reading: Chapter 4

Lecture 25. Snowball Earth vs. Slushball Earth..

In 1964, Brian Harland at Cambridge Univ.noticed Neoproterozoic glacial deposits were widely distributedon nearly every continent

Geologic evidence suggested glaciation at low latitudes

Mikhail Budyko at Leningrad Geophysical Observatoryworked on climate modelingdiscovered that ice-albedo feedback caused instability in climate

Early ObservationsQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

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Ice-Albedo Feedback

As Earth cools:1. Ice forms at lower and lower latitudes2. Albedo rises3. Reflects more solar radiation4. Cools even more(positive feedback loop)

Once ice formed at a critical latitude (30˚)the positive feedback so strong that T plummets

Creates a completely frozen planet.

Geothermal heat keeps oceans from freezing completely, so ice1km thick on average.

Albedo:0.1 seawater0.3 bare land0.6 sea ice0.9 fresh snow

A Permanent Catastrophe?

Would have already extinguished life.

Temperatures would have been -50˚C.

No means of escape.

In 1977:Discovery of hydrothermal vents. Discovery of dry valleys of Antarctica -

organisms can live in extremely cold environments.Appreciation for life in extreme environments.

also: “discovery” of plate tectonics

Evidence of Glaciation

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Glaciers carry large stones - when the glaciers melt, they leavedropstones. Characteristic of glaciers.

Glaciation at Low Latitudes (Near Equator)

PaleomagnetismWhen sediments form magnetic minerals point to the N pole. At the equator they will point horizontally at the equator.

Found that glacial rocks in the Neoproterozoic had horizontalmagnetic properties.

Suggests glacial deposits occurred at low latitudes.

Other Evidence of Large Scale Change

The return of banded iron formation in marine watersWhat do you need have have BIF?

Lots of Fe2+ - Little O2

Suggests oceans cut off from the atmosphere - ice??

Other Evidence of Large Scale Change, cont.

When Do We Have BIF?

How To Reverse a Snowball Event?

In late 1980’s, Joe Kirschvink of CalTechGave it the name Snowball Earth.

Showed that the CO2 supply would continueBUT it would be so cold that there is no liquid water at the surface.

Weathering reactions stop, sloooooow dooooowwwwn.

What happens then?

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Estimated 350x present level of CO2 needed to overcome the albedoof the Snowball Earth.

Would have taken several million years for this much CO2 to accumulate.

CO2 causes warming, which begins melting.

As melting occurs what happens to the albedo? What does this do?

Melting would have been catastrophic - sea ice gone in few hundred years.

Sea level rises dramatically!

As soon as the ice is gone, it gets hot!!

How To Reverse a Snowball Event?

The Following Hot House

After the Snowball, now have a hot house.

How hot?50˚C!!

Have a CO2-rich atmosphere, warm conditions.What happens next??

The Carbonate-Silicate Cycle Kicks Into High Gear!

1. CO2 outgassing from volcanoes (greenhouse gas)2. CO2 dissolves in rain, lakes, streams, turns into carbonic acid3. Carbonic acid reacts with rocks, making ions, quartz, and clay4. Ions and dissolved CO2 reacts to make carbonate rocks5. Carbonate rocks are subducted6. Subducted carbonate rocks turned into CO2

Extremely acidic rain.

Rapid weathering of rocks.

Since it is warm, we should make _____________ carbonaterocks.

Evidence of a Hot House

Huge, thick carbonaterock over layers ofglacial deposits-calledcap carbonates-is evidence that theywere formed veryrapidly - few hundred yrs

Snowball Earths During Critical Geologic Times

~2.3 Ga - right as oxygen becomes present in the atmosphere3 or 4 glacial events within a relatively short period of time.

Neoproterozoic - animal embryo and early sponges in betweenglacial events.

Thought to be 3 glacial events.

Both are associated with increases in oxygen.

Why Only One Snowball Event on the Early Earth?

Faint Young Sun - expect more glacial events in the Archean andPaleoproterozoic!

1. Geologic evidence of an early CO2-rich atmosphere2. Also evidence for an early CH4-rich atmosphere3. CH4 is easily oxidized to CO2

4. But CH4 is a stronger greenhouse gas5. Oxygen in the atmosphere destroyed the CH4 greenhouse6. Catastrophic entrance into a Snowball episode

reading: none - budget documents on the web site

Lecture 26. Exploration, the Federal and NASA Budgets, and Politics.