Plants fix atmospheric carbon during photosynthesis, so the level of C level for the calculation can either be estimated, or else directly compared with known year-by-year data from tree-ring data (dendrochronology) up to 10,000 years ago (using overlapping data from live and dead trees in a given area), or else from cave deposits (speleothems), back to about 45,000 years before the present.A calculation or (more accurately) a direct comparison of carbon-14 levels in a sample, with tree ring or cave-deposit carbon-14 levels of a known age, then gives the wood or animal sample age-since-formation.Atmospheric nuclear weapon tests almost doubled the concentration of The above-ground nuclear tests that occurred in several countries between 19 (see nuclear test list) dramatically increased the amount of carbon-14 in the atmosphere and subsequently in the biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease, as radioactive CO2 was fixed into plant and animal tissue, and dissolved in the oceans.
These amounts can vary significantly between samples, ranging up to 1% of the ratio found in living organisms, a concentration comparable to an apparent age of 40,000.), or other unknown secondary sources of carbon-14 production.
The presence of carbon-14 in the isotopic signature of a sample of carbonaceous material possibly indicates its contamination by biogenic sources or the decay of radioactive material in surrounding geologic strata.
Its existence had been suggested by Franz Kurie in 1934.
There are three naturally occurring isotopes of carbon on Earth: carbon-12, which makes up 99% of all carbon on Earth; carbon-13, which makes up 1%; and carbon-14, which occurs in trace amounts, making up about 1 or 1.5 atoms per 10 beta particles per second.
Carbon-14 is produced in coolant at boiling water reactors (BWRs) and pressurized water reactors (PWRs).