Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.
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.
thus introduced takes a long time to percolate through the entire volume of the ocean.
The deepest parts of the ocean mix very slowly with the surface waters, and the mixing is uneven.
The development of radiocarbon dating has had a profound impact on archaeology.