Chapter 10 - Insolation Control of Ice Sheets Flashcards
What is the Milankovitch Theory?
Proposed In order to explain the growth and melting of continental sized ice sheets. It suggests that summer insolation is the main control of ice sheets. Low summer insolation is critical in producing summers cool enough for ice to persist from one winter to the next
Why is winter not the critical season for ice sheet growth?
Because ice sheets grow at high latitudes where temperatures are always low, even during warm climate intervals. Furthermore, the sun always lays low in the winter sky regardless or orbital changes
When does low insolation occur?
- When Earths orbital tilt (obliquity) is small and poles pointed less directly at the sun
- When the northern summer solstice occurs with Earth in aphelion
- When the orbit is highly eccentric
Most sensitive latitude for low insolation?
65N because here ice sheets first accumulate and last melt
What is the initial response of bedrock to ice load?
Initially there is an elastic response in the form of quick sagging. This represents 30% of total vertical change
Over the next several thousands of years there is a viscous response in the relatively ‘soft’ layer of the upper mantle, with a response time of 3000 years.
What if ice load is abruptly removed?
The rock surface will rebound towards equilibrium, followed by a slow and viscous rebound.
Positive feedback loop is created as a depression is left, meaning remaining ice is at a lower elevation thus causing further melting
Basal slip?
slipping occurs due to meltwater at the base of the icesheet which saturates sediment to create a lubricated layer over which ice can slide. Not included in models due to unpredictability
Where do continuous records of glaciations come from?
Ocean basin sediments. Include indications of ice-rafted debris and 18O records
How do coral reefs indicate sea level?
They act as a dip stick as corals grow just below sea level, migrating up and down slope. They have a strong structural framework thus stay in tact long after death
Can also be radiometrically dated by Th/U ratio and compared with 18O signals.
How successful is the Milankovitch theory at explaining NH glaciation?
It is useful as it explains ice responses at 41,00 and 23,000 cycles and why they lag behind insolation forcing. However it does not explain the dominance of the obliquity cycle over the precession, or account for large oscillations at 100,000 (despite the fact eccentricity should only modulate, not be a direct forcing)
What is equilibrium line and why is it important?
The equilibrium line is the boundary between areas of net ice ablation and accumulation, and slopes upwards towards the atmosphere in a south direction. It is important as it can be used to model ice sheet responses to insolation changes. During high insolation, the equilibrium line is driven north and the continents are in an area of net ablation. During low insolation, the line shifts south and the continents are in an area of net accumulation
Why are the northern ice sheets likely to be more responsive to insolation changes than ice in Antarctica?
This is because the northern ice sheets are primarily located on landmasses, whereas Antarctica is surrounded by the Southern Ocean which has a high heat capacity, thus regulating insolation effects. Furthermore, the altitude of Antarctica is extremely high and temperatures are below freezing here even during summer
Why does the size of a growing or melting ice sheet lag behind changes in insolation?
This is because glaciers involve exchange with the ocean which has a very high heat capacity and responds slowly to insolation changes. For this reason, as summer insolation declines, ice accumulates. This will continue even when insolation begins to increase so the maximum ice volume occurs after the insolation minima
How does the delay in bedrock response to ice loading or unloading act as a positive feedback on ice volume?
During loading, the slow viscous response of bedrock promotes ice accumulation as ice is at a higher elevation where ablation is slower. This makes the ice mass balance more positive.
During unloading a positive feedback mechanism is set up because as ice melts, the viscous depression is still left in the bedrock, thus ice sits at a lower elevation and melting is increased.
