Download Ch 5

Greenhouse Earth: 100 Ma
GEOL 1130
Paleoclimate Research
• Two components
– Observations
• i.e. fossils, sediments, chemical proxies
– Modeling
• using observations to test climatic conditions,
• see if observations are consistent with each other,
• predict other climatic conditions that might be
recorded but not yet found.
• Observation – modeling comparisons
extremely important to moving research
forward
100 Ma observations
1. Broadleaf (breadfruit-type tree) fossils at
polar locations
a. Today found in tropical evergreen locations
2. Cold blooded animals to poles
3. Corals found between 40 N and S
a. Today found between 30 N and S
4. Sea level higher by at least 200 m
1 & 2 suggest that the atmosphere and ocean were
much warmer
3 suggests that the oceans were much warmer
Activity: Was heat distribution on Earth more or less
efficient in the Cretaceous than it is today?
Modeling Cretaceous Climate
• Activity: What are boundary conditions in
this model that would be different from
today?
– Continental position
– Sea level
– Atmospheric CO2 concentration
Modeling Results
• Ran model with
changes in
geography only
and with changes
in both geography
and CO2
– Which one does a
better job?
• Does either one get
the right amount of
latitudinal heat
transport?
• What is a potential
solution?
Cretaceous ocean circulation?
• Today’s deep oceans
filled with cold (~1-4 C),
somewhat saline water
• Cretaceous deep ocean
filled with warm (>10-15
C?) water?
• Warm water transported
from the tropics to the
poles would have
dramatically increased
the latitudinal heat
transport
Sea Level Change
• Two types of sea level change in geology
– Relative – local tectonic or isostatic effects on
coast lines
– Eustatic – global sea level changes
• In terms of climate change, eustatic sea
level change is what we are interested in
• Terminology:
– Transgression – rise in sea level
– Regression – fall in sea level
Sea Level Change
• Activity: If a continental shelf has a slope of
1:1000, how much far inland will the ocean
travel if sea level rises by 1 meter?
– 1000 m or 1 km!
• Is sea level today low or high? That is, is sea
level at the continental break in slope or up on
the shelf?
• How much would sea level rise if all the ice caps
were to melt?
• How much land would we lose if sea level rose
70 m?
Note
today’s
low sea
level!!
Has sea
level
remained
constant
over
time?
Past Sea Level
Five potential causes for sea level
change
1. Tectonic – mid ocean ridges
– Activity: How does spreading rate affect the
volume of the ocean basins?
– Thin vs fat mid ocean ridges:
•
•
•
•
Ocean floor depth (m) = original depth +
350*age1/2
Today’s ridge depth = 2500 m
Activity: What is the square root of 5, 10, 20, 40,
60, 80, 100? How much change in depth occurs
over each period?
Crust that is more than 60 Ma old has reached a
relatively stable depth of 5500 m
Plate spreading driver of sea level
• Faster spreading
makes ocean basins
more shallow, causing
high sea level (ocean
pushed up on to
continents)
• Slower
spreading…(visa
versa)
Drivers of sea level change
2. Tectonics – collisions
– When continents collide, some of the
continental land mass is thickened
(mountains and plateaus), shrinking
continents and by default expanding oceans
•
Modern example?
– Tibetan Plateau started 55 Ma
3. Volcanic Plateaus
– Take up room on ocean floor, displacing
water
Drivers of sea level change
4. Climate:
– Ice on land takes water out of the ocean
– What is total range between greenhouse
world 100 Ma and 20,000 years ago?
•
~300 m
5. Thermal expansion/contraction of sea
water
– 1 part in 7000 for each degree C