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• Geologic record àgreenhouse - icehouse cycles
• Cycles related to tectonics?- Polar position hypothesis- paleogeography- Tectonic control on atm CO2
- Changes in CO2 supply- Changes in weathering due to uplift (elevation of continents)
• Biological overprint
Plate Tectonics & Long-term Climate (Ch. 5)
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Supercontinent break-up & fm of ocean basins
260 Ma
Early Jurassic180 Ma
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Wilson cycle
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Supercontinent break-up & fm of ocean basins
550 Ma
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Polar Position Hypothesis• Continents move into different climate zones
LGM
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Polar Position Hypothesis - testable hypothesis
Does high latitude guarantee glaciation?
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Did ice sheets form in portions of Gondwana as it moved across the South Pole?
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• Gondwana moved across South Pole for ~200 Myrs
• Evidence of sporadic (not continuous) glaciations
- some were brief - n. Sahara (~430 Ma) lasted
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• Atm CO2 is also important– BLAG model 1983
Atm CO2 driven by DSFS rates– T.C. Chamberlain or Raymo/Ruddiman Model
Atm CO2 driven by uplift & weathering Ds
Ø Polar position alone ≠ glaciations (past 500 Myrs)
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BLAG has been superceded by Berner’s GEOCARB models (I, II, III, GEOCARBSULF)All C models now acct for both SFS & weathering
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GEOCARB: changes in SFS rate control atm CO2
*MOR length also important*
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SFS rates à determined from crustal ages & width btwn magnetic reversals à used to estimate CO2 outgassing through time
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*Biological processes have played an important role in transferring C from the atm to the lithosphere*
Sed C àdelamination/decarbonation à melting in subduction zones à CO2 outgassing
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GEOCARB also uses negative feedback of chemical weathering“thermostat”
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Summary of GEOCARB Model
– SFS + chemical weathering control CO2 over 10s-100s Myrs– C transported to oceans; some is buried– Sedimentary C recycled through subduction & outgassing or
mtn bldg & erosion
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Model Evaluation - works well for extensive ice sheet prediction
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Uplift Weathering Hypothesis
Tectonic uplift controls/enhances chem weathering by increasing exposure of fresh rock surfaces through physical weathering
- Assumes CO2 supply is constant- Rate of CO2 removal changes- Controlled by availability of fresh rock/mineral surfaces
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Exfoliation
granite
vertical jointing
basalts
Downward pressure
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Uplift Weathering Hypothesis
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What is the Difference?
Tectonics rule, but key factors differ:
– Uplift weathering – chem weathering drives climate change• Uplift à fragmentation & exposure of fresh rocks
à chemical weathering à CO2 sink • Big problem: run out of atm CO2
l BLAG – CO2 outgassing drives climate change;chem weathering is a negative feedback
• GEOCARB takes into acct uplift & different rock types being weathered at different times
• Polar position helps
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Combine them à Greenhouse/Icehouse Fisher cycle
1st half of Wilson cycle à Fast SFS à more CO2 à greenhouse climate
2nd half of Wilson cycle - Uplift & mtn bldg à chemical weathering à CO2 drawdown à icehouse climate
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What drives long-term (100 myr) DSL?
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SFS rate affects SL
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SFS rate affects SL & CO2
Feedbacks – albedo, thermal expansion, heat capacity
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Continental margins affect SL: large volume à displace sw
One supercontinent à less total margin area à lower SL
Many smaller continents, each w/ its own margin à higher SL
more continents + high SFS rate & MOR length = high SL
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Haq et al., 2005
Bown et al., 2004Cervato et al., 1999
Stover et al., 1996Macrae, unpub. data
CenozoicCervato et al., 1999MesozoicHarwood & Nikolaev, 1995
Miller et al., 2005
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Continental configuration and position (polar position hypothesis)Changes in CO2 supply (SFS)Changes in CO2 removal - weathering due to uplift (elevation of continents)Biosphere mediation of CO2, C export to lithosphere
What drives Greenhouse-Icehouse Cycles?
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Next timeThe Greenhouse Climate (Ch. 6, part 1)Causes of greenhouse climate, 100 Ma
Mechanisms for long-term sea-level changeSea level - climate interactions
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