Using Isotopes to Track Sea Level Change

Materials / Preparation

• A computer with internet access and a printer
• Print a copy of the Isotope Worksheet for each student
• Make a transparency of the Sea Level Curve graph

Grouping

Groups of two to four

Teacher tips

It is not necessary for students to know very much about isotopes to understand that scientists can use them as evidence for change. However, for high school students, this is a good place to reinforce concepts that they may have learned previously.

For some students the graph may appear rather confusing. It may be necessary to go through the first few questions of the student worksheet together as a class. An answer sheet is provided.

We have provided an Answer Sheet to the Isotope Worksheet.

Procedure

1. Tell students that the foraminifera that they have been studying provide even further evidence about sea level change. Many foram shells are made of calcium carbonate CaCO 3 and the oxygen contained within those molecules provide some clues to the past environment in which the forams were living. Normal oxygen contains 8 protons and 8 neutrons, so it is referred to as O 16. However, a small fraction of oxygen atoms are heavier as they contain 8 protons and 10 neutrons. It is an isotope of normal oxygen and is referred to as O 18. By comparing the amount of O 16 to O 18, we can find out information about temperature and sea level!
   The water that is trapped in snow and ice has a higher concentration of O 16, leaving a higher concentration of O 18 in the seawater and therefore in the shells of the forams. So if we see a high concentration of O 18 in foram shells, we know there is a lot of ice (rich in O 16) and sea level is low. That’s the story, so let’s take a look at the graph and see what it can tell us.
2. Pass out both the graph and the Isotope Worksheet to students. Use the transparency to help direct the class. Ask students what each of the axes represents. Ask them to describe to you what the purple line is showing. [Sea level is changing – it goes up and down] Have students work together to complete the questions on the worksheet. You may want to guide them through the first few questions and then allow them to work together in a group to answer the remaining questions.
3. Have students discuss their findings. “What does this information tell us about the last 25,000 years? What do they think might have happened to marine life when sea level changed drastically?” Ask them to record answers in their notebooks. Tell them that they will be exploring that in greater detail in the next activity.

Resources used

None