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.
- How does 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 carbon get transported down to deep ocean sediments? Which organisms are involved?
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 organism brings CO2 into the surface waters of the ocean?
How does carbon get into the ocean carbon cycle? Air-Sea Surface Exchange of CO2
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 diffusing into sea surface water than is diffusing from sea surface water out to the atmosphere. Thus, these areas are acting as 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 diffusing out to the atmosphere than is diffusing into sea surface water. Thus, this area is acting as 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 staying there for hundreds of years.
Examine the image on the right of the "Ocean's Conveyor Belt of Deep Ocean and Surface Currents" and then watch the NASA video below that animates ocean currents. As you watch the video, visualize carbon compounds moving along with these currents.
- 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. The biological pump plays a major role in:
- transforming carbon compounds into new forms of carbon compounds
- moving carbon throughout the ocean
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 above right.
- Draw a carbon pathway with the shortest time scale between two reservoirs.
- Draw a carbon pathway that would move carbon atoms from the atmosphere to a place where they would be stored for millions of years.
Food webs and marine snow - moving the carbon around.
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).
- What happens to the carbon as it moves down through the biological pump?
- What is the role of the "microbial loop" in moving that carbon?
- How much carbon actually makes it down to the deep ocean and why is this important?
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 Plankton Key and Particle Key in the upper right hand corner. Familiarize yourself with the information in these two keys.
- Then, click on the 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 and zooplankton in moving carbon to the deep ocean. (WHOI)
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
When CO2 dissolves in the ocean, it combines with water molecules and then enters into a series of reversible chemical reactions that produce bicarbonate ions(H+CO3-), hydrogen ions (H+) and carbonate (CO32-) ions. The carbonate ions are especially important to marine organisms because they combine with calcium ions (Ca2+) to form calcium carbonate (CaCO3). Shell-building organisms such as coral, oysters, lobsters, pteropods, sea urchins, and some species of plankton use calcium carbonate to build their shells, plates and inner skeletons.
Examine the image of the ocean carbonate system above right and then trace the pathway of carbon atoms from CO2 molecules to calcium carbonate (CaCO3) molecules. Now, take a big breath and then exhale. Describe how carbon atoms from the CO2 you exhaled could end up in the shells of a shell-building organism such as a lobster or clam.
Shells bring carbon down to deep sea sediments!
When shell-builders die and sink, the carbon in their shells is transported down to the deep ocean. Many shells dissolve before reaching the seafloor sediments. This carbon becomes part of deep ocean currents. Shells that do not dissolve build up slowly on the sea floor forming limestone calcium carbonate sediments . This process locks massive amounts of carbon away for millions of years.
Some of the smallest shell-builders transport the most carbon down to seafloor sediments. Microscopic, unicelluar coccolithophoes (a phytoplankton) and foraminifera (an amoeboid-like zooplankton) reproduce quickly when nutrients are available. When nutrients have been used up, trillions of these tiny shell-builders die and sink.
The White Cliffs of Dover on the coast of England in the image above are a famous example of limestone calcium carbonate sediments that were deep under the ocean millions to billions of years ago. If you examined a sample of sediments from these cliffs, you would find shells of microscopic coccolithophores and foraminifera that lived, died and then sank to the sea floor bottom millions of years ago. Over time, these sediment layers such as the White Cliffs of Dover eventually return carbon to the oceans by weathering and erosion.
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 the ocean carbon cycle:
Whale poop pumps up ocean health (ScienceDaily) or the original research at PLOS ONE: The Whale Pump: Marine Mammals Enhance Primary ...
Watch these videos: