EarthLabs > Climate and the Carbon Cycle: Unit Overview > Lab 6: Oceans: Carbon Sink or Source? > 6A: The Ocean Carbon Cycle

The Oceans: Carbon Sink or Source?

Part A: The Ocean Carbon Cycle

Oceans have a large capacity to absorb CO2, thus reducing the amount of CO2in the atmosphere and bringing carbon atoms into the ocean system. Many CO2 molecules that diffuse into ocean sea surface waters diffuse back to the atmosphere in very short time scales. However, the carbon atoms from these original CO2molecules stay in the ocean for time scales of hundreds to thousands of years. If some carbon atoms eventually make it to the bottom of the ocean sediment, they can be stored for time scales of millions of years.

In this Lab, you will learn about the capacity of the ocean carbon cycle to absorb, transport, transform and store carbon. Important questions you will investigate will be:

Lets begin by looking at the detailed illustration of the ocean carbon cycle on the right. Although quite complex, you will see similar carbon cycle components you learned about when you studied the terrestrial carbon cycle in prior Labs. Take a few moments to familiarize yourself with the ocean carbon cycle illustration and answer the discussion questions below.


  • What processes do you see that are the same in the terrestrial carbon cycle?
  • What organisms bring CO2 into the ocean carbon cycle? Using what process? What zone are they located in - euphotic zone, twilight zone, deep ocean or benthic zone? Why do you think these organisms are located only in this one zone?
  • What organisms and processes are key in bringing carbon down to the deep ocean?
  • What processes release CO2 top the atmosphere?
  • Nitrogen(N), phosphorus(P) and iron(Fe) are key elements that are critical to life in the ocean. Identify and list the sources of the elements.

How does carbon get into the ocean carbon cycle? Air-Sea Surface Exchange of CO2

First, CO2 gas enters the ocean by diffusing into the sea surface waters and dissolving - a physio-chemical process. The amount of CO2 that diffuses and dissolves in the sea surface water depends on variables such as wind, sea surface mixing, concentrations of CO2, and the temperature of the water.

The image on the right represents the movement (flux) of CO2 into and out of the sea surface of the ocean.

Take a few minutes to closely examine the image before answering the Checking-In questions below:

Checking In

  1. Which areas of the ocean are absorbing more CO2 from the air? Check all that apply
  2. Which general areas of the oceans have low amounts of CO2 diffusing into sea surface waters? Check all that apply.
  3. True or False. Check the statements below that are true.

Once dissolved in surface seawater, CO2 can enter into the ocean carbon cycle through three different mechanisms:

The Physical Carbon Pump: Downwelling and Upwelling Currents

Downwelling currents occur in areas where cold, denser water sinks. These downwelling currents bring dissolved CO2 down to the deep ocean. Once there, the CO2 moves into slow-moving deep ocean currents where it stays for hundreds of years. As a matter of fact, some of this CO2 in these deep moving currents has been there before the industrial revolution began in the 1800s!

Eventually, these deep ocean currents return to the surface in a process called upwelling. Many upwelling currents occur along coastlines. When upwelling currents bring deep, cold ocean water to the surface, the water warms and some of the dissolved CO2 is released back to the atmosphere. Downwelling and upwelling currents are important components of the deep ocean conveyor belt.

Examine the image on the right of the "Ocean's Conveyor Belt of Deep Ocean and Surface Currents" and then watch a NASA video that animates ocean currents. As you watch the video, visualize carbon compounds moving along with these currents.


  • Re-examine the Ocean CO2 flux map and compare it to the Deep Ocean Conveyor Belt Map. What patterns if any, do you see that are the same in both maps.
  • Explain how a CO2 molecule that diffuses into the ocean in the North Atlantic ocean could eventually diffuse into the atmosphere off the eastern coast of Africa hundreds of years later.

Stop and Think

1: Describe the physical pump's role in enabling the ocean to be a carbon sink.

Phytoplankton and the Oceanic Biological Carbon Pump: Tiny Organisms, Big Role!

The oceanic biological carbon pump is driven by organisms that live in the ocean. Just like the terrestrial carbon cycle, the oceanic biological carbon pump is all about photosynthesizing, respiring, eating, producing waste products, dying and decomposing.

Phytoplankton (Greek for drifting plants)are microscopic, one-celled organisms that drift in the sunlit surface areas of the world's oceans and are key to bringing carbon down into the ocean biological pump from the atmosphere via the process of photosynthesis.

Just like land plants, phytoplankton use chlorophyll and other photosynthetic pigments to capture Sun's energy for photosynthesis. Using light energy from the Sun, carbon dioxide, and important ocean nutrients such as nitrogen, phosphorus, iron and vitamin B, they convert the carbon dioxide and water into sugars and other carbon compounds. These carbon compounds enter the marine food web and some carbon eventually ends up in deep ocean currents and seafloor sediments. Phytoplankton return CO2 and O2 to the atmosphere when they respire. Over 50% of the world's oxygen needed by us to breathe is produced by phytoplankton.

Checking In

Look at the simplified image of the oceanic biological carbon pump above right.
  1. Draw a carbon pathway with the shortest time scale between two reservoirs.
  2. Draw a carbon pathway that would move carbon atoms from the atmosphere to a place where they would be stored for millions of years.

The Secret Life of Plankton

Food webs and marine snow. -

Phytoplankton are responsible for bringing carbon into the ocean food web. Once in the food web, eating, producing waste products, dying and decomposing are important food web processes that move carbon into deeper zones in the ocean. Ocean water teems with small plankton as seen in this TedEd video, "The Secret Life of Plankton." When plankton and larger marine organisms defecate, die and decompose, they produce sinking carbon-containing particles scientists call 'marine snow'.

Getting carbon into the ocean is one matter - getting it down to the deep ocean is another!

About 50 Gt (50 billion metric tons) of carbon is drawn down into the biological pump per year but only a small fraction of this carbon makes its way down into the deep ocean. (2007 IPCC Report).

To answer these questions, you will use the interactive below developed by Woods Hole Oceanographic Institute(WHOI) and then watch a video on the ocean's microbial loop.

Begin by clicking on the sequence of tabs in the interactive to follow the carbon as it moves from phytoplankton to the depths of the ocean. As you move through the WHOI interactive, pay careful attention to the role of the microbes in moving carbon to the deep ocean. (WHOI)

This content is available in flash format only

To view this interactive on an iPad, use this link to download/open the free TERC EarthLabs App.

Next, watch The Ocean's Microbial loop and read a short article with video on marine snow.


1. As the carbon moves down through the biological pump, less and less carbon actually makes it down into the deep ocean. How do microbes and zooplankton reduce the amount of carbon that eventually sinks to the ocean bottom?

2. Why are particles such as marine snow so important in bringing carbon down into the twilight and deep ocean zones?

The Ocean Carbonate System and Shell-building Organisms - Carbon Transport Down to the Deep!

The ocean naturally contains many dissolved chemicals, some of which are especially important to the ocean carbon cycle and the shell-building organisms that live in the oceans.

When CO2 dissolves in the ocean, it combines with water molecules and then enters into a sequence of chemical reactions that eventually produces carbonate (CO3-) ions. Carbonate ions (CO3-) combine with calcium ions (Ca+) in the seawater to form calcium carbonate (CaCO3). Shell builders use the calcium carbonate molecules to build their shells. Watch this process in the Ocean Chemistry video below. Note: A disruption of this ocean carbonate chemical system could result in a change in ocean pH, making the ocean more acidic. You will learn more about this in Lab 7: Ocean Acidification.

Checking In

  1. Shell-builders use calcium carbonate molecules (CaCO3) to build their shells. Which of the following could be a source for the carbon atoms in the calcium carbonate (CaCO3) molecules? Check all that apply.

Shell-building organisms such as coral, oysters, lobsters, pteropods, sea urchins, and some species of phytoplankton use calcium carbonate ions to build their shells, plates and inner skeletons. The carbon that is incorporated into the shells of these organisms can end up in deep ocean sediments as these organisms die and sink. Although some of the shells dissolve before reaching the seafloor sediments, shells slowly build up on the sea floor storing the carbon for millions of years.

Coccolithophores perhaps have had the biggest impact on the carbon cycle over time. These microscopic phytoplankton remove carbonate ions and calcium ions from seawater to build their calcium carbonate (CaCO3) plates - called coccoliths. When there is plenty of sunlight and nutrients, these tiny phytoplankton will explosively reproduce, producing a "bloom" of trillions of coccolithophores floating in the surface of the ocean. Satellites can see these blooms from space, such as the very large bloom above that appeared off the coast of Norway in the Barents Sea. In just a few days, coccolithophores gobble up the available nutrients and start to die. The trillions of coccolith plates slowly sink down to the bottom of the ocean taking the calcium carbonate in their plates with them.

The calcium carbonate in these sediments are actually chalk - the same type of chalk used to write on blackboards. Sometimes slow tectonic movements will force these chalky sediments above sea level. The White Cliffs of Dover are in England are perhaps the best known example of this. To find out more, read about Coccolithophores and White Cliffs of Dover and then play the video "Chalk Dust."

Checking In

  1. Name all of the places in the ocean that you would find carbon. Check all that apply.


Stop and Think

2. If phytoplankton populations decrease, you might expect:

A. the amount of CO2 in the atmosphere to decrease

B. The amount of CO2 in the atmosphere to increase

Explain why you chose your answer.

3. Many mountain tops contain fossils of shelled creatures that once lived in the oceans. Which of the Earth's spheres could the carbon have traveled through on its journey to these mountain tops?

A. Geosphere

B. Geosphere and biosphere

C. Geosphere, biosphere, and hydrosphere

D. Geosphere, biosphere, hydrosphere and atmosphere

4. What is the role of phytoplankton in the biological carbon pump?

5. How are marine phytoplankton and forests similar in their role in the carbon cycle?

Optional Extensions:

Read about new research on the ocean carbon cycle:

Uncovering the Oceans Biological Pump

Read about coccolithophores and their importance to the biological pump. What is a Coccolithophore? Fact Sheet : Feature Articles

If interested, you can find more videos on plankton at The Plankton Chronicles.

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