A Moment of Science

Estimating How Much Carbon Dioxide Was Available To Plankton

Scientists have reconstructed a one hundred thousand-year history of carbon dioxide.

plankton swimming

Photo: idua_japan (flickr)

Plankton swimming in the ocean.

Carbon moves in a cycle starting with carbon dioxide in the air. Plankton in the ocean use the sun’s energy to turn the carbon dioxide into complex organic molecules through the process of photosynthesis.

When the energy stored in those molecules is used, the carbon is released again as carbon dioxide.


Most of the world’s carbon is carbon-12. The twelve refers to the weight of the atom. But a small percentage is carbon-13, which has one more neutron making it a little heavier.

Either form of carbon can go into carbon dioxide, but photosynthesis occurs more readily with carbon dioxide made from the lighter carbon-12.

So, when there’s plenty of carbon dioxide, plankton use relatively little carbon-13. But when the overall level of carbon dioxide is low, they use what’s available, which means using more of the heavier carbon-13.

Carbon Dioxide And Plankton

By measuring the ratio of carbon-12 to carbon-13 in fossilized plankton at the bottom of the ocean, scientists have been able to estimate how much carbon dioxide was available when the plankton were alive.

Imagine being given a choice between apples and oranges. If you prefer oranges and there’s an unlimited supply of both, you’ll eat mostly oranges. But if food becomes scarce, you may start eating more apples than before.

Of course, plankton don’t make a conscious choice, but the ratio of carbon-12 to carbon-13 in their fossils helps indicate how much carbon dioxide was available at the time, just as the ratio of apples to oranges in your diet would indicate how much fruit was available.

Gathering A History

Using this information, these scientists have reconstructed a one hundred thousand-year history of carbon dioxide. Knowing this history could help us understand the role of this gas in our changing climate.

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