The Other Isotope of Carbon
Carbon-14 is the famous isotope of carbon. It’s the one used for identifying the age of long dead things via the technique called radio-carbon dating. Less well known is the other isotope, carbon-13. Like “regular” carbon, carbon-13 has a stable nucleus and does not decay, writes Nick Lane in Oxygen . But carbon-13 too has found to be of use in identifying the age of something: the age of life itself, i.e., when life started on earth. Here’s that story. Since neither carbon-12 nor carbon-13 decays, their ratio on or in the earth has remained constant at 99.89 to 1.11. Lane calls it the “unadulterated background ratio”. The process that can change that ratio (even by small amounts) are living things, “and so far as we know, only by living things”. Let’s see how that happens. Take photosynthesis. It prefers to use carbon-12 over carbon-13. Why? Because the lighter isotope needs (slightly) less energy to set off the reaction involved. Which leads to the following c...