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How does the C-14 method work?
With the 14The C method - also known as radiocarbon dating or radiocarbon dating - not only archaeologists determine the age of finds. Researchers also use this method to study the global carbon cycle. It is based on the decay of a specific carbon isotope that is formed in the upper layers of the atmosphere and is later taken up by all living things and the oceans on earth.
During World War II, chemist Willard Frank Libby developed radiocarbon dating while researching the effects of so-called cosmic rays. These extremely high-energy particles from space constantly hit the earth's atmosphere. Cosmic rays also contain high-energy neutrons - which, together with protons, form the building blocks of atomic nuclei. In the upper atmosphere, the neutrons meet nitrogen atoms. The nucleus of these nitrogen atoms consists of seven neutrons and seven protons - and is also called 14N denotes, where N is the element symbol for nitrogen and 14 stands for the total number of core building blocks.
Distribution of the carbon isotope C-14
"The neutrons from cosmic rays react with the nitrogen," explains Gesine Mollenhauer from the Alfred Wegener Institute in Bremerhaven. In doing so, loses 14N a proton and the carbon isotope is created 14C with six protons and eight neutrons. "14C reacts with oxygen to form CO2 and thus enters the carbon cycle, ”says Mollenhauer. But 14C is radioactive - one of the neutrons in the nucleus turns into a proton, emits an electron and an electron antineutrino and emits beta radiation. The carbon isotope decays back to nitrogen with a half-life of 5730 years.
The salary of 14C in the earth's atmosphere is very low. The ratio to the other two naturally occurring, stable isotopes of carbon is roughly one to a trillion. Still is 14C like the other two isotopes 12C and 13C is detectable on earth: Plants use it in the form of carbon dioxide for photosynthesis, animals and humans ingest it through the food chain. At the same time, carbon escapes back into the atmosphere, for example when living things breathe, microorganisms decompose waste products or gas is released from the oceans.
Based on the radioactive decay of 14C as well as its uptake in all living things, Libby realized that this cycle can be used to determine the age of preserved plant remains or bones: because if an organism dies, it does not take a new one 14C, instead the number of atoms decreases due to the radioactive decay. The half-life can then be used to calculate back when the metabolism stopped. This is possible up to an age of 50,000 to 60,000 years. Even older finds have already decayed so many atoms that the 14C content can no longer be reliably detected.
Influences on the 14C content in the atmosphere
Libby received the Nobel Prize in Chemistry in 1960 for his method. Dating has a catch 14However, C: The content of the isotope in the atmosphere is not always the same. It depends on two factors: the strength of cosmic rays and changes in the carbon cycle. Part of the cosmic radiation consists of particles that reach the earth from the sun. Since solar activity increases and decreases cyclically, the intensity of the radiation also varies. “But other cosmic phenomena such as a supernova can also lead to changes in cosmic radiation,” explains Mollenhauer. “In addition, the earth's magnetic field shields the earth from the incident radiation. In times when the earth's magnetic field is stronger, fewer neutrons penetrate into the upper atmosphere. "
In contrast, the carbon cycle is primarily influenced by the earth's climate. The last ice age, for example, changed the entire flora and fauna in Europe: Large areas of land were buried under glaciers, steppe areas expanded, forests declined. Violent volcanic eruptions also have an impact on the ratio of carbon isotopes. “A significant impact on the 14The ocean circulation also has a carbon content in the atmosphere, ”reports Mollenhauer. "Because the CO2 of the atmosphere is absorbed by the oceans and transported through the circulation into the deep water of the world's oceans. If the circulation slows down, there is more 14C in the atmosphere. So with the 14C-Method also examine the global carbon cycle. ”Humans, who have been burning considerable amounts of fossil fuels since industrialization, are also having an increasing influence. Similar to volcanic gases from deep layers of the earth, this contains CO2 almost none from industrial processes 14C, since oil and coal are created over millions of years. Your burn lowers the 14C content in the atmosphere. Due to this so-called sweet effect, the 14The C content even drops so much that at some point there is no longer enough in samples 14C exists for dating.
C-14 calibration curve
However, the Suess effect has been overlaid since the middle of the 20th century: Numerous nuclear weapons tests - such as those carried out primarily by the USA and the Soviet Union in the 1950s - produced an unusually large number of free neutrons in the upper air layers. This temporarily doubled the 14Almost C-content. Meanwhile, the content in the atmosphere has decreased significantly again after the additional 14C could distribute through the carbon cycle, where it now slowly decays. Samples from the years after 1950 therefore show a characteristically higher value 14C on. The atomic bomb tests could make dating difficult for future archaeologists if they were forgotten in 1,000 or 2,000 years.
Calibration of the measured values
As so many factors denote the 14Influence the C content, the measurements must be calibrated using other methods in order to determine the exact age of the samples. For example, so-called dendrochronology, in which the annual rings of trees are dated, is suitable for this purpose. Annual rings of trees are of different thicknesses, as the growth depends on the climate and other conditions. The characteristic sequence of thick and thin rings can be used to precisely date tree ring sequences from fossil trees. Scientists compare dated rings from different times with that 14C content in wood, calibration curves result from this, which are also applied to others 14Have C-samples applied. With the help of particularly old trees or trunks sunk in the moor, you can calculate back up to 12,000 years. The uranium-thorium method is helpful to compare older finds. "The decay of the radioactive elements in petrified corals and in stalactites extends the timetable," said Mollenhauer. “Lake Suigetsu in Japan makes a decisive contribution to the calibration curves. Its sediment shows annual layers that go back over 50,000 years and can be counted like the annual rings of a tree. "
An important prerequisite for measurements with the 14C method is that no foreign particles adhere to the sample that would falsify the age. It is not just a question of the researchers working properly, because rising groundwater can already leave deposits on wood or bones in the ground. That is why the samples are in each 14C laboratory chemically cleaned beforehand. In the end, there remains a measurement inaccuracy of several decades, which is based on statistical fluctuations and inaccuracies of the instruments. If enough sample material is available, measurements can be made longer, which at least reduces the statistical error.
But even with accurate measurements, scientists sometimes struggle to determine actual age. Because the calibration curves of the dendrochronology and the uranium-thorium method form plateaus in some places that represent longer periods of time in which the 14C-content of finds hardly varied. Such a phase last occurred between 1650 and 1950, despite the onset of the Suess effect. Therefore, the 14Do not use the C-method reliably on samples from this period.
Methods and devices
With the 14In addition to organic material, the C method can also be used to study inorganic material, the 14Contains C in the form of carbon compounds. To determine the isotope content of 14To determine the C of a sample, counter tubes such as the Geiger counter were originally used. They register the decay of the remaining ones 14C nuclei based on the beta particles emitted, which react with an inert gas in the pipe. However, a sample of a few hundred grams is required for this procedure.
Gesine Mollenhauer in the laboratory
"Depending on 14Different methods are used to determine the C content, ”reports Mollenhauer. “There are methods that are only used when the 14The C content in the sample is higher than the content in the atmosphere. ”This is the case with so-called isotope labeling: This is not about determining the age of a sample. Rather, in experiments the 14The C content of the sample is artificially increased to avoid chemical and biological processes such as the uptake of CO2 trace in algae: the algae absorbs CO2 on whose carbon atom is by the isotope 14C was replaced. To investigate how much CO2 the alga has bound after a certain time, you can now use the 14Determine the C content of the algae.
For samples with a natural 14C content is accelerator mass spectrometry - or AMS for short for the English term "accelerator mass spectrometry" - the most common method. The AMS doesn't count that 14C decay events, but the atoms themselves. For this, all of the carbon from a sample is converted to graphite and bombarded with cesium ions to generate charged carbon ions. Since the individual isotopes and thus their ions have different masses, the ions of the 14C isotopes can be separated from the other carbon ions in a mass spectrometer. However, the samples can also have other ions with the same mass as 14Contain C - about molecules from 13C and a hydrogen ion. So that these molecules are not mistakenly labeled in the mass spectrometer 14C isotopes are counted, the accelerator breaks down the molecules into their constituent parts. “The technology is now so advanced that not only solid graphite can be ionized, but also gaseous CO2“, Reports Mollenhauer. "In this way, accelerator spectrometers can analyze even the smallest samples of just a tenth of a milligram."
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