Control of atmosphere CO2 by changing sea floor spreading rate'

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Control of atmosphere CO2 by changing sea floor
spreading rate.
Taking mantle hot spots to the max the
Cretaceous Hot House
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Greenhouse Earth, The Cretaceous 100 My

• An important period for understanding potential
  anthropogenic changes in climate.
• Cretaceous was a recent example of a strong
  Greenhouse world. 12C hotter than the present
  15C.
• Geologic record is reasonably preserved, which
  indicates different pole-to-equator temperatures
  than present.
• Continental configuration is known.
• We can estimate rates of seafloor spreading.
• Do standard climate models simulate the warmth
  of this greenhouse climate? (hint no).
• If so, are high levels of atmospheric CO2
  required? (hint yes plus something else).

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Break-up of Pangea
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High Sea Level (200 m) flooded the continental
interiors
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What processes impact sea level?

• Amount of Ice on continent.
• Temperature of seawater.
• Sea floor spreading rates (shape of the ocean
  basins).
• Amount of continental margin (Mega-continent or
  lots of continental fragments).

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The impact of climate on sea level can be
intuitive particularly the change in amount of
ice sheet volume on land (NOTE sea ice doesnt
impact sea level!). But the change of albedo
that occurs when sea level covers or exposes -
continental shelves is less-intuitive. Sea water
albedo 0.06 Soil albedo 0.25 Sand albedo
0.40 So with LESS continental shelf exposed, MORE
sunlight is absorbed by the surface of the
earth. i.e., HIGH sea level absorbs more sun
light (warmer).
The impact of sea level on CLIMATE
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The change in sea floor spreading rate can also
have a dramatic change on sea level. Faster sea
floor spreading produces a hotter crust, and a
more elevated mid ocean ridge which continues
elevated even on the ridge flanks. Slower sea
floor spreading produces less elevated crust, and
a lower sea level.
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Continental margins have a lot of volume, and
displace substantial amounts of seawater. If you
have a lot of continental fragments, each will
have its own margin, and sea level will be
high. If you only have one mega-continent, you
will have less total margin area, and sea level
will be low. How does this apply to the
Cretaceous? Pangea?
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Oceanic plateaus (Large Igneous Provinces made
of basalt) can be BIG (the size of Western U.S.).
These can displace a LOT of seawater.
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• Pangaean continent broken into several smaller
  continents.
• High sea level flooded MOST continental interiors
• Note that N. Atlantic opened before the S.
  Atlantic.
• Tethyan Sea is closing India on collision path
  with SE Asia.

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Paleobotanical Evidence for Warm Climate

• Warm-adapted evergreen vegetation found above
  Arctic circle.
• Leaves of breadfruit tree found north of Arctic
  Circle.
• Today breadfruit trees found only in tropical to
  subtropical environments.
• Equator-to-pole temperature gradient different in
  Cretaceous.

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• Warm-adapted animals found at high latitudes.
• Dinosaurs, turtles and crocodiles found
  pole-wards of the Arctic and Antarctic circles.
• Coral reefs indicative of warm tropical waters
  found within 40 of equator (latitude of
  Chicago).

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Cretaceous Paleoclimate

• Faunal and floral remains provide estimates of
  Cretaceous equator-to-pole temperatures.
• NOTE that the pole-to-equator temperature
  gradient is much different than the present. WHY
  IS THIS?
• Ice distribution ocean circulation changes
  continent distribution?

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Cretaceous Paleotemperatures

• Equatorial temperatures were only a few C warmer
  than present day temperatures.
• But polar temperatures were 20-30C warmer!
• Cretaceous was an ice-free world.
• Modern Antarctic ice at high latitude is also at
  high altitude (Antarctic plateau).
• Present day polar Temperatures are very cold.
• Understanding Cretaceous climate requires
  understanding unusual equator-to-pole temperature
  gradient.
• What would this have implied for ocean
  circulation?
• For ocean stratification?

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GCM Models
Consideration of normal processes do not
replicate the Cretaceous climate or latitudinal
distribution of temperature.
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Cretaceous Climate

• CO2 at least 4x to 10x PAL
• Geography and high CO2 do not replicate global
  temperature gradient
• Higher CO2 levels increase global average
  temperature (12C).
• Questions on Cretaceous climate remain on how to
  handle
• Albedo-temperature feedback
• Water vaportemperature feedback
• Role of clouds

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Ocean Transfer of Heat

• Heat transfer through deep ocean today
• Formation of cold dense water in polar regions
  with some warm saline water from Mediterranean

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Ocean Transfer of Heat

• Deep ocean 100 My may have been filled with warm
  saline bottom water
• Cretaceous bottom water formed in tropics or
  subtropics and flowed pole-ward transferring heat

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Continental Configuration Favorable

• Large seaway covered N tropical and subtropical
  latitudes
• Seaways should have been under sinking arm of
  Hadley cell
• Dry air would have caused evaporation to exceed
  precipitation
• Increased salinity of surface water
• Explanation consistent with several large oceanic
  anoxic events (black shale formation).
• AOE may have been caused by warm saline bottom
  waters (stagnant vertical circulation, no up- or
  down-welling).

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Model Simulation

• Warm saline water could have formed in Northern
  hemisphere when salinity exceeded 37.

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Conclusions

• Attempts to model Cretaceous climate only partly
  successful.

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So we need ANOTHER process that contributes CO2
(and warmth) to the Cretaceous climate. Geologica
l Consequences of Superplumes Roger Larson
Geology, 1991. Argued that massive plumes,
arising from the core/mantle boundary during the
Cretaceous caused both an increase in Mid-Ocean
Ridge spreading rate AND built massive Large
Igneous Provinces.
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Amount of ocean crust formed vs time.
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Formation of coal, gas and oil reservoirs as a
function of geological time. Coal Pennsylvanian
(300 My) Gas pretty uniform, with some minor
peaks in production. Oil large peak in the
Cretaceous. Mostly in outcrops that surrounded
the Tethyan Sea.
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Break-up of Pangea
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Cretaceous Super-Plumes the formation of LIPS.
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Location of Large Igneous Provinces Most (but
not all) are Cretaceous in age. Rolling
Thunder age progression from East (Parana) to
West (Ontong-Java, then Kerguelen, then Deccan).
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End of an Era the end-Cretaceous event
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Asteroid Impacts and Climate

• Asteroid impacts can have apocalyptic
  consequences, but -
• Long-term climate change is not one of them

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