Reconstructing Ice Dynamics from Quaternary Sediments

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Reconstructing Ice Dynamics from Quaternary
Sediments

• Andy Evans
• Geography, Leeds University

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Till

• Could this be the most boring substance on earth?
• A diamict
• Mud
• and rocks.
• what a thrill.

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Old Till

• Old Till is even more exciting.
• It hasnt seen a glacier in 18000 years.
• Most geologists dump the whole lot in a single
  category drift.

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Well brother, Im 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 its a Tuesday.

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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 youre convince you know
  what formed it.
• Go down the pub.

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What we need is a new way of looking at old till.

• Why shouldnt till be as rigorously examined as
  anything else?
• What might it tell us about the way the world
  was?
• Ice dynamics.

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Reconstructing Ice Dynamics from Quaternary
Sediments

• Background
• Field site
• Sediments
• Micromorphology
• Model of deposition
• Reconstructing ice dynamics

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• Created by the ice.
• Travels with the ice.
• Is deposited by the ice.
• This material is the dirty fingerprint of a
  glacier.

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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?

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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 theres no aggregation.

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Field site

• Lleyn Peninsular

Rough Ice Direction
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A boulder between two tills

• Top 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.

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Micromorphology

• Three types of material
• Sand bands clean.
• Fine grained quartz.
• Melanges (mixes of silt and clay)

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Boundaries

• Suggest flow.

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Microscale fabrics

• Particles align under different situations.
• Commonly, under compression under the ice,
  particles align horizontally.

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Melanges

• Three types
• Mixed w/ varying fabrics.
• Unimodal w/ flow fabrics.
• Reverse graded beds.

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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.
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Weertman model

• Ice moves round obstacles in two ways
• Melt under pressure
• Creep under added pressure.

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Suggested origin

• The ploughing and lodgement of a clast.

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The lower till

• No sands to speak of.
• Nice strong fabric though.

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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.

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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.

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The model (isnt it a beauty!?)
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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).

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Are the results realistic?
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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.

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However...

• Bipolar behaviour interestingly between glacier
  and ice stream velocities.

Zero inflow
100 inflow
Behaviour switches quite dramatically at 43
inflow.
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Constrain with the sediment record.

• 45 of material in the gouge is sands these
  cant 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.

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In short

• The 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.

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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 whats happening under
  modern glaciers.

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More information

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