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Early Earth History
Solar system began about 4.6 Gy ago
Started with several supernova
explosions in the local neighborhood.
Sun formation from accretionary disk.
Roughly 500 planetoids (about size of
moon) in region of inner planets.
Collisions of these planetoids
produced Venus, Earth and Mars, all
with inventories of water vapor and
carbon.
There may have been early oceans on
all three of these planets.
Key Reference: Nisbet and
Sleep, The habitat and nature
of early life, Nature, 409,
1083-1091, 2001.
In discussing geological time,
1 Gyr is 109 years,
1 Myr is 106 years (the ‘ago’ is implicit and often
omitted, such that Gyr and Myr refer to both
time before present and duration).
There are four aeons.
The Hadean is taken here as the time from the
formation of the Solar System and early
accretion of the planet (4.6–4.5 Gyr), to the
origin of life (probably sometime around 4.0±0.2
Gyr).
The Archaean, or time of the beginning of life, is
from about 4–2.5 Gyr;
the Proterozoic from 2.5 Gyr to about 0.56 Gyr;
and the Phanerozoic since then.
Earth in the Hadean
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Earth formed about 4.6 billion years ago from coalescing interstellar gasses.
For 500 to 800 My, Earth was bombarded by large meteorites adding to
earth’s mass (also adding heat).
Hot spinning pre-earth mass melted, caused differentiation of materials
according to density.
Distinct earth layers begin to form
– Dense iron and nickel migrate to center (core)
– silicate material moves out to mantle
The Hadean was a time of
heavy boloid bombardment
of the earth.
No terrestrial geology
record of this: data taken
from dating of lunar
impacts.
Many impacts had
sufficient energy to boil-off
the oceans.
From: Bada, JL, How Life Began, Earth
and Planetary Science Letters 226
(2004) 1 – 15.
Boil oceans
A palaeotemperature curve for the Precambrian
oceans based on silicon isotopes in cherts
Francois Robert & Marc Chaussidon, Nature, 2006.
cold
hot
NOW
THEN
Cretaceous hot house, 100 My ago
Relative temperature of the earth, for last 600 My
The Faint Young Sun Paradox.
The Sun’s interior through out the history of its existence (4.55 Byr) has
been the site of ongoing nuclear reaction (H to He fusion). This nuclear
reaction process has caused our Sun to expand and gradually become
brighter. These models indicate that the earliest Sun shone 25% to
30% more faintly than today.
This is a problem for climate scientists.
A decrease of just few percentage in our Sun’s present strength would
cause all the water on Earth to freeze, despite the warming effect of our
present day greenhouse gases.
A positive feedback would be caused by their high albedo and it would
never get warm...
Climate models suggest that an early Earth with so weak Sun and
present level greenhouse gases would have remained frozen for the
first 3Byr of its existence.
Adding greenhouse gases to the atmosphere solves this problem.
Evidence of running water in sedimentary rocks formed during the
early Earth’s history (zircons) , means it was not frozen solid.
First evidence of ice-deposited sediments occurs in rocks dated
to about 2.3 Byr ago, probably due to glaciations localized in polar
regions, as on Earth today, and are not an indication of completely
frozen planet.
This conclusion also supported by the continued presence of life
on Earth. Primitive life forms date back at least 3.5 Byr ago.
The Problem:
With so weak a Sun, why wasn’t Earth frozen for the first two-thirds
of its history? This is known as
The faint young Sun paradox.
The early weaker sun (but similar/warmer temperatures) indicate that the
Greenhouse gas concentrations in the early atmosphere much have been
much higher than present time.
Early Earth CO2 and O2 levels
NOTE: these are determined from proxies, like BIFs and redbed formation,
isotopes of soil minerals and the presence of partially unoxidized iron minerals.
Earth’s early environment thought to have active volcanism,
causing extreme loss of volatile gases (including CO2) from its
interior.
- Earth’s surface may have been entirely molten for a few 100
million years after its formation (the ‘magma ocean’ period seen
on the moon),
- Craters on moon and other planets suggests that Earth was
once under heavy bombardment by asteroids, meteors, and
comets, triggering greater volcanism.
- radioactive elements deeper in Earth’s interior released more
heat, increasing volcanism
- increased volcanic activity would have delivered more CO2 to
the atmosphere and may have helped to make Earth hot.
What happened to all the CO2 that was in the atmosphere?
•Carbon is removed by weathering and buried in sediments and turned to rocks.
•Today, CO2 removed by weathering is deposited in ocean sediments and
becomes rocks.
•Same would have worked in the past, with a slow transfer of CO2 from the
atmosphere to the rocks.
•Most of early Earth Greenhouse atmosphere is in rocks and not in the
•atmosphere like on Venus
OXYGEN IN THE ATMOSPHERE
Microorganisms are responsible for the production of nearly all of the oxygen
we breathe.
Oxygen is produced during photosynthesis by the reaction
CO2 + H2O = CH2O + O2.
Where “CH2O” is a geochemist’s shorthand for more complex forms of organic
matter.
Most photosynthesis on land is (now) carried out by higher plants, not
microorganisms; but
Terrestrial photosynthesis has little effect on atmospheric O2 because it is
nearly balanced by the reverse processes of respiration and decay.
By contrast, marine photosynthesis is a net source of O2 because a small
fraction (about 0.1%) of the organic matter synthesized in the oceans is
buried in sediments. This small ‘leak’ in the marine organic carbon cycle is
responsible for most of our atmospheric O2.
The ‘small leak’
Early Earth CO2 and O2 levels
NOTE: these are determined from proxies, like BIFs and redbed formation,
isotopes of soil minerals and the presence of partially unoxidized iron minerals.
GLOBAL CLIMATE - in briefest summary
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4.6 to 4.0 (or 3.8) By: The Hadean; massive boloid bombardment periodically
boils ocean. Earth's core forms; geomagnetic field preserves atmosphere.
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3.8 to 2.5 By: Archean earth was ice-free and warm (60°C?) in spite of lower
sun luminosity. Must have had a very strong GHG effect (CO2, H2O, probably
methane). Occasional boloids must have made life uncomfortable. BUT life
was present at 3.8 By.
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2.3 By: End of Archean. 1st evidence of surface glaciation, continents form,
traditional rigid plate sea floor spreading begins. Oxygen is present.
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2.3 to 0.9 By: Proterozoic. Warm (30°C) during Early and Middle Proterozoic.
Life abundant.. Mega-continent Rodinia forms. Atmospheric now with lots of 02
present. Megacontinent Rodina breaks-up. Massive global glaciation starts in
Late Neo-Proterozoic (Snowball Earth).
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0.9 to 0.6 By: Neo-Proterozoic. Four possible periods of 'Snowball Earth',
where glaciation was - at sea level, at the equator. Intervals 10 My long with
ice-covered surface are followed by extremely elevated atmospheric CO2
levels, followed by (very) warm periods of inorganic carbonate precipitation.
Brief Climate Summary – continued.
•
600 My to 210 My: Climate warm to temperate, but punctuated with 2
periods of major global glaciation. Mega-continent Pangea forms from
Laurentia, Baltica and Gondwana. Development of multi-celled life, land
plants/animals.
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210 to 145 My: Jurassic climate was ‘temperate’. Pangea breaks-up.
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150 – 65 My: Cretaceous. Warm, high atmospheric CO2, high sea levels,
fast seafloor spreading, Large Igneous Provinces and mantle plumes.
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55 My: Eocene Climate Maximum. High temperature excursion within the
general cooling from the Cretaceous-warmth. Brief period of extreme
warmth followed by general cooling toward present time (methane!).
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14 My: Formation of Antarctic Ice sheet.
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3.0 My: Oscillations between periods of major glaciation and inter-glacial
warm periods. Emergence of Central American connection may have
changed global ocean circulation patterns. Mostly (90%) cool, only 10% of
the last 3 My were as warm as present.
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22,000 to 18,000 years: Last glacial maximum.
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6,000 years: present Holocene (generally) stable climate.
Major glaciations
Quaternary
Permo-Carboniferous
Ordovician
Neoproterozoic
Paleoproterozoic
Some of these glacial periods may be related to changes in
greenhouse gases, driven by biology
- OR by plate tectonics.
HOW continents form – and when they did it.
PLATE TECTONICS – a major player in global climate
The Wilson Cycle
The Wilson Cycle
The Wilson
Wilson Cycle
The
Cycle
The Wilson
Wilson Cycle
The
Cycle
The Wilson
Wilson Cycle
The
Cycle
The Wilson Cycle
The Wilson Cycle
Rodinia – the SuperContinent before Pangea (1 By ago)
N
Most of North
America “Boxed-In”
Note orientation and
neighbors of “North America”
The Appalachian Mountains – form by the closing of the paleo-Atlantic Ocean;
after Rodinia broke up and Pangea reformed.
The world we live in.
The break-up of
the Mega Continent Pangea
Why is this important to CLIMATE?
Long term (>105 year) concentrations of atmospheric CO2, O2, CH4 are set
by many different interactions, including plate tectonics.
Weathering of organics:
CH2O+O2 = CO2 + H2O
Weathering of rock:
CO2 + XSiO3= XCO3+SiO2
Burial of organics:
CO2 + H2O = CH2O+O2
Ocean
Oceanic crust
Metamorphism of rock;
XCO3+SiO2=CO2 +XSiO3
Weathered sediment from continents
Fast seafloor spreading – high CO2
input: both at mid-ocean ridges (new
CO2) and at subduction zones (re-cycled
CO2). Slow SFS, low CO2.
And this variation in CO2 input has
both positive and negative
feedbacks.
BOTH examples => are negative
feedback
As the Wilson cycle happens,
seafloor spreading and subduction
are speeding up – and slowing
down
(i.e., fast during breakup, slow
during final stages of Megacontinent formation).
This puts variable amounts of CO2
into the atmosphere,
changes the latitude of the
continents, and
changes the ratio of coast (wet) to
continental interiors (dry).
All of these are major climate
processes.
THE 'BLAG' HYPOTHESIS - WHAT IS IT?
LARGE-SCALE CLIMATE CHANGES ARE CONTROLLED BY THE
AMOUNT OF CO2 IN THE ATMOSPHERE (the thermostat), AND
ATMOSPHERE CO2 CONTENT IS, IN TURN, CONTROLLED BY THE
PROCESS OF PLATE TECTONICS, i.e.,
seafloor spreading rates,
rates of subduction,
mountain-building and
weathering.
HIGH RATES OF SEA FLOOR SPREADING
bring more new CO2 up from the mantle,
increase the amount of old CO2 emitted from subduction
volcanoes,
increase the rate of mountain building and the exposure of new
rocks to weathering,
raise sea level, changing earth's albedo.
Fast seafloor spreading – high CO2
input: both at mid-ocean ridges (new
CO2) and at subduction zones (re-cycled
CO2). Slow SFS, low CO2.
And this variation in CO2 input has
both positive and negative
feedbacks.
BOTH examples => are negative
feedback
Tracing the pathway of CO2.
(1) MOR eruptions: (2) transfer to atmosphere: (3) combined
chemically during weathering: (4) transfer to ocean via rivers: (5)
incorporated in biology
(6) Eventually sinks to seafloor as sediment: (7) seafloor is
subducted: (8) mantle heat released CO2 in subduction zone: (9)
emitted by subduction volcanoes back into atmosphere.
Then, the cycle starts over.