The Oceans: Carbon Sink or Source?
Part A: The Ocean Carbon Cycle
Oceans have a large capacity to absorb CO2 and keep that carbon out of the atmosphere for long periods of time. On average, once a carbon dioxide molecule dissolves in the ocean, the carbon atoms will stay in the ocean for more than 500 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:
- How does the carbon get into the ocean and what happens to it once there?
- How does the ocean carbon cycle compare with the terrestrial carbon cycle?
- How does the ocean carbon cycle, the ocean environment and climate influence each other?
Lets begin by looking at the detailed illustration of the ocean carbon cycle above. 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 with a peer and/or the class.
- Identify and list as many abiotic (physical, chemical) and biotic(living) components as you can.
- 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 are they located only in this one zone?
- What organisms and processes are key in bringing carbon down to the deep ocean?
How does carbon get into the ocean carbon cycle? Air-Sea Surface Exchange
The image on the right represents the movement (flux) of CO2 into and out of the sea surface of the ocean.
- Purple to blue colors indicate areas of the ocean where more CO2 is dissolving into the ocean than is "undissolving" out to the atmosphere. Thus, this area is a carbon sink.
- Green colors indicate the movement of CO2into and out of the ocean is fairly equal.
- Yellow to red colors indicate areas of the ocean where more more CO2is "undissolving" out to the atmosphere than is dissolving into the ocean. Thus, this area is a carbon source (to the atmosphere).
Once dissolved in surface seawater, CO2 can enter into the ocean carbon cycle through three different mechanisms:
- The physical carbon pump
- The biological carbon pump
- The carbonate pump
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.
- Locate the largest ocean carbon sinks on the map. These carbon sinks are areas of downwelling. Why do you think they are located there?
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, 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 InLook at the simplified image of the oceanic biological carbon pump on the right.
- Draw two pathways that can bring carbon down to ocean sediments.
- Draw a pathway that would move CO2 from the atmosphere into the food web and back to the atmosphere in the shortest time scale.
Plankton include both phytoplankton- the primary producers of the ocean- and zooplankton - the primary consumers. Find out more about plankton by watching this TedEd video, "The Secret Life of Plankton." In this beautiful video, you will see many examples of phytoplankton and zooplankton. As you watch the video, see if you can identify which organisms are phytoplankton and which are zooplankton. Remember that in marine food webs, phytoplankton photosynthesize whereas zooplankton eat phytoplankton, and each other.
Getting carbon into the ocean is one matter - getting it down to the deep ocean is another.About 50 Gt (50 million tons) of carbon is drawn down into the biological pump per year. What happens to the carbon as it moves through the biological pump? And, how much carbon actually makes it down to the deep ocean and why is that important?
To follow the carbon, click here on the Oceanic Biological Pump interactive developed by Wood's Hole Oceanographic Institute (WHOI). Take a few minutes to familiarize yourself with the interactive and the type of information you can find there. Then with a partner or in a group, use the interactive to answer the Discussion questions below.
1. Identify the euphotic, twilight and deep ocean zones.
2. What do zooplankton and microbes feed on?
3. Why are particles such as marine snow so important in bringing carbon down into the twilight and deep ocean zones? Hint: Read this short article on marine snow and watch the video at the bottom -"Diving with JAGO."
4. As the carbon moves down through the biological pump, less and less carbon actually makes it down into the deep ocean. What role do zooplankton and the ocean microbial loop have in reducing the amount of carbon sinking down into deep ocean sediments.
5. Draw a diagram that illustrates three very different timescales of the movement of carbon from the atmosphere into the biological pump and back to the atmosphere. Discuss which time scale(s) has the greatest capacity to impact the amount of CO2in the atmosphere and explain why.
- A very short time scale (hours to days)
- A medium time scale (years)
- A very long time scale ( thousands to millions of years)
- Organisms and processes that are involved in each timescale
The Ocean Carbonate Pump and Shell-Building Organisms.
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.
Shells - Carbon Transport Down to the Deep
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.
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."
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?
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?
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
Read about coccolithophores and their importance to the biological pump. What is a Coccolithophore? Fact Sheet : Feature Articles