Carbon-isotope ratios can date objects up to 100,000 years old {carbon dating}|.
instrument
Mass spectroscopy can measure isotope amounts in very small samples.
location
Lower-atmosphere carbon dioxide has radioactive carbon-14 {radiocarbon} to non-radioactive carbon-12 ratio. Living things have same carbon-isotope ratio as lower atmosphere.
time
Lower-atmosphere carbon-isotope ratio varies over time. Measuring air trapped in glaciers at different depths shows ratios at past times. Carbon-isotope ratio decreases after organisms die, because carbon-14 decays to nitrogen-14. Comparing current reduced ratio to atmosphere ratio at death indicates time of death.
age
Carbon dating is only useful up to 100,000 years ago, because almost all carbon-14 decays in 100,000 years.
changes
Older carbon-dating methods needed more mass and used fire ashes or other organic materials adjacent to formerly living things, not living things themselves. Older carbon-dating methods assumed that atmospheric carbon-isotope ratio is constant. Because ratios actually changed, carbon-dating dates in scientific literature before 1990 are typically too recent. For example, earlier-reported -9000 is actually -11000 or 13,000 years ago.
calibration
Actual lower-atmosphere carbon-isotope ratios, measured at different glacier depths, can find correct dates {calibrated carbon dating}.
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Date Modified: 2022.0224