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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BLAG has been superceded by Berner’s GEOCARB models (I, II, III, GEOCARBSULF)All C models now acct for both SFS & weathering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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