Since different materials have different δ13C values, it is possible for two samples of different materials, of the same age, to have different levels of radioactivity and different 14C/12C ratios.
The calculations given above produce dates in radiocarbon years: that is, dates which represent the age the sample would be if the 14C/12C ratio had been constant historically.
These records allow fine-tuning, or “calibration”, of the raw radiocarbon age, to give a more accurate estimate of the calendar date of the material.
One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C), and a radioactive isotope, carbon-14 (14C), also known as radiocarbon.
First, there is a long term oscillation with a period of about 9,000 years, which causes radiocarbon dates to be older than true dates for the last 2,000 years, and too young before that. Variations by Latitude Since the earth’s magnetic field varies with latitude, the rate of 14C production changes with latitude too, but atmospheric mixing is rapid enough that these variations amount to less than 0.5% of the global concentration. Variations by Sea Temperature Because the solubility of CO2 in water increases with lower temperatures, glacial periods would have led to the faster absorption of atmospheric CO2 by the oceans. Effects of human activity Coal and oil began to be burned in large quantities during the 1800s.
Dating an object from the early 20th century hence gives an apparent date older than the true date; and for the same reason, 14C concentrations in the neighbourhood of large cities are lower than the atmospheric average.