Transcript 6_GC1_AtmosOceanCon..

Evolution of the Atmosphere, Oceans,
Continents
Evolution of Atmosphere, Ocean, & Life
We will address the following topics....
• Evolution of Earth’s atmosphere, continents, and
oceans
Early Earth had small continents, no ocean and a thin,
inhospitable primordial atmosphere. How did the modern
atmosphere and ocean come about, and what role did life
play?
• What was the Timing of Life and what was its impact
on the composition of the Earth?
State of Early Earth
Very Early Earth…a vision of Hell?
• Hot: from primordial heat,
impacts, decay of radioactive
elements
• Violent: frequent impacts
• Unstable: constant volcanism;
thin, unstable basaltic crust
• Inhospitable: scalding
atmosphere devoid of oxygen
MECHANISMS FOR CREATING FELSIC CONTINENTAL CRUST
Fig. 13.05 a, b
W. W. Norton
How did we date the age of Continents
When did continents form?
• Ratio of Nb/U tracks the
creation of continental crust
• U is preferentially extracted
during the creation of
continental crust
• Causes the mantle Nb/U ratio
to increase
• Today the ratio is 47
• Examining past Nb/U ratio of
mid-ocean ridge basalts
provides evidence of
continental crust production
Hoffman et al. (1997)
AGE OF CONTINENTAL CRUST -- CRATONS
Fig. 13.08
W. W. Norton
The Growth of the Continents
By investigating the Nb/U ratio, geologists have
found:
• 4.5-4.0 Ga: Slow production of
continental crust
• 4.0-2.5 Ga: Rapid growth of
continents, 70% of continental volume
was achieved by 3.0 Ga
• 2.5-0 Ga: Slow production of
continental crust
Primordial atmosphere: Earth’s first early atmosphere
• Primordial atmosphere
- Composed of H2 and He gas
from protoplanetary disk
- Gravity on the terrestrial
planets was too low to retain
these light gases
- Also driven off by planetary
heat, solar wind, and violent
impacts
SO WHERE DID THE ATMOSPHERE COME FROM?
HOW DO YOU GET GAS??
IF YOU’VE GOT GAS, HOW DO YOU GET RID OF IT?
State of Early Earth
Secondary atmosphere: a new atmosphere formed
early in Earth’s history
• Secondary atmosphere
- Initially composed of CO2,
N2O, and H2O and ?CH4?
- Impact degassing:
vaporization of planetesimals
during period of heavy
bombardment would have
contributed CO2, H2O, NH3
- Volcanic outgassing: output
of gases by volcanic eruptions Gas compositions from 3 volcanoes
(H2O, CO2, N2, HCl, other
H2O
CO2
SO2
H2S
HCl
volatiles)
95
1.1
1.5
0.07
0.006
96
1.9
2.3
0.08
0.004
97
1.1
1.5
0.07
0.006
Composition of Early Atmosphere
Modern
• Secondary atmosphere composition
- No O2: No photosynthetic organisms to
produce free O2
CO2
O2
N2
- Some N2: Inert gas, so all N2 from volcanic
and impact degassing would have remained
in atmosphere
- Lots of CO2: Chemical weathering rates
would have been lower because continents
would have been smaller– 30,000x present
value!
- Lots of H2O: Due to vaporization of oceans
• Global warming!
- Due to high CO2, surface temperatures
may have been 80-90°C
H2O
N2
Other
O2
~4.56 Ga
H2O
CO2
N2
N2
H2O
CO2
Other
O2
How long have we had Oceans?
Oceans: formed soon after Earth’s temperature fell to
levels where liquid water was stable
• Oceans may have condensed
and then been vaporized many
times as impacts bombarded
early Earth
• Size of impactor matters
- Diameter of ~100km will
vaporize photic zone (upper
100m)
- Diameter >440 km will
vaporize entire ocean
- Last ocean-vaporizing event
probably occurred at 4.1-4.3
Ga
Elemental Composition of the Ocean
Rise of Oxygen
Rise of oxygen: essential to the rise of multicellular
eukaryotic organisms- organisms whose cells have
nuclei
• Rise of oxygen
- Requires O2 source > O2 sink
• Earth Earth had a reducing atmosphere
• Reduced gases from volcanic eruptions
(H2 and CO) reacted with free oxygen (O2)
to form H2O and CO2
• Result: early atmosphere had low oxygen
concentrations
• Sink of oxygen
Redox Conditions
Redox conditions: whether environment is conducive
to oxidation or reduction
• Oxidation: loss of electrons by a
molecule or atom
• Reduction: gain of electrons by a
molecule or atom
•E.g., Fe2+  Fe3+
= oxidation
•Oxygen is a Great “oxidizing” agent
Rise of Oxygen
Prebiotic atmosphere: oxygen levels were very low
• Source of O2
• Photochemical reactions: chemical reactions induced by light
- Photolysis of CO2 and H2O leads to production of H and O2
- H escapes to space
- In reducing atmosphere, O2 source < O2 sink, no accumulation of
atmospheric O2
Photolysis
But what about me??
Role of Early Life and Atmosphere Evolution
Earliest know life is ~3.8 billion years old
• Source of O2?
• Evidence of early life
- Microfossils: preserved
remains of single-celled
prokaryotic organisms (3.5 Ga)
Microfossils from 3.5 Ga Warrawoona
Formation in Australia
Early Life
Earliest know life is ~3.8 billion years old
Modern stromatolites, Australia
• Source of O2?
• Evidence of early life
- Microfossils: preserved
remains of single-celled
organisms (3.5 Ga)
- Stromatolites: layered
structures formed by trapping,
binding, and cementation of
sediments by cyanobacteria
(3.2 Ga). Blue-green algae
- Organic carbon in ancient
sedimentary rocks (3.8 Ga)
Ancient
stromatolites
Rise of Oxygen
Great Oxidation Event: rise in atmospheric oxygen
levels between 2 and 2.2 Ga
• Cyanobacteria (prokaryotes)
develop ability to photosynthesize
- Appeared 1 billion years before
rise of oxygen
- Increase O2 source
• Oxidation of mantle
- Decrease O2 sink
• Switch from mainly submarine to
subaerial volcanoes
- Due to development of thick
continental crust
- Decrease O2 sink
Great Oxidation Event
Rise of Oxygen
Great Oxidation Event: rise in atmospheric oxygen
levels between 2 and 2.2 Ga
• Cyanobacteria (prokaryotes)
develop ability to photosynthesize
- Appeared 1 billion years before
rise of oxygen
- Increase O2 source
• Oxidation of mantle
- Decrease O2 sink
• Switch from mainly submarine to
subaerial volcanoes
- Due to development of thick
continental crust
- Decrease O2 sink
Cyanobacteria- first
organisms to produce O2
by photosynthesis
Photosynthesis:
CO2 + H2O --> CH2O + O2
Preferentially uses 12C
CO2 + H2O --> 12CH2O + O2
Results in an shift in 13C/12C
preserved in limestones
The Oxygen Cycle
(Bio)geochemical cycle: pathway through which a
molecule moves through compartments of the
natural world (including biotic and abiotic)
• Geochemical cycles
- Reservoir: compartment where
chemical species resides
- Flux: rate of transfer of chemical
species between reservoirs
- Source: origin of chemical species
in reservoir
- Sink: destruction of chemical
species in reservoir
• Carbon cycle, water cycle, oxygen
cycle, nitrogen cycle, phosphorus
cycle
Rise of Oxygen
Great Oxidation Event: rise in atmospheric oxygen
levels between 2 and 2.2 Ga
• Cyanobacteria (prokaryotes)
develop ability to photosynthesize
- Appeared 1 billion years before
rise of oxygen
- Increase O2 source
• Oxidation of mantle
- Decrease O2 sink
• Switch from mainly submarine to
subaerial volcanoes
- Due to development of thick
continental crust
- Decrease O2 sink
Oxidation of mantle changed
composition of volcanic outgassing-- to
less reducing
Rise of Oxygen
Great Oxidation Event: rise in atmospheric oxygen
levels between 2 and 2.2 Ga
• Cyanobacteria (prokaryotes)
develop ability to photosynthesize
Archaean
- Appeared 1 billion years before
rise of oxygen
- Increase O2 source
• Oxidation of mantle
- Decrease O2 sink
• Switch from mainly submarine to
subaerial volcanoes
- Due to development of thick
continental crust
- Decrease O2 sink
Switch from mainly submarine to
combination of submarine/subaerial
volcanoes
BANDED IRON FORMATION -- BIFs
COMPOSED OF REDUCED IRON MINERALS
Evidence for Rise of Oxygen
Evidence from Rock Record of Low O2 until 2.2 Ga
• Rocks provide evidence of the
oxidation state of the atmosphere/ocean
• Presence of detrital minerals, uraninite
and pyrite
- These minerals are insoluble (can’t be
dissolved) in absence of oxygen
- Uraninite and pyrite disappeared after
2.3 Ga
• Banded iron formation
- Marine sedimentary rocks consisting
of layers of iron-rich minerals and chert
- Iron is only soluble in seawater in its
reduced form (Fe2+)- indicating low O2
- BIFs become scarce after ~2.2 Ga
BIF
Oxygen Levels and BIF Deposits
Fig. 13.12
W. W. Norton
Formation of Ozone Shield
Rise of ozone (O3): critical to the evolution of life
• Ultraviolet radiation is harmful to
eukaryotes (cells w/nucleus)
• Ozone absorbs ultraviolet (UV)
radiation, providing a protective shield
to life
• Ozone
- Absent in early earth
- Formed by the interaction of UV
and O2
- As atmospheric O2 rose, ozone
layer would have accumulated
Structure of Earth’s Atmosphere
Earth’s atmosphere is divided into layers based on
the lapse rate
• Lapse rate: change in
temperature with altitude
• Troposphere: temperature
decreases with height
• Stratosphere: temperature
increases with height
• Mesosphere: temperature
decreases with height
Evidence for Rise of Oxygen
Evidence from Rock Record of High O2 after 2.2 Ga
• Rocks provide evidence of the
oxidation state of the atmosphere/ocean
• Presence of red beds
- Reddish-colored sedimentary rocks
- Red color comes from oxidation of iron
(rusting)
• Iron-rich paleosols
- Prior to 1.9 Ga, iron in soils was in
reduced form (Fe2+), soluble, and
weathered away
- After 1.9 Ga, iron in soils was in
oxidized form (Fe3+), insoluble, and
retained in soil
Red Beds
Rise of Oxygen
Rise to modern atmospheric levels
• Modern oxygen levels (21%) were
not reached until about 400 Ma
• Reasons for slow rise
- Oxidation of mantle
- Evolution of higher plants and
increase in photosynthesis
Rise to modern levels
Decline of CO2 and H2O
Earth’s early atmosphere had high levels of CO2
and H2O. Where did they go?
• Atmospheric H2O would have declined as Earth’s atmosphere cooled
• Atmospheric CO2 would have declined due to chemical (silicate)
weathering
- CO2 + H2O  H2CO3 (carbonic acid)
- CaSiO3 + 2H2CO3  Ca2+ + 2HCO3- + SiO2 + H2O (silicate weathering)
- Ca2+ + 2HCO3-  CaCO3 + H2CO3 (carbonate precipitation)
- Net: CaSiO3 + CO2  CaCO3 + SiO2
Conversion of CO2 gas to CaCO3 mineral!