Week 1 Flashcards
Components of the Cryosphere (6)
Snow Sea Ice Ice Shelves Ice Sheets Ice Caps Permafrost (discontinuous, continuous, isolated).
Where are the majority of glaciers today?
Mostly in the Arctic north of 60 degrees (Svalbard, Greenland, Norway, N. Canada)
Topographically unconstrained ice masses:
Ice sheet Ice Cap Ice Shelf Ice Stream Ice Tongue
Topographically constrained ice masses:
Highland ice field Valley Glacier Piedmont Glacier Cirque Glacier Ice Apron/hanging glacier Rejuvinated glacier
Cirque Glacier
A Mountainout basin that has good yearly snow accumuation. Favourable at the highest peaks, with steep relief that means adjacent peaks have no glaciers.
Valley Glacier
Climate allows ice to extend to lower elevations, (valleys) where flow is constrained and directed by topography.
Ice Fields:
If climate cools or accumulation increases, glaciers can coalesce into an ice field with some peaks appearing through the field.
Ice Caps
<50,000km sq, ice may subsume mountain peaks, and this starts to go beyond topographically directed ice flow.
Ice Sheets
> 50,000 km sq consumes mountain ranges, flow may be entirely independent of topography. ‘organsed flow’ is common at margins as ice streams and outlet glaciers.
Outlet Glaciers
Incision of fjords organises and directs ice flow.
Ice Streams
Fast moving Ice that fractures perpendicular to flow.
Ice Shelves and Tongues
Floating ice that extends offshore until calved away.
Tropical glaciers
Found at extremely high elevation only within the tropics.
Polar/Continental glacier
Climate dictates ice is below the freezing point year-round, except for a thin surface layer in summer.
Temperate/Maritime glacier
Climate dictates that ice is above the freezing point throughout the year except for a thin surface layer in winter.
Accumulation Area Features:
Ice domes, divides and dispersal centres (large ice masses), cirque basins and ice fields for smaller ice masses.
Ablation area features
Ice streams, outlet glaciers, ice shelves, ice bergs, tongues, piedmont lobes, calving terminus.
Evidence for Glacial Theory
Glacial features downstream of current glaciers, e.g. moraines.
Agassiz (1837) used evidence of striations, sediment deposition to suggest N. America had been glaciated over.
Astronomical Theory of Ice Ages
- Eccentricity of Earth’s orbit, circular to ellipsoid over time, tilt and precession (wobble) also change with time, changing solar forcing on Earth, dictating climate.
Recent Methodological Advances
Shells of marine organisms are a proxy for global temp (oxy-18 record).
ice cores are a proxy for temperature record from air bubbles.
Advancements in description and classification of sediments recently.
Satellite based remote sensing.
Dating Methods
TCN - Terrestrial cosmogenic nucleide - dates boulders and bedrock surfaces, dating when the rock was exposed from ice.
OSL - Optically stimulated luminescence dating of grains of sand.
Snowball Earth Theory
Evidence of widespread glaciation in Africa, we think there was a supercontinent in tropics, where various factors produces a low CO2 atmos, creating snowball earth.
Glacier Formation
- Snow remains on ground for multiple years, gradually compressing and recrystallizing (firn) crystals grow, air pockets shrink and restricts to bubbles, Glacier ice is formed.
- Density increases with depth, profiles of this illustrate the rate at which snow turns to ice, which depends on accumulation rate and presence of meltwater.
- Once dense enough, ice absorbs red light leaving a bluish tint in the ice.
What is Mass Balance?
The difference between mass gains and losses to a glacier or part of it.
Positive = net gain
Negative = net loss
Melting Processes (Ablation)
- Surface melting most important for land-terminating glaciers.
- Subaqueous melt at lake- and ocean-terminating glaciers.
- Basal melt through friction heating, geothermal heating and rainfall runoff.
- Meltwater that refreezes elsewhere doesn’t count.
- Highest melt rates where steep mass balance gradients enable glaciers to extent into relatively warm low altitudes.
Sublimation
Transition from solid ice straight into water vapour, occurs in cold, dry climates.
Solid Ice Discharge (Ablation)
- Either directly in a lake or ocean (iceberg calving)
- Or motion of ice across grounding line (begins to float)
- Can result in rapid loss of several km3 of ice (e.g. Helheim Glacier, Greenland)
Important for Greenland (50%) and Antarctic (90%) Ice sheets as well as tidewater glaciers. - Calving from ice sheets dont count as its already floating.
Accumulation Processes
- Snowfall
- Deposition of other forms of ice (freezing rain, rime ice (frozen fog))
- Avalanches
- Wind blown snow
- Basal freeze on
- Superimposed ice (freezing of water saturated snow).
What produces the highest and lowest accumulation rates?
Highest: Mountainous regions with frequent onshore winds and much orographic precipitation e.g. West coast NZ and Patagonia)
Lowest: Very dry and cold regions such as interior of the Antarctic.
What is Surface Mass Balance?
Mass balance process occurring at the glacier surface. This is partly dependent on surface energy balance: the net energy produced by solar radiation, longwave radiation, sensible heat from atmosphere, latent heat related to phase changes.
If energy balance is + after surface is at melting point, ablation processes will occur.
Typical Temperature and Snowfall Lapse Rates
Temp decreases 6.5 celcius per km altitude under normal conditions.
Snowfall normally increases with elevation : orographic precipitation and high proportion of snow vs rain at higher altitude. Reduces past a certain point as air cannot hold sufficient moisture.
Mass Balance Gradient
This is the rate at which mass balance changes with elevation, related to temp and precipitation lapse rates.
High gradient = high ablation and accumulation, e.g. Maritime glaciers such as NZ.
Calculate as the average of the accumulation and ablation gradients.
Typical Seasonal Mass Balance Pattern:
Most accumulation in winter, little in summer.
Most ablation in summer, little in winter.
Quantifying Accumulation
Seasonal acc measured by reference to upper surfave of the previous season’s firn.
Multi-year acc measured by digging a snow pit, or extracting shallow ice core and identifying key seasonal variation in structure and density.
Short term acc measured by ablation stakes.
Quantifying Ablation
Direct field measurements of stakes drilled into the ice surface.
Quantifying Net Mass Balance
Compare ablation with accumulation over the balance year.
Quantifying Glacier Volume Change
GEODETIC METHODS
- Repeat topographic surveys or DEMS to compare contours. Terrestrial photogrammetry too.
- Must convert volume change to mass change (need to know density of ablated/accumulated material.)
- We can calibrate surface elevation changes with snow pits and cores, or use average densities for snow firn and ice.
Quantifying Glacier Volume Change
HYDROLOGICAL METHOD
Used in valley catchment glaciers, use stream gauges to measure glacier catchment and runoff, R. Measure precipitation, P, on and off ice.
Estimate or measure evaportation, E.
Balance = P - R - E
Quantifying Mass Balance
MASS FLUX METHOD for ice sheets
Compare the mass flux (depth averaged ice flow and thickness) through a downstream vertical cross section (often at grounding line) with upstream observed or modelled accumulation & melt.
Weighing Ice Sheets?
Satellite gravimetry measures changes in Earths gravitational field every 30 days. Temporal changes in mass distribution of ice and underlying bedrock can be extracted after moving the effects of tides, changes in circulation and atmospheric loading.
How does mass balance relate to climate?
Knowledge of mass balance of current glaciers and its relationshp with climate can be used to reconstruct past climates, and predict impacts of changing mass balance on water resources.
Contemporary mass balance proxies
- Snow line mapping of glaciers as a proxy for mass balance.
- ELA estimated by mappign end of the summer snow line.
- Relative position of snow line and firn line can be use to assess longer-term trends in mass balance.
- Snow line adjacent to bare ice = positive mass balance relative to last year.
- Snow line separated from bare ice by firn = negative mass balance relative to last year.
Palaeo Mass Balance Proxies
- Maximum altitue of lateral moraines delineate the ELA based on evacuation of debris by ice flow in ablation zone.
- Accumulation area ratio (AAR) where we determine the relative size of palaeo glacier extent (ELA lies below ≈60% of former glacier area).
- Area Altitude Balance Ratio (AABR) varies between glaciers based on hypsometry. Balance ratio improves AAR by accounting for shape and mass balance gradient (Rea, 2009).
Glacier Response times
Time taken for glaciers geometry to adjust to a new steady state after a change in glacier mass balance. Depends on glacier volume and area and mass balance gradient.
What controls the extent of mass balance change?
- speed of change depends on glacier’s response time and rate of climate change.
- size of change depends on elevation range and hypsometry of glacier (greater proportion of area close to ELA = bigger response to change in ELA)
