Greenhouse Earth

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Greenhouse Earth
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Why was it warm 100 Myr?

• Middle Cretaceous was characterized by
• a global sea level 200 meters higher than today
• Fossil evidence of warm-adapted vegetation
• Dinosaurs!

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More shallow seas, less land
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Cretaceous climate and todays climate

• Main difference?
• Warmer temps at the poles
• Models have tried to match climate 100 Myr using
  Cretaceous geography. Didnt match data
• Model tried increasing CO2 by 4 times. Still
  didnt work.
• Why?

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Solution 1 blame the data

• Maybe modern organisms dont correctly represent
  fossils (modern species less cold tolerant?)
• Why would they do this?? Unlikely
• Shallow seas 100 Myr would moderate climate.
• Best preservation of record in shallow oceans
• Underrepresentation of cold climates?
• Post-depositional alteration of materials
• Isotopes have been changed/overprinted
• Recent studies indicate tropical oceans may have
  been warmer than we think. SO. . . Given that we
  can ramp up CO2 and warm poles. Possible solution

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Solution 2 blame the model

• GCMs dont represent ocean circulation very well
• Maybe oceans circulated more heat to the poles?
  Ocean heat transport hypothesis. This would
  resolve the data
• Maybe deep water formed differently? Maybe in
  shallow salty seas?

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Solution 2 blame the model

• GCMs dont represent ocean circulation very well
• Maybe oceans circulated more heat to the poles?
  Ocean heat transport hypothesis. This would
  resolve the data
• Maybe deep water formed differently? Maybe in
  shallow salty seas?
• Fossil data shows palmtrees at high latitudes.
  Models freeze high latitudes
• Not enough fossil data?
• Lakes made for a warmer climate at high latitudes?

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Changes in sea level over time

• Sea level in Cretaceous 200 meters higher than
  today-why?

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How to change sea level change the volume of the
ocean basins

• How do you do that?
• 1) Change volume of ocean ridges
• 2)

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How to change sea level change the volume of the
ocean basins

• How do you do that?
• 1) Change volume of ocean ridges
• 2) collide continents

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How to change sea level change the volume of the
ocean basins

• How do you do that?
• 1) Change volume of ocean ridges
• 2) collide continents
• 3) Make volcanic plateaus

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How to change sea level change the climate

• 1) Change the amount of water stored in ice
  sheets
• 2) cooler water contracts, warmer water expands

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Meteorite impacts, extinctions, and climate change
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The K-T extinctionWhy? Climate change?
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Geologic evidence at the K-T boundary

• Sediments with layer enriched in iridium (rare on
  earth, abundant in some meteorites)
• Distinct clay layer with shocked quartz

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Chicxulub, Yucatan Peninsula, Mexico

• The Chicxulub impact (CHEECH-uh-loob) is
  coincident with one of the largest mass
  extinction events in Earth's history, which
  occurred 65 million years ago.
• Are these two events connected? Could an impact
  event have caused the deaths of so many plants
  and animals, and, if so, how?

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Impact effects (much worse than Katrina)

• Shock wave flattened everything for hundreds of
  miles
• Force of impact generated equivalent of magnitude
  11 earthquake
• Giant tsunamis
• Hot debris may have caused widespread wildfires
• Soot blocked incoming radiation, cooling earth.
• But then excess CO2 (from burning) would have
  warmed Earth
• But. . .there is not a major change in climate
  that corresponds with this event

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Timescales of climate change
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The last 55 Myr

• Estimates of temperature trends using shapes of
  fossil leaves
• Progressive cooling of the mid latitudes of the
  N. Hemisphere during last 55 Myr

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Oxygen Isotopes and Climate

• Oxygen isotopes provide a very important record
  of past changes in climate

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Planktonic foraminifera and oxygen isotopes

• PLANKTON are primary source of data on changes in
  climate in oceans
• The oxygen in shells consist of two isotopes 18O
  and 16O
• Isotopic composition of shells changes in
  response to 2 things
• 1) Changes in ICE VOLUMES in continental ice
  sheets
• 2) the local OCEAN TEMPERATURE where the shell
  formed
• SO. . .shell isotope values tell us about changes
  in ice volume and changes in temperature

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Oxygen Isotopes Ratios

• Delta 18O values range from 4 to -2
• Large amounts of 18O ? positive 18O values18O
  enriched (or 16O-depleted)
• Small amounts of 18O ? negative 18O values ? 18O
  depleted (or 16O-enriched)

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Planktic and Benthic foraminifera

• Planktic foraminifera live in upper 100 meters
  (oxygen in shells is from surface water)
• Benthic foraminifera live on sea floor (oxygen
  in shells is from deep water)
• So what? Planktic forams tell you about
  temperatures in upper ocean, Benthic tell you
  about deep ocean temperatures

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ocean temperature and oxygen isotopes

• As ocean temperature increases, the d 18O ratio
  decreases (less 18 O and more 16O)
• For each 4.2 0C increase in temperature, the d18O
  ratio decreases by 1
• So. . .cant we just measure these ratios and
  reconstruct temperature?
• Noice volumes also change isotope ratios

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Ice volume and oxygen isotopes

• 16O is lighter than 18O and evaporates more
  easily
• 16O evaporates from tropics, gets transported to
  poles
• 18O is more likely to get rained out. .
• So atmosphere gets richer in the lighter 16O and
  ocean gets richer in heavier 18O This is called
  FRACTIONATION

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• Polar snow and ice water molecules are very
  depleted in 18O

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Ice volume and oxygen isotopes

• 16O is lighter than 18O and evaporates more
  easily
• SNOW and GLACIERS at high latitudes are enriched
  in 16O
• more negative ratiosup to -55 for Antarctic
  ice sheet
• If all the 16O enriched ice melted, ocean water
  would be 1 less positive (more 16O) than it
  is today.

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Combined effects of temperature and ice volumes

• So. . If your ocean 18O values became more
  positive, this could indicate either
• 1) the oceans are cooling (because for each 4.2
  0C decrease in temp, d18O increases by 1 )
• 2) more 16O is being stored in ice, so ocean
  ratios are more positive

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So. . . How do you separate the effects of
temperature from ice volume?

• You cant, completely.
• But, you can combined geologic evidence to tell
  you if there was ice around at allif not,
  changes in oxygen isotope ratios tell you about
  water temperature changes

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70-40 Myr

• No evidence of ice
• d18O in forams increased from -0.75 to 0.75
  (change of 1.5 )
• Increase means temps must have cooled
• 1.54.2 0C/change in temp of gt6 0C

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40 Myr to today

• 35 Myr
• Evidence of ice on Earth. . But we dont know how
  much. So we dont know how much
• Between 40 Myr-to today, d18O in forams increased
  by 2.75 )
• Increase means ocean temps must have cooled,
• BUT If all the 16O enriched ice melted, ocean
  water would be 1 less positive (more 16O)
  than it is today.
• So 2.75- 1 1.75
• 1.75 4.2 0C/change in temp of gt14 0C
• Over last 40 Myr, ocean temps have cooled 14 0C

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The last 55 Myr

• Estimates of temperature trends using shapes of
  fossil leaves
• Progressive cooling of the mid latitudes of the
  N. Hemisphere during last 55 Myr
• WHY?

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BLAG?

• Does a slowing of spreading rates account for
  global cooling?
• Ummm. . Not really.
• Slow down of spreading rates to 15 Myr, but then
  rates speed back up

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Uplift weathering?

• Must have large amount of high terrain lots of
  fragmented rock lots of weathering

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Uplift weathering?

• Must have large amount of high terrain lots of
  fragmented rock lots of weathering

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Uplift weathering?

• So lots of high terrain, but is it unusually
  high?
• No continent/ continent collisions from 100-65
  Myr
• What about Rocky Mountains?

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Uplift weathering?

• Rocky mtns flooded 100-70 Myr ago.
• 1) upwarping of entire West 20 Myr due to deep
  heating?
• 2) early uplift before this, 70-45 Myr
• Only area proven to have uplifted is Yellowstone

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Sediment yields
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Sediment yields, Mg/km2/yr
Rio Grande
St. Lawrence
Colorado
Chiang Jiang
Huang He (Yellow)
Indus
Ganges
Brahmaputra
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Denudation rates, mm/kyr
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Uplift weathering?

• Weathering should have been rapid in SE Asia over
  last 55 Myr.

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Ocean gateways

• 1 opening of circulation around Antarctica
• Opening Drakes passage 25-20 Myr allowed
  circumpolar circulation
• Promotes cooling, formation of ice sheet

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Ocean gateways

• 2 closing of the Isthmus of Panama
• Occurred just before 4 Myr
• Large scale glaciation 2.7 Myr ago
• Closing Isthmus redirects warm salty water
  northshould this promote glaciations?
• (suppressing sea ice making more moisture
  available to grow ice sheets on continents?

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Last 70 million years
55-35mya Antarctica makes it to its current
location and ice builds 15mya uplift of
Himalayan mountains 2.5mya closing of the
Isthmus of Panama
From Bartlein, 1997
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