Paleozoic/Mesozoic Climate

I. Paleozoic History
Organisms with hard shells= proxy information
Only 5% if Cambrian sedimentary rocks are preserved
Age of seafloor

A. Overall Conditions
Icehouse/Greenhouse record
Positions of continents (correlation with icehouse/greenhouse)
Sea level variations- relationship to spreading rates
CO2 record (Faint Young Sun)

B. Continental position
Two exceptions:
1. Snowball Earth late Proterozoic
2. Cretaceous Greenhouse

= global climate change, not just continental positioning
Variables that can be changed:
* solar constant
* albedo
* greenhouse gases
Also- redistribution of heat by oceans

II. Snowball Earth
All continents at low latitude
Evidence of glaciation- tills overlying carbonates

Ice albedo effect at equator very strong- result= oceans freeze over completely

Theory for recovery related to the carbon cycle.
Major source for CO2 = volcanoes
Major sinks = weathering, burial of organic matter (ocean, plants)
Sinks shut off by ice
Source remains and CO2 builds up.

Controversial

III. Mesozoic History
General warming trend throughout the era.  Ending with Cretaceous warmth
Conditions:
* S. Atlantic just starting to open (Cretaceous Reconstruction)
* Sea level 200m higher than today
* Western US interior seaway
* Circum-equatorial circulation
* Continents more equally distributed between northern and southern hemispheres
* Equitable climate- low gradient between equator and poles

Global Mean Average Temperature ~ 6-12°C warmer, mostly at poles
Cretaceous Temperature Gradient

Evidence of warmth:
· Lack of glacial deposits
· Distribution of fauna and flora
· Oxygen isotope record

A. Cretaceous Sediment
1. Chalk (Crete)- warm water, high productivity.  White Cliffs of Dover
2. Black Shales- very common in marine sediments.  Theory- warm saline bottom water, less oxygen.
3. Reefs in Gulf of Mexico
4. Bauxite/laterite- highly weathered soils

B. Cretaceous Sealevel
Very high.
Related to seafloor spreading and outflow of basalt provinces (superplumes)
Also, thermal expansion
Interior seaways and lakes would have helped to modulate interior climates

C. Model Results
Affects of:
1) Paleogeography
· Paleogeography- land/sea distribution- more even distribution across the equator
· Lack of ice - albedo
· Sealevel - albedo
· Ocean circulation- circumequatorial circulation rather than circumpolar
Tropics in middle of expected range, poles too cold

2) Paleogeography + 4X CO2
· 4x CO2 conservative estimate for 4-10X
Tropics overheat, poles still too cold

3) Increased poleward heat transport
· 4x poleward heat transport

Paleogeography can account for ½ of the temp change (accounts for 4.8°C warming).
Remaining ½ attributed to 4X CO2
Add poleward heat transport to redistribute

Sources of CO2:
- More rapid seafloor spreading
- Large basalt provinces (subsea volcanism)
- Less land exposed for weathering

Poleward Heat Transport
Paleogeography shifts Gulf Stream as far north as Nova Scotia

Deep Water Circulation-
Modern deep water = cold and salty (mostly cold- forms at high latitudes)
Cretaceous Deep Water= warm, salty bottom water- forms in desert latitudes (like Mediterranean today)
* Would transport heat from the tropics to the poles in the deep ocean
* Resulted in less stratified ocean more sluggish circulation and less O2

Increased latent heat transport

D. Summary
Mean Global temperature was warmer than today by ~6-12°C based on :
* biological data
* geological data
* geochemical data

Assume ice free
Lower pole to equator temperature gradient (equitable)

Warming caused by:
* paleogeography (minor)
* albedo
* increased CO2
* increased poleward heat transport