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Why does carbon-14 dating work only for relatively recent biological materials?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

Carbon-14 dating is effective only for relatively recent biological materials—typically those between 500 and 50,000 years old—because of the specific half-life of the carbon-14 isotope, which is 5,730 ± 40 years.123

As a radioactive isotope, carbon-14 decays at a constant rate into stable nitrogen-14. While an organism is alive, it continuously replenishes its carbon-14 levels through the carbon cycle. Upon death, this intake ceases, and the amount of carbon-14 in the remains steadily decreases through beta decay.123

After approximately 50,000 years, the remaining amount of carbon-14 is so minuscule that it becomes extremely difficult to measure accurately. At this point, the technique’s margin of error becomes too large to provide useful chronological data. Furthermore, for very old samples, even trace amounts of modern contamination can significantly skew the results, making the method unreliable for materials beyond this age range.34