Chapter 11: Evolution of the Earth

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Chapter 11 Evolution of the Earth

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Early history

• Earliest evidence for oceans?
• - oldest whole rock samples, 4 billion yrs
  (northern Canada)
• - composites of basalts typical of ocean
  crust, rock somewhat like continents.
• Oxygen record
• A. Zircons (zirconium sulfate) very hard
  gem like quality, dated by U-Pb system.
• - resistant to melting
• - dates 3.9 4.3 billion
• - ratio of O-18 / O 16 formation
  environment, particularly presence of water.
• - 1 of zircons, dated 4.3 billion yrs, show
  evidence that water was circulating in crust.

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• B. Cherts Sedimentary form for silica (SiO2)
• (small cyrstals or glassy
  forms)
• - isotopic ratio of oxygen is
  sensitive to type of environment in which chert
  formed
• - for ocean sediments, oxygen
  isotope ratio increases with increasing
  temperature (deduced from experiments).
• - uncertainties local measure
  of T, not global oceanic values of ratio
  affected by large scale glaciations
• - find has been a general
  decrease in ocean temperature. Earliest oceans
  much hotter than today, so only more heat
  resistant microbes would be selected.

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• Early Earth Atmosphere(s).
• First Atmosphere
• - Composition - Probably H2, He
• - These gases were probably lost to space
  early in Earth's history because
• Earth's gravity is not strong enough to hold
  lighter gases
• Earth still did not have a differentiated core
  (solid inner/liquid outer core) which creates
  Earth's magnetic field (magnetosphere) which
  deflects solar winds.
• - Once the core differentiated the heavier
  gases could be retained
• Second Atmosphere
• Produced by volcanic out gassing.
•  
• - Gases produced were probably similar to
  those created by modern volcanoes (H2O, CO2, SO2,
  CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4
  (methane)
• - No free O2 at this time (not found in
  volcanic gases)
• 
  

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Earliest evidence for life

• Need a good clock for biochemical processes.
• - these rely on carbon uptake so look at
  long-lived isotopic ratio.
• - 13C/12C is preferred since living things
  take up the lighter isotope 12C
• Banded iron formation (BIF) layers of iron
  rich sediment interspersed with chert, show
  evidence of high isotopic ratio
• Embedded zircons provide age 3.85 billion
  years.
• - iron sedimentation varies strongly with
  oxygen content. perhaps as consequence of oxygen
  production such as early photosynthesis?

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• Banded iron formation
• Red layers are iron cherts
• (from http//geology.about.cotmm/library/bl
  /images/blbif.htm )

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• Banded Iron Formation (BIF) - Deep water deposits
  in which layers of iron-rich minerals alternate
  with iron-poor layers, primarily chert. Iron
  minerals include iron oxide, iron carbonate, iron
  silicate, iron sulfide.
• BIF's - major source of iron ore, b/c they
  contain magnetite (Fe3O4)
• Common in rocks 2.0 - 2.8 B.y. old, but do not
  form today.

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Fossils of early micro-organisms?

• Stromatolites layered remnants of biological
  activity in bacterial colonies enriched light
  carbon found in these
• - rocks lt 3 billion yrs old show widespread
  stromatolites and other biomarkers.
• - some evidence, although disputed, is that
  this is seen also at 3.5 Byr.
• Best evidence life began at least 3.5 Byr ago.

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Oxygen history of early Earth.
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CO2 and a massive early Green house effect

• Equate the energy absorbed by the Earth from
  incoming radiation, with energy by the Earth
  (this is the black body radiation condition
  again)
• where A is reflectivity of the atmosphere, or
  albedo.
• - For A 0.33 find T256 K below freezing
  point of water!
• - Atmosphere must have some way of retaining
  heat allowing warmer temperatures.
• Greenhouse effect optical light penetrates
  atmosphere and warms surface of Earth
• - Reradiation is at infrared wavelengths
  which is absorbed by atmosphere
• - Lack of transparency due to carbon
  monoxide, methane, and water . Gives T288K for
  Earth

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Evolution of the Sun and effect upon Earth

• When sun started burning on main sequence,
  luminosity was lower (70 of present value)
• So flux from Sun was lower so that temperature
  of Earth would be lower as well
• T255K then (33K lower than
  todays average temperature)
• How much CO2 was needed and how much was
  around?
• - CO2 is mostly locked up in sedimentary
  rock in carbonates. Amount present comparable
  to total amount that is in Venus atmosphere
• - the above are 105 more massive than
  amount of CO2 in atmosphere..

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• This amount of CO2 placed in atmosphere,
• can keep ocean warm. Remember, early sun less
  luminous, and Earth is 30 farther from Sun than
  Venus.
• Sustain over long time need to decrease levels
  with time ie dissolve CO2 in ocean depositing
  carbonates.

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Carbon-silicate cycle plate techtonics and
weathering

• Removal time for CO2 out of atmosphere is only
  a million yrs! conversion of calcium and
  bicarbonate accomplished by shell-forming
  organisms
• What replaces such rapid loss?
• - plate techtonics Subduction of plates
  beneath others heats up rock, which melts at T
  around 1000K.
• Calcium carbonate reacts with silicates and
  water in this high pressure environment,
• releasing CO2. which gets back into
  atmophere by escape through volcanoes near
  subduction region.
• Cycle time 60 million yrs.

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• Plate tectonics
• (map of earthquake activity)
• Plates are buoyant and float on underlying
  plastic mantle
• Two types of plates oceanic (basalt) and less
  dense continental (granite)

Subduction zones located generally near edges of
continents (eg. Japan, Alaska,..)
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Stabilizing the Earths weather systems

• When climate unusually warm
• - higher rainfall -gt increased erosion
• -gt increased
  binding of CO2 from atmosphere into carbonates
• -gt temperature
  drops
• When climate unusually cold
• - lower rainfall and lower rate of loss
  of CO2 due to erosion,
• - CO2 input into atmosphere from
  volcanic activity -gt raises temperature
• Together, this constitutes negative feedback
  loop
• Life affects this plants accelerate trapping of
  CO2
• decreasing it in atmosphere,
  increasing cycling rate of CO2 into mantle

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Origins of continents

• Large continents - arose less than 3Byr ago
  rock record shows ancient material very small
  fraction of total stable crust even taking into
  account cycling time of continents.
• Less than 0.2 of Earths volume has been
  transformed into granite crust associated with
  continents
• Formation of iron core leads ultimately to mantle
  depleted in iron which when melted produced
  basalt.
• Making granite is different not well
  understood. As it builds up -gt larger
  continents, and fewer of them.
• Collisions of these create supercontinents every
  500 million yrs. -gt reduces area of ocean floor
  that is subducted
• reduces mountain building -gt less erosion
  from rainfall engendered by high mountains
• Breakup of continents -gt much more rapid
  scrubbing of CO2 from atmosphere -gt cooling -gt
  Snowball earth episodes?

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The rise of oxygen

• Today largest source for oxygen
  photosynthesis largest sink is respiration and
  decay (Table 11.2)
• When was atmosphere first oxygen rich
• 2.2 - 2.4 Byr ago
• Earliest appearance of oxygen 3.5 Byr ago..
• Why the difference? Oxygen produced was first
  absorbed into minerals which deposited as ocean
  sediments before it could build up and
  accumulate in atmosphere
• Oxygen added to ferrous iron (FeO) in water
  produces
• Fe2O3 - which is much less soluble and
  precipitates out
• Probably origin of BIFs. These have ages 3.5
  1.8 Byr ago..
• Bottom Line appearance of oxygen producing life
  had profound geochemical effects, as well as
  chemical warfare on earliest organisms which
  were poisoned in these new, oxygen rich
  environments