The ocean and the carbon cycle

The ocean plays an important role in the global carbon cycle. Carbon moves in and out of the ocean daily, but it is also stored there for thousands of years.

The carbon cycle

Carbon is a chemical element vital to all living things and is present in many non-living things. Carbon circulates around the planet in what is called the carbon cycle. Parts of the cycle store carbon for different lengths of time, and there are many processes that move carbon in and out of the stores.

Find out more about the carbon cycle in the interactive.

Carbon moves through the carbon cycle in stops and starts. An individual atom might pass through plants, animals and the atmosphere in a matter of days, yet stay trapped in rocks for millions of years.

Plants exchange carbon with the atmosphere every day as they make and use carbohydrates for energy, but plants also store carbon because they use it to build their structures – some trees store carbon for hundreds of years. This carbon only moves on again if the tree dies and decays or is burnt. Carbon can be stored for much longer periods too – for thousands of years in the ocean or for millions of years in rocks.

Changes to the carbon cycle

Systems do not stay the same – when one part is changed, other parts respond and change to restore a balance. Over the last 200 years or so, there has been a detectable change in the carbon cycle. Humans have been burning carbon-rich fossil fuels, releasing carbon dioxide into the atmosphere. This has been releasing carbon that would normally be stored in the geosphere. Even the carbon dioxide put into the atmosphere by all the volcanoes that erupt in a year is only a hundredth of that released by human activity.

The ocean removes about half the extra carbon dioxide from the atmosphere and increased plant growth removes more through photosynthesis, but the level of carbon dioxide in the atmosphere is still rising. Carbon dioxide is one of the greenhouse gases that stops heat escaping from the planet. More carbon dioxide in the atmosphere, means more heat trapped, resulting in an increase in temperature – this is contributing to climate change.

The ocean and carbon

The ocean plays an important part in the carbon cycle. Overall, the ocean is called a carbon ‘sink’ because it takes up more carbon from the atmosphere than it gives up.

Carbon dioxide from the atmosphere dissolves in the surface waters of the ocean. Some of the carbon dioxide stays as dissolved gas, but much of it gets turned into other things. Photosynthesis by tiny marine plants (phytoplankton) in the sunlit surface waters turns the carbon into organic matter. Many organisms use carbon to make calcium carbonate, a building material of shells and skeletons. Other chemical processes create calcium carbonate in the water. The using up of carbon by biological and chemical processes allows more carbon dioxide to enter the water from the atmosphere.

Biological pump

Living things in the ocean move carbon from the atmosphere into surface waters then down into the deeper ocean and eventually into rocks. This action of organisms moving carbon in one direction is often called a biological pump.

Carbon gets incorporated into marine organisms as organic matter or structural calcium carbonate. When organisms die, their dead cells, shells and other parts sink into deep water. Decay releases carbon dioxide into this deep water. Look at the carbon cycle interactive to see how much carbon is in the deep ocean compared to other stores. Some material sinks right to the bottom, where it forms layers of carbon-rich sediments. Over millions of years, chemical and physical processes may turn these sediments into rocks. This part of the carbon cycle can lock up carbon for millions of years.