You just learned about the importance of carbon in the oceans. Now let’s think about how human activities can change ocean chemistry, and what this means for marine organisms.
Part I: Use of CO
2 in the oceans
As you know, CO
2 from the atmosphere dissolves into seawater. Let’s look at this process in more detail:
First, CO
2 reacts with water to form carbonic acid (H
2
CO3-):
(1) CO
2
+ H
2
O  H
2
CO
3
-
Carbonic acid can then dissociate into bicarbonate (HCO3-):
(2) H
2
CO
3-
 H
+
+ HCO
3-
Bicarbonate can then dissociate into carbonate ions (CO
32-
)
(3) HCO
3-
 H
+
+ CO
32-
There are two very important biological and chemical processes that are dependent upon the above compounds.
First, some marine organisms use carbonate to form calcium carbonate (CaCO
3
). Can you list two examples of vital uses of calcium carbonate to marine life?
Second, carbonate ions regulate the pH of seawater. If an acid is added to seawater, the carbonate ion reacts with excess hydrogen ions to produce bicarbonate (see equation 2). If a base is added to seawater, this bicarbonate will donate hydrogen ions to neutralize the base.
(4) 2CO
32-
+ H
2
SO
4
(an example of an acid)  2HCO3- (bicarbonate) + SO
42-
Questions for Part I
1. The above reactions are all reversible, meaning that a chemical change can cause them to occur in the opposite direction. a. Write the reverse reaction of equation (3). b. Thinking about your answer from question (a), if excess CO
2 results in an excess of hydrogen ions in seawater what will happen to carbonate? c. What effects do you think this could have on the formation of skeletal structures for marine life?

Part II: Testing the effect of increased acidity on calcium carbonate
Your teacher has provided you with two pieces of chalk and two cups, one filled with water and the other with vinegar. The chalk is made of calcium carbonate and will serve as our representation of shells and corals. The vinegar is a weak acid similar to carbonic acid. What do you think will happen to the chalk if it is left in the mildly acidic vinegar? Do you expect the same outcome as the chalk placed into the water?
Forming your hypothesis:
1. Write your hypothesis about what you think will happen to chalk placed in the vinegar compared to the chalk placed in the water.
Testing your hypothesis:
2. Place one piece of chalk into the cup of water, and the other into the cup of vinegar. Let them sit for 10 minutes. Observe and record any changes as you are waiting.
3. If you have computer access while you are waiting, research some marine organisms that build shells and exoskeletons from calcium carbonate. This might help you answer the questions.
4. After ten minutes, remove both pieces of chalk from your cups. Write down your observations about what happened to the chalk in both the treatment and control solutions and answer the questions below.
Synthesis questions
1. Based on the outcome of the chalk experiment, what do you think will happen to shells and corals in a slightly more acidic ocean over time?
2. What specific organisms and marine ecosystems do you think might be affected by the human activities you saw in the PowerPoint?
Know these Definitions
Carbon sink:
Carbon sequestration:
Bicarbonate:
Carbonate ion: pH:

Make the connections!
Make sure you understand the following relationships from this section.
1. As the amount of hydrogen ions increases in a solution, the pH level decreases
2. As temperature OR salinity increases, the amount of gas the ocean can absorb decreases
Why does it matter?
The concepts you learn are almost always related to human interaction with the oceans. Here are just a few of the human-ocean interactions from this lesson. Can you think of others?
When shelled organisms are affected by ocean acidification, larger species that need them for food may decline in number. This may affect the availability of some human food sources like fish and shellfish.
Coral reef ecosystems may decline because of increasing ocean acidity. If coral reef quality is poor, fewer people will want to visit the reefs. As a result, money from tourism may decline in coral reef areas.
Check out this ocean acidification learning tool online: http://www.whoi.edu/oceanus/viewFlash.do?fileid=55243&id=38052&aid=52990