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Average Composition of the Troposphere
Gas Name
Nitrogen
Oxygen
*Water
Argon
*Carbon Dioxide
Neon
Helium
*Methane
Hydrogen
*Nitrous Oxide
*Ozone
*variable gases
Formula
Abundance (%)
N2
O2
H2 O
Ar
CO2
Ne
He
CH4
H2
N 2O
O3
78.08%
20.95%
0 to 4%
0.93%
0.0360%
0.0018%
0.0005%
0.00017%
0.00005%
0.00003%
0.000004%
Residence time
(approx)
42,000,000 years
5,000 years
10 days
~Infinite
4 years
~Infinite
~Infinite
10 years
3 years
170 years
20 days
EVOLUTION OF THE ATMOSPHERE
• Earth thought to have formed about 4.5 billion years ago:
Atmosphere probably consisted of gases then abundant in the solar
system -> hydrogen and helium.
Most of these gases were lost to space
• Over time a secondary atmosphere was formed:
(Current atmosphere doesn't contain much hydrogen or helium).
outgassing from cooling magma
Volcanoes efflux: H20, CO2, SO2, N2, H2, Cl2
• Upon cooling of this prehistoric atmosphere:
Water vapor condensed and precipitated to form oceans.
Some carbon dioxide dissolved in droplets also precipitated out.
• Eventually anaerobic bacteria developed some 3.5 billion years ago:
Could survive in the absence of oxygen.
Began the conversion of carbon dioxide to oxygen while removing
Carbon Dioxide which is now stored primarily in carbonate rocks.
Plankton and shellfish continue this process more effeciently.
• As oxygen started to become abundant, some of it broke down by the
sun’s radiation into atomic oxygen and eventually formed ozone .
• Ozone absorbed most of harmful ultraviolet radiation to make Earth
suitable for life.
The atmosphere we know was produced by biological processes.
Geological carbon cycle
In the geological carbon cycle, carbon moves between
rocks and minerals, seawater, and the atmosphere.
Carbon dioxide in the atmosphere reacts with some
minerals to form the mineral calcium carbonate
(limestone). This mineral is then dissolved by
rainwater and carried to the oceans. Once there, it can
precipitate out of the ocean water, forming layers of
sediment on the sea floor. As the Earth’s plates move,
through the processes of plate tectonics, these
sediments are subducted underneath the
continents. Under the great heat and pressure far
below the Earth’s surface, the limestone melts and
reacts with other minerals, releasing carbon dioxide.
The carbon dioxide is then re-emitted into the
atmosphere through volcanic eruptions. (Illustration
by Robert Simmon, NASA GSFC)
Slow time scale - controls atmospheric carbon dioxide on time scales
of hundreds of millions of years
Biological/Physical carbon cycle - shorter than geologic cycle
Land plants ~ 50 years
atmosphere ~ 4 years
soils ~ 25 years
Fossil fuels ~ 650 years
oceans ~ 100s to 1000s years
carbonates ~ 150 years
Addition of O2 to the Atmosphere
Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the
atmosphere?
Revisit the oxygen cycle:
* Oxygen Production
o Photochemical dissociation - breakup of water molecules by ultraviolet
+ Produced O2 levels approx. 1-2% current levels
+ At these levels O3 (Ozone) can form to shield Earth surface from UV
o Photosynthesis - CO2 + H2O + sunlight = organic compounds + O2 - produced by
cyanobacteria, and eventually higher plants - supplied the rest of O2 to atmosphere.
Thus plant populations->
* Oxygen Consumers
o Chemical Weathering - through oxidation of surface materials (early consumer)
o Animal Respiration (much later)
o Burning of Fossil Fuels (much, much later)
Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence
levels). What little was produced by cyanobacteria, was probably consumed by the weathering
process. Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in
the atmosphere.
During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10 %.
Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3
Ga. Present levels of O2 were probably not achieved until ~400 Ma.
Vertical structure of atmospheric pressure
Atmospheric temperature: vertical structure
Vertical structure as a function of latitude
Venus
Mars
The Ocean: composition
• Dissolved salts comprise 3.5% by volume of sea water
• Originate from weathered rocks, volcanic, and atmosperic sources
Composition of sea water remarkably uniform and constant
over time
Density of ocean water varies by < 7%
Variations of density as a function of salinity and temperature
 (T,P,S)  (T,P,S) 1000kgm3

Vertical structure of ocean in different latitudes