- All living things contain carbon, and when a dead organism lays on a rock, an extremely thin layer of carbon is deposited onto the rock over time. As the hydrogen, oxygen and nitrogen in the organism's body disappear -- usually being dissolved and vaporized under a body of water -- the only remaining material is this layer of carbon. This decaying process is called carbonization or distillation.
- In contrast to imprint fossils, which can be used to create a three-dimensional cast that is a copy of the organism's true shape, a carbon film fossil appears as a two-dimensional image imprinted delicately into rock. They are usually black or brown, standing out in contrast to the color of the rock. Carbon film fossils are therefore not as "flashy" or prominent as fossils formed by other methods, but they can sometimes demonstrate intricate surface detail.
- Because carbon films are usually left by specimens preserved under a body of water, the most common fossils are of fish, crustaceans, and leaves. These specimens probably sank and adhered to rock under bodies of slow-moving water where they were allowed to settle rather than being ripped or crushed by a current. In the case of leaves, internal components of the leaf such as cell walls and internal cell structures are usually lost, but cells are sometimes filled with mineral-rich water that solidifies to preserve these miniscule features.
Dating Fossils Using Carbon
- Carbon-14 is a radioactive particle formed when cosmic radiation from the sun collides with an atom to release a neutron, which is then added to a nitrogen atom to create carbon-14. This atom is absorbed by plants from the air and eaten by animals in the form of plant material. When an animal dies, carbon-14 atoms continue to decay at a predictable rate. Scientists know that the half-life of Carbon-14 -- the time it takes for the atoms to be reduced by half -- is about 5,700 years. Scientists measure the quantity of carbon-14 isotopes to determined how much time the organism has had to decay and approximate its age.