Precambrian Climates

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					Precambrian Climates
           A Quick Review
• Archean Tectonics - Small Protocontinents

• Proterozoic - Amalgamation into larger
  continents

• Rodinia, Iapetus, and Mirovia
Rodinia
~ 1 Ga
          Rifting of Rodinia
• Initial Rifting ~700 Ma




• Iapetus ~650 Ma
               GRENVILLE Orogeny
               1.3-1.0 billion years ago
• Prior to this orogeny, a westward-dipping subduction zone
  existed along the east coast of Proto-North America,
  adjacent to the Pre-Grenville Ocean.
• As subduction of the Pre-Grenville Ocean occurred, a huge
  continent from the opposite site of this ocean slammed into
  Proto-North America, creating a Grenville supercontinent.
• This was a very active tectonic environment, affecting an
  area from Labrador, Canada, south through Georgia and
  Texas into Mexico, forming mountains as high as the
  Himalayas.
• The resulting mountains, the Grenville Plateau, were a
  source for sediment all over North America.
  RIFTING of the Grenville Supercontinent
          660-550 million years ago
• A rift zone formed along the suture zone where
  continents had joined during the Grenville
  Orogeny.

• The splitting of the Grenville Supercontinent
  formed the Iapetus Ocean.

• About 550 million years ago, the Taconic Island
  Arc formed in the Iapetus Ocean, as a result of a
  subduction zone that developed to the east.
             Iapetus Ocean
• In Roman mythology, Iapetus was the father
  of Atlas.

• Atlas is the namesake for today's Atlantic
  Ocean.
         Panthalassic Ocean
• Great Ocean that covered the most of the
  planet.

• Extension of the Mirovia
Archean & Proterozoic Climates
• Rocks

• Sediments

• Glaciers

• Ice Sheets
Archean & Proterozoic Climates
• Rocks

• Sediments

• Glaciers

• Ice Sheets
                   Rocks
• Tell us that the Earth’s surface was cool
  enough for rocks to solidify

• Prior to the rock formation, may have had
  magma ocean
 Oldest Rocks
Isua Greenland
The Acasta Gneiss Complex, Northwest
         Territories, Canada
             The story of zircon




The Jack Hills region of   Microscopic view of a zircon
Western Australia, where   (zirconium silicate) crystal
the zircons were           determined to be 4.4 billion
discovered.                years old.
The story of zircon
         Earth's Oldest Sedimentary Rocks




Earth's oldest sedimentary rocks, found in Greenland, are about 3.9
billion years old. Unusual chemical traces in these rocks may suggest
that life existed when they formed. Image courtesy of Dr. Graham
Ryder.
Glaciations
The Gowgonda Glaciation
     The Gowgonda Glaciation
• Tillite

• Varved Sediments

• 2300 Ma

• Overlays 2600 Ma igneous rock and is cross
  cut by 2100 Ma dike
Snow Ball Earth Hypothesis

   When the Earth Froze Over
             Earth’s Climate
• 18,000 years ago were experienced one of
  the largest glaciations of the last 500 million
  years.
Reconstructed Ice Sheet at 18 kyr
            Earth’s Climate
Over the past million years we have
 experience 10 of these glacial-interglacial
 cycles.
(In terglacial)
     Warm




                      Present
(Glacial)
  Co ld




                  0      50     100   150     200    250     300     350   400   450   500

                                            Thousands of years ago
             The Evidence
• While these changes are dramatic, they pale
  in comparison to glacial intervals that
  occurred during the late Neoproterozoic.

• The following is based largely on a
  Scientific American article by Paul
  Hoffman and Dan Schrag at Harvard.
               The Evidence
• Dropstones

• Paleomagnetics

• Sedimentology

• Scenario
              Glaciations
• In 1964, Brian Harland at Cambridge
  University postulated that the Earth had
  experienced a great Neoproterozoic ice
  age.
• He pointed out that Neoproterozoic
  glacial deposits, similar in type to those
  of the Pleistocene, are widely
  distributed on virtually every continent.
                         Glaciations




QuickTime™ and a TIFF (Unco mpressed) d ecompressor are needed to see th is pictu re.
               Glaciations
• Harland could only speculate on the positions
  of continents in Neoproterozoic time and
  could not rule out the possibility that various
  continents were glaciated at different times as
  they drifted close to the poles.
• Nevertheless, he inferred that ice lines
  penetrated the tropics from the occurrence of
  glacial deposits within types of marine
  sedimentary strata characteristic of low
  latitudes.
             Glaciations
• In response to the Nuclear
  Winter Hypotheses in the
  1960s, Mikhail Budyko
  made some calculations
  concerning climate. These
  had interesting
  implications for the
  postulated tropical Ice
  sheets.
              Climate Notes:
• The Earth's climate is fundamentally controlled by the
  way that solar radiation interacts with the Earth's
  surface and atmosphere.
• We receive ~343 watts per square meter of radiation
  from the Sun.
• Some of this is reflected back to space by clouds and
  by the Earth's surface, but approximately two thirds is
  absorbed by the Earth's surface and atmosphere,
  increasing the average temperature.
          Climate Notes:
• Earth's surface emits radiation at longer
  wavelengths (infrared), balancing the
  energy of the radiation that has been
  absorbed.

• If more of the solar radiation were
  reflected back to space, then less
  radiation would be absorbed at the
  surface and the Earth's temperature
  would decrease.
           Climate Notes:
Surface albedo = radiation is reflected
Snow has a high albedo (~0.8)
Seawater has a low albedo (~0.1)
Land surfaces have intermediate values that
  vary widely depending mainly on the types
  and distribution of vegetation.
When snow falls on land or ice forms at sea,
  increased albedo causes greater cooling,
  stabilizing the snow and ice.
This is called ice-albedo feedback.
Glaciations
Glaciations
             Glaciations
• Where were the continents?

• Tropics

  – Sediments are carbonates

  – paleomagnetism
 Where were the continents?
• Namibia




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   Tropics

 Paleolatitude
(calculation:

   tan () =
  0.5*tan(I)
           Paleomagnetic - Problem

NRMs are acquired when rocks formed, so long as they had not
  subsequently been heated above their Curie temperature (580-
  680 degrees Celsius). However, it was subsequently learned
  that many rocks, particularly sedimentary rocks, may be
  chemically remagnetized at much lower temperatures if
  subjected to prolonged groundwater percolation. Many of the
  early paleomagnetic measurements were shown to be from
  remagnetized samples and the rest were suspect.
    Paleomagnetic - Problem Solved
Joe Kirschvink reexamined South Australian
  Neoproterozoic glacial deposits giving shallow
  inclinations.

These had the least chance of being
 remagnetized because South Australia was
 never at low latitude in the last 400 million
 years.
    Paleomagnetic - Problem Solved
Another reason to look at South Australia:
Glacial deposits formed close to sea level;
 sediments of tidal origin.
Modern tropical glaciers are found but not below
 5000 meters above sea level. During LGM,
 equatorial ice lines in the Andes were no lower
 than 4000 meters above sea level.
    Paleomagnetic - One more Point
• Linda Sohl at Lamont-Doherty Earth
  Observatory documented as many as six
  polarity reversals in the South Australian
  glacial deposits. The frequency of polarity
  reversals of the Earth's magnetic field is such
  that the glacial deposits must represent a
  minimum of several 100,000s and more likely
  millions of years.
• Consistent with the time scale of a "snowball"
  Earth
                                                                                   Namibia - Then


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                                                                                                             QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.




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           Glaciations
• Budyko's model results helped stimulate
  interest in the science of climate
  modeling, but few believed that the
  Earth had ever actually experienced a
  runaway ice albedo.

• Extinctions

• Escape clause
            Glaciations
• The first of these objections began to
  fade in the late 1970s with the discovery
  of remarkable communities of
  organisms living in deep-sea
  hydrothermal (hot water) vents, and
  later in the extremely cold, dry,
  mountain valleys of East Antarctica.
           Glaciations
• Some of these organisms appeared
  capable of surviving global
  glaciation and their existence in the
  Neoproterozoic was unquestioned
  — molecular studies showed that
  they disproportionately represent
  the oldest branches in the universal
  tree of life.
            Glaciations
The key to the second problem—

reversing the ice-albedo feedback

—is plate tectonics.
            Glaciations
• In the late 1980s, Joe Kirschvink at the
  California Institute of Technology
  pointed out that during a global
  glaciation, what he termed a "snowball"
  Earth, the supply of carbon dioxide to
  the atmosphere and oceans from
  volcanism would continue because of
  plate tectonics.
            Glaciations
• However, if the Earth were so cold that
  there were no liquid water on the
  continents, weathering reactions would
  effectively cease, allowing carbon
  dioxide to build up to incredibly high
  levels.
• Eventually, the carbon-dioxide-induced
  warming would offset the ice albedo,
  and the glaciation would end.
               Glaciations
Ken Caldeira and Jim Kasting at Pennsylvania
  State estimated that roughly 0.12 bar of
  carbon dioxide (about 350 times the present
  concentration) would have been required to
  overcome the albedo of a snowball Earth.

Assuming current rates of volcanic carbon
  dioxide emissions, a Neoproterozoic
  "snowball" Earth would have lasted for
  millions to tens of million of years before the
  sea ice would begin to melt at the Equator.
Glaciations
        More About Iron




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          More About Iron
• Iron is the paleomagnetists’ favorite element,
  and iron gave Joe Kirschvink another reason
  to favor the "snowball" Earth. Several
  examples of Neoproterozoic glacial
  deposition in marine waters are unusually rich
  in iron oxides and sulfides.
• In fact, they were the object of international
  iron-ore exploration after World War II, and
  belong to a class of sedimentary ore deposits
  called banded iron-formation or BIF, which is
  otherwise restricted to a much earlier time in
  Earth history.
          More About Iron
Modern seawater contains less than one part
 per billion of iron because iron in its oxidized
 form (Fe 3+ ) is quite insoluble. However, in
 its reduced form (Fe 2+ ), iron is relatively
 soluble.
Most BIF occurs in rocks older than 1850 million
 years and is believed to have formed at a
 time when the atmosphere had little free
 oxygen, and seawater in the deep ocean
 contained abundant iron.
More
About
 Iron
         Cap Carbonates
• Neoproterozoic glacial intervals are
  blanketed by peculiar "cap" dolostones
  (equimolar calcium-magnesium
  carbonate).
• The transition from glacial deposits to
  "cap" dolostone is abrupt and lacks
  evidence of significant hiatus.
• Neoproterozoic glacial epochs closed
  with "abrupt climatic warmings".
Carbonate-Silicate Cycles
    • How to get into and out of
      an Ice Age
  Carbonate-
Silicate Cycles
       Carbonate-Silicate Cycles
• Recall the transient conditions unique to the
  end of a "snowball" Earth. An ultra-high
  carbon dioxide atmosphere is needed to raise
  temperatures to the melting point at the
  Equator.
• Once melting begins, the ice-albedo feedback
  is reversed and combines with the extreme
  greenhouse atmosphere to drive surface
  temperatures upward.
        Carbonate-Silicate Cycles
• Warming proceeds rapidly because the
  change in albedo begins in the tropics, where
  insolation and surface area are maximal.
• Evaporation resumes adding, water vapor to
  the atmosphere adds powerfully to the
  greenhouse effect.
• Tropical sea-surface temperatures may have
  approached 50 °C in the aftermath of a
  "snowball" Earth
        Carbonate-Silicate Cycles
• Intense chemical weathering of silicate rocks
  and dissolution of carbonate rocks would
  follow, the low pH of carbonic acid rain, and
  the large surface area of frost-shattered rock
  and rock "flour" produced by the grinding
  action of glaciers.
• The products of chemical weathering
  reactions, cations and bicarbonate, would be
  delivered by rivers to the ocean, where they
  would neutralize the acidity of the surface
  waters.
        Carbonate-Silicate Cycles
• Massive precipitation of inorganic carbonate
  sediment in the rapidly warming surface
  ocean.



• "Cap" dolostones are no paradox; they are the
  expected consequence of the "ultra-
  greenhouse" conditions unique to the transient
  aftermath of a "snowball" Earth
Carbonate-Silicate Cycles



    QuickTime™ and a TIFF (Unco mpressed) d ecompressor are needed to see th is pictu re.
Carbonate-Silicate Cycles



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           Carbon Isotopes
• Volcanic Input = -7‰

• Output = Input

• CaCO3 = 80%            ~5‰
• Organic matter = 20%   ~-25‰
           Carbon Isotopes
• The carbonate rocks beneath the glacial
  deposits are enriched in 13C by as much as
  15‰ relative to volcanoes, much more than
  modern carbonate sediments. This enrichment
  extends over many 100s of meters of section,
  interpreted as representing at least 10 million
  years. This implies that burial of organic
  carbon in the Neoproterozoic accounted for
  nearly 50% the total carbon removed from the
  ocean.
  Carbonate-
Silicate Cycles
           Carbon Isotopes
• But just before the glacial deposits, 13C
  values decrease to levels equivalent to the
  volcanic source (-7‰).

• This drop persists through the cap carbonates
  atop the glacial deposits, and then slowly
  rebounds to higher 13C values several
  hundred meters above.
 Carbon
Isotopes   Qui ckTi me™ and a TIFF (Uncompressed) d eco mpressor are nee ded to se e th is p ictu re .
           Carbon Isotopes
• The drop in 13C just before the glaciations
  can be explained as a drop in biological
  productivity as ice formed over the oceans at
  high latitudes, and the Earth teetered on the
  edge of a runaway ice-albedo feedback. Once
  ice covered the oceans entirely, biological
  productivity would have essentially ceased.
  Carbonate-
Silicate Cycles
Carbonate-Silicate Cycles
Carbonate-Silicate Cycles
                Pop Quiz
What process or force is postulated as the
 mechanism that allowed the Earth to get
 out of the Snow Ball Earth scenario?

A. Gravity
B. Plate Tectonics
C. Photochemical dissociation
Putting all
 together

				
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