The top number is the Mass Number for each isotope.The Mass Number for any isotope is the addition of all the protons and neutrons in the nucleus.To the left side of each C (C is the symbol for Carbon) are two numbers, the bottom number indicates the Atomic Number or the number of protons in the nucleus.
Three of the Carbon isotopes (C) are found in nature.
The rest of the carbon isotopes are only of laboratory interest.
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In another very important paper, scientists from the RATE group summarized the pertinent facts and presented further experimental data. This effectively limits the age of all buried biota to less than (at most) 250,000 years.