Carbon dating has a certain margin of error, usually depending on the age and material of the sample used. Carbon has a half-life of about. Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of. Known as radiocarbon dating, this method provides objective age estimates for . within a small margin of error, demonstrated the accuracy of the technique.
The carbon atoms that make up, for example, a tree's annual growth ring, or the cob from an ear of corn, therefore contain the same isotope ratio as does the atmosphere. When the plant dies, the carbon starts to decay. Measuring the ratio of the two isotopes therefore determines how long it has been since the constituent carbon atoms were absorbed from the atmosphere.
The carbon in animals comes entirely from eating plants, or from eating other animals that eat plants. 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. Radiocarbon dating is also susceptible to contamination.
- Radiocarbon dating
- Calibration of radiocarbon dates
If the ground in which an object is buried contains particles of coal or other ancient sources of carbon, radiocarbon testing may indicate that the object is far older than it really is. Conversely, contamination by newer plant matter carried by flowing water or intruding plant roots may result in a date that is much too young.
Radiocarbon dating - Wikipedia
Archaeologists are acutely aware of these and other potential difficulties, and take extreme care in the selection and handling of objects to be dated. Animals take up atmospheric 14C indirectly, by eating plants or by eating other animals that eat plants. Measuring the proportion of 14C as opposed to 12C remaining in a sample then tells us how long ago the sample stopped taking up 14C — in other words, how long ago the thing died.
Carbon dating has a certain margin of error, usually depending on the age and material of the sample used. Carbon has a half-life of about years, so researchers use the process to date biological samples up to about 60, years in the past. 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.How Does Radiocarbon Dating Work? - Instant Egghead #28
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.