Lecture 8: Snowball Earth

Question 1

For most of the questions you need to get the picture of the model on the lecture to answer the questions the questions that require it will be marked with an “M” at the start

Answer 1

Cool

Question 2

What is a snowball earth event?

Answer 2

The commonly understood definition is when the earth is in a complete ice state

Question 3

M: Who produced the model that simulated snowball earth events?

Answer 3

Budyko (1968) and Seller (1969)

Question 4

M: What does the model represent?

Answer 4

How ice cover changes with alterations to solar flux and carbon dioxide concentration

Question 5

M: If the ice level is located at 90 degrees latitude, then what does this mean, as illustrated by the blue line?

Answer 5

That there is no ice present at any latitude

Question 6

M: If the ice line is located at 0 degrees latitude, then what does this mean, as illustrated by the white line?

Answer 6

That ice is present across the whole earth surface

Question 7

M: What is the term given to the ice state if the front line is located between 90 and 30 degrees latitude?

Answer 7

Partial Ice

Question 8

M: What are the three points points of stability?

Answer 8

No ice, partial ice, all ice. These points are when the amount of energy coming in equals that going out and so as long as the solar flux and carbon dioxide concentration remain, the ice will remain in this position

Question 9

M: What does the model predicts will happen once the ice line advances to 30 degrees latitude? and what is it?

Answer 9

If the line reaches 30 degrees latitude there will be a runaway effect. This means that regardless of changes in CO2 and solar flux the ice will advance towards the equator from this point

Question 10

M: In the modelled snowball earth what does Donnadieu et al. (2003) believe the temperature was at the equator and poles?

Answer 10

Equator: -20
Poles: -80 (like mars)

Question 11

M: In order to break out of a snowball earth event, because the earth cannot alter the solar luminosity, what needs to change?

Answer 11

The carbon dioxide concentration needs to increase by several degrees of magnitude

Question 12

M: How would the carbon dioxide concentration be allowed to increase?

Answer 12

The snowball earth event would shut down the silicate weathering and carbonate burial processes while at a slow rate carbon continues to be released in to the atmosphere by volcanic and degassing through cracks i sea ice. This would allow a GHG layer to develop which would over time melt the ice

Question 13

M: Where on the diagram will the carbon dioxide concentration reach a critical point and start to reverse the snowball earth event?

Answer 13

When it starts to reach a concentration level on the bottom axis that registers on the white line. i.e where the white line ends with the red arrow above it corresponds to a CO2 concentration below it which is the critical point for when the snowball earth event will start to reverse

Question 14

M: Describe the rate of the transition from a snowball earth event to deglaciation and then back to normal and explain why

Answer 14

Rapid - because as CO2 increases it speeds up the melting process which allows more carbon - its essentially a runaway/feedback

Question 15

What and when are the three most recent snowball earth events?

Answer 15

Marinoan: ~0.64Ga
Sturtian: ~0.71Ga
Makganyene: ~2.2Ga

Question 16

How long did the Sturtian possibly last for?

Answer 16

Tens of millions of years

Question 17

How long did the Marinoan possibly last for?

Answer 17

Few millions of years

Question 18

What events coincide with the snowball earth events?

Answer 18

Oxygen rises

Question 19

What event happened before the Makganyene snowball and also what event happened just before the Sturtian and Marinoan?

Answer 19

Great Oxidation - pre Makganyene

Lesser Oxygenation - pre Sturtian and Marinoan

Question 20

What palaeomagnetism evidence do we have for the snowball earth events?

Answer 20

The metal elements in liquid rock also orientate themselves according to latitude. At the poles they at positioned directly vertical and at the equator they are directly horizontal. Combined with looking at dropstones that are deposited by glaciers in to sedimentary rocks, we can tell what latitude a rock is from, date it and then find out whether it had a glacier present at that time

Question 21

What BIF evidence do we have for snowball earth events?

Answer 21

During snowball earth events oceans and atmosphere loose oxygen concentrations because there are less plants alive to produce it. This means rock formations will appear darker as they are not being oxidised

Question 22

What are cap carbonates?

Answer 22

Types of rock that occur after snowball earth events

Question 23

What evidence do cap carbonates provide to past snowball earth events?

Answer 23

After snowball earth events the conditions are wet (due to melting) and hot (due to GHG layer) which means weathering rates increase for continental rocks like cap carbonates. This allows them to release magnesium and calcium which react with the atmosphere to form carbonate rocks with those metals as the base. These are large formations that are a visual testament to the snowball earth event

Question 24

Who proposed the theory about cap carbonates and deglaciation?

Answer 24

Hoffman

Question 25

What are dropstones?

Answer 25

These are deposits that a glacier leaves behind in rocks as they scratch over them. They therefore serve as a memory of a snowball earth event in different places

Question 26

What do critics of the snowball earth event state is the reason why a snowball earth event could not have occurred?

Answer 26

Because life was created before them and they believe it would not have been able to survive the conditions and so the snowball events could not have happened

Question 27

What do the critics of a snowball earth event believe was an alternative idea to the snowball earth event?

Answer 27

Slush ball model - ice would have extended down to a critical point and then the remaining part would have been like a slush in which life would have been able to survive

Question 28

How does the breaking up of a supercontinent lay the foundations for a snowball earth event?

Answer 28

The Rodina supercontinent was in the later stages of its breakup and getting scattered across the earths surface, but mostly only within the mid latitudes. these mid latitudes had large igneous provinces below that created basalts

Question 29

How could the creation of basalts by a large igneous provinces led to a snowball earth event?

Answer 29

The basalts are sensitive to weathering in relatively warm latitudes which will further increase this weathering rate. this weathering will draw down carbon dioxide from the atmosphere rapidly to reduce a GHG layer and cause rapid cooling eventually leading to a snowball earth event

Question 30

What is the limitation of the basalt weathering explanation of a snowball earth event?

Answer 30

Donnadieu et al. (2004) had to really stretch the data for this conclusion

Question 31

What was the theory proposed by Tim Lenton and a student for a snowball earth event?

Answer 31

Lichen could have intensified the weathering rates of rocks they were on to speed up the bringing down of carbon dioxide from the atmosphere.

Question 32

What was the limitation of Tim Lenton and student's hypothesis?

Answer 32

It may be a self-limiting feedback - as temperature cools the potential for land colonization diminishes and so the weathering rates would no longer continue to increase. It would take too long for explaining the contrasting rapid rate at which ice grows and retreats