Paleoclimate Review

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Paleoclimate Review

• Miriam Jones
• February 3, 2006

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Scales in Paleoclimate

• Tectonic scale (Millions of years) faint young
  sun, snowball earth, BLAG vs. uplift weathering
  hypothesis
• Orbital Scale (focusing on the last 3 Myr)
  Monsoon, ice sheet, CO2, and CH4 cycle
• Deglacial and Millennial Scale LGM, YD, Heinrich
  event, and D-O cycle
• Historical climate change little ice age, the
  Medieval Warm Period etc.

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How does plate tectonics influence climate?

• 1. Location of continents
• 2. Mountain building- alters atmospheric flow
• 3. Open/close ocean gateways
• 4. Sea-level change- modifies ratio of land to
  ocean
• 5. Altering weathering rates- linked to
  concentration of CO2 in atmosphere
• 6. Altering rates of outgassing- linked to
  concentration of CO2 in atmosphere

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BLAG hypothesis Plate tectonics influence global
climate by moderating atmospheric CO2
concentrations
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Uplift weathering hypothesis Uplift accelerates
chemical weathering, drawing down CO2, and
cooling the global climate.
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Faint Young Sun Paradox

• The Sun's luminosity has increased through
  geologic time due to a nuclear reaction in the
  Sun's interior that fuses nuclei of hydrogen
  together to form helium. This nuclear reaction
  has caused the Sun to expand and become brighter.
  Consequently, the early Sun shone 25-30 less
  brightly than it does today.
• This raises a paradox. At such a low solar
  luminosity, we would expect all water in Earth to
  have been frozen. Yet, sedimentary rocks provide
  evidence of running water at least 4 billion
  years ago. Some mechanism must have kept Earth
  warm. Yet, wouldn't the same mechanism cause the
  Earth to be intolerably hot today?
• It has been hypothesized that the solution to the
  faint young sun problem is that outgassing from
  volcanoes was high due to vigorous seafloor
  spreading. At the same time, weathering was very
  low due to a dearth of continents. Thus,
  atmospheric CO2 was much higher than today,
  providing a healthy greenhouse effect to keep the
  early Earth warm.

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Long-term carbon cycle

• Carbon added to atmosphere through metamorphic
  outgassing and outgassing of volcanoes and
  mid-ocean ridges
• Hydrolysis-weathering of silicate minerals in
  continental crust
• CaSiO3 H2CO3 CaCO3 SiO2 H2O
• The products of continental weathering are
  transported to the oceans by rivers, where they
  are used to make CaCO3 and SiO2 shells of marine
  organisms. When these organisms die, many of them
  are deposited and buried on the seafloor. The
  carbon cycle is completed upon subduction and
  melting of these sediments. The melt may rise as
  magma, providing volcanoes and MORs with a source
  of recycled CO2.

Important flows of carbon on 100,000 year time
scales
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Long-term carbon cycle

• Chemical weathering can also occur through a
  process called dissolution, the chemical
  weathering of carbonate sediments (CaCO3)
  (limestone, for example). Dissolution can be
  described by the following reaction
• CaCO3 H2CO3 CaCO3 H2O CO2
• Note, however, that the net removal of
  atmospheric CO2 is 0. CO2 is taken from the
  atmosphere to make carbonic acid, but is released
  to the atmosphere during the creating of CaCO3
  shells.

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Factors that control chemical weathering

• 1. Temperature- chemical weathering increases
  with increased temperatures
• 2. Precipitation- increased precipitation raises
  the level of groundwater in soils, promoting the
  production of carbonic acid
• 3. Vegetation- plants extract CO2 from the
  atmosphere and deliver it to soils, where it
  combines with groundwater to make carbonic acid

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Atmospheric CO2 through Earth history
How to explain long-term changes in CO2?
According to Berner (1994) 1. increase in solar
radiation has caused gradual drop in atmospheric
CO2 2. high CO2 during Mesozoic and decrease in
Cenozoic are due to high Mesozoic relief and
Cenozoic mountain uplift combined with decreasing
metamorphic/volcanic degassing of CO2 during
Cenozoic 3. variable degassing, due to changes in
seafloor spreading was not a major control on CO2
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Milankovitch theory orbital-scale control of ice
sheets

• In the last 3 million years, the ice sheets over
  North America grew and melted over short
  intervals.
• Summer insolation controls North Hemisphere ice
  sheet growth. Ice growth occurs during times when
  summer insolation is low in high northern
  latitude.

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Orbital forcing Milankovitch Theory

• Obliquity 41, 000 yr cycle

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Orbital forcing Milankovitch Theory
Eccentricity 100,000 years
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Orbital forcing Milankovitch Theory
Precession 23,000 years
The major axis of each planet's elliptical orbit
also precesses within its orbital plane, in
response to perturbations in the form of the
changing gravitational forces exerted by other
planets. This is called perihelion precession.
It is generally understood that the
gravitational pulls of the sun and the moon cause
the precession of the equinoxes on Earth which
operate on cycles of 23,000 and 19,000 years.
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Orbital scale insolation change
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The 100k orbital cycle dominates this time period
(record of dust, CO2, and temperature
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Northern Hemisphere Ice sheet History
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Orbital monsoon hypothesis

• Changing seasonal insolation will change the
  strength of the monsoons. Stronger summer
  radiation will strengthen the summer monsoon.
  Weaker winter radiation will strengthen the
  winter monsoon. It turns out that the African
  monsoon is very sensitive to insolation
  variations.
• The African monsoon is responsible for
  precipitation over northern Africa. Today, the
  summer solstice occurs at aphelion. So, the
  summer insolation is near its minimum. As a
  consequence, northern Africa summer monsoon is
  weak.
• Although the strength of the winter monsoon also
  varies, it has less impact on the African
  environment because the winter monsoon has little
  affect on precipitation over Africa.

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Evidence for an orbitally-controlled monsoon
1. Lake levels across North Africa 2.
Mediterranean circulation and deposition of
marine sediments 3. Freshwater diatoms (small
plant plankton) in the tropical Atlantic 4.
Upwelling in the equatorial Atlantic
Relationship between summer radiation and
African monsoon (from Earth's Climate Past and
Future by W.F. Ruddiman).
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Paleoclimate proxies

• Lithologic indicators
• Packrat middens
• Tree rings
• Corals
• Ice Cores
• Foraminifera other marine organisms
• Oxygen isotopes (temperature and ice
  volume)
• Terrestrial fauna flora

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Millenial Scale Climate Change

• Last glacial maximum (LGM) 21kya
• Bolling/Allerod warming- Younger Dryas
  cooling13-11.9kya
• Heinrich events
• Dansgaard-Oeschger events

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Last glacial maximum

• Cold
• Continent-sized ice sheets (Laurentide ice sheet
  over North America)
• 110m lower sea level than present
• Dry and windy

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Tropical debate over LGM cooling

• Small tropical cooling (2C ) CLIMAP
  reconstruction based on the changes in planktic
  fauna and flora in the low-latitude oceans. Other
  evidences biochemical composition of plankton
  shells (double bonds of alkenones), del18O
  measurements on the CaCO3 shells of plankton.
• Large tropical cooling (5C ) Mountain glacial
  ice line change, noble gases dissolved in
  glacial-age groundwater.
• GCMs can only get level of ice sheet and tropical
  glacier growth with 5ºC shift in tropical
  temperature

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Abrupt climate change

• Heinrich event ice-rafting event, terrigenous
  material found in deep-sea cores, corresponding
  to Greenland ice core low del18O.
• Dansgaard-Oeschger cycle A series of warm-cold
  oscillation punctuated the last glaciation from
  15 to 110 Kyr BP. The D-O cycles have been marked
  by abrupt terminations, and often by abrupt
  onsets.

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Heinrich and D-O events
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Antarctic Record v. Greenland
An absence of D-O events in Antarctica
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Younger Dryas
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Younger Dryas

• The Younger Dryas was first detected from layers
  in north European bog peat, and named for the
  alpine/tundra plant Dryas octopetala.
• It was a brief (approximately 1300 /- 70year
  1) cold climate period following the
  Bölling/Allerød interstadial at the end of the
  Pleistocene, and preceding the Preboreal of the
  early Holocene.
• It is dated approximately 12,900-11,500 BP
  calibrated, or 11,000-10,000 BP uncalibrated, but
  dating is difficult because it occurs during a
  radiocarbon plateau

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Younger Dryas

• The prevailing theory holds that the Younger
  Dryas was caused by a significant reduction or
  shutdown of the North Atlantic thermohaline
  circulation in response to a sudden influx of
  fresh water from Lake Agassiz and deglaciation in
  North America. The global climate would then have
  become locked into the new state until freezing
  removed the fresh water "lid" from the north
  Atlantic Ocean. This theory does not explain why
  South America cooled first.

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Younger Dryas

• A problem with this hypothesis is the timing of
  meltwater pulses that are supposed to have
  triggered the THC shutdown it was found that a
  second meltwater pulse, albeit slightly smaller
  than the first one, occurred at the end of the YD
  (Fairbanks, 1989) why didn't it also trigger a
  similar chain of consequences in the climate
  system?
• An alternate explanation (Clement et al., 2001)
  invokes the abrupt cessation in the El Nino
  -Southern Oscillation in response to changes in
  the orbital parameters of the Earth, although how
  such a change would impact regions away from the
  Tropics remains to be explained.
• For further discussion, see Broecker, WS., Does
  the trigger for abrupt climate change reside in
  the oceans or in the atmosphere? Science 300
  (5625) 1519-1522 JUN 6 2003.

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Medieval Warming

• 10th century-14th century in Europe global
  extent has been questioned
• Coincided with a peak in solar activity

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Little Ice Age

• A period of cooling from approx. 14th-19th
  century, occurs after the medieval warming,
  though there seems to be little global agreement
  on the timing.
• Most evidence in Europe and north America
• Hypotheses of the cause include decreased sunspot
  activity (Maunder minimum) and increased volcanic
  activity, others claim it had to do with a
  decrease in population resulting from the black
  death and thus a decrease in agricultural activity