Fractionation radiocarbon dating
The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died.
The method was developed by Willard Libby in the late 1940s and soon became a standard tool for archaeologists. Libby, a Professor of Chemistry at the University of Chicago, predicted that a radioactive isotope of carbon, known as carbon-14, would be found to occur in nature.Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere.At the present time, for a 1 milligram sample of graphite, this limiting age is about ten half-lives, or 60,000 years, if set only by the sample size.
However, limiting ages or "backgrounds" are also determined by process blanks which correspond to the method used to extract the carbon from the sample.
Craig (1953) first identified that certain biochemical processes alter the equilibrium between the carbon isotopes.
Some processes, such as photosynthesis for instance, favour one isotope over another, so after photosynthesis, the isotope C13 is depleted by 1.8% in comparison to its natural ratios in the atmosphere (Harkness, 1979).
It was discovered in 1934 by Grosse as an unknown activity in the mineral endialyte. This half life has later been re-determined by Godwin. Libby recognized that due to its occurrence in natural materials, either by simple mixing processes or by carbon exchange.
In the same year, Kurie (Yale) exposed nitrogen to fast neutrons and observed long tracks in a bubble chamber. The mean life time of roughly 8000 years is ideal for dating of reservoirs that are a few decades to a few ten thousand yeas old.
The challenge in C content of groundwater at the time of recharge, i.e., at the time when groundwater is isolated from exchange with the soil air and moves away from the water table.