Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years, which is very short compared with the above isotopes.In other radiometric dating methods, the heavy parent isotopes were produced by nucleosynthesis in supernovas, meaning that any parent isotope with a short half-life should be extinct by now.When an organism dies, it ceases to take in new carbon-14, and the existing isotope decays with a characteristic half-life (5730 years).The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time elapsed since its death.While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured.The scheme has a range of several hundred thousand years.The carbon-14 dating limit lies around 58,000 to 62,000 years.
Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead.
The uranium-lead radiometric dating scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.
An error margin of 2–5% has been achieved on younger Mesozoic rocks.
Carbon-14, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth.
The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2). Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals.