reconstructing ice dynamics from quaternary sediments andy evans geography, leeds university
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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/