Oceans and the Carbon Cycle

Part A: Down to the Deep - The Ocean's Biological Pump

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 sea surface waters diffuse back to the atmosphere on very short time scales. However, some of the carbon atoms from these original CO₂ molecules 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:

• 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?

Let's begin by looking at the detailed illustration of the ocean carbon cycle on the right. Click to enlarge. Although quite complex, you will see carbon cycle processes and biosphere components similiar to those 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 questions below.

Carbon dioxide (CO₂) diffuses into the ocean carbon cycle via the air-sea surface exchange

Molecules of CO₂ enter 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.

1. Take a few minutes to closely examine the image below. This image 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 that the movement of CO₂ into and out of the ocean is fairly equal.
   • Yellow to red colors indicate areas of the ocean where more more CO₂ is diffusing out to the atmosphere than is diffusing into sea surface water. Thus, this area is acting as a carbon source to the atmosphere.
2. Then, answer the Checking In questions.

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

 

In the physical carbon pump, carbon compounds can be transported to different parts of the ocean in downwelling and upwelling currents

Downwelling occurs when surface waters converge (come together), pushing the surface water downwards; regions of low primary productivity because nutrients get used up and are not continuously resupplied by the cold, nutrient-rich water from below the surface. 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.

Eventually, these deep ocean currents return to the surface in a process called upwelling. Upwelling currents occur when surface waters diverge (move apart), enabling upward movement of water; bring water to the surface that is enriched with nutrients important for primary productivity (phytoplankton growth) that in turn supports richly productive marine ecosystems.. 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 and are important in physically transporting carbon compounds to different parts of the oceans.

1. Watch the NASA video below that animates ocean currents. As you watch the video, visualize carbon compounds moving along with these currents.
2. Next, examine the image below of the "Ocean's Conveyor Belt of Deep Ocean and Surface Currents." Look for areas where currents are sinking and rising. 

 

Phytoplankton are small photosynthetic organisms that move carbon into the oceanic biological pump

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
• moving carbon down to sea floor sediments

Phytoplanktonmostly microscopic, unicellular photosynthetic organisms that live in the upper sunlit layers of oceans and other bodies of water; mainly unicellular algae but also includes cyanobacteria. (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. 

Ocean food webs —move the carbon around the ocean's biological pump

 

Phytoplankton are responsible for bringing carbon into the ocean planktonic food web which teems with small plankton as seen in this TedEd video, "The Secret Life of Plankton." As you watch the video, make note of:

• different types and size of phytoplankton and zooplankton
• feeding relationships that would pass carbon through members of the ocean food web

Once in the food web, important food web processes such as feeding, producing waste products, dying and decomposing move carbon down into the twilight and deep zones in the ocean. When plankton and larger marine organisms eat, defecate, die and decompose, they produce sinking carbon-containing particles called 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 visit an interactive developed by Woods Hole Oceanographic Institute (WHOI) and then watch a video on the ocean's microbial loop.

1. Begin by visiting the interactive Carbon in the Ocean. On the home page, click the green Ocean button. 
2. Click through and read each of the slides: Dissolved Gas, Plants, Animals, Detritus, Deep Ocean, and Humans. Take note of the arrows indicating timescales for the changes. See if you can 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. 
3. Next, watch the video below on the ocean's microbial loop. As you watch, make note of:

• the ecological role of microbes in the ocean food web
• the role of microbes and the microbial loop in reducing the amount of carbon that eventually makes its way down to the bottom of the ocean.

The ocean carbonate system is essential to marine organisms such as coral, oysters, clams and lobsters building their shells

The ocean naturally contains many dissolved chemicals which are especially important to the ocean carbon cycle and the shell-building organisms that live in the oceans. The ocean carbonate system is linked to the biological pump and plays a very big role in transporting carbon down to deep ocean sediments where it is stored for very long time scales of millions of years.

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 (CO₃^2-) 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.

 

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Microscopic shells of coccolithophore and foraminifera

Provenance: Coccolithophore - Allison R Taylor (University of North Carolina Microscopy Facility) Micrograph of Foraminifera courtesy of Lynn M. Hansen, Great Valley Museum, Modeasto California.
Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.

Sinking shells bring carbon down to the deep ocean

When shell-builders die and sink, the carbon in their shells is transported down to the deep ocean where the carbon can become part of deep ocean currents and seafloor sediments. Many shells dissolve before reaching the seafloor sediments, a process that releases CO₂into deep ocean currents. Shells that do not dissolve build up slowly on the sea floor forming calcium carbonate (CaCO₃) sediments. Eventually, tectonic processes of high heat and pressure transform these sediments into limestone. 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 shell-building unicelluar coccolithophores tiny unicellular marine phytoplankton that live in large numbers throughout the upper layers of the ocean; build outer plates (shells) of limestone calcium carbonate(CaCO₃) and foraminifera mostly microscopic, unicellular amoeboid protists that live in the ocean; produce shells of organic material, sediment grains, or calcium carbonate; can be found in just about every marine environment from the deep sea to shallow reefs, buried in sediment or floating in the water column. reproduce quickly when nutrients are available. When nutrients have been used up, trillions of these tiny shell-builders die and sink, bringing carbon down to the deep.

The White Cliffs of Dover on the coast of England pictured in the image on the right are a famous example of limestone calcium carbonate sediments that were deep under the ocean millions of years ago. Over very long time scales, tectonic forces have pushed these sediments above water. 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 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.

 

Optional extensions

Want to learn more about the oceanic carbon cycle? Check out these resources.

• Learn more about the microbial loop at this link http://coolclassroom.org/micro_test/web_lessons/ML/ml.html
• Whale poop pumps up ocean health (ScienceDaily) or the original research at PLOS ONE: The Whale Pump: Marine Mammals Enhance Primary ...
• Ocean Bacteria get pumped up! Team discovers new factors impacting fate of sinking carbon
• Uncovering the Oceans Biological Pump: Scientists reveal the hidden movement of chemicals and particles in the sea
• What is a Coccolithophore? Fact Sheet : Feature Articles
• Watch this TED talk video: Why You Should Care about whale poo!
• Watch The Plankton Chronicles
• Watch From Mud to Molecules - What Deep Sea Sediments Can Tell Us About Past Climates