Palaeoglaciology 2: ice sheet reconstruction

Question 1

Why are ice sheet reconstructions needed?

Answer 1

1. determine maximum extents
2. determine patterns and rates
3. determine forcings and behaviour
   4 determine past contributions to sea level and climate change
4. Project future perspectives

Question 2

What are the 5 ingredients for ice sheet reconstructions?

Answer 2

1. ice sheet footprint - determine spatial coverage
2. ice sheet discharge patterns - locations of dynamic elements and make thermal zonations
3. ice sheet margins - determine spatial extent
4. ice sheet thickness - determine vertical extent
5. chronology - variation through time

Question 3

How do you determine ice sheet footprint?

Answer 3

using subglacial landforms

Question 4

What features can you use to determine ice sheet footprit?

Answer 4

Transverse = moraines, corrugation ridges, composite ridges, 
Linear = drumlins, flutes, MSGLs, eskers, moraines

Question 5

What other glacial features can you use to determine ice sheet footprint that are not necessarily subglacial?

Answer 5

Till and erratic distribution

Question 6

What is the process for identifying ice sheet footprint?

Answer 6

1. Identify landform flow traces
2. Interpret occurring patterns
3. Reconstruct ‘fans’ using dates of landforms to better show the landform flow traces significance

Question 7

How do you determine discharge patterns?

Answer 7

Using subglacial landforms to determine thermal regime and organisation

Question 8

What can the thermal zonation and regime of an ice sheet tell us about the glacier discharge patterns?

Answer 8

Then we can zonate in to different sections based on its different flow speeds which then tells us what part of the ice sheet it is (middle, end, side etc.)

Question 9

What are shear margin moraines?

Answer 9

These are shear margins between different types of landform that can be used to determine the boundary between different parts of the glacier

Question 10

What do elongated and non-aligned features highlight?

Answer 10

Elongated tells us where ice is used to flow at significant speeds
Non-aligned tells us where the ice did not exhibit much flow

Question 11

What does a boundary between elongated and non-aligned bedform possibly tell us?

Answer 11

This marked the boundary between fast and slow ice flow which could be a difference between the middle ice stream and the side.

Question 12

What can elongated lateral features tell us?

Answer 12

This might show us the previous glacier margin extent

Question 13

What might the bedform of a frozen-to-bed glacier look like?

Answer 13

There is little erosion when the ice is frozen to the bed which means that there is likely to be very few landforms

Question 14

What are the different ways that we can determine ice sheet margins?

Answer 14

Terminal moraines, hummocky moraines,

Meltwater channels and lateral channels - eskers

Question 15

How can eskers be used to determine the previous ice margin?

Answer 15

Eskers fill in previous channels. Meltwater channels such as R-channels terminate at the glacier margin so where we can see these channels we can see where the glacier used to end. Similarly, lateral channels could mark transverse channels to ice flow that can be used to determine retreat.

Question 16

What type of subglacial channel terminates at the ice margin?

Answer 16

R-channel

Question 17

What things can we use to determine ice sheet thickness?

Answer 17

Trimlines - show maximum former extent of deposition
Weathering zones - show vertical extents of previous erosion lines
Identifying the altitude of thermal zones and land forms such as subglacial channels can tell us where the base of the glacier used to extend to
Similarly, presence of till and eskers on mountains can tell us where the glacier base used to be

Question 18

What are the different methods that we can use to date the different glacial landforms?

Answer 18

Radiocarbon dating, schmidt hammer, lichen measurements, cosmogenic exposure

Question 19

Explain how cosmogenic exposure works

Answer 19

Earth is bombarded by cosmic rays that are received by the top surface layer. Silica and oxygen in the rocks of these top surface layer react with the cosmic rays to produce isotopes such as Biridium 10. These isotopes can be measured and compared against the known rate of production to determin how long that surface has been exposed i.e. how long a surface has been exposed since it was revealed during glacier retreat.

Question 20

What can we use cosmogenic exposure to show graphically?

Answer 20

Plot age and height against each other we can create a previous vertical extent and associated age against each other