Biological Carbon Pump: How the Ocean Stores CO2

Key Takeaways

Phytoplankton in sunlit surface waters absorb dissolved carbon dioxide as they photosynthesize and grow, which lets the surface ocean draw down more CO₂ from the atmosphere.
Carbon captured by phytoplankton passes up the food web to zooplankton and larger animals, and a portion of it sinks toward the deep sea.
The fraction of carbon that sinks deep enough can stay sequestered for hundreds to thousands of years, keeping it out of the atmosphere.

The biological carbon pump is the living ocean’s way of moving carbon from sunlit surface waters into the deep sea, where it can stay locked away for hundreds to thousands of years, helping regulate the amount of carbon dioxide in the atmosphere.

What is the biological carbon pump?

When sunlight hits the ocean’s surface waters, it stimulates tiny marine plants called phytoplankton to photosynthesize. This process removes carbon dioxide dissolved in the water as phytoplankton incorporate the carbon as they grow. As carbon dioxide levels in surface waters decrease, water is then able to absorb more carbon dioxide from the atmosphere. Small marine animals called zooplankton feed on phytoplankton and are, in turn, eaten by larger marine organisms. Although much of the carbon taken up by phytoplankton is recycled in the upper layers of ocean, the remaining portion sinks, eventually reaching depths at which the carbon will remain sequestered, or removed, for hundreds to thousands of years. Scientists generally consider carbon to be sequestered once it reaches a depth of 500 meters (1,640 feet). At this point, it is unlikely to return to the atmosphere for hundreds or years or more. The ocean’s so-called biological carbon pump removes carbon from the atmosphere and stores it deep in the ocean on timescales that are important to the lifespan of humans. The solubility carbon pump, which stores much larger amounts of carbon, operates on timescales in the thousands of years and is a much slower mixing process.

How does the biological carbon pump export carbon to the deep ocean?

Once incorporated into an organism’s body, most carbon that reaches the deep sea falls as marine snow: tiny pieces of carbon-rich material that sink under the force of gravity. These organic scraps typically result from messy feeding, release of fecal pellets, dead organisms, or shedding of scales and other tissues. The rate of sinking varies depending on the size and density of the material. Small organisms, such as dead phytoplankton or zooplankton, might take as long as two weeks, or even months to reach the sea floor. Larger, denser organisms, such as fish or marine mammals, will sink much more quickly when they die. Jelly-like salps seem to play a particularly important role in the rapid export of carbon. They feed on phytoplankton and subsequently release dense fecal pellets that can reach the sea floor in a matter of days.

Diagram comparing sinking rates of five marine snow types, from fish poop at 50 m/day to a quartz sand grain at 5000 m/day.

Denser material sinks faster. Loose fish waste drifts down at about 50 meters a day, taking 10 weeks to reach the seafloor, while compact salp pellets and quartz sand grains plunge more than 1,000 meters a day—reaching the average ocean depth of 3,500 meters in under four days. (Illustration by Charin Park, © Woods Hole Oceanographic Institution)

How does the world’s largest animal migration affect the biological carbon pump?

Every evening in the ocean, animals that spend their days in the deep, dark waters of the ocean’s twilight zone swim to the surface to feed. The twilight zone, also known as the ocean’s midwater, reaches up to 1,000 meters (3,280 feet) deep. Rising in the dark after sunset, these animals feast on phytoplankton, zooplankton, and other surface-dwelling organisms throughout the night, then return to depth as light returns at dawn. Called the diel vertical migration, this movement plays an important role in the biological carbon pump. By feeding at the surface before returning to deeper waters, these animals actively carry carbon deeper into the water column. Although some carbon is recycled within the twilight zone as animals and bacteria feed, any dead animals, discarded tissues, or fecal matter that are not consumed sink to the deep ocean, taking their carbon with them.

What role does the pump play in helping mitigate the climate crisis?

The biological carbon pump plays a huge role in the ocean’s ability to remove carbon dioxide from the atmosphere. Without it, the amount of carbon dioxide added to the atmosphere would be twice as large as what humans have already added. This would return global climate to a state not seen in 50 million years—a time with no ice, high sea levels, and warmer temperatures than today. The global ocean absorbs about one-quarter of the carbon dioxide released into the atmosphere. Through the biological carbon pump and other mechanisms, some of that carbon is then sequestered in the deep ocean for hundreds to thousands of years.

Why should we study and protect the biological carbon pump?

Understanding how the biological carbon pump works to export carbon to the deep sea can help researchers improve models of the ocean’s role in climate. The ocean’s ability to absorb carbon dioxide varies over time and space and is predicted to decline over the rest of this century. A more detailed understanding of the pump’s ability to remove carbon will improve climate models and the ability to forecast the potential impacts of global heating. This could allow cities to selectively develop areas that are less likely to flood in the future or to better invest in flood defense projects. Such improvements in predictive modeling could save up to $500 billion worldwide by reducing damage to property and infrastructure through better planning. The twilight zone is estimated to contain about 15 billion metric tons of fish. Despite how far offshore and difficult to reach the twilight zone is, recent technology innovations have begun to make it a more attractive location for commercial fisheries. Although there are currently no active commercial twilight zone fisheries, it is imperative that we understand the potential impact fishing could have before the twilight zone is irreparably changed. We need to understand the impact such activities would have not only on the ecosystem, but also on the biological carbon pump and its ability to help us fight the climate crisis.