Early Atmosphere and Evolution of the Atmosphere
The Early Atmosphere
The Earth is approximately 4.6 billion years old. For the first billion years, intense volcanic activity shaped the planet's surface and released gases that formed the early atmosphere.
Composition of the Early Atmosphere
Scientists believe the early atmosphere was similar to the atmospheres of Mars and Venus today:
- Mainly carbon dioxide (CO₂) — possibly up to 95%
- Water vapour (H₂O)
- Small amounts of nitrogen (N₂), methane (CH₄) and ammonia (NH₃)
- Little or no oxygen
The evidence for this comes from analysis of gases trapped in ancient rocks, volcanic emissions, and comparison with other planets.
How the Atmosphere Evolved
Stage 1: Oceans Form (about 4 billion years ago)
As the Earth gradually cooled, water vapour in the atmosphere condensed to form the oceans. This was a crucial step because:
- It removed large amounts of water vapour from the atmosphere
- CO₂ began to dissolve in the oceans, reducing CO₂ levels
Stage 2: CO₂ Levels Decrease
Carbon dioxide was removed from the atmosphere by several processes:
1. Dissolving in the oceans — CO₂ is soluble in water
2. Photosynthesis by early organisms — algae and later plants used CO₂:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
3. Formation of sedimentary rocks — dissolved CO₂ reacted with minerals to form insoluble carbonates (e.g. calcium carbonate, CaCO₃), which settled on the ocean floor and became limestone over millions of years
4. Formation of fossil fuels — dead organisms were buried and compressed, locking carbon into coal, oil and natural gas
Stage 3: Oxygen Levels Increase (about 2.7 billion years ago)
The development of photosynthetic organisms (first cyanobacteria, then algae, then plants) was the key event:
- These organisms absorbed CO₂ and released oxygen (O₂)
- Over hundreds of millions of years, oxygen accumulated in the atmosphere
- This allowed more complex life to evolve
The first photosynthetic organisms appeared about 2.7 billion years ago. It took a very long time for oxygen to build up because it initially reacted with iron and other minerals (forming rust-like deposits called banded iron formations).
Stage 4: Nitrogen Accumulates
Nitrogen built up in the atmosphere because:
- It was released by volcanic activity
- It is a very unreactive gas (strong N≡N triple bond), so it did not get removed by chemical reactions
- Denitrifying bacteria also release nitrogen from nitrogen compounds
Today's Atmosphere
The atmosphere has been roughly stable for about 200 million years:
| Gas | Approximate percentage |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon (Ar) | 0.9% |
| Carbon dioxide (CO₂) | 0.04% |
| Water vapour | Variable (0–4%) |
Plus trace amounts of other noble gases, methane, and other gases.
Evidence for Atmospheric Change
- Fossils — show when photosynthetic organisms appeared
- Iron compounds in rocks — banded iron formations show when oxygen first appeared and reacted with iron
- Trapped gas bubbles in ancient rocks and ice cores
- Limestone and fossil fuels — show where carbon was locked away
Common Misconceptions
- The early atmosphere was NOT like today's atmosphere — it had almost no oxygen
- Plants did NOT create the first oxygen — it was cyanobacteria and algae
- The change from CO₂-rich to O₂-rich took billions of years, not a sudden event
Exam Tips
- Know the approximate composition of today's atmosphere: 78% N₂, 21% O₂, 0.04% CO₂, ~1% Ar
- Be able to describe at least three ways CO₂ was removed from the early atmosphere
- Photosynthesis is the reason oxygen levels increased — always mention this
- The formation of limestone and fossil fuels locked carbon away
- You may be asked to describe how evidence from rocks supports our understanding of atmospheric change