Transcript Lecture 25. Snowball Earth vs. Slushball Earth..

Lecture 25. Snowball Earth vs. Slushball Earth..
reading: Chapter 4
Early Observations
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TIFF (Uncompressed) decompressor
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In 1964, Brian Harland at Cambridge Univ.
noticed Neoproterozoic glacial deposits were widely distributed
on nearly every continent
Geologic evidence suggested glaciation at low latitudes
Mikhail Budyko at Leningrad Geophysical Observatory
worked on climate modeling
discovered that ice-albedo feedback caused instability in climate
Qu ickT ime ™ a nd a
TIF F (U nco mpre sse d) de com pres sor
are nee ded to s ee th is pi cture .
Ice-Albedo Feedback
As Earth cools:
1. Ice forms at lower and lower latitudes
2. Albedo rises
3. Reflects more solar radiation
4. Cools even more
(positive feedback loop)
Albedo:
0.1 seawater
0.3 bare land
0.6 sea ice
0.9 fresh snow
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 ice
1km thick on average.
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 leave
dropstones. Characteristic of glaciers.
Glaciation at Low Latitudes (Near Equator)
Paleomagnetism
When 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 horizontal
magnetic properties.
Suggests glacial deposits occurred at low latitudes.
Other Evidence of Large Scale Change
The return of banded iron formation in marine waters
What 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?
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In late 1980’s, Joe Kirschvink of CalTech
Gave it the name Snowball Earth.
Showed that the CO2 supply would continue
BUT it would be so cold that there is no liquid water at the surface.
Weathering reactions stop, sloooooow dooooowwwwn.
What happens then?
How To Reverse a Snowball Event?
Estimated 350x present level of CO2 needed to overcome the albedo
of 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!!
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.
2.
3.
4.
5.
6.
CO2 outgassing from volcanoes (greenhouse gas)
CO2 dissolves in rain, lakes, streams, turns into carbonic acid
Carbonic acid reacts with rocks, making ions, quartz, and clay
Ions and dissolved CO2 reacts to make carbonate rocks
Carbonate rocks are subducted
Subducted carbonate rocks turned into CO2
Extremely acidic rain.
Rapid weathering of rocks.
Since it is warm, we should make _____________ carbonate
rocks.
Evidence of a Hot House
Huge, thick carbonate
rock over layers of
glacial deposits
called
cap carbonates
is evidence that they
were formed very
rapidly - few hundred yrs
Snowball Earths During Critical Geologic Times
~2.3 Ga - right as oxygen becomes present in the atmosphere
3 or 4 glacial events within a relatively short period of time.
Neoproterozoic - animal embryo and early sponges in between
glacial 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 and
Paleoproterozoic!
1.
2.
3.
4.
5.
6.
Geologic evidence of an early CO2-rich atmosphere
Also evidence for an early CH4-rich atmosphere
CH4 is easily oxidized to CO2
But CH4 is a stronger greenhouse gas
Oxygen in the atmosphere destroyed the CH4 greenhouse
Catastrophic entrance into a Snowball episode
Lecture 26. Exploration, the Federal and NASA
Budgets, and Politics.
reading: none - budget documents on the web site