Radiocarbon measurements are always reported in terms of years `before present' (BP).This figure is directly based on the proportion of radiocarbon found in the sample.Our statisticians helped produce the radiocarbon calibration curves Int Cal04, Int Cal09 and, most recently, Int Cal13.The curves allow scientists and archaeologists to obtain precise age estimates for discoveries.The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen.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 wood in these rings once laid down remains unchanged during the life of the tree.
The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.
To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP.
For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.
Here are two women and two men near the top of our range.
As for photos at the bottom of the curve, it didn’t feel right to write someone and say “can I use you to illustrate the concept of ugliness on my blog? The above featured users have graciously agreed to let me post their pictures, so please don’t make them regret it.
But carbon-14 levels have fluctuated over time, so to allow accurate dating, an adjustment must be made using a calibration curve.