http://hostmaster.wecan-group.com/22741.php They do not, however, give "absolute" dates because they merely provide a statistical probability that a given date falls within a certain range of age expressed in years. Chronometric methods include radiocarbon, potassium-argon, fission-track, and thermoluminescence.
The most commonly used chronometic method is radiocarbon analysis. It measures the decay of radioactive carbon 14C that has been absorbed from the atmosphere by a plant or animal prior to its death. Once the organism dies, the Carbon begins to decay at an extremely predictable rate. Radioactive carbon has a half-life of approximately 5, years which means that every 5, years, half of the carbon will have decayed.
This number is usually written as a range, with plus or minus 40 years 1 standard deviation of error and the theoretical absolute limit of this method is 80, years ago, although the practical limit is close to 50, years ago. Because the pool of radioactive carbon in the atmosphere a result of bombardment of nitrogen by neutrons from cosmic radiation has not been constant through time, calibration curves based on dendrochronology tree ring dating and glacial ice cores, are now used to adjust radiocarbon years to calendrical years. The development of Atomic Absorption Mass Spectrometry in recent years, a technique that allows one to count the individual atoms of 14C remaining in a sample instead of measuring the radioactive decay of the 14C, has considerably broadened the applicability of radiocarbon dating because it is now possible to date much smaller samples, as small as a grain of rice, for example.
Dendrochronology is another archaeological dating technique in which tree rings are used to date pieces of wood to the exact year in which they were cut down. In areas in which scientists have tree rings sequences that reach back thousands of years, they can examine the patterns of rings in the wood and determine when the wood was cut down. This works better in temperate areas that have more distinct growing seasons and this rings and relatively long-lived tree species to provide a baseline. Data collection and analysis is oriented to answer questions of subsistence, mobility or settlement patterns, and economy.
Data collections based on study of hard tissues bones and teeth , usually the only remains left of earlier populations, which include:. After death the radioactive C14 is not replenished from the atmosphere.
There occurs disintegration at a constant rate. The quantity is halved after 5, years which is the Libby Value. There is a practical limitation of radiocarbon dating because the certain minimum quantity of organic carbon must be available in the sample specimen.
Calcined bone is undateable, whereas charred bone is potentially dateable. The absolute age of a skull or mandible is usually obtained indirectly when it comes from a deposit containing more suitable for radiocarbon dating.
Either from the same site or some other area if they have been preserved under comparable conditions. As soon as bones are buried their composition is subject to chemical changes, some of which are slow, some fairly rapid. Fats, protein collagen and the fatty composition are lost quite rapidly.
The protein disappears much more slowly. Under some conditions, such as permanently frozen soil or exclusion of air and bacteria, the protein may persist for tens of thousands of years. The appearance and texture of bone is not a reliable guide to how much organic matter it contains. The organic content of fossil bones has become widely regarded as an unreliable criterion for their antiquity. Some bones that appear to be well fossilised do in fact contain considerable amounts of protein.
Relative ages can be determined by comparison of the contents that include nitrogen, carbon, and chemically bound water. Also the quantity of mineral ash after burning which increases with age. It is convenient to assess the residual organic matter in fossil bone or dentine by determination of the nitrogen content.
As bone protein or collagen decays it becomes fossilised and broken down into amino-acids. These are leached out or retained according to local conditions. This depends on the composition of the percolating ground water which is of two kinds. Alteration of the phosphatic material of which bones are mainly composed is hydroxyapatite. The addition of new mineral matter, such as lime or iron oxide, changes the latter and leads to an increase in weight. The most valuable change however is the irreversible substitution of one element for another in the hydroxyapatite.
These two elements are thus fluorine and uranium. The fluorine is distributed is soluble fluorides in trace quantities in all ground waters. Over the passage of time bones and teeth in permeable deposits progressively accumulate fluorine. The fluorine becomes fixed in the bone and is not readily removed which provides the fluorine dating method.
With regard to uranium mineral phosphates, including bones, all contain uranium. Also the determination of morphological evidence in terms of the hominid phylogeny. Also the correct ecological information of the sites. Considerations of fossil age raises two questions. Firstly the relationship of geological, faunal, archaeological sequences at the site in terms of chronological age in years BP.
The association of faunal, climatic and archaeological information derived from the site can be the same as the specimen. This allows assessment of relative age and answers the second question or determination of absolute or chronometric age.
So carbon is the most common. So most of the carbon in your body is carbon But what's interesting is that a small fraction of carbon forms, and then this carbon can then also combine with oxygen to form carbon dioxide. And then that carbon dioxide gets absorbed into the rest of the atmosphere, into our oceans. It can be fixed by plants. When people talk about carbon fixation, they're really talking about using mainly light energy from the sun to take gaseous carbon and turn it into actual kind of organic tissue.
And so this carbon, it's constantly being formed. It makes its way into oceans-- it's already in the air, but it completely mixes through the whole atmosphere-- and the air. And then it makes its way into plants. And plants are really just made out of that fixed carbon, that carbon that was taken in gaseous form and put into, I guess you could say, into kind of a solid form, put it into a living form.
That's what wood pretty much is. It gets put into plants, and then it gets put into the things that eat the plants. So that could be us.
Now why is this even interesting? I've just explained a mechanism where some of our body, even though carbon is the most common isotope, some of our body, while we're living, gets made up of this carbon thing. Well, the interesting thing is the only time you can take in this carbon is while you're alive, while you're eating new things. Because as soon as you die and you get buried under the ground, there's no way for the carbon to become part of your tissue anymore because you're not eating anything with new carbon And what's interesting here is once you die, you're not going to get any new carbon And that carbon that you did have at you're death is going to decay via beta decay-- and we learned about this-- back into nitrogen So kind of this process reverses.
So it'll decay back into nitrogen, and in beta decay you emit an electron and an electron anti-neutrino. I won't go into the details of that. But essentially what you have happening here is you have one of the neutrons is turning into a proton and emitting this stuff in the process.
Now why is this interesting? So I just said while you're living you have kind of straight-up carbon And carbon is constantly doing this decay thing. But what's interesting is as soon as you die and you're not ingesting anymore plants, or breathing from the atmosphere if you are a plant, or fixing from the atmosphere. And this even applies to plants. Once a plant dies, it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue. The carbon in that tissue gets frozen. And this carbon does this decay at a specific rate.
And then you can use that rate to actually determine how long ago that thing must've died.
So the rate at which this happens, so the rate of carbon decay, is essentially half disappears, half gone, in roughly 5, years. And this is actually called a half life. And we talk about in other videos. This is called a half life. And I want to be clear here.