Radiocarbon dating - Wikipedia
Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively. Carbon dating is a technique used to determine the approximate age of once- living materials. It is based on the decay rate of the radioactive carbon isotope 14 C. Radiocarbon dating has transformed our understanding of the past years. Professor Willard Libby produced the first radiocarbon dates.
The carbon in animals comes entirely from eating plants, or from eating other animals that eat plants.
Carbon dating - RationalWiki
The technique can therefore be used to date human and animal remains. Why doesn't the carbon in the air decay along with terrestrial carbon? The trick is that radioactive carbon is continually replenished in a complex reaction that involves high-energy cosmic rays striking the upper atmosphere. In this process, nitrogen 7 protons and 7 neutrons gains a neutron and loses a proton, producing carbon 6 protons and 8 neutrons.
The proportion of carbon to carbon in the atmosphere therefore remains relatively stable at about 1. One of the implied assumptions in radiocarbon dating is that levels of atmospheric carbon have remained constant over time. This turns out not to be exactly true, and so there is an inherent error between a raw "radiocarbon date" and the true calendar date. To correct for this, scientists have compared radiocarbon dates from objects who's age is known by other means, such as artifacts from Egyptian tombs, and growth rings from ancient trees.
In this way, calibration tables have been developed that eliminate the discrepancy. Despite its usefulness, radiocarbon dating has a number of limitations. First, the older the object, the less carbon there is to measure. Radiocarbon dating is therefore limited to objects that are younger than 50, to 60, years or so. Since humans have only existed in the Americas for approximately 12, years, this is not a serious limitation to southwest archaeology.
Beyond that timespan, the amount of the original 14C remaining is so small that it cannot be reliably distinguished from 14C formed by irradiation of nitrogen by neutrons from the spontaneous fission of uranium, present in trace quantities almost everywhere.
For older samples, other dating methods must be used. The level of atmospheric 14C is not constant. Atmospheric 14C varies over decades due to the sunspot cycle, and over millennia due to changes in the earth's magnetic field.
On a shorter timescale, humans also affect the amount of atmospheric 14C through combustion of fossil fuels and above-ground testing of the largely defensive weapon of the thermonuclear bomb.
Therefore dates must be calibrated based on 14C levels in samples of known ages. A favorite tactic of Young-Earthers involves citing studies which show trace amounts of 14C in coal or diamond samples, which — being millions of years old — should have no original atmospheric 14C left.
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.
Explainer: what is radiocarbon dating and how does it work?
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.