Transcript 22.3 – Formation of the Atmosphere and Oceans

22.3 – Formation of the
Atmosphere and Oceans
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
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
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
Oxygen in Atmosphere
Only negligible amounts of free oxygen
in Earth’s atmosphere until the early
Proterozoic
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
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
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
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.
Importance of Oxygen
Most animals require it for respiration
Also provides protection from harmful
ultraviolet radiation (UV) from the Sun
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
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
Rain
As liquid water formed, rain began to
fall
It filled the low-lying basins and
eventually formed the oceans
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
In-class
Assignment/Homework