What do banded iron formation deposits reveal about the evolution of the atmosphere?
Submitted by Bosiljka Glumac, Smith CollegeHow to use this question in the classroom
In my introductory-level geology courses BIFs are the focus of one of the topics on the evolution of the atmosphere. I first share with students information about stratigraphy spatial and temporal distribution of BIFs. The students also use magnets to examine BIF samples. From this introduction to BIFs the students conclude that these deposits were likely marine in origin judging from their substantial thickness and lateral extent and that they contain iron oxide magnetite among other minerals.
Next I challenge the students to make a simple concept sketch to illustrate the difference between the pre-BIF Earth and post-BIF Earth. A horizontal line in the middle of the sketch symbolizes the interface between the hydrosphere oceans below and the atmosphere above. Two vertical lines mark the time periods where abundant BIFs first appeared about 3.5 to 3 Ga and disappeared about 1.8 Ga. With my help the students identify the key components to include in their sketch such as the presence versus absence or paucity of free oxygen abundant iron and continental red beds.
From this exercise the students conclude that:
1. free oxygen was rare or absent on the pre-BIF Earth
2. oxygen was combining with abundant iron in oceans of the BIF Earth to form BIFs and
3. formation of abundant BIFs stopped once the majority of iron from oceans was used up which resulted in buildup of oxygen in the atmosphere as also suggested by the first appearance of common continental red beds of the post-BIF Earth.
This BIF story also serves as an introduction to the topic of the origin of life on Earth including the importance of photosynthesizing cyanobacteria in oxygenation of the early atmosphere.
Even though this more-or-less standard approach to teaching about BIFs makes for a nice story I am concerned that it might represent a gross oversimplification. After all BIFs are one of the least understood and most controversial deposits on Earth. Their temporal distribution is more complicated than stated above: for example BIFs first appeared about 3.8 Ga and after they disappeared about 1.8 Ga their reappearance between 0.8 and 0.6 Ga suggests a link to Snowball Earth conditions. Opinions regarding the alternating BIF banding or the role of microbial and other processes in the formation of BIFs vary greatly. For these reasons I look forward to the opportunity to consult with experts at the workshop about strategies for developing effective approaches for teaching this and other important and fascinating but potentially poorly understood and controversial subjects.