Oceans and the Carbon Cycle
Investigation
15B
15B Oceans and the Carbon Cycle
What role do the oceans play in the carbon cycle?
CO2 is important to living things, but it is also a major
greenhouse gas that traps energy from the Sun. Together
with water vapor and methane, CO2 helps keep Earth’s
surface at an average temperature of about 15°C. In the last
investigation, you learned that CO2 is produced during
cellular respiration. It is also being produced in larger and
larger quantities by the burning of fossil fuels and
deforestation. The oceans absorb CO2 from the atmosphere,
accumulating and storing it for an indefinite period.
In this investigation, you will explore how the oceans absorb
CO2 and the chemistry involved in that process.
Materials
• 2 500mL beakers
• Red cabbage juice
solution
• BTB (bromothymol blue) solution
• Simulated seawater
• Distilled water
• pH paper (range 4.5 to 9)
• 250 mL graduated cylinder
• 2 straws
Safety tip: Wear safety goggles and an apron during
this investigation.
A
Thinking about the carbon cycle
a.
List the carbon sources in the diagram.
The carbon cycle has carbon sources and carbon sinks. Anything that releases CO2 into the
atmosphere is a carbon source. Anything that absorbs CO2 and holds it is a carbon sink. Study
the diagram and then answer the questions below.
b. List the carbon sinks in the diagram.
1
Investigation
15B
Oceans and the Carbon Cycle
Over time, carbon sources and sinks tend to stay in balance. Today, CO2 levels are on the
rise. What are the sources for this extra CO2?
c.
d. Based on the diagram, do you think the oceans are a net carbon producer or sink? Explain
your answer.
B
CO2 absorption and pH
Water and other solutions like seawater absorb CO2 through a chemical reaction. When CO2
diffuses into water, carbonic acid forms and then ionizes to form hydrogen, bicarbonate, and
carbonate ions.
We measure the acidity of a solution using a value called pH. The pH scale is shown below. As
you can see, the lower a solution’s pH number, the greater its acidity. In this experiment, you
will determine the effects of CO2 absorption on the pH of a solution.
What do you think will happen to the acidity of a solution as it absorbs more CO2. Will it
become more acidic, less acidic, or will the acidity stay the same? State your answer in
the form of a hypothesis.
a.
2
Oceans and the Carbon Cycle
Investigation
15B
1. Put on your safety goggles and apron.
2. Obtain a 500 mL beaker containing 100 mL of red cabbage juice solution. Red
cabbage juice changes color as pH changes.
3. Record the initial color and pH of the solution. Use the pH chart on the left side of the
table below to find the pH value that matches the color you observe.
4. Choose a “bubbler” from your group. Other members will observe the beaker and
record observations.
5. Using the straw, the bubbler will gently blow bubbles into the solution. Do not inhale
the solution. Observe and record each color you observe in the table. Use the chart
provided by your teacher to record the pH. Keep recording until the color appears to
stop changing.
Table 1: Color and pH
Red cabbage juice pH scale
color
pH
blue
7-8
violet
5-6
dark red
3-4
pink
1-2
C
a.
Data from the experiment
Color
pH
Initial
Analyzing the results
Was the initial pH of your solution acidic, neutral, or basic?
b. Was your hypothesis correct? Explain why or why not based on your results.
c.
What happened to the pH of the solution as CO2 was absorbed?
3
Investigation
D
15B
Oceans and the Carbon Cycle
Comparing CO2 absorption in freshwater vs. seawater
BTB turns from dark blue to green to yellow depending on the concentration of ionized
carbonic acid in the solution. As the solution absorbs more CO2, more carbonic acid forms and
ionizes which lowers the pH. The color of the solution gets lighter as the pH gets lower.
In this experiment, you will bubble CO2 into distilled water
(representing fresh water) and then simulated seawater using your
breath and a straw. You will then observe and compare the color
change in both solutions.
1. Put on your safety goggles and apron.
2. Fill one 500 mL beaker with 100 mL of distilled water and
label it “distilled water.”
3. Fill another 500 mL beaker with 100 mL of seawater and
label it “seawater.”
4. Add 20 drops of BTB solution to each beaker.
5. Using pH test strips, measure the pH of each beaker and record it in the first row of
Table 2.
6. Place each beaker on a blank white sheet of paper.
7. Choose a “bubbler”, “timer”, and “observer” from your group.
8. Using a straw, the bubbler will gently blow bubbles into the distilled water beaker
while the timer measures the time each minute for 6 minutes and the observer
watches for color changes in the beaker.
9. Record your observations in the data table below.
10. Use a pH test strip to measure the final pH and record in the last row of Table 2.
11. Repeat steps 7 and 8 for the seawater beaker using a clean straw. If the seawater
does not change to a pale yellow, record the lightest color change you observed within
6 minutes of gentle bubbling.
Table 2: Time vs. color change
Time (min)
Initial color and pH
1
2
3
4
5
6 min (final color and pH)
4
Distilled water
Seawater
Oceans and the Carbon Cycle
E
a.
Investigation
15B
Analyzing the data
Which was the control in this experiment, distilled water or seawater? Why did you need
a control?
b. Based on your results, which do you think has the ability to absorb more CO2 before
becoming acidic, distilled water or seawater? Explain your answer.
c.
Might the results of this experiment explain part of the reason why oceans are a carbon
sink? Justify your answer.
d. What other processes in the oceans store CO2? What processes release CO2?
e. Based on your results, what effect might continued CO2 absorption have on the pH of
seawater?
f.
Based on the results of the experiment, could the release of excess CO2 from the burning
of fossil fuels and deforestation have an effect on the pH of the oceans? Explain your
reasoning.
5
Investigation
F
15B
Oceans and the Carbon Cycle
Impact of ocean acidification
Although the natural absorption of CO2 by the oceans helps
absorb excess CO2, many scientists believe the decrease in pH
will have a negative effect on marine calcifying organisms. These
are organisms that construct shells out of calcium carbonate and
include
some
phytoplankton,
coccolithophores,
corals,
crustaceans, and some molluscs. Calcifying organisms help store
carbon in their calcium carbonate shells and many absorb CO2
for photosynthesis. When these organisms die, their shells sink
to the bottom of the ocean and store carbon in calcium carbonate
sediments. In fact, England’s famous white cliffs of Dover are
made mostly of the calcium carbonate shells of coccolithophores.
a.
Conduct research on ocean acidification and its effect on
calcifying organisms. Try to find answers to these
questions: (1) What is the pH of seawater in the oceans and has it changed since the
Industrial Revolution? (2) How could rising CO2 levels impact calcifying organisms?
(3) What are some steps humans need to take in order to lower our impact on the pH of
oceans?
b.
Suppose you are a marine biologist who is designing an experiment to test the effects of
ocean acidification on a species of coral. Answer these questions: (1) What is your
research question and hypothesis? (2) What are the control variables in your experiment?
What is your experimental variable? (3) What materials would you need for your
experiment? (4) Briefly describe your procedures and the data you would need to collect.
6