1. 22.3 – Formation of the Atmosphere and Oceans

2. Formation of the Atmosphere
Atmosphere began to form as Earth formed
Objects that collided with Earth probably contained water that would have vaporized on impact
forming a haze around the planet

3. Formation of the Atmosphere
Atmosphere changed with addition of volcanic gases
Volcanic eruptions release large quantities of gasses
Lots of volcanic activity during the Precambrian

4. Outgassing
Present-day volcanoes release large amounts of water vapor, carbon dioxide, and trace amounts of nitrogen and other gases in a process called outgassing
Earth’s early atmosphere probably contained the same gases that vent from volcanoes today

5. Oxygen in Atmosphere
Only negligible amounts of free oxygen in Earth’s atmosphere until the early Proterozoic

6. First Oxygen Producers
Fossils preserved in rocks that are about 3.5 billion years old have traces of tiny, threadlike organisms called cyanobacteria
Ancient cyanobacteria used photosynthesis and produced the nutrients they needed to survive
Oxygen gas is a waste product of the process of photosynthesis

7. Cyanobacteria
Could have produced enough oxygen to change the composition of the atmosphere that existed on Earth during the Archean
By the early Proterozoic, large, coral reef-like mounds of cyanobacteria called stromatolites dominated the shallow oceans that covered most of Earth’s continents

8. Evidence in Rocks Late Archean rocks do not contain iron oxides
This indicates that no free oxygen gas was in the atmosphere at that time
Cyanobacteria increased oxygen gas levels enough for iron oxides to form

9. Iron Oxides Formed in localized areas called banded-iron formation
Consist of alternating bands of iron oxide and chert (an iron-poor sedimentary rock)
Red beds in sedimentary rock deposits provide evidence that free oxygen existed in the atmosphere during the Proterozoic.

10. Importance of Oxygen Most animals require it for respiration
Also provides protection from harmful ultraviolet radiation (UV) from the Sun

11. Ozone
As oxygen accumulated in Earth’s atmosphere, an ozone layer began to develop
Ozone filtered out much of the UV radiation, providing an environment where new life-forms could develop

12. Formation of the Oceans
Water in oceans probably originated from two sources
Volcanic outgassing
Objects bombarding Earth
As Earth cooled, the water vapor in the atmosphere condensed to form liquid water

13. Rain As liquid water formed, rain began to fall
It filled the low-lying basins and eventually formed the oceans

14. Water and Life
Precambrian began with an environment not suitable for life
It ended with much of Earth was covered with oceans that were teeming with tiny cyanobacteria and other life-forms.
Life as it exists on Earth today cannot survive without liquid water
The search for life elsewhere in the solar system and universe today is centered on the search for water

15. In-class Assignment/Homework