Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar datingas use of the word "absolute" implies an unwarranted certainty of accuracy.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates coins and written history. Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge Absolute dating can be determined by methods such as thermoluminescence dating of glazed ceramics.
In historical geologythe primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young radiocarbon dating with 14 C to systems such as uranium—lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth.
Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
One of the most widely used and well-known absolute dating techniques is carbon or radiocarbon dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay.
Carbon moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon stops.
It takes 5, years for half the carbon to change to nitrogen; this is the half-life of carbon After another 5, years only one-quarter of the original carbon will remain. After yet another 5, years only one-eighth will be left. By measuring the carbon in materialscientists can determine the date of death of the organic matter in an artifact or ecofact.
The relatively short half-life of carbon, 5, years, makes the Absolute dating can be determined by only up to about 50, years.
The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating. An additional problem with carbon dates from archeological sites is known as the "old wood" problem.
It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating particular tree does not necessarily indicate when the fire burned or the structure was built.
For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry AMS dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium—argon dating K—Ar dating. Potassium is a radioactive isotope of potassium that decays into argon The half-life of potassium is 1. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argona noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay.
The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice.
K—Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing also dates items to the last time they were heated.
This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to degrees Celsius or higher releases the trapped electronsproducing light.