Reconstructing Ice Dynamics from Quaternary Sediments

Andy Evans

Geography, Leeds University

TillCould this be the

most boring substance on earth?

A diamict: Mud… …and rocks.

…what a thrill.

Old TillOld Till is even

more exciting.It hasn’t seen a

glacier in 18000 years.

Most geologists dump the whole lot in a single category “drift”.

Well brother, I’m here to tell you…Till is fab.Till is great.

Till can wash your car, reduce your taxes, feed your cat, entertain surprise guests, organise parties, power national grids, remove stains from sheets, hide embarrassing odours, resolve international conflicts, speak Japanese (kanichiwa!), hold congress on matters of structuralist anthropology, straighten hair and visit relatives for you at christmas. All in a day, as long as it’s a Tuesday.

Well, no. But…

It might, possibly, just possibly, be marginally more useful than we thought.

Traditionally till interpretation is something of an art.Look at lots of till forming.Look at old till until you’re convince you know what

formed it.Go down the pub.

What we need is a new way of looking at old till.Why shouldn’t till be as

rigorously examined as anything else?

What might it tell us about the way the world was?

Ice dynamics.

Reconstructing Ice Dynamics from Quaternary Sediments

BackgroundField siteSedimentsMicromorphologyModel of depositionReconstructing ice dynamics

Created by the ice.Travels with the ice. Is deposited by the ice.

This material is the dirty fingerprint of a glacier.

Qualitative model (Boulton, 1974)Prow builds up and “stops” clast. Clasts collide and stop.

If the sediment is soft enough to deform, how can it stop anything?

Why do clasts stop when they collide?

Quantitative models (Brown, 1987)

Occurs when the force on a clast drops below that needed for sediment failure.

Not a steady state model.Assume perfectly plastic till.Inevitably lead to models where the whole

bed deforms and there’s no aggregation.

Field site Lleyn Peninsular

Rough Ice Direction

A boulder between two tillsTop till is a flow till.Bottom till is a water-lain clay with clasts lodged in it.Also a resistant band of till and sands.

MicromorphologyThree types of materialSand bands: clean.Fine grained quartz.Melanges (mixes of silt

and clay)

Boundaries

Suggest flow.

Microscale fabrics

Particles align under different situations.

Commonly, under compression under the ice, particles align ~horizontally.

MelangesThree typesMixed w/ varying fabrics.Unimodal w/ flow fabrics.Reverse graded beds.

What does it all mean?

Evidence for: Suggests:

Small scale flow bodies. Slumping of material.

Sands without smaller grains.

Water based separation and washing.

Smaller quartz grains in beds between units.

Winnowing of materials.

Blocks in melanges with strong fabrics.

Reworking of consolidated sediments.

Weertman modelIce moves round obstacles in two ways:

Melt under pressure

Creep under added pressure.

Heat dragged from down-ice

Water moves up-ice

Suggested originThe ploughing and lodgement of a clast.

The lower tillNo sands to speak of.Nice strong fabric

though.

Forces The force from sediment increase as contact area with sediment

increased. Melt out sediment + inflow pushes ice off clast. Transferred to a smaller and smaller area of ice contact increasing

stress (force / unit area) and thus melt.

Prow causes greater sedimentresistance and more meltout

Weak cohesive sediment forceMassive force from sediment acting on a very small area of ice contact accelerating meltout and creep.

What does this give us

A steady state model.

A model that produces fine sediments and clasts.

A model where force is transferred between the ice and the bed (and the bed and the ice).

A model that builds up till even when the till fails.

A model that can be turned into numbers and compared with reality.

The model (isn’t it a beauty!?)

Modelled stuff

Weertman equations for flow around the clast.Till has a fixed residual strength:

Realistic estimates are 0.5 – 50kPa.

Slumping modelled using angle of rest of sediment.Till flow around the clast can be zero (very stiff till)

to 100% (very soupy till).Clast 1m x 1.75m x 1.75m cuboid.Stop when ice movement passed clast = ice

velocity (initial estimate 20ma-1).

Are the results realistic?

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300 350 400

Time (days)

Dis

tanc

e pl

ough

ed (

m)

50000Pa

5000Pa500Pa

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

Distance ploughed (m)

He

igh

t o

f m

ate

ria

l de

po

site

d b

eh

ind

th

e c

last

(m

)

50000Pa

5000Pa

500Pa

0

5000

10000

15000

20000

25000

0 50 100 150 200 250 300 350 400

Time (days)

Fo

rce

tra

ns

fere

d b

etw

ee

n c

las

t a

nd

ic

e (

N)

50000

5000Pa

500Pa

So what can we do?

We know how far it ploughed: calculate all possible combinations of velocity and till strength.

Seems to produce realistic ice velocities for realistic tills.

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70

Ice velocity (m per year)

Till

res

idua

l str

engt

h (k

Pa)

However...Bipolar behaviour: interestingly between glacier

and ice stream velocities.

0

5

10

15

20

25

30

35

40

45

50

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Ice velocities (m per year)

Till

res

idua

l str

engt

hs (

Pa)

0

5

10

15

20

25

30

35

40

45

50

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Ice velocity (m per year)

Till

res

idua

l str

enth

(P

a)

Zero inflow 100% inflow

Behaviour switches quite dramatically at 43% inflow.

Constrain with the sediment record.

45% of material in the gouge is sands: these can’t be from reworking.

In addition, we might suggest at least another 10% is meltout material (the quartz beds, some of the clays).

Seems likely therefore that we fall well below the 43% inflow.

0

10000

20000

30000

40000

50000

60000

70000

0 10 20 30 40 50 60 70

Ice velocity (m per year)

Ma

xim

um

n t

ran

sfe

red

fo

rce

(N

)

0

10000

20000

30000

40000

50000

60000

70000

0 10 20 30 40 50 60

Till residual strength (Pa)

Max

imum

n tr

ansf

ered

for

ce (

N)

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60

Till residual strength (Pa)

To

tal

se

dim

en

t v

olu

me

(m

etr

es

cu

be

d)

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 70

Ice velocity (m per year)

To

tal s

ed

ime

nt

vo

lum

e (

me

tre

s c

ub

ed

)

In shortThe glacier was

moving at 5 – 60 ma-1.

Maximum transferred force before lodgement was 10 – 60 kN.

Total volume of meltout material is reasonably constant at ~1.5m3.

Where does this get us?We have a reasonable model that

allows us to look at force and material transfer.

Material uncouples with the ice and couples with the bed, transferring force.

We can make quantitative estimations of something that happened 18000 years ago.

This gives us more solid data for climate models and a better idea about what’s happening under modern glaciers.

More information

http://www.geog.leeds.ac.uk/people/a.evans/ http://www.geog.leeds.ac.uk/projects/a.evans/