One of the most common dating techniques uses carbon’s radioactive isotope: radiocarbon. If the time scale you are interested in is less than ~50,000 years, and your sample contains carbon, we have the technique for you. How does it work?
There are three natural isotopes of carbon, carbon-12 (12C), carbon-13 (13C) and carbon-14 (14C). 12C accounts for ~99.8 %, 13C accounts for ~1% and ~1 in every 1 billion is 14C. 14C is radioactive and has a half-life of 5730 years. This is the time that it takes for the amount of 14C in a sample to halve, and due to this constant decay we have a “clock” that can measure the elapsed time. A unique characteristic of 14C is that it is constantly formed in the upper atmosphere. Cosmic rays bombard nitrogen-14 atoms, and when a proton is knocked out 14C is formed. These newly formed 14C atoms oxidize to form 14CO2 and are rapidly mixed in the atmosphere, becoming part of the earth’s carbon cycle. Photosynthesis incorporates 14C into plants, and the animals that eat the plants (yes including us!). 14C gets into the dissolved inorganic carbon pool in the oceans, lakes and rivers where it is incorporated into shells, corals and other marine organisms. When a plant or animal dies it no longer exchanges CO2 with the atmosphere or surrounding water, effectively being cut off from incorporating any more 14C. This starts the radioactive decay “clock”. We then measure the amount of 14C left in the sample using an Accelerator Mass Spectrometer. We happen to have one downstairs, which is quite handy…….
Another application of radiocarbon dating in the more recent past uses the “bomb curve”. The figure below depicts the pulse in atmospheric 14C produced from above ground weapons testing that began in the 1950’s, and its decay since the implementation of the Test Ban Treaty in 1963. From these very distinct markers, the inflection, the peak and then our date of collection we are able to pinpoint where in time we are in our archive. We have used this technique to be the first team to age lungfish and also to date our deep-sea coral archives. Rather ironic that the fall out from nuclear weapons testing would be used in conservation and climate studies, don’t you think?
For more information, visit the RSES website.