THE CRYOSPHERE - Notes
GLACIERS AS CLIMATE INDICATORS
Fundamentals of glaciology pt1
- ‘Accumulation’ is the gains/inputs in a glacier system
- Accumulation includes: snowfall, wind-blown drift, avalanches, condensation (rime), freezing
rain/external water
- Ablation is the losses/outputs in a glacier system
- Ablation includes calving, melting, evaporation/sublimation
- Snowfall requires sub-zero temperatures + atmospheric moisture
- The highest accumulation rates occur in mountainous maritime regions
- Such as alaska, western patagonia, west coast of nz south island, and japan
- The lowest rates of accumulation occur in the continent interiors
- Such as central east antarctica
- Local variations in accumulation occur due to wind-blown snow, and avalanches (only
mountainous regions obviously)
- firn/nevé is snow that is partially compressed towards ice - it is found under the snow that
accumulates at the head of a glacier
- The density of F or N is at 400-830kg-m3
- The glacial mass balance of a system depends on its inputs minus its outputs
- Accumulation and ablation together, cause ice to flow and therefore the mass balance to
change
- A “steady state” assumes a state of equilibrium whereby accumulation is matched by ablation
- A steady state can adjust a little over time, allowing for fluctuations in temperature etc.
- The ELA is the equilibrium line altitude - also known as the line/zone of equilibrium
- This line is found directly between the zones of accumulation and equilibrium
- Glacial mechanisms of motion include: Internal ice deformation (UF), sliding at the bed (US),
and shear in underlying deformable sediments (UD)
Fundamentals of glaciology pt2
- Glacier sliding refers to slip between the glacier and its bed
- For frozen bed glaciers, sliding is negligible: less than ~4 mm/year (ignored)
- The factors controlling sliding include bed roughness, amount of debris embedded in base of
glacier, and the quantity, distribution and ‘delivery rate’ of water at the bed
- Regelation is pressure melting - whereby pressure forces ice to melt and refreeze on the
other side of the obstacle
- Enhanced basal creep is due to stress concentration when ice encounters an obstacle
- Many glaciers are underlain by soft unlithified sediments (rather than crystalline bedrock)
- If sediments are deformable, this contributes to glacier flow speeds measured at the surface -
difficult to study due to access, difficult to observe
- If sediment deformation occurs, this allows ice to flow very fast
- Sediment deformation depends on water content (and pressure) of the sediments
- With higher water pressure, the sediment is weaker and less able to resist
deformation
- Factors affecting the melting/freezing temperature of ice include
- Pressure (increases with ice thickness), below 2000m of ice, Melt = -1.27•C
- Impurities (e.g. salinity), sea water freezes at ~-2•C due to salt content
- Surface energy balance
- Geothermal heat flux
- Frictional heat due to ice deformation and sliding
- Milankovitch cycles (eccentricity, precession, obliquity)
Fundamentals of glaciology pt3
, - SW net is controlled by albedo (= a)
- Clean ice = high a
- Dirty ice = intermediate a
- Debris-covered ice = low a
- Geothermal heat flux depends on various factors
- Crustal thickness (thin crust = higher heat flux)
- Tectonic history / proximity to boundary
- Local subglacial topography (locally higher at sides of valleys)
- Average for Greenland is ~60mW m-2, and for Antarctica it is ~50 mW m-2
- Frictional heat can be generated by ice deformation, sliding at the bed, and at the
shear margins
- We can define glaciers by its ‘thermal’ or ‘temperature regime’
- Cold based glaciers only have internal deformation - frozen to the base, eg they are
thin, high latitude glaciers
- Warm based glaciers are at pressure melting point, therefore there is water present.
These are common in mid-latitude ‘maritime’ glaciers
- Polythermal glaciers are a mixture of both cold and warm glaciers
- Sliding, and therefore significant erosion can only occur where the ice is at the
pressure melting point
- Cold-based ice preserves underlying surfaces
Glacier erosion and transport
- Erosion
- Glaciers have a very high capacity for erosion
- Glacial environments produce characteristic landforms
- The thermal regime of glaciers: affected by atmospheric temperature, and also pressure
melting point (base of glacier - pressure of ice)
- Basal sliding - caused by the melting at base of glacier
- Subglacial erosion primarily occurs via two processes - Abrasion and Plucking (Boulton 1979)
- Subglacial erosional formations include: striae and polishing, and chattermarks and gouges
- Striae (striations) are scratches incised into bedrock or clast surfaces; it is a direct evidence of
abrasion (therefore evidence of basal sliding) - direction of ice-flow can be observed if scratch
gets wider (as particle gets more blunt from friction)
- Chattermarks and gouges are crescent-shaped fractures, concave side facing down-ice
(usually a few centimetres across), the spacing is often consistent
- Plucking (also known as quarrying), is extended abrasion processes that lead to isolation of
fragments and rock fracture, it is strongly influenced by pre-existing joints
- Transport
- There are many ways that debris is transported, it depends on glacier configuration and
conditions
- Debris may undergo many different cycles of ‘modification’ due to ice/water/wind/gravity etc.
- This leads to a very large variability in form and appearance - which means it is hard
to understand what caused the different landforms found
- Supraglacial debris entrainment: valley glaciers, ice caps (nunataks, volcanic ash), ice sheets
(nunataks)
- Subglacial debris entrainment: mostly dependent on basal flow, however ‘freeze-on’ process
(regelation) has a huge impact on basal flow. Basal flow affects
- Ice motion - debris content changes rheology
- Rates of subglacial erosion
- Debris transportation distances (eg. beyond glacier margins)
- ‘Freeze-on’? = where basal ice freezes onto the frozen glacier bed
- Frost wedging: weathering of rocks due to repeated freeze-thaw cycle
, - Effect of transport on debris - Active transport versus passive transport (Boulton 1978)
- Active transport in basal transport zone: sediment transported in the basal tractive zone,
abrasion and crushing progressively modifies particles
- Passive transport ‘high-level transport’: supraglacial and englacially transported particles
transported with little or no modification
- Sorting (of sediment) gives evidence of transport, and energy (type).
- Clast morphology gives evidence of transport duration and process - shape (sphericity),
roundness
- Glaciofluvial transport includes suspended load and bedload
- Suspended load: for a given water flow, grains settle at a rate proportional to diameter;
generally finer than sand
- Bedload: larger particles roll/slide across the stream bed
Depositional environments and landforms
- Glacial deposition
- Glacial sediments can be categorised by the processes which deposited them (genetic
classification)
- Glacigenic deposits
- Glaciofluvial deposits
- Gravitational mass movement
- Primary glacigenic deposits - moraines
- Moraines are the key glacigenic ice marginal landform - they help outline the previous
configuration of glaciers, and are diagnostic of particular behaviours
- The main types of ice-marginal moraines are:
- Glaciotectonic moraines
- Annual push moraines
- Dump moraines
- Ablation moraines
- Glaciotectonic landforms - glaciotectonic is the main deformation process at ice margins
- Glaciotectonic structures occur where stress transferred from the glacier is greater than (>)
the strength of the material subjected to stress, which = failure
- Push moraines - pushed from behind (shoving)
- Morphology - asymmetric and arcuate
- Locally, it can be winding, reflecting the prophology of the glacier snout
- Distinctive structure single asymmetric fold, tilted axis dipping up-glacier
- Push moraines can link glacier recession (mass balance) to climate
- Beedle et al. (2009) showed that rate of ice-front retreat correlates to summer
air temperature
- Seasonal push moraine patterns can provide a high-resolution ice-climate
proxy
- Valuable in reconstructing (and understanding the forcing of) recently
deglaciated areas
(Bennett, 2001)
- Dump moraines - ridges of supraglacial and subglacial material that accumulates at a
stationary glacier front that has steep margins, and is then deposited by slump and flow
processes
- Common to temperate glacier systems
- Frontal and lateral examples; often latero-frontal
- Depositional-constructional features
- Material is not deformed (as in glaciotectonic moraines)
- Moraine size depends on the: ice flow velocity, debris content, duration of stationary event
