Chapter 7 - Greenhouse to Icehouse

Question 1

Factors influencing 18O variations? (i.e. what does 18O tell us about climate)

Answer 1

Water temperature and ice sheet size

Question 2

Why is the last half of the Cretaceous useful for comparison?

Answer 2

Abundant evidence in the geological record, and this was a greenhouse era.

Question 3

How well does BLAG explain cooling in the last 50 Myr?

Answer 3

The requirement for this hypothesis to be true is that the mean global rate of spreading has slowed in the last 50myr to account for the cooling. However, evidence suggests this is not true. In fact spreading has levelled out or even increased, which according to this hypothesis should correspond to climate warming.
An alternative hypothesis suggests the type of sediment being subducted plays a crucial role. If subduction was to occur in the Atlantic, vast amounts of carbon would be delivered and melted in the mantle.

Question 4

How well does the uplift hypothesis explain cooling in the last 50 Myr?

Answer 4

The requirements include unusually high terrain, physical and chemical weathering in todays world. The later two cannot be definitively proved.
The likely explanation is decreased CO2 levels caused by a combination of decreased sea floor spreading and increased removal by chemical weathering.

Question 5

Explain how chemical weathering could either moderate or deepen the long term cooling

Answer 5

It could moderate cooling by removing less co2 from the atmosphere due to decreased evaporation and hence precipitation. This means carbonic acid cannot act on silicates.
However it could deepen cooling if there are short term size fluctuations; fresh debris created along margins will be rapidly weathered, drawing more co2 out of the atmosphere