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

Carbon enters and moves throughout the oceans by both a physical process and a biological process. Scientists call these processes the physical carbon pump and the biological carbon pump because they transport carbon down into the deep ocean and sea floor sediments for long term storage.

First, carbon dioxide gas enters the ocean by dissolving in the sea surface waters. The amount of CO2 that dissolves in the sea surface depends on variables such as wind, sea surface mixing, and temperature of the water. The colder the sea surface water, the more CO2 will dissolve. This is why more carbon dioxide will dissolve in the sea surface at the higher, colder latitudes. In warmer oceans, less CO2 will dissolve in the sea surface water.

Examine the image, right, of the ocean carbon cycle. As you examine the image, make note of the following:

Remember that one gigaton (Gt) = 1 billion metric tons. 1 metric ton = 2,200 lbs.

  • The white arrows represent gigatons of carbon moving into and out of the ocean per year.
  • The red arrow represent gigatons of carbon that have come from human carbon dioxide emissions.
  • The numbers in ( ) represent gigatons of carbon stored in a reservoir.
  • Net ocean uptake indicates that the amount of carbon in the ocean is increasing by 2 gigatons per year.
  • The Atmospheric Carbon Net Annual Increase indicates that the amount of carbon in the atmosphere is increasing by 4 gigatons per year
  • Reactive sediments refer to layers of sediment at the bottom of the ocean


Checking In

  1. What process transfers carbon from the atmosphere into the ocean? Check all that apply.
    [INCORRECT]
    [INCORRECT]
    [CORRECT]
    [CORRECT]
  2. What biosphere processes are the same as in the terrestrial carbon cycle? Check all that apply.
    [CORRECT]
    [CORRECT]
    [CORRECT]
    [INCORRECT]
  3. Which reservoir stores the most carbon?
    [INCORRECT]
    [CORRECT]
    [INCORRECT]
  4.   

What happens to carbon dioxide once it dissolves in the ocean?

Some carbon dioxide stays as a dissolved gas in the ocean and may be transported down to deep ocean. If the water stays at the surface and warms up as it moves around the globe, the carbon dioxide will "undissolve" and move back into the atmosphere. However, if the water sinks to the deep ocean (downwelling), the carbon goes with it and can be stored for hundreds of years in slow moving deep ocean currents. Eventually, these deep ocean currents return to the surface along coastlines in a process of upwelling. Once reaching the surface, some carbon can be released back to the atmosphere. This process is what scientists call the "physical carbon pump."

The ocean's conveyor belt is critical to the physical carbon pump. Examine the image below of the "Ocean's Conveyor Belt of Deep Ocean and Surface Currents" and then watch a NASA animation Notice that the white arrows represent surface currents and the blue arrows represent deep ocean currents.

Checking In

  • Locate the largest ocean carbon sinks on the map. Why do you think they are located there?
    The largest carbon sinks are located in the Northern Atlantic and off Antarctica for two reasons:

    The sinks are located in higher latitudes because cold water is denser and sinks. Because large amounts of carbon dioxide dissolve in cold waters, the carbon dioxide is brought down to deep ocean currents as the denser, colder water sinks.

  • What happens to the "old" carbon dioxide dissolved in deep ocean water that eventually rises to the surface?
    Some of the carbon dioxide can be released into the atmosphere and some will stay dissolved in the ocean.

Stop and Think

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


The Biological Carbon Pump: Small Organisms, Big Effect!

Compared to the physical pump, the biological carbon pump plays a much bigger role in making the oceans a strong carbon sink. In the biological carbon pump, most carbon dioxide is chemically transformed by marine organisms into other carbon compounds which can then be recycled and transported to different parts of the ocean or buried in sea floor sediments.

Examine the image below of the biological and physical pump below and compare the physical pump on the right with the biological pump on the left.

Checking In

  1. What of the Earth's four spheres are involved in the physical carbon pump? Check all that apply.
    [CORRECT]
    [INCORRECT]
    [CORRECT]
    [INCORRECT]
  2. What of the Earth's four spheres are involved in the biological carbon pump? Check all that apply.
    [CORRECT]
    [CORRECT]
    [CORRECT]
    [CORRECT]
  3. What type of organisms in the biological pump move carbon from the atmosphere into the ocean?
    [INCORRECT]
    [INCORRECT]
    [INCORRECT]
    [CORRECT]
      

Phytoplankton-the key driver of the ocean's biological pump.

If you live near a pond, lake or ocean and you have seen green scum on the water's surface, you are most likely looking at algae - just one of the many different types of phytoplankton that exist on Earth. Like land plants, phytoplankton contain chlorophyll and other photosynthetic pigments they use to capture sunlight's energy needed to power photosynthesis.

Phytoplankton are mostly microscopic and uni-cellular and come in many shapes and sizes - from extremely small photosynthetic cyanobacteria to larger eukaryotic protists such as the different types of algae in the image on the right. Some phytoplankton, such as Diatoms and Coccolithophores take chemicals out of sea water to make hard outer or inner shells. Coccolithophores are very important to the carbon cycle because they make their hard outer plates from calcium carbonate.

All phytoplankton photosynthesize. They can obtain CO2from the air overlying the surface of sea water and from dissolved CO2 in the water. Using energy from the Sun, carbon dioxide, and important ocean nutrients such as nitrogen, phosphorus and iron, they convert the carbon dioxide and water into sugars and other carbon compounds. These carbon compounds enter the marine food web and eventually find their way into deep ocean currents and seafloor sediments. To help you visualize this process, view the PowerPoint on the ocean's biological pump below. You can click on the image to watch it in slideshare mode or download it by clicking on the download link.

Ocean Biological Pump
Click to view

download link
When you view the power point, make note of the following:


Don't forget to answer the two Stop and Think questions embedded in the PowerPoint. You will also find these questions at the end of this lab. When you are done with the PowerPoint, watch this short animation Ocean Biological Pump.

If you have the time and the interest, watch this TedEd video The Secret Life of Plankton or directly on YouTube YouTube- The Secret Life of Plankton. In this beautiful video, you will see many examples of phytoplankton and zooplankton. Remember that in marine food webs, phytoplankton photosynthesize and zooplankton eat phytoplankton, and each other.

Checking In

  1. Name all of the processes that transfer carbon from the atmosphere to deep seafloor sediments. Check all that apply
    [CORRECT]
    [CORRECT]
    [CORRECT]
    [CORRECT]
  2. Name all of the places in the ocean that you would find stored carbon. Check all that apply.
    [CORRECT]
    [CORRECT]
    [CORRECT]
    [CORRECT]
  

Stop and Think

2. (Powerpoint Question)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. (Powerpoint question) 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 "Whale Poop" - an Upside-down Biological Pump.

Whale poop pumps up ocean health (Science Daily) or the original research at

PLOS ONE: The Whale Pump: Marine Mammals Enhance Primary ...

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

WHOI : Oceanus : How Long Can the Ocean Slow Global Warming?


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