During the lifetime of an organism, the amount of c14 in the tissues remains at an equilibrium since the loss (through radioactive decay) is balanced by the gain (through uptake via photosynthesis or consumption of organically fixed carbon).However, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].The person who wrote these words lived in the 1800s, many years before archaeologists could accurately date materials from archaeological sites using scientific methods.This discovery meant that there are three naturally occurring isotopes of carbon: Whereas carbon-12 and carbon-13 are stable isotopes, carbon-14 is unstable or radioactive.Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.I have tried here to answer some of the frequently asked questions that I receive from students via email, as well as providing some basic information about scientific dating methods."Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.
Scientists use a technique called radiometric dating to estimate the ages of rocks, fossils, and the earth.Atoms are made up of much smaller particles called protons, neutrons, and electrons.Protons and neutrons make up the center (nucleus) of the atom, and electrons form shells around the nucleus.They used pottery and other materials in sites to date 'relatively'.They thought that sites which had the same kinds of pots and tools would be the same age.The carbon in their bodies at the time of their death will remain in their bodies until they decompose, or if they become fossilized, then forever. This allows scientists to look at the amount of decay in a fossil’s radioactive carbon and determine a relative date.Radiocarbon dating is only effective for objects and fossils that are less than 50,000 years old.The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere.Plants take up c14 along with other carbon isotopes during photosynthesis in the proportions that occur in the atmosphere; animals acquire c14 by eating the plants (or other animals).Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.In contrast, methane made from petroleum products had no measurable radioactivity.