C 14 dating method
To check on the method before applying it on various historical and paleontological material, Libby chose material of Egyptian archaeology, under the assumption that no other historical material from over 2,000 years ago is so secure as to its absolute dating.
The correctness of the method depends greatly on the condition that in the last 40 or 50 thousand years the quantity of water in the hydrosphere (and carbon diluted in it) has not substantially changed. The method depends also on the condition that during the same period of time the influx of cosmic rays or energy particles coming from the stars and the sun has not suffered substantial variations.Radiocarbon dating works by precisely measuring the ratio of radiocarbon to stable carbon in a sample. The tree-ring chronologies have been constructed by counting the annual rings in living trees and matching patterns in these rings to older wood and dead trees.By cross-matching tree-ring sequences in individual specimens a long, continuous tree-ring chronology is constructed with very little dating uncertainty. for more information on tree-ring chronologies.) By measuring radiocarbon concentrations in these tree-rings of known age a calibration table is constructed giving the true date of a sample versus its raw radiocarbon date.Part of the result of these collisions is the production of radiocarbon (C, pronounced "c fourteen"), carbon atoms which are chemically the same as stable carbon, but have two extra neutrons.Radiocarbon is not stable; over time radiocarbon atoms decay into nitrogen atoms.First, it was noticed that, when radiocarbon dated, wood grown in the 20th century appears more ancient than wood grown in the 19th century.
Suess explained the phenomenon by the fact that the increased industrial use of fossil carbon in coal and in oil changed the ratio between the dead carbon C12 and the C14 (radiocarbon) in the atmosphere and therefore also in the biosphere.
The radiocarbon dates diverge from the historical dates by several hundred years (often 500 to 700), and, interestingly, in the Egyptian samples more so than in samples from most other ancient civilizations.
This led Libby to write in 1963: The data [in the Table] are separated into two groups Egyptian and non-Egyptian. The combined efforts of several researchers led them to believe that one of the conditions stipulated by Libby for a flawless functioning of his method was not historically sustained; it is claimed that the influx of cosmic rays varied with time.
With initial large margin of error and anything that did not square with expectation, judged as contaminated, the method appeared to work and was hailed as completely reliablejust as the atomic clock is reliableand this nobody doubted.
But as the method was refined, it started to show rather regular anomalies.
Thus, the ratio of radiocarbon to stable carbon in a living plant is the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.