MSGL/Continuum Flashcards
Groove-ploughing for MSGL formation is an attractive idea, what issues can you think of?
- Can keels be produced at appropriate scale?
- Can they survive once they move onto soft till and survive downstream transport of 100km?
- Friction of keels being dragged through sediment
- Cannot observe subglacial processes. SO modelling?
Ice-basal roughness for MSGL formation:
- What factors should models account for to get rid of keels?
- What may ‘flow stripes’ on ice streams suggest? (Author?)
- Are there any observations in support of groove ploughing (HINT: iceberg)?
- Thermal (frictional) losses, 2. Ice-deformational losses, 3. Mechanical degradation
- Gudmundsson e.a (1998) - regard ‘stripes’ as a reflection of reaction to bumpy bedrock, with similar scale to MSGL, if modelling allow for these then potential for MSGL 100’s km long. - Polyak e.a (2001) - Deep Arctic - grooves found too deep for iceberg ploughing - downstream thick drape sediment found - partially grounded ice shelf.
- Norway - MSGL grade perfectly into ploughmarks - strong indication of governing mechanism by keels.
Main points on Subglacial Floods theory for MSGL (Author?)
Fannon et al., (2017)
- formed by water erosion - but unlikely flooding - likely to be bed rilling instabilities where water acts as self organising erosive force that develops RM –> Drumlins –> MSGL over timesteps.
- model does show bedforms produced of correct size/morphology, and streamflow becomes localised.
- BUT, controversial as model predicts MSGL diagonal to ice flow.
Conclusion on formation of MSGL:
- Fast flow
- Theories
- MSGL exist as largest subglacial bedform
- Fast flow arguement confirmed - valuable geomorph landform to infer paleo IS/behaviour
- Theories - Deformation (unlikely as no real basis), Subglacial Megafloods (NO), Groove-plough (NOT PROVEN WRONG), Instability theory extended to include water flow via rilling. (possible)
Is there a unifying theory that could explain a possibility of continuum?..
What does Barchyn e,a (2016) have to say about subglacial bedforms?
- enigmatic repetitive flow-parallel and flow-transverse landforms
- growth and evolution critical to understanding of basal roughness evolution beneath IS
- likely to influence subglacial hydrological systems, and in turn IS velocity.
Outline traditional views upon subglacial bedforms as a morphological continuum: Aario (1977) - Rose (1987) - Clark (1993) - Stokes e.a (2013) -
Aario (1977) - ‘transition zones’ where bedforms get progressively elongate downflow.
Rose (1987) - 1st to measure - found distinction between flutes and drumlins
Clark (1993) - upon discovering MSGL - thought may be separate population due to great length.
Stokes e.a. (2013) - noticed MSGL adjacent to drumlins, parallel, so same ice motion. Noticed no distinction between drumlins and MSGL - are they separate?
Summarise Ely e.a (2016) - do subglacial bedforms comprise a shape/size continuum?
- mapping from previous studies
- ~100000 bedforms across Canda, Iceland, UK, Norway etc.
- Length/width estimated (elongation ratio)
- ran clustering algorithm to test for continuum
- findings:
1. flutes separate - own scale, narrow & elongate (>1.5km) , exceeding length of drumlins but narrower, superimposed on drumlins - distinct diff in morph
2. drumins and MSGL overlap - continuum of subglacial lineations. - no distinction upon morph
3. ribs and mega-ribs overlap - continuum of subglacial ribs - no distinction upon morph
4. A larger continuum? - as quasi-circular bedforms linking ribs-drumlins, raises possibility of a single bedform continuum ribs - quasi forms - lineations
5. bedforms seen to evolve along ice flow trajectories - can take form of increased lineation elongation or switch from ribs-drumlins. King et al., 2009 - evidence to support this.
6. local conditions may correspond to morph differences. e.g hard bedrock and soft deformable sediment, often find a corresponding increase in elongation = acceleration in ice flow.
Does a morphological continuum = a process continuum?
- flutes likely separate
- spatial transitions - unlikely two forms next to eachother produce same result.
- Shaw ‘megaflood’ debunked - water supply.. SO
- -> Instability theory - different instabilities diff bedforms. EVIDENCE: Fowler and Chapwanya (2014) - combined equations from previous papers for diff bedforms. FOUND: possible. sediment in drumlins can be preserved by transporting sediments only on top-layer, beneath preserved.
Potential controls on how single process = variety of forms/sizes:
- sediment/lithology
- flow velocity
- Sediment/Lithology - no clear relationship of composition shape. Dowling e.a. 2015 - no diff in morph between different ‘core’ drumlins.
BUT - some abrupt regional changes where abrupt boundaries in sediment properties - linked to velocity?
Greenwood and Clark, (2010) - no relatione between sediment and bedform size on ice-sheet scale.
- Flow Velocity - shown to relate to greater elongation, potential proxy? EVIDENCE: Stokes e.a (2013) - proposes velocity as main control.
Summarise Batchyn et al., (2016) - Subglacial bedform morph controlled by ice speed and sediment thickness
- Method:
- Primary control of form variability
- Finding 1. Lee side deposition..
- Finding 2. Timescale
- Finding 3. Velocity = elonagation. 3 reasons (a,b,c)
provides evidence that…
Issue:
- Cellular model employing several core mechanisms hypothesised to exist subglacially. (e.g. ice advects down-flow, deformation over topography, Pw at bed, pressure drives flow until bed deformation.)
- Ice velocity
- Lee side deposition needed to elongate ribs to subglacial bedform. Models sediment load to travel with ice and entrains/deposits as a function of pressure = drumlins/MSGL develop.
- broad pattern characteristics develop after ~200 years. - typical
- a) increased speed = stoss pressure on bed bumps, as ice forced to deform faster rate. Accelerating bed form development and amplifying differences in
(b) lee tails elongate due to larger lee-side cavities and greater ice-entrained sediment flux (due to faster ice)
(c) locking sediment in lee tails has similar effect to reducing sediment thickness = increasing spacing, elongation, and restricts lateral linking. - as does increased speed.
provides evidence that features in shallow sediment likely to form from same mechanism. Also, ice speed and sediment/cavity length have role in elongation.
Issue: largely abstract model due to lacking understanding of relevant phyics subglacially - modelled in simple manner for computational efficiency
summary on continuum:
- quasi
- Flutes
Drumlins/MSGL, Ribs/mega-ribs, form a process & morph continuum where quasi-circular bedforms bridge later two continua.
Flutes - separate due to differed size and shape.
