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Back into the Icehouse

GEOL 3100

Christina Gallup

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Slipping into an Icehouse

• Evidence for cooling in Antarctica?– Ice-rafted debris at 35 Ma show beginning of

ice cap• Big jumps in ice cap size at 13 and 7-5 Ma• Today 90% of continent covered in ice

– Beech trees and ferns lived there at 40 Ma • lichen and summer algae today

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Antarctic cooling

55 Ma Today

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Slipping into an Icehouse

• Evidence for cooling in the Arctic?– Ice caps?

• NO!! Ice caps cannot form in the open ocean

– Breadfruit until 60 Ma– Boreal forest by 20 Ma– Tundra last few million years

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Arctic cooling

Broadleaf trees 55 Ma

Tundra today

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Slipping into an Icehouse

• Evidence for cooling in the mid to upper latitudes?– Greenland ice cap starting 7-3 Ma– N. American and Eurasian ice sheets starting

2.7 Ma• Increased in size starting at 0.9 Ma

– Mountain glaciers in the Andes starting 7-4 Ma

– Leaf outlines in western North America transitioned from smooth to more serrated in the last 55 Ma

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Cooling estimated from leaf shape~10-15 C in last 50 Ma

Activity: What would allow you to evaluate the precision of this estimate?ERROR BARS

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Oxygen Isotope evidence for deep water cooling over last 70 Ma – the deep ocean has cooled by at least 14 degrees C

Assignment: Read the handout and start the homework and we will go over oxygen isotope systematics next time

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What caused the cooling: Testing Hypotheses

• Activity: What are the main hypotheses that we need to test?– Polar position Hypothesis– Spreading Rate Hypothesis– Uplift Weathering Hypothesis– Ocean Heat Transport Hypothesis

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Polar Position Hypothesis:

When there are continents in polar position, there will be an icehouse and when there are not continents in polar position, there will be a greenhouse.

Where was Antarctica at 100 Ma?

Where is it today?

How does the polar position hypothesis do?

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Spreading Rate Hypothesis:

When spreading is slower, there will be cooling or in an icehouse and when spreading is faster, there will be warming or a greenhouse.

Were spreading rates at 50 Ma faster or slower than today?

Are spreading rates for the last 50 Ma consistent with cooling for the last 50 Ma?

How does the spreading rate hypothesis do?

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Uplift Weathering Hypothesis

• To evaluate, need to look at 1. Evidence for uplifted terrain

2. Evidence that terrain was rapidly physically weathered

3. Evidence that physical weathering led to rapid chemical weathering

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High Terrains today

4 main uplifted regions today that are not covered by ice

Most of these include uplifted 100-65 Ma marine sediments

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Mountain building geologic processes

• Activity: What are they?– Continent-continent collision– Subduction– Continental rifting

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Collision of India with Asia

Tibetan plateau includes 2,000,000 sq. km with average elevation > 5 km

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Continental Collision History

• India-Asia collision first continental collision since 250 Ma

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Subduction related mountains

• Andes– Subduction started before 100 Ma– Altiplano plateau and Eastern Andes created

in last 55 Ma

• Western North America– Subduction for last 200 Ma– Rockies timing questionable

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Continental rifting

• East African rift in last 30 Ma

• Similar or greater African rifting at 100 Ma

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Uplift Weathering 1: uplift

• Where do we stand?- For the three different processes of mountain

building we identified, which argue for increased uplift in the last 55 Ma?

- Continent-continent collision!

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Uplift Weathering 2: Physical Weathering

• What is the best evidence for increased physical weathering in the geologic record?– Sediments!– The more physical weathering, the more

detritus is created and delivered to basins

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Sediments in Indian Ocean from weathering of Himalayan mountains

Would sediments coming off of subduction related mountains be as well preserved?

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Delivery of sediments increased by monsoon rains that are amplified by the high plateau

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Uplift Weathering 3: Chemical Weathering

• What is the best evidence for increased chemical weathering?– Dissolved ions in rivers– Hard to measure today, even harder to

quantify in the geologic past!!!

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Uplift Weathering Hypothesis

1. Sufficient uplift to drive cooling in last 50 Ma? Yes, Himalayas and Tibetan Plateau

2. Sufficient physical weathering to drive cooling in last 50 Ma?

Probably, though sediments from subduction zones lost to trenches

3. Sufficient hydrolysis to drive cooling in last 50 Ma?

Probably, by inference from #2

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A more complete climate feedback loop for the uplift-weathering hypothesis

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Ocean Heat Transport Hypothesis

• Background: Increased ocean heat transport kept poles warmer during greenhouse

• Hypothesis: Continued high ocean heat transport contributed to polar cooling during the last 55 Ma

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Opening of Drake’s Passage ~25-20 Ma may have decreased oceanic poleward heat transport

After opening, circumpolar current started

No major climate shifts at this time however

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Closure of Isthmus of Panama ~4 Ma caused increased poleward heat transport

Major Northern Hemisphere glaciations started ~1 Ma later

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Ocean Heat Transport Hypothesis

• The history of oceanic circulation and the role of gateways in oceanic circulation still too poorly known to evaluate the hypothesis well.

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The Schrag Hypothesis

• Hypothesized by Dan Schrag (Harvard) in the last few years

• The more limestone bearing oceanic crust being subducted, the more CO2 released into the atmosphere – Greenhouse

• The less limestone bearing oceanic crust being subducted, the less CO2 released into the atmosphere - Icehouse

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How does the Schrag Hypothesis stack up?

• The ocean crust that was subducted while India was moving towards Asia was very limestone rich

• Most subduction zones today are not subducting limestone bearing oceanic crust

• The cooling started after India collided with Asia, 55 Ma ago.– Coincidence? Maybe not

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Many possible hypotheses and feedbacks!

-where would the Earth go next on a tectonic timescale, a deeper icehouse or a greenhouse?

-will anthropogenic global warming affect this future?

Paleoclimatology is a very young field and we have a lot to learn!!

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