Humans are taking carbon out of the ground by burning fossil fuels deposited millions of years ago and putting it into the atmosphere as carbon dioxide.
Organic carbon in the form of dead plants, algae and animals is mostly eaten by other creatures, mainly bacteria, in both the ocean and in forest soils.
Seashells, which are made of calcium carbonate, sequester significant amounts of carbon.
This is about the same as emissions from burning fossil fuels on the continent today, within errors of calculation.
The Earth stores carbon dioxide when rocks are altered at the surface and as seashells accumulate on the seabed.
About 4-8 million years ago, the climate was warmer, carbon dioxide levels were similar or even higher than today, and the ocean was more acidic.
Rivers and the wind deliver nutrients to the ocean, especially during extreme weather events, and changes can occur over short time scales.
Our work highlights and quantifies the important role the ocean, and particularly the microscopic life within it, will eventually play in restoring balance to our planet.