Phenomenon
The oxygen content of the atmosphere changed over Earth’s history from nearly 0% to it’s present level of nearly 21%.
Question
Where did the oxygen in the atmosphere come from? Why did the level change?
Model Ideas
- Oxygen is one of the most abundant elements on Earth, but it didn’t occur as a gas in the early atmosphere.
- Some kinds of life release oxygen as a gas as a result of some life processes.
- Oxygen is a highly reactive substance and combines readily with other molecules to form new compounds.
- As life became more abundant on Earth oxygen was released into the atmosphere but most of it quickly reacted with other substances and therefore did not build up in the atmosphere for a long time (as much as a billion years).
- Around 2.5 billion years ago it began accumulating in the atmosphere as evidenced by banded iron formations
- As life continued to become more abundant and moved onto land the atmospheric oxygen increased and stabilized to modern levels of approximately 21%.
- At the end of the triangle, we'll more broadly recognize that Earth's systems or "spheres" influence one another.
Overview
Transition In: After imagining an Earth with both atmosphere and oceans, we recognize the stage has been set for the evolution of life. But we're still wondering when and how life comes to out planet.
In searching for the origins of life, we review the state of the early Earth. In comparing to where we are now, we recognize that Earth's systems have changed over time. We investigate one particular change, the increase in atmospheric oxygen from almost nothing to it's current level making up nearly 21% of the mass of the air we breathe. The pattern of change shows an initial increase billions of years ago, followed by a long state of low oxygen and then a more recent climb to the levels present over the past hundreds of millions of years. How can we explain this pattern of change?
We offer some initial ideas but realize we need to learn more about oxygen. We learn it is the most abundant element on our planet, but that it is highly reactive. In studying oxidation and considering photosynthesis as an explanatory mechanism for putting oxygen into the air, we begin to piece together a story that explains our increase-plateau-increase pattern. The model invokes both oxygen's reactivity and the origin and diversification of life on Earth.
In this way we see not only that Earth's "spheres" change over time, but that they influence one another.
Transition Out: Recognizing the advent of life on the planet leads us to finally move into our study of biology, which begins with a consideration of extant biodiversity.
Advanced Planning
There are several options for demos and labs depending on how long you want to spend and how you want to structure this unit for your students. This model includes whole class processing of two ideas related to chemical processes--photosynthesis and oxidation--and then small group processing of some additional data about timing of life and info from the geologic record (card sorts). There are easy labs or demonstrations that could be done for photosynthesis and oxidation if you want your students to have more hands-on lab time. We provide supports in the curricular materials for a "rust demo" and a link to a video demonstrating the role of oxygen in photosynthesis. The materials needed for each can be found on the "Supplies Details and Ordering" tab on the Advanced Planning spreadsheet available from the Advanced Planning page on this site.
1: An Pattern of Increase in Oxygen Over Time
Overview: We recognize that Earth has changed over time and focus in on one very specific change: the amount of oxygen in Earth’s atmosphere has increased from 0% to 21% over billions of years. We then develop a driving question that asks how and why oxygen levels have changed over time.
What we have figured out...We came up with our driving question: Why/how did the atmosphere’s oxygen increase over time?
2: Initial Ideas on the Origin of Oxygen in the Atmosphere
Overview: We offer some initial ideas about where the oxygen came from and then revise our phenomenon to look at a more detailed pattern of changes in oxygen in the atmosphere.
What we have figured out... We’ve explored some initial ideas and examined a bit more information on the pattern of change in oxygen in the atmosphere.
3: Two Processes Involving Oxygen
Overview: We work as a class to examine two chemical processes that involve oxygen: photosynthesis and oxidation. We then work with more data about both Earth’s geology and the history of life on Earth in order to piece together an apparent correlation.
What we have figured out... We figured out that oxygen was always on Earth but not as a free gas in the atmosphere. We learn about some processes that cause oxygen to change form. We have more detail about the timeline of events on Earth.
4: Addressing Our Question
Overview: We return to the driving question and answer it.
What we have figured out...We put all the pieces together to see how we can explain the pattern of change in the oxygen in the atmosphere over time.
5: Thinking About the "Spheres" and Change
Overview: We have a quick discussion about how the change we see in oxygen in the atmosphere is just one example of how the different spheres of Earth have changed over time. We also recognize a broader idea that the spheres are interconnected.
What we have figured out...We realized that our spheres framework allows us to see and think about change over time on Earth and interaction among different systems on the planet.
Optional A*: Oxygen from Water?
Overview: We engage in a conversation in order to evaluate a competing model that oxygen in the atmosphere came from water molecules.
*This learning segment should follow Learning Segment 02 if (and only if) your students insist that the oxygen came from splitting water molecules.
What we have figured out...We've recognized that obtaining the atmosphere's abundance of oxygen from water doesn't make sense as the required energy for this path is much too high to have lead to the increases we are trying to explain.