Transcript Week 2A Figures ()

Early Earth History
Solar system began about 4.6 Gy ago
Started with several supernova
explosions in the local neighborhood.
Sun formed from an accretionary disk
of dust.
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 geological time, 1 Gyr is 109 years.
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;
• The Phanerozoic is the period since then.
Earth in the Hadean
For 500 to 800 My after formation, bombardment 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 boiloff the oceans.
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
hot
cold
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 => 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 such a weak Sun and
the present level greenhouse gases would have remained frozen for the
ENTIRE first 3 Byr of its existence.
Adding greenhouse gases to the atmosphere solves this problem.
The addition of GHGs makes our
world warmer, like turning up a
thermostat.
Evidence of running water in sedimentary rocks (zircons) formed
during the early Earth’s history, 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 Banded Iron Formations and
redbed formation, isotopes of soil minerals and the presence of partially
unoxidized iron minerals.
Until recently, CO2 levels were
high compared to present, and
O2 levels did not reach present
values until ‘about’ 600 My.
Earth’s early environment thought to have very 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 early earth atmosphere?
•Carbon was first removed by weathering and buried in sediments and turned to
rocks (via inorganic carbonate precipitation).
•Today, CO2 removed by weathering is deposited in ocean sediments and
becomes rocks via biological processes!
•Same processes would have worked in the past abiologically, with a slow
transfer of CO2 from the atmosphere to the rocks – of the seafloor and
continents.
•Most of early Earth Earth Greenhouse atmospheric CO2 is stored in rocks and
not still in the atmosphere - like on Venus
Present day carbon
reservoirs
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 + biology = 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 NET 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.
GLOBAL CLIMATE - in briefest summary
•
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.
•
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; maybe earlier.
•
2.3 By: End of Archean. 1st evidence of surface glaciation, continents form,
traditional rigid plate sea floor spreading begins. Oxygen is present.
•
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).
•
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 complex multi-celled life, land plants/animals.
•
210 to 145 My: Jurassic climate was ‘temperate’. Pangea breaks-up.
•
150 – 65 My: Cretaceous. Warm, high atmospheric CO2, high sea levels, fast
seafloor spreading, Large Igneous Provinces and mantle plumes.
•
55 My: Eocene Climate Maximum. High temperature excursion within the general
cooling from the Cretaceous-warmth. Brief period of extreme warmth (methane!),
followed by general cooling toward present time.
•
35 My: 1st formation of Antarctic ice sheet; 14 My: 2nd Antarctic Ice sheet.
•
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.
•
1.0 My: Large Northern Hemisphere glaciations, with oscillations at orbital cycles.
•
22,000 to 18,000 years: Last glacial maximum. Begin warming to present climate.
•
6,000 years: present Holocene (generally) warm 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.
Growth of Continents vs Time
Age, in By
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
Play Movie Here
The Wilson
Wilson Cycle
The
Cycle
Old Pacific Crust (~160 to 175 My old, and very dense)
The old, heavy Pacific Plate
is sinking into the lighter
mantle, and subducting
beneath the Phillipine Plate.
The melting of the wet
sediments from the Pacific
Plate causes the formation
of the Mariana Island Arc.
The Wilson
Wilson Cycle
The
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 the supercontinent Pangea reformed.
The world we live in.
The break-up of
the Super Continent
Pangea
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 (on long time-scales), i.e.,
• seafloor spreading rates,
• rates of subduction,
• mountain-building and
• rate of weathering.
HIGH RATES OF SEA FLOOR SPREADING
1. bring more new CO2 up from the mantle,
2. increase the amount of old CO2 emitted from subduction volcanoes,
3. increase the rate of mountain building and the exposure of new
rocks to weathering,
4. raise sea level, changing earth's albedo.
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.
Fast seafloor spreading – high CO2 input:
both at mid-ocean ridges (new CO2) and
at subduction zones (re-cycled CO2).
Slow SeaFloor Spreading, low CO2.
And this variation in CO2 input has
both positive and negative
feedbacks.
BOTH examples => are negative
feedback
• CARBON SINKS (from the surface of the earth)
• WEATHERING OF ROCKS (pulls CO2 out of the atmosphere)
• RIVERS – transports soluble carbon to ocean.
• OCEAN / PHYTOPLANKTON (converts soluble carbon to insoluble
solids – cell walls and fecal pellets). Which then fall to the seafloor.
• SEDIMENTS (carbon is buried and temporarily out of the loop).
• CARBON RECYCLED
•
SEDIMENTS ARE SUBDUCTED, ORGANIC COMPOUNDS BROKEN DOWN
BY HEAT AS OCEAN SLAB IS CARRIED INTO MANTLE,
•
CO2 EMITTED BY SUBDUCTION VOLCANOS.
Weathering – a primary
sink of CO2 from the
atmosphere!
<= Note: carbonate
weather produces no
net sequestering of
CO2!
<= Note: 2 molecules
of CO2 are consumed,
and only 1 is given
back to atmosphere.
Net sink of CO2 from
atmosphere.
Largest Reservoir of carbon