using the current amount of carbon-14 in a sample and relating it to the decay rate of the atom. Obtaining these values will provide an estimation of the samples age, taking into account 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. It is imperative to avoid contamination of the artifacts, since any carbon-14 from the non-sample material, such as roots or other decaying remains possibly from a different time period, could significantly distort the results (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. After the meticulous cleaning, the artifacts are packed in chemically neutral materials to ensure that the sample’s ratio of C-14 to C-12 remains undisturbed. 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 multiple tests on the artifact to confirm methodical precision. 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 constant 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: A = A0* e^(-λt) (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, “t” represents the time it takes to reduce the original amount of the isotope to its present amount, 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 and can be extended to determine the amount of carbon-14 that would diminish over a given time period.

An important assumption used by archaeologists and scientists in this formula is that the production of radiocarbon in the atmosphere and the carbon-12 and carbon-14 ratio has remained constant over time. These principles have paved the way for the various methods used today. The establishment of carbon-14 dating arose when Libby devised a method to date organisms and artifacts; however, over time several other methods of radiocarbon dating have emerged. 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 emissions interact with the Geiger counter, the device clicks, indicating radioactivity. After, scientists start their process of determining the samples historical age. For example, if scientists want 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 have been removed prior to testing. The next step separates the carbon-14 atoms from everything else in the wood. To do this, scientists place the wood in a heavy glass tube and burn it. When this combustion reaction occurs, the wood breaks down to some ash, carbon dioxide, and water, so the gases released are collected in a purification vacuum system. Once separated from the water, the carbon dioxide gas containing carbon-14 enters a glass jar, which then runs through the carbon-14 counter. This device is comprised of a ring of Geiger counters inside a casing, and completely surrounded by lead or iron bricks to filter out radiations from the atmosphere. The sample is then tested and an electronic panel counts and records the time elapsed as each carbon-14 atom decays.

From this, scientists can calculate the approximate age of the sample by manipulating the formulas mentioned previously and determining 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, a particularly popular technique during the 1960’s. The Liquid Scintillation counting method requires a sample of carbon dioxide created 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 bombarded with photons and has high precision.

Advancement in technology has led to the third method of radiocarbon dating, which is more precise than the former two methods: accelerator mass spectrometry, also known as 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 happens at a concentration of one carbon-14 atom for every 1016 stable carbon-12 atoms. This sensitivity is roughly the equivalent of being able to detect a unique grain of sand along a hundred 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 have an enormous footprint and often take up space in entire buildings; they 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). Radiocarbon dating, as a tool, has helped date artifacts in several recent archaeological discoveries and defines cultures throughout time, labeling the methodology under a specific theoretical school in archaeology. Various scientists use radiocarbon dating as a device to measure ages of artifacts; consequently, 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 this field, archaeologists focus on the distribution of artifacts that can define traits, which leads to defining cultures and changes over time. Carbon-14 dating falls under this field because its purpose is to date artifacts, which can then be related to other artifacts and aid in defining a culture or changes in culture over time (Urban and Schortman, 70).

A recent case where carbon dating has been particularly useful occurred while attempting 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 second 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 carbon-14 dating benefits came from evidence of human migration 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 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, scientists created a Clovis model that suggested that people should not have lived in the Americas prior to 13,000 years ago. The Ice Age made travel to the Americas difficult very difficult before this time; however, archaeologists have claimed to discover sites that predate the Clovis points proposing that humans lived in the Americas earlier than most scientists hypothesized they did. 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).

As with all discoveries, Radiocarbon dating contains limitations in its measurements that lead people to point out controversies over this technique. Controversies over atmospheric changes, climate changes, and contamination often arise when discussing C-14 dating. These disputes surround the Meadowcroft site, which is near Canada, where remains of oak and hickory trees were found in the oldest layers. Scientists debate the likeliness of deciduous trees surviving in this area during the time period of the ice age; yet, the excavators counter this argument saying the area was sheltered which made the climate milder. Meadowcroft skeptics also suggest that the samples might be contaminated since the site is in the heart of coal country. If any of the surrounding materials, which no longer had carbon-14 after decaying mixed with charcoal from the environment, it would dilute the C-14 fraction and results from the dates would yield as too old.

The archaeologists countered this by saying the dates in each layer of excavation were in order of age. As to Monte Verde, a skeptic said the artifacts found were near a stream, negating the reliability of the evidence since the mixture of material could come from a range of different times (Hedman 2007:89-91). These criticisms point out some of the shortcomings of radiocarbon dating on the one hand, and on the other hand, they show the valuable for approximating the chronology of history when used with other dating techniques. Carbon-14 dating successfully establishes the age of artifacts, however, several drawbacks arise when using this method. First, it can only date objects up to about 70,000 years ago since the half-life is 5,730 years (Wheatley 2004:98). In addition, some scientists debate the accuracy, specifically, they 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 time and that living organisms have a constant ratio of C-14 to C-12 based on the C-14 content in the atmosphere. To support this rebuttal, 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 classified as younger than their true age since the results would yield a higher C-14 to C-12 ratio than if real atmospheric concentrations were considered. Similarly, 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 with C-14. Because of the increased levels, it would cause things to appear older than in actuality. 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, dendrochronology – a separate dating technique using the ring patterns in trees.

Pairing these methodologies has greatly improved the accuracy (Wheatley 2004:98). The earth’s magnetic field can also affect 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: a strong magnetic field reduces the number of cosmic rays. This in turn affects the amount of C-14 in the atmosphere. 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 has stayed constant in the living organisms of the past and those of today. Some living organisms can accumulate more carbon-14 in their bodies than others can, this phenomenon is known as mass fractionation.

For example, the photosynthesis process in corn creates a 2-3% higher carbon-14 fraction than the fraction in 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 because of the chronological information this method provides.

Specifically, it helped revise the human timetable when evidence proved the Ice Age was as recent as 11,000 years ago. Radiocarbon dating has also aided in confirming previously established dates. Some may discredit this dating technique because of its assumptions, however, refinements to this carbon dating process and collaboration with other dating techniques, such as dendrochronology, confirms that carbon-14 is still an important yardstick in measuring time. By continually improving the technique and accuracy, archaeologists have and will continue to use radiocarbon dating to redefine past cultures and create a chronological history of the human world.

Bibliography
“Dating.” Encyclopedia Britannica. 2009. Encyclopedia Britannica Online. 09 Sept. 2009 <http://www.britannica.com/EBchecked/topic/152243/dating/69778/Carbon-14-dating-and-other-cosmogenic-methods>. DeYoung, Don. Thousands, Not Billions: Challenging an Icon of Evolution: Questioning the Age of Earth. Green Forest: New Leaf, 2005. 13-62.

Hedman, Matthew. The Age of Everything: How Science Explores the Past. Chicago: University of Chicago P, 2007. 49-95. Higham, Thomas. “The 14C Method.” Radiocarbon WEB-info. 9 Aug. 2002. 26 Sept. 2009 <http://www.c14dating.com/int.html>. L’Annunziata, Michael. Willard F. Libby. Radioactivity Introduction and History. Amsterdam: Elsevier B.V., 2007. 518-28. Poole, Lynn, and Gary Poole. Carbon-14. New York: McGraw-Hill Book Company, Inc., 1961. Urban, Patricia, and Schortman, Edward. Archaeological Theory In Practice. Walnut Creek: Left Coast Press, Inc., 2012. 67-76.

Wheatley, Abigail, and Struan Reid. Radioactive Dating. The Usborne Introduction to Archaeology. London: Usborne Ltd., 2004. 98-99. Willard, Libby F. Radiocarbon Dating. Chicago: University of Chicago, 1952. 69-111.

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Carbon 14 Dating. (2017, Feb 11). Retrieved from http://studymoose.com/carbon-14-dating-essay

Carbon 14 Dating
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