Planet Earth/3d. Radiometric dating: Using chemistry to tell time

Planet Earth
3d. Radiometric dating: Using chemistry to tell time

3e. The Periodic Table and Electron Orbitals

Radiometric dating to determine how old something is – the hour glass analogy

The radioactive decay of isotopes and use of excited electron energy states have come to dominate how we tell time from the quartz crystals in your wrist watch and computer, to atomic clocks onboard satellites in space. Measuring radioactive isotopes and electron energy states is the major way we tell time in the modern age. It also enables scientists to determine the age of an old manuscript a few thousand years old, as well as uncovering the age of the Earth itself at 4.6 billion years old. Radioactive decay of isotopes has revolutionized how we measure time, from milliseconds up to billions of years, but how is this done?

First, imagine an hour glass filled with sand which drops from two glass filled spheres connected by a narrow tube. When turned over, sand from the top portion of the hour glass will fall down to the bottom. This rate of sand falling is a linear rate, which means only sand positioned near the opening between the glass spheres will fall. Over time the ratio of sand in the top and bottom of the hour glass will change, so that after 1 hour all the sand will have fallen to the bottom. Note that an hour glass cannot be used to measure years, nor can it be used to measure milliseconds, since in the case of years, all the sand will have fallen, and in measuring milliseconds, not enough sand would have fallen in that short length of time. This ratio is measured by determining the amount of sand in the top of the hour glass, and the amount of sand in the bottom of the hour glass. In chemistry dealing with radioactive decay, we call the top sand the parent element and the bottom sand the daughter element of decay.

An “hourglass” where sand drops from the top (Parent) to the bottom (Daughter), which can be used to measure the passage of time up to 11 minutes. The graph shows the ratio of sand at each moment, with an arrow pointing to the moment when half of the sand is in the top and half on the bottom (this is known as half-life, which is 6 minutes). This is an example of linear decay.

Radiometric dating to determine how old something is – the microwave popcorn analogy

Radiometric decay does not work like an hour glass, since each atom has the same probability to decay, whereas in an hour glass, only the sand near the opening will fall. So a better analogy than an hour glass is to think about popcorn, in particular microwave popcorn. A bag of popcorn will have a ratio of kernels to popped corn, such that the longer the bag is in the microwave oven, the more popped corn will be in the bag. You can determine how long the bag was cooked by measuring the ratio of kernels and popped corn. If most of the bag is still kernels, the bag was not cooked long enough, while if most of the bag is popped corn, then it was cooked for a longer time.

Microwave popcorn where kernels (Parent) pop into popcorn (Daughter), which can be used to measure the passage of time up to 11 minutes. The graph shows the ratio of kernels to popcorn at each moment, with an arrow pointing to the moment when half of the kernels have popped (this is known as half-life, which is 2 minutes). This is an example of exponential decay similar to that used in radioactive dating.

The point in which half of the kernels have popped is referred to as half-life. Half-life is the time it takes for half of the parent atoms to decay to the daughter atoms. After 1 half-life the ratio of parent to daughter will be 0.5, after 2 half-lives, the ratio of parent to daughter will be 0.25, after 3 half-lives the ratio will be 0.125, and so on. Each half-life the amount of parent atoms is halved. In a bag of popcorn, if the half-life is 2 minutes, you will have half un-popped kernels and half popped popcorn, and after 4 minutes the ratio will be 25% kernels and 75% popcorn, after 6 minutes, only 12.5% of the kernels will remain. Each 2 minutes the number of kernels will be reduced by one half.

You can leave the bag in the microwave longer, but the amount of kernels will drop by only half for each additional minute, and likely burn the popcorn, leaving a few kernels still left un-popped. Radiometric dating works the same way.

What can you date?

The first thing to consider in dating Earth materials is what precisely you are actually dating. There are four basics moments that determine the start of the clock in measuring the age of Earth materials:

A phase transition from a liquid to a solid, such as the moment liquid lava or magma cools into a solid rock or crystal.
The death of a biological organism, the moment an organism (plant or animal) stops taking in new carbon atoms from the atmosphere or food sources.
The burial of an artifact or rock, and how long it has remained in the ground.
The exhumation of an artifact or rock, so how long it has been exposed to sunlight.

Radiocarbon dating or C-14 dating

There are two stable isotopes of carbon (carbon-12 and carbon-13), and one radioactive isotope of carbon (Carbon-14), a radioactive carbon with 6 protons and 8 neutrons. Carbon-14 decays, while carbon-12 and carbon-13 are stable and do not decay. The decay of carbon-14 to nitrogen-14 involves the loss of a proton. For any sample of carbon-14 half of the atoms will decay to nitrogen-14 in 5,730 years. This is the half-life, which is when half of the atoms in a sample have decayed. This means carbon-14 dating works well with materials that are between 500 to about 25,000 years old.

Radiocarbon dating was first developed in the 1940s, and pioneered by Willard Libby, who had worked on the Manhattan Project in developing the atomic bomb during World War II. After the war, Libby worked at the University of Chicago developing carbon radiometric dating, for which he won the Nobel Prize in Chemistry in 1960. The science of radiocarbon dating has been around for a long time!

Radiocarbon dating measures the amount of time since the death of a biological organism, the moment an organism (plant or animal) stopped taking in new carbon atoms from the atmosphere or food sources. It can only be used to date organic materials that contain carbon, such as wood, plants, un-fossilized bones, charcoal from fire pits, and other material derived from organic material. Since the half-life of carbon-14 is 5,730 years, this method is great for material that is only few hundred or thousand years old, with an upper limit of about 100,000 years. Radiocarbon dating is mostly used in archeology, particularly in dating materials during the Holocene Epoch, or the last 11,650 years. The first step is to collect a small piece of organic material to date, being very careful not to contaminate the sample with organic material, such as the oils on your own hands. The sample is typically wrapped in aluminum foil to prevent contamination. In the early days of radiometric dating before the 1980s, labs would count the decay in the sample measuring the radioactivity, the more radioactivity, the younger the material was. However, a new class of mass spectrometers were developed in the 1980s giving the ability to directly measure the atomic mass of atoms in these samples. The steps are complex, but yield a more precise estimate of age. The steps involve determining the amount of carbon-14, as well as the two stable types of carbon-13 and carbon-12. Since the amount will depend on the amount of material, scientists look at the ratio of carbon-14 to carbon-12, and carbon-13 to carbon-12. The higher the ratio of carbon-14 to carbon-12 the younger the material is, while the carbon-13 to carbon-12 ratio is used to make sure there is not an excess of carbon-12 in the first measurement, and provide a correction if there is.

One of the technical problems that needed to be overcome was that traditional mass spectrometers measure only the atomic mass of atoms, and carbon-14 has the same atomic mass as nitrogen-14. Nitrogen-14 is a very common component of the atmosphere, and air that surrounds us. And this is a problem for labs. In the 1980s a new method was developed called the Accelerated Mass Spectrometry method, which deals with this problem.

The first step of the process is to take your sample and combust the carbon in a stream of pure oxygen in a special furnace or react the organic carbon with copper oxide, both of which produces carbon dioxide, a gas. The gas of carbon dioxide (which is often cryogenically cleaned) is reacted with hydrogen at 550 to 650 degrees Celsius, with a cobalt catalyst, which produces pure carbon in the form of power graphite from the sample, and water. The graphite is held in a vacuum to prevent contamination from the nitrogen-14 in the air. The vacuumed graphite powder is then purged with ultra-pure argon gas to remove any lingering nitrogen-14 which would ruin any measurement, in a glass vial. This graphite, or pure carbon is ionized, adding electrons to the carbon and making it negatively charged. Any lingering nitrogen-14 will not be negatively charged in the process, because it has an additional positive charged proton. An accelerated mass spectrometer spins the negatively charged atoms passing them through the machine at high speeds as a beam. This beam will have carbon-14, but also ions of carbon-12 bonded to 2 hydrogen, as well as carbon-13 bonded to 1 hydrogen all of which have an atomic mass of 14. To get rid of these carbon atoms bonded with hydrogen, the beam of molecules and atoms with atomic mass of 14 is passed through a stripper that removes the hydrogen bonds, and then through a second magnet, resulting in a spread of atomic mass of carbon-12, carbon-13 and carbon-14 on the detector for each mass. The ratio of carbon-14/carbon-12 is calculated as well as the ratio of carbon-13/carbon-12 and compared to lab standards. The carbon-13/carbon-12 ratio is used to correct the ratio of carbon-14/carbon-12 in the lab and to see if there is an excess of carbon-12 in the sample, due to fractionation. To find the actual age in years, we need to find out the initial amount of carbon-14 that existed at the moment that the organism died.

Now carbon-14 is made naturally in the atmosphere from nitrogen-14 in the air. In the stratosphere these atoms of nitrogen-14 are hit by cosmic rays from the sun, which bombards the nitrogen-14 with thermal neutrons, producing a carbon-14 and an extra proton, or a hydrogen atom. This process is dependent on the magnetic field from the Earth and solar energy, which vary slightly in each hemisphere, and when solar anomalies happen, such as solar flares. Using tree ring 14-carbon/12-carbon ratios, where we know the year of each tree ring, we can calibrate 14-carbon/12-carbon ratios to absolute years for the last 10,000 years.

There are two ways to report the age of materials dated this way: One is to apply these corrections, which is called the radiocarbon calendar age or you can report the raw date determined solely from the ratio, called Carbon-14 dates. Radiocarbon calendar ages will be more precise than simple carbon-14 dates, especially for older dates.

There is one fascinating thing about determining the initial 14-carbon/12-carbon ratios for materials during the last hundred years. Because of the detonation of atomic weapons in the 1940s and 1950s, the amount of 14-carbon increased in the atmosphere dramatically after World War II, as seen in tree ring data and measurements of isotopes of carbon in carbon-dioxide of the atmosphere.

This fact was used by neurologist studying brain cells, leading to the medical discovery that new brain cells are not formed after birth, as people born before the 1940s have lower levels of 14-carbon in their brain cells in old age, than brain cells of people born after the advent of the nuclear age, which have much higher levels of 14-carbon in their cells. However, over the past few decades, neuroscientists have found two brain regions, the olfactory bulbs (where you get the sense of smell) and the hippocampus (where the storage of memories happen) that do grow new neuron cells throughout life, but the majority of your brain is composed of the same cells throughout your life.

Radiocarbon dating works great, but like a stop watch, it is not going to tell us about things much older than 100,000 years. For dinosaurs and older fossils, or rocks themselves the next method is more widely used.

Potassium-argon (K-Ar) Dating

Potassium-argon dating is a great method for measuring ages of materials that are millions of years old, but not great if you are looking to measure something only a few thousand years old, since it has a very long half-life.

Potassium argon dating measures the time since a phase transition from a liquid to a solid took place, such as the moment liquid lava or magma cools into a solid rock or crystal. It also requires that the material contain potassium in a crystal lattice structure. The most common minerals sampled for this method are biotite, muscovite, and the potassium feldspar group of minerals, such as orthoclase. These minerals are common in volcanic rocks and ash layers, making this method ideal for measuring the time when volcanic eruptions occurred.

If a volcanic ash containing these minerals are found deposited within or near the occurrence of fossils, a precise date can often be found for the fossils, or a range of dates, depending on how far stratigraphically that ash layer is found from the fossils. Potassium-40 is radioactive, but with a very long half-life of 1.26 billion years, making it ideal for determining ages in most geologic time ranges measured in millions of years. Potassium-40 decays to argon-40, as well as calcium-40, argon-40 is a gas, while calcium-40 is a solid, and very common, hence we want to look at the amount of argon-40 trapped in the crystal and compare that amount to potassium-40 contained in the crystal (both of which are fairly rare).

Ancient volcanic ash layers preserved in sediment can be dated using Potassium-Argon dating.

This requires two steps, first to find out how much potassium-40 is contained within the crystal, and second how much argon-40 gas is trapped in the crystal. One of the beautiful things about potassium-argon dating is that the initial amount of argon-40 in the crystal can be assumed to be 0, since it is a gas. Argon-40 was not present when the crystal was a liquid and cooled into a solid. The only argon-40 found within the crystal would be formed by radioactive decay of potassium-40 and become trapped inside the solid crystal after this point. One of the problems with potassium-argon dating is that you have to do two different lab methods to measure the amount of potassium-40 and the amount of argon-40, and within a single crystal and not destroy the crystal in the process of running those two separate tests. Ideally, we want to sample the exact spot on a crystal for both measurements with a single analysis. And while potassium-argon dating came about in the 1950s, it has become less common compared to another method, which is easier and more precise, and only requires a single test.

⁴⁰Ar/³⁹Ar dating method

This method uses the potassium-argon dating technique but makes it possible to do a single lab analysis on a single point on a crystal grain, making it much more precise than the older potassium-argon method. The way it works is that a crystal containing potassium is isolated, and studied under a microscope, making sure it is not cracked or fractured in any way. The selected crystal is subjected to neutron irradiation, which converts any of the potassium-39 isotopes, to argon-39 isotopes a gas that will be trapped within the crystal (this is similar to what the sun does to nitrogen-14 to change it to carbon-14). These argon-39 isotopes join any of the radiogenic argon-40 isotopes in the crystal as trapped gases, so we just have to measure the amount of argon-39 to argon-40.

The argon-39 number will determine about how much potassium was in the crystal. After being subjected to neutron irradiation the sample crystal will be zapped with a laser, that will release both types of argon gas trapped in the crystal. This gas is sucked up within a vacuum into a mass spectrometer to measure atomic masses of 40 and 39. Note that Argon-39 is radioactive, and decays with a half-life of 269 years, so any argon-39 measured was generated by the irradiation done in the lab. Often this method requires large, unfractured and well-preserved crystals to yield good results. The edges of the crystal and near cracks within the crystal, may have let some of the argon-40 gas to leak out, and will yield too young of a date. Both potassium-argon and argon-argon dating tend to give minimum ages, so if a sample yields 30 million years within a 1-million-year error, the actual age is more likely to be 31 million years, than 29 million years. Often potassium-argon and argon-argon dates are younger than other evidence suggests, and likely were determined from fractured crystals with some leakage of argon-40 gas. Studies will often show the crystal sampled and where the laser points are, and the dates calculated from each point in the crystal. The maximum age is often found near the center and far from any edge or crack within the crystal. Often this will be carried out with multiple crystals in a single rock, to get a good range, and taking the best resulting maximum ages. While potassium-argon and argon-argon are widely used it does require nicely preserved crystals of such fragile minerals grains as biotite, which means that the older the rock, the less likely good crystals can be found. It also does not work well with transported volcanic ash layers in sedimentary rocks, because the crystals are damaged in the process. Geologists were eager to use other minerals, more rugged minerals, that could last billions of years yet preserve the chemistry of radioactive decay—the mineral that meets those requirements is zircon.

Zircon Fission track dating

Zircons are tough and rugged minerals, found in many igneous and metamorphic rocks, and are composed of zirconium silicate (ZrSiO₄), which form small diamond-like crystals. Because these crystals are fairly rugged and can survive transport they are also found in many sandstones. These transported zircons in sedimentary rocks are called detrital zircons. With most zircon dating, you are measuring the time since the phase transition from a liquid to a solid, when magma cooled into a solid zircon crystal.

Zircon fission track dating is more specifically measuring the time since the crystal was cooled to 230 to 250 °C, which is called the annealing temperature. Between 900 °C to 250 °C the zircons are somewhat mushy. Zircon fission dating dates the cooler temperature when the crystal became hard, while another method dates the hotter temperature when the crystal became a solid. Zircons are composed of a crystal lattice of zirconium bonded to silicate (silica and oxygen tetrahedrals), the zirconium is often replaced in the crystal with atoms of similar size and bonding properties, including some of the rare earth elements, but what we are interested in is that zircon crystals contain trace amounts of uranium and thorium. Uranium and thorium are two of the largest naturally occurring atoms on the periodic table. Uranium has 92 protons, while thorium has 90. Both elements are radioactive and decay, with long half-lives. These atoms of uranium and thorium act like mini-bombs inside the crystal, and when one of these high atomic mass atoms decay, it sets off a long chain reaction of decaying atoms, the fission of which causes damage to the internal crystal structure.

The older the zircon crystal is the more damage it will exhibit. Fission track dating was developed as an independent test of potassium-argon and argon-argon dating, as it does not require an expensive mass spectrometer, but simply looking at the crystal under a powerful microscope and measuring the damage caused by the radioactive decay of uranium and thorium. Zircon fission track dating is also used to determine the thermal history of rocks, as they rose up through the geothermal gradient, recording the length of time it took to cool to 250° C.

Uranium–Lead dating of Zircons

Uranium-lead dating is the most common way to date rocks and used to determine the age of the Earth, meteorites, and even rocks from the Moon and on Mars. It has become the standard method for radiometric dating, as new technology has made this method much easier. In the 1950s and 1960s, geologists were eager to figure out a way to use the uranium and lead inside zircons to get a specific date, more precise than estimates based on fission track dating, which was somewhat subjective. The problem was that all those radioactive tiny atomic bombs causing the damage to the zircon crystals over millions of years was also causing the loss of daughter product that would escape during those decay events, such as the gas Radon. The decay of the two most common isotopes of uranium (Uranium-235 and Uranium-238) is a complex chain of events, during which the radioactive gas radon is produced as one of the steps. If there are cracks or fractures in the crystal, the radon gas escapes from the crystal and as a result the ratio would yield too young of a date. If the radon gas is still held within the crystal, it would decay back to a solid, eventually as lead. Lead is not found initially within zircon crystals, and lead would only be found within zircons from the decay of uranium isotopes, allowing radiometric dating.

During the 1940s and 1950s a young scientist named Clair Cameron Patterson was trying to determine the age of the Earth by dating zircons and meteorites. Rather than look at zircons, he was trying to date meteorites, which contain the stable isotope lead-204, and used a type of uranium-lead dating simply called lead-lead dating. Clair Patterson used an isochron, which graphically compares the ratios of lead produced through the decay of uranium and thorium, lead-206 and lead-207 with stable lead-204 (an isotope not produced by radioactive decay of uranium and thorium), by plotting these ratios on a graph the resulting slope would indicate the age of the sample, the line is called an isochron, meaning the same age. Using lead isotopes recovered from the Canyon Diablo meteorite from Arizona, Patterson calculated that the Earth was between 4.5 and 4.6 billion years old in 1956.

To acquire these ratios of lead, Clair Patterson developed the first chemical clean room, as he quickly discovered abundant lead contamination in the environment around him traced to the widespread use of lead in gasoline, paints, and water pipes in the 1940s and 1950s. Patterson dedicated much of his later life fighting corporate lobbying groups and politicians to enact laws prohibiting the use of lead in household products, such as fuel and paints. The year 1956 was also the year that the solution to the uranium-lead problem was solved by the brilliant scientist George Wetherill who published a solution to the problem, something called a Concordia diagram, or sometimes called the Wetherill diagram, which allowed direct dating of zircons. There are two types of Uranium isotopes in these zircons: Uranium-238 (the most common) which decays to Lead-206, and Uranium-235 (the next most common) which decays to Lead-207 (with different half-lives).

An example of a concordia diagram which overcomes the problem of missing daughter products lost during the gas (radon) stage of decay to lead.

If you could measure these two ratios, in a series of zircon crystals and compare the ratios graphically, you could calculate the true ratios of the zircons as if they had not lost any daughter products. Using this set of ratios, you can determine where the two ratios would cross with a given age, and hence where they would be in accordance with each other. It was a brilliant solution that solved the issue of daughter products escaping from zircon crystals. Today geologists can analyze particular points on individual zircon crystals, and hence select the best spot on the crystal that has the minimum amount of leakage of daughter products. Using the Concordia diagram allows a correction to these resulting ratios.

Uranium-Lead dating requires you to determine two ratios, Uranium-238 to Lead-206, and Uranium-235 to Lead-207. The Uranium-238 to Lead-206 has a half-life of 4.46 billion years, while Uranium-235 to Lead-207 has a half-life of 704 million years, making them great for both million-year and billion-year scales of time. To do Uranium-Lead dating on zircons, rock samples are grounded, and zircons are extracted using heavy liquid separation. These zircons are analyzed under a microscope. Zircons found in sedimentary rock will yield the age when the zircon initially formed from magma, not when it was re-deposited in a sedimentary layer or bed. Zircons found in sedimentary rocks are called detrital zircons, and will yield maximum ages; for example, a detrital zircon that is 80 million years old, could be found in sedimentary rock deposited 50 million years ago, and the 30-million-year difference is the time when the zircon was exhumed and eroded from igneous rocks and transported into the sedimentary rock. A 50-million-year old zircon will not be found in sedimentary rocks that are in fact 80 million years old, so detrital zircons will tell you only that the rock is younger than the zircon age.

Zircons deposited or forming within igneous rock or volcanic ash layers that contain fresh zircons can yield very reliable dates, particularly when the time between crystallization and deposition is minimal.

The first step of Uranium-Lead dating is finding and isolating zircon crystals from the rock, usually by grinding the rock up, and using heavy liquid to separate out the zircon crystals. The zircons are then studied under a microscope to determine how fresh they are. If zircons were found in a sedimentary rock, they are likely detrital, and the damage observed in the crystal will tell you how fresh they are. Detrital zircons are dated in studies to determine the source of sedimentary grains by sedimentary geologists, however, they often lack the resolution for precise dates, unless the zircon crystals were deposited in a volcanic ash, and have not be eroded and transported. Once zircons are selected, they are analyzed using laser ablation inductively coupled plasma mass spectrometry, abbreviated as LA-ICP-MS, which zaps the crystals with a laser, the ablated material is sucked up and ionized in the mass spectrometer under extremely hot temperatures, and a plasma is created which passes the atoms along a tube at high speed measuring the atomic mass of the resulting atoms scattered along the length of the plasma tube. LA-ICP-MS measures larger atomic mass atoms, such as lead and uranium. LA-ICP-MS does not require much lab preparation, and zircons can be analyzed quickly resulting in large sample sizes for distributions of zircons, giving very precise dates. Zircon Uranium-Lead dating is the most common type of dating seen today in the geological literature, exceeding even the widely used Argon-Argon dating technique. It is also one of the more affordable methods of dating, requiring less lab preparation of samples.

Dating using electron energy states

One of the things you will note about these dating methods is that they are used either to date organic matter that is less than 100,000 years old, or volcanic or igneous minerals that are much older between 1-million to 5-billion years old.

That leaves us with a lot of materials that we cannot date using those methods, including fossils directly that over 100,000 years old, and sedimentary rocks, since detrital zircons will only give you the date when they turned into a solid crystal, rather than the age the sedimentary rocks they are found in. Also using these methods, we cannot determine the age of stone or clay pottery artifacts, the age of glacial features on the landscape, or fossilized bone directly.

One place that is notoriously difficult to date are cave deposits that contain early species of humans, which are often older than the limits of radiocarbon dating. This problem is exemplified by the controversial ages surrounding the Homo floresiensis discovery, a remarkably small species of early humans found in 2003 in a cave located on the island of Flores in Indonesia. Physical anthropologists have argued that the species shares morphological similarities with Homo erectus, which lived in Indonesia from 1.49 million years ago to about 500,000 years ago. Homo erectus was the first early human to migrate out of Africa, and fossils discovered in Indonesia were some of the oldest, as determined from potassium-argon and zircon fission track dating. However, radiocarbon dating from the cave where the tiny species Homo floresiensis was found were much younger than expected, yielding radiocarbon dates of 18,700 years and 17,400 years old, which is old, but not as old as the anthropologists had suggested if the species was closely related to Homo erectus. Researchers decided to conduct a second analysis, and they turned to luminescence dating.

Luminescence (optically and thermally stimulated)

There are two types of luminescence dating, optically stimulated and thermally stimulated. They measure the time since the sediment or material was last exposed to sunlight (optical) or heat (thermal). Luminescence dating was developed in the 1950s and 1960s initially as a method to date when a piece of pottery was made. The idea was that during the firing of clay in a pottery kiln to harden the pottery, the quartz crystals within the pottery would be subjected to intense heat and energy, the residuals of this energy would dim slowly long after the pottery was cooled down. Early experiments in the 1940s on heating crystals and observing the light emitted after subjecting the crystals to heat or light, showed that materials could fluorescence (spontaneously glow) and phosphorescence as a delay of light given off by the material for a longer period of time, long after the material was subjected to the initial light or heat.

A glow in the dark figure of an eagle, which needs to be exposed to light.

If you have ever played with glow in the dark objects, you can see this when you expose the object to light, then turn off the light, there is a glow to the object for a long while until it dims to the point you cannot see it anymore. This effect is called phosphorescence. It was also known that material near radioactive-materials would also give off either spontaneous fluorescence and phosphorescence which would last, so it does not have to be heated or in light, radioactive particles can also excite material to glow as well.

What causes this glow is that by exciting electrons in the atom with intense heat or exposure to sunlight (photons) or even radioactivity, the electrons move up in energy levels, however these electrons quickly drop down in energy levels, and in doing so emit photons as observable light as the object cools or was removed from the light. In some materials these electrons become trapped at these higher energy levels, and slowly and spontaneously pop back down to the lower energy levels over a more extended period of time. When electrons drop down from their excited states they emit photons, prolonging the glow of the material over a longer time period and perhaps thousands of years.

Scientists wanted to measure the remaining trapped electrons in ancient pottery. The dimmer the glow observed the older the pottery would be. Earlier experiments were successful, and later this tool was expanded to materials exposed to sunlight, rather than heat. The way it works is to determine two things, first is the radiation dose rate, as this will tell you how much radiation the crystal is absorbing over time. This is usually is done by measuring the amount of radioactive elements in the sample and surrounding it. The second thing to measure is the total amount of absorbed radiation, which is measured by exposing the material to light or heat, and measuring the number of photons emitted by the material. Using these two measurements you can calculate the age since the material was subjected to the initial heat or light. There are three types of Luminescence dating.

The first is TL (or thermal luminescence dating) using heat to measure the amount of photons given off of the material. The second is infrared stimulated or IRSL, and the third is optical stimulated or OSL, both of these methods refer to how the photons are measured in the lab by stimulating them with either infrared light or visible optical light. The technique works well, but there is a limit to how dim the material can be to give you useful information, so it works well for materials that are 100 to 350,000 years old, similar to ranges found with radiocarbon dating, but can be carried out on different material, such as pottery, stone artifacts, and the surfaces of buried buildings and stone work.

Researchers in addition to determining the radiocarbon age of Homo floresiensis, used luminescence dating and found a TL maximum date of 38,000 years old, and an IRSL minimum date of 14,000 years old, suggesting that the 18,000 years old date was correct for the skeletons found in the cave. These ages are when these sediments were last exposed to sunlight, and not when they were actually deposited in their current place in the cave, so there is likely a lot of mixing going on inside the cave.

Uranium series dating

As a large atom, uranium decays over a very long half-life to lead, and that there are two uranium decay chains, one for uranium-235 which decays to the stable isotope lead-207 and one for uranium-238 which decays to stable isotope lead-206. Scientists look at it just as a segment of that long decay chain, the decay of uranium-238 to uranium-234, which is the first part of uranium-238 decay.

And just measure the amount of uranium-234 decaying to thorium-230. Uranium-238 decays to thorium-234 with half-life of 4.27 billion years, thorium-234 decays to protactinium-234 with a half-life of 27 days, then protactinium-234 decays to uranium-234 also with a half-life of 27 days and finally Uranium-234 with a half-life of 245,500 years decays to thorium-230.

The decay between Uranium-234 and thorium-230 can be used to measure things within a few hundred thousand years. There is a problem with this method, since scientists do not know the initial amount of uranium within the bone or sediment that we are measuring. There is an unknown amount of uranium-234 starting out in the bone or sediment. Uranium-oxide is often carried by groundwaters moving into and out of the fossil and pores between sediment grains.

So unlike other dating methods, were the initial amount of daughter product was assumed to be zero, or a way to determine it experimentally, such as in carbon-14 dating, we cannot make that case. So we have to build a diffusion model, often this is called modeled ages.

The way this is done is that the bone is sectioned, cleaned and laser ablated at various points across its depth measuring the ratios between uranium-234 and thorium-230. Because the bone absorbed more uranium-234 over time, the outer layers of the bone will be enriched in uranium-234 compared to the internal part of the bone, using a gradient of uranium-238, uranium-234 and thorium-230 a diffusion model can be made to determine the amount of uranium-234 likely in the bone when the fossil organism died, and the amount of thorium-230 resulting from the decay of this uranium-234 as additional uranium-234 was added during the fossilization process. Because of this addition of uranium-234, and the fact that uranium-234 is very rare, as it is produced only by the decay of uranium-238, this method is reserved for difficult cases, such as dating fossils deposited in hard-to-date cave deposits, especially in the upper limits of radiocarbon dating, between 100,000 to 500,000 year-old fossils.

Uranium series dating was used to reexamine the age of Homo floresiensis by looking at the actual fossil bone itself. Uranium series dating of the bone of Homo floresiensis resulted in ages between 66,000 and 87,000 years old (link to revised age), older than the radiocarbon dates from the nearby charcoal (17,400-18,700 years old) and luminescence dating of sediment in the cave (14,000-38,000 years old), but modeled on the actual bones themselves. These are modeled ages, since you have to determine the diffusion of uranium into the pores of the bone as it was fossilized in the cave, which can yield somewhat subjective dates.

Uranium series dating was also done for another problematic early human cave discovery, the age of Homo naledi from the Rising Star cave in South Africa. Fossil teeth were directly dated using uranium series dating, yielding a minimum age of 200,000 years old (link to paper on the age of the fossil), which had been predicted to be about 1,000,000 years old.

Although, uranium series dating tends to have large error bars, due to the modeling of the diffusion of uranium into the fossils and rocks. Depending on how quickly and how much uranium-238 and uranium-234 was added to the fossil over time in the cave. Uranium series dating is used really only in special cases where traditional dating such as radiocarbon dating and uranium-lead dating cannot be done.

Electron Spin Resonance (ESR)

In April of 1986 the Chernobyl Nuclear Power Plant suffered a critical meltdown resulting in an explosion and fire that released large amounts of radioactive materials into the nearby environment. The accident led to the death of 31 people directly from radiation, and 237 suffered acute radiation sickness. Worry spread across Europe as to how to measure the exposure to radiation from the accident, and electron spin resonance was developed by Soviet scientists to measure the exposure to radiation by looking at teeth, particularly baby teeth of children living in the area.

Electron spin resonance is the measurement of the number of unpaired electrons within atoms. When exposed to radiation, electrons will unpair from their typical covalent bonds, and become unpaired within the orbitals resulting in a slight difference in the magnetism of the atom. This radiation damage, results in the breaking of molecular bonds, and the reason radiation causes cancers and damage to living cells. At the atomic level radiation can break molecules, resulting in abnormally high errors in DNA and proteins within living cells. Electron spin resonance measures the amount of free radical electrons with a material.

Using this measurement, scientist measured the amount of electron spin resonance in teeth from children who lived near the accident to determine the amount of exposure they had to radiation fall-out from the Chernobyl accident. The study worked, which led to the idea of using the same technology in fossilized teeth, exposed to naturally occurring radiation in the ground.

The loss of baby teeth, allowed scientists to measure the amount of radiation exposure to childern using lost baby teeth in children living near the nuclear plant.

Dating using electron spin resonance requires that we know the amount of uranium and radioactivity that is in the surrounding material through its history, and calculate the length of exposure time to this radiation. The issue however is that you have to model the amount of uranium uptake within the fossil over time, similar to the model you develop with uranium series dating. This is because in both methods of dating you cannot assume that uranium (and amount of radioactivity) in the material remained the same, as the uptake of fresh uranium over time likely occurred. Often scientists will focus on the dense crystal lattice structure of enamel, a mineral called hydroxyapatite, as it is less susceptible to the uptake of uranium.

Electron spin resonance is often paired with uranium series dating, since it has a similar range of ages that it can be used for from a 100 up to 2,000,000 years. Unpaired electrons within atoms is a more permanent state than electrons at higher energy levels seen in Luminescence dating, so older fossils can be dated, up to 2 million years old. This dating method cannot be used for the vast majority of fossils older than 2 million years old, but can be used to date the length of time a fossil or rock was buried up to that limit. Note that electron spin resonance dating is determining the length of time a fossil was buried in sediment that has a background radiation that can be measured.

Surface Exposure Dating or Beryllium-10 dating

This dating method has revolutionized the study of past Ice Ages over the last 2.5 million years, and study of the glacial and interglacial cycles of Earth’s recent climate. Surface exposure dating can determine the length of time a rock has been exposed to the sun. Ascertaining the length of time, the rock has been exposed to the sunlight allows geologists to discover the age of when that rock or boulder was deposited by a melting glacier, and the timing of the extent of those glaciers on a local level throughout past ice age events.

The way it works is that when rocks are exposed to sunlight, they are bombarded by cosmic rays from the sun, which contain neutrons. These rays result in something called spallation of the atoms in mineral crystals, resulting in the build-up of cosmogenic nuclides.

There are a number of different types of cosmogenic nuclides. For example, we had previously talked about potassium-40 being hit with neutrons in a lab setting and producing Argon-39, in argon-argon dating. The same thing happens in nature, when rocks are left in the sun for a long time, and you could measure the amount of argon-39. Most geologists instead look for atoms which form solids as they are easier to extract from the rock, including Beryllium-10, one of the most widely used cosmogenic nuclides to measure.

Beryllium-10 is not found in quartz minerals common in rocks and boulders when they form, but will accumulate when the oxygen atoms within the crystal lattice structure are exposed to cosmic rays containing short-lived free neutrons. The beryllium-10 will build up within the crystals, as long as the rock is exposed to the sun. Beryllium-10 is an unstable, radioactive isotope, with a half-life of 1.39 million years, making it ideal for most applications of dating during the Pleistocene Epoch. Most rocks studied so far have exposure ages of less than 500,000 years, indicating that most rocks get re-buried within half a million years.

Surface Exposure Dating is different because we are looking at the amount of beryllium-10 building up within the surface of the rock over time, so the more beryllium-10 is within the rock the longer it has been exposed to sunlight. If the rock becomes obscured from the sun through burial, or a tree grows next to it, then the build-up of the beryllium-10, will be slowed or turned off, and over time will decay to boron-10, emptying the beryllium-10 out of the rock, and resetting the clock for the next time it is exposed to the sunlight. Geologists have to be sure that the rock has been well exposed to sunlight and not shaded by any natural feature in the recent past, like trees.

Geologists will select a boulder or rock, and carefully record its location, as well as the horizon line surrounding the rock, to account for the length of sunlight exposure of any given day at that location.

A small explosive charge is drilled into the rock, and rock fragments are collected of the surface edge of the rock. The sample is grounded into a powder back in the lab, and digested with hydrofluoric acid, to isolate quartz crystals, which is turned into a liquid solution within a very strong acid. This solution is reacted to various chemicals to isolate the beryllium into a white powder, which is then passed through a mass spectrometer to measure the amount of Beryllium-10 in the rock. This amount is then compared to a model of how much sunlight the rock was exposed to at that location, with the topography of the surrounding features, and determine the length of time that rock has sat there on the surface of the Earth. It’s a pretty cool method which, has become highly important in understanding the glacial history of the Earth through time.

Magnetostratigraphy

Magnetostratigraphy is the study of the magnetic orientations of iron minerals within sedimentary rocks. These orientations record the direction of the magnetic pole when the sedimentary rocks were deposited. Just like a magnetic compass, iron minerals when transported in lava, magma, or in sediment will orient to the current Earth’s magnetic field. The Earth magnetic field is not stationary, but moves around. In fact, the orientations of the poles switch randomly every few hundred thousand years or so, such that a compass would point toward the south pole rather than the north pole. This change in the orientation of the iron minerals is recorded in the rock layers formed at that time. Measuring these orientations between normal polarity, when the iron minerals point northward, and reversal polarity, with the iron minerals point southward, gives you events that can be correlated between different rock layers.

The thickness of these bands of changing polarity can be compared to igneous volcanic rock as well, which record both the absolute age using potassium-argon dating for example, as well as the polarity of the rocks at that time, allowing correlation between sedimentary and igneous rocks. Magnetostratigraphy is really important because it allows for the dating of sedimentary rocks, that contain fossils, even when there is no volcanic ash layers present.

There are a couple problems with magnetostratigraphy. One is that rocks can become demagnetized in sedimentary rocks. For example, the rock being struck by lightning will scramble the orientations of the iron grains. It can be also difficult to correlate layers of rock if the sedimentation rates vary greatly or you have unconformities you are unaware of. However, it really works well for many rock layers, and is a great tool to determine the age of rocks by documenting these reversals in the rock record. It was also one of the key technologies to demonstrate the motion of Earth’s tectonic plates.

Rock samples are collected in the field, by recording their exact orientation and carefully extracting the rock not to break it. The rock is then taken to a lab, where the rock is placed in an iron cage to remove the interference of magnetic fields from the surrounding environment. The rock sample is cryogenically cooled down to extremely cold temperatures just above absolute zero, where the residual magnetism of the rock will be easier to measure, because sedimentary rocks are not very magnetic. More magnetic rocks, like igneous rocks do not have to be cooled. The rock is slowly demagnetized and the orientations vectors are recorded and spatially plotted.

These data points will fall either more toward the north or south, depending on the polarity of the Earth at the time of deposition. The time span between polar reversal events is short, a few hundred years, with the majority of time the polarity is either in normal or reverse state. Sometimes the polarity changes rapidly, while other times the polarity does not change for millions of years, such long intervals with lack of change occurred during the Cretaceous Period, during the age of dinosaurs for 40 million years, which is called the Cretaceous Superchron where the polarity stayed normal for a very long time, and geologists do not know why this happened.

Overview of Methods

Method	Range of Dating	Material that can be dated	Process of Decay
Radiocarbon	1 - 70 thousand years	Organic material such as bones, wood, charcoal, and shells	Radioactive decay of 14C in organic matter after removal from biosphere
K-Ar and 40Ar-39Ar dating	10 thousand - 5 billion years	Potassium-bearing minerals	Radioactive decay of 40K in rocks and minerals
Fission track	1 million - 10 billion years	Uranium-bearing minerals (zircons)	Measurement of damage tracks in glass and minerals from the radioactive decay of 238U
Uranium-Lead	10 thousand - 10 billion years	Uranium-bearing minerals (zicrons)	Radioactive decay of uranium to lead via two separate decay chains
Uranium series	1 thousand - 500 thousand years	Uranium-bearing minerals, corals, shells, teeth, CaCO3	Radioactive decay of 234U to 230Th
Luminescence (optically or thermally stimulated)	1 thousand - 1 million years	Quartz, feldspar, stone tools, pottery	Burial or heating age based on the accumulation of radiation-induced damage to electron sitting in mineral lattices
Electron Spin Resonance (ESR)	1 thousand - 3 million years	Uranium-bearing materials in which uranium has been absorbed from outside sources	Burial age based on abundance of radiation-induced paramagnetic centers in mineral lattices
Cosmogenic Nuclides (Beryllium-10)	1 thousand - 5 million years	Typically quartz or olivine from volcanic or sedimentary rocks	Radioactive decay of cosmic-ray generated nuclides in surficial environments
Magnetostratigraphy	20 thousand - 1 billion years	Sedimentary and volcanic rocks	Measurement of ancient polarity of the earth's magnetic field recorded in a stratigraphic succession

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c. The Chart of the Nuclides.	d. Radiometric dating, using chemistry to tell time.	e. The Periodic Table and Electron Orbitals.