Bog oak at Wicken Fen: carbon dating

Trees are carbon-based lifeforms. During their life they constantly take in carbon dioxide from the air. Now some of that carbon dioxide contains a radioactive isotope carbon-14 which is made when high energy solar particles come into the atmosphere. The tree maintains a regular level of carbon-14 during its lifetime. When it dies it can no longer replace lost carbon-14. The carbon-14 starts to decay. By guessing that the level in a tree in the past would be the same as it is now, we compare to the amount in the wood dug up from the bog and work out how many half-lives have passed. For carbon-14, one half-life is 5600 years. The carbon-14 level in this one has not yet halved as it is only 4200 years old.