Question

The basis of C-14 dating method is that the:

• A. C-14 fraction is the same in all objects
• B. C-14 is highly unstable and is readily lost from the objects
• C. The ratio of C-14 and C-12 in our atmosphere has always been constant.
• D. Living tissue will not absorb C-14 but will absorb C-12 from its source of carbon

Answer 1

The correct option is C The ratio of C-14 and C-12 in our atmosphere has always been constant.

Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
Cosmic rays – high energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14. Carbon-14 is considered a radioactive isotope of carbon. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes. Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.
Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere. They have the same ratio of carbon-14 to carbon-12 as the atmosphere, and this same ratio is then carried up the food chain all the way to apex predators, like sharks.
But when gas exchange is stopped, be it in a particular part of the body like in deposits on bones and teeth, or when the entire organism dies, the ratio of carbon-14 to carbon-12 begins to decrease. The unstable carbon-14 gradually decays to carbon-12 at a steady rate.
And that’s the key to radiocarbon dating. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive.