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 consequence:

“The faster rate at which carbon-12 bonds are cracked means that organic matter becomes enriched in carbon-12 relative to carbon-13.”

In turn, that means:

“When the remains of plants, algae or cyanobacteria are buried in sediments, their extra carbon-12 is buried with them.”

 

Now flip that point around: if organic matter took some excess carbon-12 to its grave (literally), and the ratio of carbon-12 to carbon-13 is constant, it means:

“More carbon-13 is left behind as carbonates in the oceans or rocks, or as carbon dioxide in the air.”

That’s just maths: If there’s more of one isotope in one part (organic matter), then there has to be more of the other isotope in another part (inorganic matter).

 

Organic matter doesn’t survive for too long, certainly not on the scale of billions of years. That means organic remains can’t be found or used to detect how long back life may have started on earth. But rocks do survive for long periods. The oldest sedimentary rocks on Earth have been reliably dated to an age of 3.85 billion years old. And those rocks were found to have a 2-3% higher concentration of carbon-13 than the “unadulterated background ratio”.

 

Combine that with everything you read in this blog, and you understand why scientists came to the conclusion that “life existed on earth more than 3.85 billion years ago”. (Of course, there’s other data from other sources too which aligns with this date).

 

The ingenuity of the thought process behind all this is very impressive indeed. It also shows that life, no matter how primitive or complex, from its very beginning, always changed the environment. (This isn’t a defence for the damage we humans are inflicting on the environment today; it’s just an observation).

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