Dating in 1950
Using very old trees (such as the Bristlecone Pines in the western U. A.), it is possible to make measurements back to a few thousand years ago.
To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.
For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.
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.
The pair of blue curves show the radiocarbon measurements on the tree rings (plus and minus one standard deviation) and the red curve on the left indicates the radiocarbon concentration in the sample.In practice this is complicated by two factors: These effects are most clearly seen by looking at a specific example.This plot shows how the radiocarbon measurement 3000 -30BP would be calibrated.The information from measurements on tree rings and other samples of known age (including speleothems, marine corals and samples from sedimentary records with independent dating) are all compiled into calibration curves by the Int Cal group.
These are the basis for the calibrations performed by the programs like CALIB and Ox Cal. Calibration of radiocarbon determinations is in principle very simple.This is very useful as a record of the radiocarbon concentration in the past.