From these records a “calibration curve” can be built (see figure 2, below).A huge amount of work is currently underway to extend and improve the calibration curve.Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.Radioactive decay can be used as a “clock” because it is unaffected by physical (e.g. For instance, the amount varies according to how many cosmic rays reach Earth.This is affected by solar activity and the earth’s magnetic field.Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
Tree rings can be counted and their radiocarbon content measured.
Radiocarbon dates are presented in two ways because of this complication.
The uncalibrated date is given with the unit BP (radiocarbon years before 1950).
If you ever wondered why nuclear tests are now performed underground, this is why.
Most radiocarbon dating today is done using an accelerator mass spectrometer, an instrument that directly counts the numbers of carbon-14 and carbon-12 in a sample.