Of all the carbon stores on Earth, one dwarfs the rest. The ocean holds roughly 38,000 gigatonnes of carbon, most of it dissolved as inorganic carbon in the water itself. By comparison, soil and sediment hold around 4,000 gigatonnes, the atmosphere around 900, and all living things put together around 550. The ocean store is, very roughly, ten times larger than soil and sediment, and forty times larger than the atmosphere.
But the ocean isn't just a store. It is a carbon engine: constantly moving carbon between the surface, the water column, and the seabed, through three distinct sets of processes scientists call "pumps."
The interactive diagram below shows all three pumps at work in a single coastal scene, colour-coded so you can trace each one through the water column.
The physical pump moves carbon through the mechanics of water and gravity. Carbon-rich freshwater runs off the land into the sea. At the surface, carbon dioxide exchanges directly between the atmosphere and the ocean, dissolving into the water. Currents and turbulence mix that dissolved carbon down through the water column.
At high latitudes, cold water absorbs more CO2 and becomes denser, so it sinks, carrying dissolved carbon down into the deep ocean, where it can remain for centuries. The same physical logic applies to solid matter: the bodies and faeces of marine animals simply fall, carrying carbon down with them under gravity alone.
The biological pump moves carbon through the activity of living things. Phytoplankton near the surface photosynthesise, drawing dissolved CO2 out of the water and locking it into their cells. Zooplankton feed on phytoplankton, transferring that carbon up the food web. All of these organisms also respire, releasing some carbon back into the water as CO2.
This pump is fast and biologically driven. It operates on the timescale of feeding, growth, and reproduction, recycling carbon through the living community of the water column before any of it has a chance to sink.
The chemical pump operates on a far longer timescale than the other two. When organic material from the water column reaches the seabed and is buried rather than decomposed, the carbon it contains can be locked away in sediment. Over millions of years, and under the right conditions, this buried organic matter is gradually transformed by heat and pressure into fossil fuels and sedimentary rock.
This is the slow carbon cycle: carbon that has effectively left circulation, removed from the ocean-atmosphere system for geological timescales rather than years or centuries. It is also, notably, the same process that produced the fossil fuels now being extracted and burned, returning ancient, safely stored carbon to the atmosphere within a human lifetime.
Together, these three pumps explain why the ocean is not just the planet's largest carbon store, but one of its most active. Disrupting any one of them, through warming, acidification, or seabed disturbance, has consequences for how much carbon stays out of the atmosphere.