And so it's kind of a record of the atmosphere up to 10,000 years. Those are the speleothems that are coming from the top of the cave.If you want to go even further back, you can look at cave deposits, and the fancy word for these cave deposits are speleothems. But the reason why these are useful is these are formed by calcium carbonate, so they have carbon in them, and slowly over, really, tens of thousands of years, the water in the cave deposits that calcium carbonate.
(This 5,730-year period is called the half-life of radiocarbon, Figure 1).1 2 At this decay rate, hardly any carbon-14 atoms will remain after only 57,300 years (or ten half-lives).
The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. This makes it possible to tell the age of substances that contain carbon. Dates obtained are usually written as before present ('present' is 1950).
Atmospheric nuclear weapon tests almost doubled the concentration of Radiocarbon dating, also known as the C14 dating method, is a way of telling how old something is. Plants take up atmospheric carbon dioxide by photosynthesis, and are eaten by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality.
Most laboratories measure radiocarbon with a very sophisticated instrument called an accelerator mass spectrometer, or AMS.
It is literally able to count carbon-14 atoms one at a time.3 This machine can theoretically detect one radioactive carbon-14 atom in 100 quadrillion regular carbon-12 atoms! AMS instruments need to be checked occasionally, to make sure they aren’t also “reading” any laboratory contamination, called background.