"C14"

The basic science behind radiocarbon dating is straightforward. The element carbon exists naturally on Earth in two stable forms, or isotopes, known as carbon-12 and carbon-13. The nucleus of the C-13 atom contains one extra sub-atomic particle called a neutron. A third isotope of carbon with two extra neutrons, and is known as carbon-14. It is produced in the atmosphere when atoms of nitrogen are blasted by high-energy cosmic rays streaming in from space. The atom is unstable — radioactive — and eventually decays back into nitrogen-14. Because some of the radioactive atom always exists at a low level in the atmosphere and on Earth, it becomes incorporated into the cells and tissues of all living things, from bacteria to plants to people.

After things die, they no longer take in C-14, and so as the isotope decays back to nitrogen-14 and its abundance steadily decreases over time. Measuring the amount of C-14 in something originally derived from organic material, then tells the amount of time that has passed since the organism it came from was last alive.

However, this means that any contamination of an artifact with a material that was more recently alive will raise the relative abundance of C-14 atoms in the artifact, yielding an apparently younger age. In the case of the Shroud of Turin, some say the younger contaminants were bacteria. Over many hundreds of years  the Shroud has come in contact with thousands of species of bacteria and fungi and some were able to grow for periods of time. Some of these organisms would be more recent and be incorporating more recent radiocarbon material. As long as the organisms grew and incorporated carbon dioxide, they could actually make the Shroud appear more recent in origin.

Scientists have found several different species of bacteria colonizing pieces of the Shroud, including some organisms that had never been seen before. Testing of samples from an outside strip of the cloth removed during the 1988 dating effort showed that one of the microbe species seemed to be producing an unusual material that coats the linen threads “with a brittle plastic-like material that made the linen difficult to cut, much like trying to cut through dry pasta.”  This polymer may have actually helped preserve the Shroud linen through time.