OpenStax Biology 2e
Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and methane. Carbon-14 (14C) is a naturally occurring radioisotope that is created in the atmosphere from atmospheric 14N (nitrogen) by the addition of a neutron and the loss of a proton because of cosmic rays. This is a continuous process, so more 14C is always being created. As a living organism incorporates 14C initially as carbon dioxide fixed in the process of photosynthesis, the relative amount of 14C in its body is equal to the concentration of 14C in the atmosphere. When an organism dies, it is no longer ingesting 14C, so the ratio between 14C and 12C will decline as 14C decays gradually to 14N by a process called beta decay—electrons or positrons emission. This decay emits energy in a slow process.– What is a naturally occuring radioisotope that is created in the atmosphere from nitrogen-14?
– What is the process when carbon-14 decays gradually to nitrogen 14?
After approximately 5,730 years, half of the starting concentration of 14C will convert back to 14N. We call the time it takes for half of the original concentration of an isotope to decay back to its more stable form its half-life. Because the half-life of 14C is long, scientists use it to date formerly living objects such as old bones or wood. Comparing the ratio of the 14C concentration in an object to the amount of 14C in the atmosphere, scientists can determine the amount of the isotope that has not yet decayed. On the basis of this amount, the image above shows that we can calculate the age of the material, such as the pygmy mammoth, with accuracy if it is not much older than about 50,000 years. Other elements have isotopes with different half lives. For example, 40K (potassium-40) has a half-life of 1.25 billion years, and 235U (Uranium 235) has a half-life of about 700 million years. Through the use of radiometric dating, scientists can study the age of fossils or other remains of extinct organisms to understand how organisms have evolved from earlier species.– Can we determine the age of a dead material that is more than 50,000 years old using carbon dating?
Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e