Before radiocarbon dating Adulttv
It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years.
For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.
This plot shows how the radiocarbon measurement 3000 -30BP would be calibrated.
The left-hand axis shows radiocarbon concentration expressed in years `before present' and the bottom axis shows calendar years (derived from the tree ring data).
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.
To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.
By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region.
This requires a computer since the calculations are more complicated.
It gives the time range, from which you can be 95% sure the sample came.
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The first indicates the proportion of radiocarbon atoms in the sample as compared to samples modern in 1950.