Radiocarbon dating also referred to as carbon dating or carbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbona radioactive isotope of carbon.
The method was developed in the late s by Willard Libbywho received the Nobel Prize in Chemistry for his work in It is based on the fact that radiocarbon 14 C is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14 C combines with atmospheric oxygen to form radioactive carbon dioxidewhich is incorporated into plants by photosynthesis ; animals then acquire 14 C by eating the plants.
"Carbon dating applications"
When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14 C it contains begins to decrease as the 14 C undergoes radioactive decay. Measuring the amount of 14 C 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 14 C there is to be detected, and because the half-life of 14 C the period of time after which half of a given sample will have decayed is about 5, years, the oldest dates that can be
Carbon dating applications measured by this process date to around 50, years ago, although special preparation methods occasionally permit accurate analysis of older samples.
Research has been ongoing since the s to determine what the proportion of 14 C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age. Other corrections must be made to account for the proportion of 14 C in different types of organisms fractionationand the varying levels of 14 C throughout the biosphere reservoir effects.
Additional complications come from the Carbon dating applications of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the s and s. Because the time it takes to convert biological materials to fossil fuels is substantially longer than the time it takes for its 14 C to decay below detectable levels, fossil fuels contain almost no 14 Cand as a result there was a noticeable drop in the proportion of 14 C in the atmosphere beginning in the 19th century.
Conversely, testing increased the amount of 14 C in the atmosphere, which attained a maximum in about of almost twice what it had been before the testing began. Measurement of radiocarbon was originally done Carbon dating applications beta-counting devices, which counted the amount of beta radiation emitted by decaying 14 C atoms in a sample.
More recently, accelerator mass spectrometry has become the method of choice; it counts all the 14 C atoms in the sample and not just the few that happen to decay during the measurements; it therefore be used with much smaller samples as small as individual plant seedsand gives results much more quickly.
The development of radiocarbon dating has had a profound impact on archaeology.
In addition to permitting more accurate dating within archaeological sites than previous methods, it allows comparison of dates of events across Carbon dating applications distances.
Histories of archaeology often refer to its impact as the "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice ageand the beginning of the Neolithic and Bronze Age in different InMartin Kamen and Samuel Ruben of the Radiation Laboratory at Berkeley began experiments to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research.
They synthesized 14 C using the laboratory's cyclotron accelerator and soon discovered that the atom's half-life was far longer than had been previously thought. Korffthen employed at the Franklin Institute in Philadelphiathat the interaction of thermal neutrons with 14 N in the upper atmosphere would create 14 C.
InLibby moved to the University of Chicago where he began his work on radiocarbon dating. He published a paper in in which he proposed that the carbon in living matter might include 14 C as well as non-radioactive carbon. By contrast, methane created from petroleum showed no radiocarbon activity because of its age. The results were summarized in a paper in Science inin which "Carbon dating applications" authors commented that their results implied it would be possible to date materials containing carbon of organic origin.
Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages. For example, two samples taken from the tombs of two Egyptian kings, Zoser and Sneferuindependently dated to BC plus or minus 75 years, were dated by radiocarbon measurement to an average of BC plus or minus years. These results were published in Science in In nature, carbon exists as two stable, nonradioactive isotopes: The half-life of 14 C the time it takes for half of a given amount of 14 C to decay is about 5, years, so its concentration in the atmosphere might be expected to reduce over thousands of years, but 14 C is constantly being produced in the lower stratosphere and upper troposphereprimarily by galactic cosmic raysand to a lesser degree by solar cosmic "Carbon dating applications." Once produced, the 14 C quickly combines with the oxygen in the "Carbon dating applications" to form first carbon monoxide CO and ultimately carbon dioxide CO 2.
Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is Carbon dating applications up by plants via photosynthesis. Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere. The ratio of 14 C to 12 C is approximately 1. The equation for the radioactive decay of 14 C is: During its life, a plant or animal is in equilibrium with its surroundings by exchanging carbon either with the atmosphere, or through its diet.
It will therefore have the same proportion of 14 C as the atmosphere, or in the case of marine animals or plants, with the ocean. Once it dies, it ceases to acquire 14 Cbut the 14 C within its biological material at that time will continue to decay, and so the ratio of 14 C to 12 C in its remains will gradually decrease. The equation governing the decay of a radioactive isotope is: Measurement of Nthe number of 14 C atoms currently in the sample, allows the calculation of tthe
Carbon dating applications of the sample, using the equation above.
The above calculations make several assumptions, such as that the level of 14 C in the atmosphere has remained constant over time. The calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" Carbon dating applications the sample: Calculating radiocarbon ages also requires the value of the half-life for 14 C.
Radiocarbon ages are still calculated using this half-life, and are known as "Conventional Radiocarbon Age". Since the calibration curve IntCal also reports past atmospheric 14 C concentration using this conventional age, any conventional ages calibrated against the IntCal curve will produce Carbon dating applications correct calibrated age.
Carbon dating applications a date is quoted, the reader should be aware that if it is an uncalibrated date a term used for dates given in radiocarbon years it may differ substantially from the best estimate of the actual calendar date, both because it uses the wrong value for the half-life of 14 Cand because no correction calibration has been applied for the historical variation of 14 C in the atmosphere over time.
Carbon is distributed throughout the atmosphere, the biosphere, and the oceans; these are referred Carbon dating applications collectively as the carbon exchange reservoir,  and each component is also referred to individually as a carbon exchange reservoir.
The different elements of the carbon exchange reservoir vary in how much carbon they store, and in how long it takes for the 14 C generated by cosmic rays to fully mix with them. This affects the ratio of 14 C to Carbon dating applications C in the different reservoirs, and hence the radiocarbon ages of samples that originated in each reservoir. There are several other possible sources of error that need to Carbon dating applications considered.
The errors are of four general types:. To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. Over time, however, discrepancies began to appear between the known chronology for the oldest Egyptian dynasties and the radiocarbon dates of Egyptian artefacts. The question was resolved by the study of tree rings: Coal and oil began to be burned in large quantities during the 19th century.
Dating an object from the early 20th century hence gives an apparent date older than the true date. For the same reason, 14 C concentrations in the neighbourhood of large cities are "Carbon dating applications" than the atmospheric average. This fossil fuel effect also known as the Suess effect, after Hans Suess, who first reported in would only amount to a reduction of 0.
A much larger effect comes from above-ground nuclear testing, which released large numbers of neutrons and created 14 C. From about untilwhen atmospheric nuclear testing was banned, it is estimated that several tonnes of 14 C were created. The level has since dropped, as this bomb pulse or "bomb carbon" as it is sometimes called percolates into the rest of the reservoir. Photosynthesis is the primary process by which carbon moves from the atmosphere into living things. In photosynthetic pathways 12 C is absorbed slightly more easily than 13 Cwhich in turn is more easily absorbed than 14 C.
This effect is known as isotopic fractionation. At higher temperatures, CO 2 has poor solubility in water, which means there is less CO 2 available for the photosynthetic reactions. The enrichment of bone 13 C also implies that excreted material is depleted in 13 C relative to the diet. The carbon exchange between atmospheric CO 2 and carbonate at the ocean surface is also subject to fractionation, with 14 C in the atmosphere more likely than 12 C to dissolve in the ocean.
This increase in 14 C concentration almost exactly cancels out the decrease caused by the upwelling of water containing old, and hence 14 C depleted, carbon from the deep ocean, so that direct measurements of 14 C radiation are similar to measurements for the rest of the biosphere. Correcting for isotopic fractionation, as is "Carbon dating applications" for all radiocarbon dates to allow comparison Carbon dating applications results from different parts of the biosphere, gives an apparent age of about years for ocean surface water.
The CO 2 in the atmosphere transfers to the ocean by dissolving in the surface water as carbonate and bicarbonate ions; at the same time the carbonate ions in the water are returning to the air as CO 2. The deepest parts of the ocean mix very slowly with the "Carbon dating applications" waters, and the mixing is uneven.
The main mechanism that brings deep water to the surface is upwelling, which is more common in regions closer to the equator. Upwelling is also influenced by factors such as the topography of the local ocean bottom and coastlines, the climate, and wind patterns.
Carbon dating applications, the mixing of deep and surface waters takes far longer than the mixing of atmospheric CO 2 with the surface waters, and as a result water from some deep ocean areas has an apparent radiocarbon Carbon dating applications of several thousand years.
Upwelling mixes this "old" water with the surface water, giving the surface water an apparent age of about several hundred years after correcting for fractionation.
The northern and southern hemispheres have atmospheric circulation systems that are sufficiently independent of each other that there is a noticeable time lag in mixing between the two. Since the surface ocean is depleted in 14 C because of the marine effect, 14 C is removed from the southern atmosphere more quickly than in the north.