1. Origin of the Atmosphere and Oceans

2. Stromatolite bacteria mounds
Figure 12.6
Over 4 billion years ago volcanic activity was producing Earth’s primitive atmosphere and oceans.
Stromatolite bacteria mounds

3. This process continues today.
Earth’s primitive atmosphere, which consisted mainly of H2O vapor and CO2, was formed by a process called “outgassing”, in which gases trapped in the planet’s interior are released by volcanic eruptions.
Figure 12.7
This process continues today.

4. Origin of the Atmosphere and Oceans
Water vapor in the primitive atmosphere cooled and condensed to form clouds. Precipitation produced rainwater that contributed to the oceans.

5. Origin of the Atmosphere and Oceans
Scientists now believe that most of the ocean’s water was brought to Earth on comets and icy asteroids.

6. Origin of the Atmosphere and Oceans
About 3.5 billion years ago, photosynthetic bacteria in the oceans began to release oxygen.

7. Origin of the Atmosphere and Oceans
Oxygen levels steadily increased

8. Origin of the Atmosphere and Oceans
Eventually oxygen levels were sufficient for ozone to develop in the atmosphere.

9. These layered, iron-rich rocks, called banded iron formations, were deposited during the Precambrian. Much of the oxygen generated as a by-product of photosynthesis was readily consumed by chemical reactions with iron to produce these rocks.

10. Origin of the Atmosphere and Oceans
Outgassing produced acidic conditions that caused an accelerated rate of weathering of Earth’s rocky surface.

11. Origin of the Atmosphere and Oceans
Products of this weathering were carried to the oceans, thus increasing the salinity of the oceans..

12. Origin of the Atmosphere and Oceans
The oceans also serve as a depository for carbon dioxide CO2, which is readily soluble in water.

13. CO2 reacts with other substances in seawater to form calcium carbonate (limestone.) Eventually, marine organisms would removed calcium carbonate from the seawater to make their shells. When these organisms died, their remains were deposited on the seabed in the form of chalk. These chalk beds later became exposed, as in this picture of the White Cliffs of Dover, in southern England.

14. Today, Earth’s oceans are fairly stable in their salt content (salinity), and the atmosphere exists in a 78% – 21% mix of nitrogen and oxygen gases, respectively.

15. Earth… or Water?

16. e-Research Paper The Origins of the Atmosphere and Oceans
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