List three methods of radiometric dating and explain the age range for which they are most effective

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You’ll hear it often in stories about palaeontology and archaeology: “the wood was dated using radiocarbon dating”, “the rock was dated to 100 million years ago”.

But how is it dated? What does radiometric dating actually mean? And what methods of dating can be used to date which kinds of items?

What is radiometric dating?

Radiometric dating is a method of establishing how old something is – perhaps a wooden artefact, a rock, or a fossil – based on the presence of a radioactive isotope within it.

The basic logic behind radiometric dating is that if you compare the presence of a radioactive isotope within a sample to its known abundance on Earth, and its known half-life (its rate of decay), you can calculate the age of the sample.

Radiometric dating is useful for finding the age of ancient things, because many radioactive materials decay at a slow rate.

What is radioactive decay?

Radioactive atoms are unstable, meaning they decay into “daughter” products. The number of protons or neutrons in the atom changes, leading to a different isotope or element. The time it takes for one half of the atoms to have decayed is referred to as a “half-life”.

We know the half-lives of the radioactive isotopes found on Earth, and so we can trace how long a radioactive material within an object has been decaying for, and therefore how long (within a range of error) it’s been since the object was formed.

Some radioactive materials decay into daughter products that are also radioactive, and have their own half-life: the result is called a “decay-chain”, which eventually decays into a non-radioactive substance.

Types of radiometric dating

Radiocarbon (14C) dating

You’ve almost definitely heard of “carbon dating”. It’s a very common method used mostly by archaeologists, because it can only date relatively recent materials.

Radiocarbon dating is possible because all living things take in carbon from their environment, which includes a small amount of the radioactive isotope 14C, formed from cosmic rays bombarding nitrogen-14.

When an animal or plant dies, it will not take in any more carbon, and the 14C present will begin to decay. We can thus measure how long it’s been since the animal or plant died by comparing the presence of 14C with the known half-life.

This can raise complexities in archaeology when, for example, a society uses a piece of wood that was felled hundreds of years prior. There are also issues because the rate of cosmic ray bombardment of the planet over time has not always been stable: but this problem is largely redressed by a calibration factor.

Radiocarbon dating is not suitable for dating anything older than around 50,000 years, because 14C decays quickly (its half-life is 5,730 years) and so will not be present in significant enough amounts in older objects to be measurable.

Radiocarbon dating identified Ötzi, the Italian-Alps Iceman, as a 5,300-year-old traveller. More recently, Australian scientists used radiocarbon dating to figure out the age of wasp nests in rock art, and thereby establishing a date range for the art.

Potassium-argon and argon-argon dating

Potassium-argon dating is a method that allows us to calculate the age of a rock, or how long ago it was formed, by measuring the ratio of radioactive argon to radioactive potassium within it.

Radioactive potassium (40K – a solid) decays to radioactive argon (40Ar – a gas), at a known rate. When volcanic rocks are formed and cooled, all argon within the rock is released into the atmosphere, and when the rock hardens, none can re-enter.

This means that any argon present in a volcanic rock must have been produced by the decay of radioactive potassium, so measuring the ratio can enable a scientist to date the sample.

This method is limited, because it’s only applicable to volcanic rocks, but is useful for older archaeology because it has a date range of about 4.3 billion to 100,000 years ago.

However, there are potential issues with potassium-argon dating. For example, deep-sea basalts retain some argon after formation due to high hydrostatic pressure, and other rocks may incorporate older “argon-rich” material during formation.

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Argon-argon dating is an updated method, based on the original K-Ar dating technique, that uses neutron irradiation from a nuclear reactor to convert a stable form of potassium into the argon isotope 39Ar, and then measures the ratio of 40Ar to 39Ar.

Argon-argon dating was used to determine that the Australopithecus Lucy, who rewrote our understanding of early hominin evolution, lived around 3.18 million years ago.

Uranium-lead dating

This technique involves measuring the ratio of uranium isotopes (238U or 235U) to stable lead isotopes 206Pb, 207Pb and 208Pb. It can be used to determine ages from 4.5 billion years old to 1 million years old. This method is thought to be particularly accurate, with an error-margin that can be less than two million years – not bad in a time span of billions.

U-Pb dating can be used to date very old rocks, and has its own in-built cross-checking system, since the ratio of 235U to 207Pb and 238U to 206Pb can be compared using a “concordia diagram”, in which samples are plotted along a straight line that intersects the curve at the age of the sample.

U-Pb dating is most often done on igneous rocks containing zircon. It’s been used to determine the age of ancient hominids, along with fission-track dating.

Fission-track dating

This method involves examining the polished surface of a slice of rock, and calculating the density of markings – or “tracks” – left in it by the spontaneous fission of 238U impurities.

The uranium content of the sample must be known; this can be determined by placing a plastic film over the polished slice and bombarding it with slow neutrons – neutrons with low kinetic energy. This bombardment produces new tracks, the quantity of which can be compared with the quantity of original tracks to determine the age.

This method can date naturally occurring minerals and man-made glasses. It can thus be used for very old samples, like meteorites, and very young samples, like archaeological artefacts.

Fission-track dating identified that the Brahin Pallasite, a meteorite found in the 19th century in Belarus – slabs of which have become a collectors item – underwent its last intensive thermal event 4.26–4.2 billion years ago.

Chlorine-36 dating

This method involves calculating the prevalence of the very rare isotope chlorine-36 (36Cl), which can be produced in the atmosphere through cosmic rays bombarding argon atoms. It’s used to date very old groundwater, from between around 100,000 and 1 million years old.

Chlorine-36 was also released in abundance during the detonation of nuclear weapons between 1952 and 1958. It stays in the atmosphere for about a week, and so can mark young groundwater from the 1950s onwards as well.

Luminescence dating

Luminescence dating methods are not technically radiometric, since they don’t involve calculating ratios of radioactive isotopes. However, they do use radioactive material.

These methods date crystalline materials to the last time they were heated – whether by human-made fires or sunlight.

This is possible because mineral grains in sediments absorb ionising radiation over time, which charges the grains in “electron traps”. Exposure to sunlight or heat releases these, removing the charges from the sample.

The material is stimulated using light (optically stimulated luminescence) or heat (thermoluninescence), which causes a signal to be released from the object, the intensity of which can provide a measure of how much radiation was absorbed after the burial of the material – if you know the amount of background radiation at the burial site.

This method can date archaeological materials, such as ceramics, and minerals, like lava flows and limestones. It has a normal range of a few decades to 100,000 years old, but some studies have used it to identify much older things.

Other types of radiometric dating

There are several other radioactive isotopes whose ratios can be measured to date rocks, including samarium-neodymium, rubidium-strontium, and uranium-thorium. Each of these have their own advantages and idiosyncrasies, but they rely on the same logic of radioactivity to work.

The Royal Institution of Australia has an Education resource based on this article. You can access it here.

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Unlike relative dating methods, absolute dating methods provide chronological estimates of the age of certain geological materials associated with fossils, and even direct age measurements of the fossil material itself. To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events. Geologists also use other methods - such as electron spin resonance and thermoluminescence, which assess the effects of radioactivity on the accumulation of electrons in imperfections, or "traps," in the crystal structure of a mineral - to determine the age of the rocks or fossils.

All elements contain protons and neutrons, located in the atomic nucleus, and electrons that orbit around the nucleus (Figure 5a). In each element, the number of protons is constant while the number of neutrons and electrons can vary. Atoms of the same element but with different number of neutrons are called isotopes of that element. Each isotope is identified by its atomic mass, which is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon 12 (12C), carbon 13 (13C), and carbon 14 (14C) (Figure 5a).

Figure 5: Radioactive isotopes and how they decay through time.

(a) Carbon has three isotopes with different numbers of neutrons: carbon 12 (C12, 6 protons + 6 neutrons), carbon 13 (C13, 6 protons + 7 neutrons), and carbon 14 (C14, 6 protons + 8 neutrons). C12 and C13 are stable. The atomic nucleus in C14 is unstable making the isotope radioactive. Because it is unstable, occasionally C14 undergoes radioactive decay to become stable nitrogen (N14). (b) The radioactive atoms (parent isotopes) in any mineral decay over time into stable daughter isotopes. The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope.

Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both. This change is called radioactive decay. For example, unstable 14C transforms to stable nitrogen (14N). The atomic nucleus that decays is called the parent isotope. The product of the decay is called the daughter isotope. In the example, 14C is the parent and 14N is the daughter.

Some minerals in rocks and organic matter (e.g., wood, bones, and shells) can contain radioactive isotopes. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1.

The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay (argon, a gas, escapes into the atmosphere while the lava is still molten). When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral.

The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope (Figure 5b). When the quantities of the parent and daughter isotopes are equal, one half-life has occurred. If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated.

For example, if the measured abundance of 14C and 14N in a bone are equal, one half-life has passed and the bone is 5,730 years old (an amount equal to the half-life of 14C). If there is three times less 14C than 14N in the bone, two half lives have passed and the sample is 11,460 years old. However, if the bone is 70,000 years or older the amount of 14C left in the bone will be too small to measure accurately. Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past. Luckily, there are methods, such as the commonly used potassium-argon (K-Ar) method, that allows dating of materials that are beyond the limit of radiocarbon dating (Table 1).

Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material. Dating methods like thermoluminescence, optical stimulating luminescence and electron spin resonance, measure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material. If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material. These methods are applicable to materials that are up to about 100,000 years old. However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged.