Download Oxygen in the Early Atmopshere

C1.1a Oxygen in the early atmosphere
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How do we know that there was little or no oxygen in the atmosphere when the Earth was young, and that
it has increased? Some of the most important evidence comes from iron compounds in rocks.
The Earth is thought to be about 4.5 billion years old. The earliest rocks that have been found date from
about 4.1 billion years ago. These rocks contain iron compounds that would form if there was no oxygen
around – such as iron sulfide and iron carbonate. Scientists therefore believe there was little or no oxygen
in the atmosphere at this time.
Rocks that contain iron oxide date from around 3.5 billion years ago. These rocks contain a form of iron
oxide (Fe3O4) that forms if there is only a little oxygen available. The oldest fossils date from 3.5 million
years ago as well and they are of organisms that could photosynthesise. Most scientists think this shows
that oxygen was formed by photosynthesis when life began on Earth.
Rocks dating from about 1.8 billion years ago contain a different form of iron oxide (Fe2O3). This form of
iron oxide forms when there is more oxygen present. Scientists use this as evidence that a significant
amount of oxygen had built up in the atmosphere 1.8 billion years ago.
1
Use the information above to write a paragraph to explain why scientists think that there was little or
no oxygen in the Earth’s early atmosphere.
2
Use the information above to write a paragraph to explain why scientists think the amount of oxygen in
the Earth’s atmosphere has increased.
3
Use the information above to write a paragraph to explain why scientists think that it was the start of
life on Earth that led to changes in the amount of oxygen in the atmosphere.
© Pearson Education 2010. Edexcel GCSE Science Activity Pack.
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