1.2: Carbon from the Past
What is the pattern for carbon dioxide (CO2) and temperature in Earth's past?
In Lesson 1.1, you saw that CO2 can enter and exit water. The activities were a good model for carbon moving between the ocean and air in the carbon cycle. Now think about the geologic past. Do you think there were times when more CO2 was leaving than entering the ocean, or vice versa? How might you, or a geoscientist, be able to investigate CO2 in Earth's past? This is an important question. Perhaps you've heard that the temperature of the atmosphere is related to CO2 contents of the air.
Think of the atmosphere as a key part of the Earth system. If CO2 is leaving the oceans, it is probably moving to the atmosphere. Or, if CO2 is entering the ocean, it is probably coming from the atmosphere. So, how could you get samples of that prehistoric air?
You're in luck. Geoscientists have a clever way to get tiny samples of "old" air. Can you think of a natural material on Earth that captures a small sample of air as it forms? It is not obvious, but there is an earth material that does this: Snow!
In this lesson you will see that:
Later in Carbon Connections, you will return to see how these concepts closely connect to the carbon cycle. In this lesson, you will focus on understanding this question better.
Think about how snow might trap air. When snow falls, the flakes stack up. Between stacked flakes, tiny pockets of air get trapped. It's like the model with cereal flakes that your teacher showed you. If that snow melts, the air is released back into the atmosphere. For example, this happens each spring in cities and towns across the U.S., such Denver, Chicago, New York, or Boston, or maybe in your own town. No air inside the snow is preserved because it's melted.
Other places on Earth are cold all year. The snow does not melt, it piles up, layer upon layer. The best examples are near Earth's poles, such as on the ice sheets of Greenland or Antarctica. There, the temperature is almost always below freezing. As snow gets deeper, it hardens into glacial ice and preserves small pockets of air from the time when the snow fell. Deeper in the snowpack, the pockets of air contain older air. View the Air Between Flakes video to see how air gets preserved in snow.
The air bubbles image to the right shows what ice looks like deep in an ice sheet. Under the increased weight of overlying snow and ice, a layer progressively changes from packed snow to ice. The largest ice sheets today are on Antarctica and Greenland. The layers in these ice sheets are of particular interest to geologists who study Earth's past climate. Part of understanding climate is retrieving and analyzing the CO2 in those trapped air bubbles.
How can you measure CO2 contents of air in the past? To do this, geologists drill a core into the ice sheet; the deeper they drill, the older the ice. The Young Ice, Old Ice animation shows how geologists get an ice core. The bubbles in that old ice have samples of old air. Then, the scientists measure the CO2 content in the bubbles from different depths.
The graph below shows the CO2 levels in the air bubbles of an ice core from Antarctica. The age of the ice (x-axis) as you go down the core goes from recent times (about 1,000 years ago) back to 650,000 years ago. It may not seem like 1,000 years ago is "recent," but this is only a small fraction of 650,000 years. On that scale it is relatively recent. The core penetrates more than three kilometers (nearly two miles) into the ice sheet. This distance is like going around your school track nearly eight times. That's deep! It takes many years to drill and recover a core like this.
The y-axis on the graph shows the CO2 levels in air when the snow fell. The units are in parts per million (ppm) of CO2. You can think of parts per million like this: Think about visiting a large city like Dallas, Texas, which has a population of about one million people. As one person in a million people, you would represent one part per million.
Explore this graph further while answering questions about CO2 levels in the atmosphere in Earth's past. Record your answers in your science notebook.
So, why would geologists study the CO2 levels in the atmosphere? Maybe you have heard that CO2 levels in the atmosphere are related to Earth's climate. The ice core that you just explored also contains evidence for the climate and air temperature at the time the snow was falling. Using data from this ice core, continue with the next section to explore whether the air temperature at Antarctica is related to CO2 levels in the atmosphere.
Scientists have a detailed record of air temperatures above the ice sheet from the past. Levels of CO2 come from the bubble, but temperature is measured from the ice (H2O) immediately around the bubble. This is possible because the chemistry of ice differs very slightly depending on the average air temperature.
The Temperature Variation graph shows temperature changes for air above the Antarctic ice sheet. As with the CO2 graph, the age of the ice (x-axis) goes from recent times (about 1,000 years ago) to 650,000 years ago. The y-axis shows how average air temperature above the ice sheet has changed during that time. Even though Antarctica is always cold, the data show that the average temperature has varied. In this graph, the value of 0°C is the mean value for the past 1,000 years. Along the length of the core, temperature values are either below or slightly above that value. Explore temperature changes further in the questions below.
Carbon Cycle Connections
In Lesson 1.1, you investigated carbon as CO2, moving in and out of water. In this lesson, you saw how CO2 levels have changed in Earth's past in the atmosphere. For a period where CO2 is increasing in the air, does the CO2 just come from the oceans? What changes might be occurring on land?
Maybe you have explored photosynthesis in your science class. Photosynthesis is the process where plants and other CO2 producers use water (H2O), carbon dioxide (CO2), and energy from the sun to produce organic molecules (e.g., C3H6O3) and oxygen (O2). This process moves carbon from the atmosphere (in CO2) to living organisms.
You saw from the Antarctica ice core that there are times when CO2 was decreasing in the atmosphere. Perhaps producers account for these dramatic changes in CO2 in the air. Recall that primary producers, or autotrophs, include a variety of plants, algae, and bacteria in food webs. Photosynthesis moves carbon from the atmosphere to the oceans or land. The photosynthesis reaction is:
3CO2 + 3H2O + energy → C3H6O3 + 3O2
carbon dioxide + water + energy → organic molecules + oxygen
Explore how carbon on the land changes as carbon in the atmosphere changes by looking at ecosystems in the past. Changes between glacial and interglacial periods is a good timeframe to compare. Do the following steps to investigate further carbon cycling in Earth's past.
|Factor||Does the factor increase (↑) or decrease (↓) from 20,000 years ago to today?||Does the factor increase (↑) or decrease (↓) from 125,000 to 20,000?|
|carbon in atmosphere (as CO2)|
|average temperature over Antarctica|
|mean temperature in North America and Eurasia|
|amount of vegetation on land|
|amount of carbon on land|
Summarize your ideas for Lesson 1.2, using the figures in the lesson to help you. If helpful, work with a partner; he or she might have some ideas to help you get the big picture.