Influence of the world's ocean currents on the carbon cycle

Category: Simulation

Materials, equipment and/or facilities:
1- Activity Sheets
2- Information Sheets
Per group of 4-5 students

1. clear plastic tub, aquarium or other container (4-8L)
2. 4 beakers (250 mL)
3. table salt
4. food coloring
   
5. a supply of warm and cold water
   
6. ice and a container to make an ice bath for a 250mL beaker

Sequence and duration of each part of lesson:

PART 1 ( 30-45 min )
I.Students should perform ACTIVITY 1 (groups of 4-5)

1. Make a saturated salt solution by slowly adding salt to 400 mL of warm water (35-40 degrees Celsius) until no more salt will dissolve.
   **(The teacher may want to make a quantity of warm, saturated water. Each group needs 400mL.)
   
2. Put 3-4 drops of food coloring in the salt water solution.
   
3. Pour 200 mL of the salt solution into one beaker and 200mL into a 2nd beaker.
   
4. Put one of the beakers in an ice bath.
   
5. Prepare two beakers exactly as above but omit the salt.
   
6. Use thermometers and adjust the water temperatures so that the beakers containing warm water are as close to the same as possible. Do the same with the beakers of cold water.
   
7. Fill the clear plastic container with clear, cool water.
   
8. Choose one of the beakers and gently pour its contents down the inside of the plastic container.
   
9. Observe and record your observations.
   
10. Empty the contents of the large, clear container.
    
11. Repeat steps 8-10 for the three remaining beakers.
    
12. Clean up all materials.
    
13. Use your data to suggest a reason for the results you obtained.

II. After performing Activity 1, the teacher should pull the class together to report on their activity results and form some general conclusions.

III. Students should then read Appendix A: "Circulation of the Oceans."
As a class, discuss how the article relates to the activity.

PART 2 ( 20-30 min )
I. Have students read Appendix B: "Student Information Sheet" which provides background information.

As a class , discuss the articles. How does the ocean absorb carbon dioxide? What factors influence how much carbon dioxide the ocean absorbs?

PART 3 ( 60 min )
I. The teacher should group students and explain that they are now working as scientific teams. Using the following information each team will identify oceanic areas to determine whether they act as a sink or a source of carbon dioxide.

II. Scientific teams

1. Use Figure 02.1 ( the long term circulation of water through the deep oceans ) and Figure 02.2 ( the distribution of primary plant production in the world's oceans), in conjunction with Facts 1-12 on the Fact Sheet.
2. If you think the fact suggests a source, put a "+" by the fact. If you think the fact suggests a sink, put a "-" by the fact.
3. Now go back through your facts and identify areas on the blank map (Figure 02.3) where these +'s and -'s should go.
   NOTE: It may be that you will find locations where both a + and - might be appropriate, if this happens you need to make a decision as to whether the net result would be a + or a -.
4. Form a class group to compare map results.
5. Where students have different results ask them "What data do you have to support your results?"
6. Show students the map compiled by scientists. Figure 02.6 on page 3301.
7. Discuss implications of their data. How do oceans afffect the carbon cycle? How do they effect other Earth Systems?

Evaluation: Teacher to evaluate participation of the student teams and evaluate the results of their map comparing it to the one on page 3300 (White book).

Ask students to write a half page paper explaining how oceans affect the carbon cycle.

Related Reference:
OCEANUS MAGAZINE, ARTICLE "Balancing the Budget", Vol 35, Number 1, Spring 1992, pp. 18-28. Woods Hole Oceanographic Institution, Woods Hole, Mass.

Fact Sheet

Fact 1
In biologically active surface waters carbon dioxide is consumed by primary production of phytoplankton (algae).

Fact 2
Decomposition and dissolution of particulate matter that has originated from biological processes in surface waters enriches deepwater in carbon dioxide.

Fact 3
Sinking of water in near polar regions takes carbon dioxide with it followed by a flow towards the equator.

Fact 4
Upwelling of equatorial waters releases carbon dioxide and the surface water then flows towards the poles.

Fact 5
In rough waters ocean overturning causes carbon-rich water from intermediate depths to be mixed with water of less carbon content than surface waters.

Fact 6
In areas of low productivity (i.e. few algae or low photosynthetic activity), the partial pressure of carbon dioxide is greater than that in the atmosphere and carbon dioxide is released from the surface.

Fact 7
The role of the oceans in the carbon cycle is much dependent on the rate of overturning and mixing. This overturning and mixing takes a long time. For example, it takes a few hundred years in the Atlantic Ocean and around 1500 years in the Pacific Ocean.

Fact 8
Water temperature changes geographically from:1.9 degrees c in the polar seas to 30 deegrees c in the equatorial oceans and also seasonally by as much as 15 degrees celsius at a given place.

Fact 9
The effect of warming seawater (pCO2) overwhelms the lowering (absorbtion) effect of CO2 caused by photosynthesis.

Fact 10
In contrast to most high-latitutde areas, those of the northeast Pacific are, on a yearly average, a strong carbon dioxide source.

Fact 11
The upwelling of deep waters brings up nutrient salts which support photosynthesis.

Fact 12
During the summer, the surface water pCO2 in the north Pacific is reduced because of the intense photosynthetic activity. This lowering effect on the pCO2 far surpasses the increasing effect of the summer warming. However, on an annual basis the winter source condition wins out over the summer sink condition.

NOTE: The following reading may be challenging to students. The teacher may need to guide student's reading to ensure comprehension.

CIRCULATION OF THE OCEANS
Water increases in density with a decrease in temperature and an increase in salinity. Surface waters temperatures in the tropics are relatively constant while those of the temperate regions warm in the summer. The temperatures of polar waters are often below freezing especially those in deep water and can be as low as -2 degrees Celsius due to the high salinity. There is a slow exchange between the oceanŐs warm surface layers and the cold deep ocean a mile or so below. Essentially these ocean currents are driven by cooling in the polar regions. In the tropics, oceans are relatively stable and water layers are thermally stratified. The upper layers are separated from the cooler layers by a density barrier called the thermocline.

Temperatures decline rapidly towards the thermocline (50-300 meters) . Below it temperatures continue to drop but much more slowly. This feature is a barrier to water movement and consequently dissolved carbon dioxide, but not to particulate carbon bearing matter such as dead phytoplankton. In the middle latitudes, winds can cause considerable mixing of surface and intermediate waters when at certain times of the year the thermocline is nonexistent. In the polar seas, other than on the continental shelf, the thermocline is nonexistent. In rough waters carbon dioxide is dissolved more readily. If it is then cooled, the dense water sinks taking with it the carbon dioxide into the deep ocean.

The effect of winds and ocean currents in equatorial regions can have a pronounced effect on up welling and the thermocline. Thus nutrients are brought to the surface which increase primary production. This complex of movements in the oceans are still not fully understood but clearly they have a marked influence on the fate of carbon dioxide.

STUDENT INFORMATION SHEET - GENERAL INFORMATION
Scientists have attempted to construct a global budget to balance estimates of carbon dioxide production and absorption. Until recently it was assumed the land was a net producer of carbon dioxide. Carbon dioxide production in the world exceeds its absorption and there is a global excess of around 5 gigatonnes* of carbon per year. Scientists estimate that carbon dioxide is increasing in the atmosphere at a rate of around 3.4 gigatonnes of carbon per year. Where is the 1.6 gigatonnes of carbon dioxide being absorbed? One hypothesis is that the ocean is absorbing the carbon oxide. Is the ocean capable of absorbing more carbon dioxide? Is the land an important source for carbon dioxide absorption? Two different views are currently being investigated. Pieter Tans at the University of Colorado proposes that a large amount of excess carbon dioxide is being absorbed by terrestrial ecosystems. The more commonly accepted view is that the oceans are probably the carbon dioxide absorber. If an area absorbs carbon dioxide it is called a sink.
*1 gigatonne = 1 Gt = 1,000 million tonnes. 1 tonne = 1000 kilograms

The oceans capacity to retain carbon dioxide is determined by 3 major factors:

1. the chemical properties of carbon dioxide in sea water
2. the presence of a "biological pump" that transports carbon dioxide from the surface to the deep ocean
3. the circulation of the deep ocean water with water on the surface

Carbon Dioxide and Sea Water
Carbon dioxide has a greater solubility in sea water than do oxygen and nitrogen, the two major components of the air. The amount of carbon dioxide dissolved in sea water rapidly increases up to 1000 meters and then it increases more slowly. This property is due to the biological pump process, discussed in the reading on The Biological Pump. Carbon from the remains of dead algae and other organisms, is released in the deeper waters as carbon dioxide. For carbon dioxide to be removed from the atmosphere by the sea, the partial pressure of carbon dioxide in the sea water has to be less than that in the atmosphere. For carbon dioxide to be drawn down to deep waters the partial pressure of carbon dioxide has to be greater than that in the surface waters. These processes will of course work in the opposite direction dependent upon the relative partial pressure values. The surface water partial pressoure of carbon dioxide values determine whether the ocean produces or absorbs carbon dioxide.

The Biological Pump
During photsynthesis marine phytoplankton (algae) absorb light from the sun and combine carbon dioxide, dissolved in the sea, with water to produce carbohydrates: a process which is called primary production. When primary production is high, many algae are dying per unit volume of sea water, particularly in the surface waters where photosynthesis is greatest. These algae decompose and the particulate matter sinks enriching the deeper waters in carbon dioxide. This process is often termed the biological pump (pumping carbon to the bottom of the sea). One very common example of this is the way carbon dioxide that is dissolved in surface waters is incorporated into microscopic shells or tissues of living organisms and is then carried to the bottom in the remains of the dead organism. This uptake of dissolved carbon dioxide from the surface waters lowers the partial pressure of carbon dioxide. So where the biological pump is active it lowers the partial pressure in deep water not in contact with the atmosphere. In surface waters of low productivity, the partial pressure is often greater than the atmosphere and carbon dioxide is released from the surface.