Carbon Dating (Radiocarbon Dating) - Windows to the Universe
Radiocarbon, or carbon 14, is an isotope of the element carbon that is unstable and weakly radioactive. The stable isotopes are carbon 12 and carbon constant value of 1 carbon atom to every carbon atoms by cosmic rays that turn A famous artifact dated using radiocarbon dating is the Shroud of Turin, which was long . Related End-of-Chapter Exercises: 12, 26, 27, 35, 58, Carbon dating (also called "radiocarbon dating") is used to determine the age of materials that 14C, as compared to the normal isotope of carbon (carbon or 12C), is still around. Last modified August 26, by Randy Russell.
Earth's upper atmosphere is constantly being bombarded by cosmic rays usually protons travelling at nearly the speed of light.
When those speedy protons hit atoms you end up with a few stray neutrons zipping around the place. And when one of those energetic neutrons hits a nitrogen atom, the nitrogen spits out a proton.Carbon- 14 Dating Explained in Detail
With an extra neutron and one less proton, that's no longer a nitrogen atom — six protons plus eight neutrons spells carbon The newly formed carbon atoms end up in carbon dioxide, which ends up in plants, which end up on our dinner plates as fruit, veg or a highly processed version of plants known as meat.
So the proportion of carbon inside living things is the same as the proportion of carbon in the atmosphere at that time. But when we stop eating, or when plants stop photosynthesising, our carbon levels no longer get topped up.
From the moment we die the proportion of carbon compared to non-radioactive carbon in what's left of our bodies starts to drop as it gradually turns to nitrogen. And the longer dead things lie around, the lower the carbon levels get. If you know the rate that carbon decays at, and how much of the carbon in a shroud, iceman or piece of old wood or bone is radioactive, you can work out how long ago they stopped breathing or photosynthesising.
It just involves a bit of maths. We even know that in a gram of carbon, 14 carbon atoms turn into nitrogen every minute. The 14 is a coincidence! But the value that's used to calculate the age of an object isn't an absolute figure, it's a statistical term called half-life.
The half-life of a radioactive isotope is the amount of time it takes for half of the atoms in a sample to decay. Carbon has a half-life of 5, years. That means that no matter how many carbon atoms were present when something died, after 5, years only half of them are left — the rest have decayed to nitrogen.
And after 11, years two half-livesonly a quarter of the original carbon atoms are left. That's why radiocarbon dating is only reliable for samples up to 50, years old. But old age isn't the only thing that affects the accuracy of carbon dating.
Radiocarbon dating - Wikipedia
The level of radiocarbon in the atmosphere has varied over time — it was about two per cent higher 3, years ago, possibly due to factors affecting cosmic rays like changes in solar cycles or the Earth's magnetic field. And nuclear reactions have seen a leap in carbon activity since It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron.
This can reduce the problem of contamination. In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample.
For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.
This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.
Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero.
The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system.
These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes.
Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis. The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No.
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.
This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded.
The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization.
On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams. Uranium—lead dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.
Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. Samarium—neodymium dating method[ edit ] Main article: Samarium—neodymium dating This involves the alpha decay of Sm to Nd with a half-life of 1. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable.
Potassium—argon dating This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1.
Rubidium—strontium dating method[ edit ] Main article: Rubidium—strontium dating This is based on the beta decay of rubidium to strontiumwith a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocksand has also been used to date lunar samples. Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.
Uranium—thorium dating method[ edit ] Main article: Uranium—thorium dating A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years. It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedimentsfrom which their ratios are measured.
The scheme has a range of several hundred thousand years. A related method is ionium—thorium datingwhich measures the ratio of ionium thorium to thorium in ocean sediment.
Radiocarbon dating method[ edit ] Main article: