Many human-made chemicals are derived from fossil fuels (such as petroleum or coal) in which  is greatly depleted because the age of fossils far exceeds the half-life of . The relative absence of  is therefore used to determine the relative contribution (or mixing ratio) of fossil fuel oxidation to the total carbon dioxide in a given region of the Earth's atmosphere.
Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of carbon-14. 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 years. This may indicate possible contamination by small amounts of bacteria, underground sources of radiation causing the (n,p) reaction, direct uranium decay (although reported measured ratios of /U in uranium-bearing ores would imply roughly 1 uranium atom for every two carbon atoms in order to cause the / ratio, measured to be on the order of 10), 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. In connection with building the Borexino solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low  content. In the Borexino Counting Test Facility, a / ratio of 1.94×10 was determined; probable reactions responsible for varied levels of  in different petroleum reservoirs, and the lower  levels in methane, have been discussed by Bonvicini et al.