The C14 will undergo radioactive decay, and after 5730 years, half of it will be gone. So, if we find such a body, the amount of C14 in it will tell us how long ago it was alive. The method doesn't work on things which didn't get their carbon from the air.This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock.
For object over 4,000 years old the method becomes very unreliable for the following reason: Objects older then 4,000 years run into a problem in that there are few if any known artifacts to be used as the standard.Carbon first began to dilute the marker material at the beginning of the Industrial Revolution, researchers state.Concentrations climbed once again during the 1950s and '60s as nuclear tests released the radioactive atoms into the environment.( A new study found that fossil fuel emissions could soon make radiocarbon dating techniques less accurate.The research, authored by Heather Graven of Imperial College London, was published in the Proceedings of the National Academy of Sciences and it presented simulations of carbon and carbon-14 dynamics in the future.