IS Hydrology Flashcards
Overview of GRIS:
MB - Negative = 250 gigatons per annum.
GRIS loses more than AIS, despite smaller size.
Sources of Mass Loss:
- GRIS
GRIS:
- recent, due to increased discharge and decrease in SMB (more). - mostly surface processes.
- summer: ablation> accumulation –> large area bare ice forms, low permeability (so surface meltwater high) and water retention capacity (in contrast to firn)
- SE plentiful snow - neutralising MB.
Sources/network of Meltwater Drainage on the GRIS:
- low permeability of ice = surface meltwater
- self organises into complex surface drainage network. features include river, lakes, moulins.
What did Smith et al., (2015) have to say regarding this surface meltwater drainage system?
METHOD: Sat Mapping, in situ of SW GRIS July 2012
- Post huge melt: 97% surface thaw.
FINDINGS:
- 523 streamed efficient network, moulin (hydrofracture) terminating (rapid).
- moulins formed irrespective of depressions of large drained lake basins.
- meltwater runoff accounts for 1/2 or more of total mass loss to global ocean.
- complex ponds/stream/rivers formed when below 1300 asl.
CONCLUSIONS
- network, exclusive or topographic lows and lakes, and 100% river termination in moulins signify efficient new routing of meltwater to compensate with melt event.
- DEM’s cannot alone imply/describe supraglacial drainage and subglacial systems.
- relying upon outflow predictions from climatic model alone, without recognising subglacial processes, overestimated true meltwater export.
Outline occurrence of supraglacial lakes in Greenland:
- GRIS largest supraglacial lake discharge?
- influencing of routing/streaming on surface?
- effect of overlying lakes on cracks/crevasses?
- Greenland: Largest discharge from a Supraglacial lake 17.72 m3s-1 (relatively small)
- streams feed surface lakes formed in closed surface basin, often lakes form on top of surface cracks/crevasses. – high density of water (to ice), crack propagates to bed by hydrofracture where tensile strength of ice is overcome.
Example of Supraglacial Lake (Greenland)- e.g. Igneczi et al., (2016):
- used surface elevation/regional climate model to show at end of 21st C increase in 113% compared to 1980-2009 of meltwater stored as supraglacial lakes.
Formation: accumulation of meltwater in surface depressions on a glacier/ice sheet above impermeable snow/ice. - occur below the ELA across region, principally in same positions, rather than migrating, indicates location controlled by transfer of bed undulations to surface (instability).
- rapid transport (hydrofracture) to subglacial networks (unless efficient) –> increase water pressure, thus less basal friction and flow increase.
According to Igneczi et al., (2016) - what 3 effects (besides on ice flow) can surface-to-bed connections and efficient subglacial drainage occur:
- transfer of melt to ice sheet margin, reducing retention and refreezing of melt on surface.
- transfer heat into ice sheet, reducing viscosity and promoting fast flow.
- affect magnitude and timing of freshwater and nutrient delivery to ocean.
How did Ingeczi et al., (2016) predict future supraglacial lake distribution and secondary effects?
- consider all closed surface depression a potential SGL. Assuming controlled by bed topography, unlikely to change over <100 year period.
- closed depressions surveyed using a DEM
Describe and explain surface depression occurence on ice sheets, in relation to supraglacial lake formation in GL:
Ingeczi et al., (2016)
hints:
1. how many below ELA?
2. how many closed surface depressions?
3. distribution of depressions above ela?
4. future?
- 81% located below ELA, agrees with previous observations.
- 25,140 closed surface depressions: close to ice margin, where thinner and large basal slip ratios permit transfer of short wavelength bed undulations to surface.
- DISTRIBUTION - N, NE & E more depressions (44%, 69%, 88% above ELA).
- NW, W, SW - fewer depressions above ELA, implying current supraglacial lake dist close to the topographical limit of depressions - important as future advance of SGLs could be limited in W GRIS.
Outline the predicted distribution of large volumes of water in GL in regards to supraglacial lakes:
Ingeczi et al., (2016
ALSO: how would a negative feedback system occur in W GRIS
- greater volumes of meltwater infill expected, NW, W, SW (most depressions) increased volume likely to initiate supraglacial lakes from depressions.
- expansions will impact ice dynamics and mass balance.
ALSO - below ~1500m in W GRIS - lake drainge = negative feedback on flow - where increase volume = efficient subglacial network development.
THUS, in a warming climate -greater/longer melt - this efficient sub system will be primary control on ice flow in W GRIS.
Outline the atmospheric and surface controls on meltwater drainage and supraglacial lake location:
- climatic
- surface
- primary control
- Temp, Irradiation, Precip (climatic)
- Firn layer, Surface Permeability, Albedo (surface)
- Primary control of surface drainage: surface topography, routing. - same place most years - indicates ice surface topography fixed in space, on scales controlling structure of drainage.
Outline how the topography at the surface is controlled in regards to depressions/supraglacial lake drainge: - linked to basal what? - surface topography are on what scales? mesoscale undulations are... 1. shorter than.... but longer than..... 2. \_\_\_\_\_\_\_, modulated by \_\_\_\_ and \_\_\_\_
- basal topography
- surface topography on scales comparable to the ice thickness is controlled by basal variability.
- mesoscale undulations are:
1. shorter than ice sheet length but longer than forms from surface processes.
2. relatively permanent, controlled by basal topo and ice thickness.
What does Ingeczi et al., (2018) say regarding ‘transfer of basal variability’?
- theory used to predict surface relief of the sheet from bed topography, ice thickness and basal slip ratio data.
- using radar data - gives roughness/slipperiness AT BASE
- surface relief
- controlled by bed-to-surface transfer of basal variability (TOBV), preconditions of which are large spatial structures of surface drainage, which other factors modulate the actual drainage system through influencing the temporal evolution of meltwater features.
e.g. hydrofracture, crevasses/moulins
CONCLUSION - surface topographic relief controlled predominantly by basal topographical perturbations, while ice thickness, basal slip ratio and surface slope modulate this control.
- linear regression significant relationship between these relief variables, the general method and pattern was successful, BUT considerable error (14.9m) between observed VS predicted mean surface relief.
Outline the main reasons of difference between predicted/observed surface relief/depressions:
Ingeczi et al., (2018)
- method basd off TOBV theory - doesnt incorporate other surface relief production processes e.g. redistribution of snow/firn by strong winds - small wavelength relief forms.
- underestimates true basal slip ratio - underestimates surface relief. - if faster slip you get more efficient transfer of basal variations.
- assumption of constant linear viscosity - exhibits linear stress/strain relationship. - glens law says otherwise (strain up/viscosity decrease) - deformation smooths out where should be bumpy, didnt consider.
- Assume ice is fully temperate at melting point, but towards centre ice freezing but colder than expected.
- 3D Effect - big convergence happening (e.g. ice streams) messes up effect. Laterally confined = dampen transfer
What are the main points on pattern of supraglacial lake controls from Ingeczi et al., (2018):
- mescoscale
- relief?
- run off?
- moulin/rivers
- Mesoscale undulations –> primary control on surface drainage structure.
- Supraglacial Lakes –> strongly controlled by surface relief; occur in moderate relief.
- Run off - little effect, always peaks where lakes form.
- Moulins/rivers –> increase in density towards lower elevations. Moulins where relief high.
CONCLUSION: Ingeczi et al., (2018)
- long term implications - predictability
- long term implications - distribution
- success of predictions vs observed
- controls and preconditions for surface drainage system.
- Use of depressions as proxy for lakes, ELA rises with warming - more water available and at higher elevations (NE/NW) - especially in interior of sheet (lots of depressions)
- ELA rises - lakes at higher elevations. NW/NE GL huge potential for increase.
- mismatch between observed/predicted relief, arising from unknown basal slipperiness, uncertainties of bed topography, basal slip ratios, surface processes, assumption of linearly viscous medium, 3D. Underestimations 1% by 2100, but 13% by 2300.
- basal topography as primary precondition, other factors e.g. surface runoff generation and crevassing influence temporal evolution.
According to Ingeczi et al., (2018), what are the main controls for surface topographical undulations?
- dimensional wavelength - a ratio between undulation wavelength and ice thickness.
- ice surface slope
- basal slip ratio
- transfer more efficient where basal slip ratio and surface slope larger.
- inefficient at short dimensional wavelengths
Summarise AIS ice surface hydrology:
- little precipitation (< 1cm p/yr) ==> linked to continentality, not enough moisture to accumulate in centre.
- below freezing across ice sheet most of year
- aeloian processes present - high winds.
According to Kingslake et al., (2017): Summarise AIS Ice Surface Hydrology: Hints - number of systems/lakes - 3 triggers - temporal/spatial scale
- 696 meltwater systems/lakes connected by surface streams
- surface meltwater drains across ice sheet, forming melt ponds/streams - can trigger (1) shelf collapse, (2) acceleration of grounded ice flow and (3) increased SLR.
- drainage as high as 1300m a.s.l.
- temporal/spatial scale - decades, transport 120km from grounded ice onto/across shelves. Making shelves vulnerable to increased melt rates.
According to Kingslake et al., (2017): What is the general structure of surface hydrology across AIS?
- High elevations
- moderate elevations
- Low elevations (200m a.s.l)
- 2/3 originate from ice flow <120m yr-1 and near low albedo areas (e.g. nunataks)
High elevation: characterised by meltwater production near rocks at glacier margin, flows through streams to marginal melt ponds
Moderate elevation: water drains through streams running parallel to surface lineations
Low elevations: streams join to form braided network that cross grounding line, entering Ross Ice Shelf.
According to Kingslake et al., (2017): What is the primary control on surface melt? (AIS)
- albedo processes.
- low albedo blue ice (snow removed by high winds) nunataks, surface debris facilitate melt.
- therefore, melting and wind erosion lower ice surface, enlarging areas of exposed rock - i.e coupling between melt, rock and blue ice.
- results in close spatial association between drainage, blue ice and nunataks. –> further S than 75* = stronger association due to lower air temps so albedo greater influence.
Detail how a southerly glacier (e.g. Shackleton, 85* S) gets water (despite low air temp):
hint - 2 reasons
- nunataks/exposed rock/moraine systems
- near ice shelf grounding lines and rock/darker ice.
- low albedo - heats up - melted water absorbs heat and ponds further –> darkens futher = POSITIVE FEEDBACK - Katabatic winds
- scour snow in dark areas, lowering albedo.
- draws in hot air from above, removing snow from below.
- sometimes surface lakes found beneath.
What are the implications associated with enhanced melt in southerly regions of AIS/ice shelves?
- example: Larsen B
Break up of ice shelves
1st effect - hydrofracture. initiated by ponding in crevasses.
2nd effect - weighs down shelf - denser - induced flexural stresses, creating more crevasses, water, hydrofracture, instigate break up.
Larsen B
- collapse 2002, 6-weeks.
- collapse - relevant to warming signal - retreat of AP shelves, rapid regional warming - surface melt - hydraulic fracture/crevassing
According to Shepard et al., (2018) - what would the widespread implications of ice shelf collapse result in?
GOOD STATS on enhanced AIS melt.
- raise global sea level - 58m (AIS)
- e.g. 1992-2017
- 2720 +- 1390 Gt loss
= 7-10mm SLR - ocean driven melting has causes rapid rates of ice loss from W AIS. = ~160 Gt/yr in 2010s
- Ice shelf collapse has driven Antarctic Peninsula ice loss massively = ~45 gt/yr 2010s - from ~13 gt/yr 1990s. —> wont contribute to SLR –> but removes buttress, allowing grounding ice to speed up. - increasing discharge to oceans. (De Rydt et al., (2015)).
According to Rignot et al., (2004) - What followed the LarsB collapse of 2002?
GENERAL - accelerated discharge from AP following LarsenB in 2002.
Details:
- mass loss associated with flow acceleration exceeding 27km^3/year, ice is thinning at rates of tens of metres per year.
- attribute to loss of buttressing ice shelves.
- compared 1996/2000 ERS data, revealing a 20% acceleration (100m /yr) =–> but no acc of glacier.
According to Rignot et al., (2004) - What followed LarsA collapse 1995? - - - - Hint Zwally et al., (2002)
- 3x fold acceleration of Drygalski Glacier (500m/yr)
- results indicated thinning rates of 10s m/yr.
- assuming increase in grounding line outflow is proportional to velocity increase.
Zwalley et al., (2002) - ‘regional climate warming certainly caused change. Warmer air temperatures increase surface melt water production, which may increase basal lubrication near grounding lines’ (i.e. warm, speeds up basal slip near point at which becomes ice shelf) - as close to grounding line, surface melt likely little factor in acceleration.
Detail the subglacial hydrology of AIS:
- drivers of basal system
- ice sheet characteristics
- basal pressure
Main drivers of basal system - periodic lake drainage, subglacial lakes and background hydrological flow.
Ice sheet characteristics: thick ice, low accumulation rates and geothermal flux.
basal pressure - 55% bed above PMP - hence lakes.
Outline the effect of geothermal flux and elevated melt rates in AIS:
e.g. Loose et al., (2018)
- elevated melt rates, found active volcanic heat beneath WAIS, upstream of fast melting pine island ice shelf.
- EVIDENCE
1. Sea water helium isotope ratios at front of ice shelf
2. direct measurements of heat flux beneath Whillhans exceed background geothermal gradient
Conclusions:
- location and extent of activity debated. BUT localisation of mantle helium to glacial meltwater indicates volcanism induced heat flux melts ice and feeds hydrological system beyond grounding line.
- definite proof still missing (e.g. thick ice hard to investigate)
Presently, what is the greatest contributor to ice shelf instability?
- circumpolar deep water currents are the primary heat source for melting glacial ice.
- implicated to be cause of rapid melting and grounding line retreat beneath Pine island glacier and atmo warming around West AP.
Outline how on top of CDW, volcanism is contributing to enhanced melt of ice shelves in WAIS?
- hint 1 - helium isotope signature
- hint 2 - rate
- observations of helium isotope signatures, suggest volcanic heat sources lie within the Hudson Mt range, and is driving a subglacial melt that subsequently crosses the ice shelf grounding line.
- quantified at a rate of - 2500 +-1700 MW.
What could be the causes of variations in magnitude etc of volcanic heat flux to WAIS?
- internal magma migration,
or - increase in volcanism as a result of ice thinning.
- potentially impacting Pine Island futher.
Outline Wingham et al., (2006) on rapid discharge connecting subglacial lakes:
- Observations
Observations:
- ice sheet surface elevation changes in central EAIS, interpreted to represent rapid discharge from a subglacial lake.
- 16 months, 1.8km^3 water, over 290km to at least two other subglacial lakes.
- series identified along Adventure subglacial trench (4km so PMP), from radio-echo sounding.
- altimeter - revealed two clustered anomalies of ice-sheet elevations
- hydraulic gradient 5.1 Pa m-1, discharge 50m^3 s-1, supported by single tunnel 4km radium and 290km.
Outline Wingham et al., (2006) on rapid discharge connecting subglacial lakes: - Discussions/Conclusions hints - instability - expectations - mechanism - termination
- the intrinsic instability of system suggested such events common mode of basal drainage - if large enough may reach ocean.
- conflicted with expectations, that SGL occupy long residence times & slow circulations.
- flushed periodically
- mechanism: rapid transfer of basal water, where the potential energy released by flow melts the tunnel ice walls, a positive feedback causes flow to rise abruptly and drains lake.
- for lake to exist, must be a local reverse in hydraulic gradient, if filling, this will be overcome and a hydraulic connection between lakes is possible.
- termination of discharge: explained as result of tunnel closing by ice flow as pressure drops. Bed and ice close former cavity left by lake.
According to Wingham et al., (2006)
- what is the largest present day lake
Lake Vostok - 5400km^3 of water.
- possible lake experienced past drainage and is currently filling.
What is the importance of understanding SGLs?
- Influence on water motion beneath ice sheet.
- Effect on ice-sheet stability and thermal regime
- Drainage and implications on oceanic circulation
- Archive of Quaternary climate change.
- Paleo-biology of extreme forms
How has theory developed in terms of channel development and connectivity of SGL beneath AIS?
-hint 1 - Hydrological connections and pathways beneath IS. pr
- 1.5 year rapid filling/drainage
- connected hydro pathways between lakes towards margins - as far in as interior
- real question is, what initiates channel development?
R channels? Channels cut into sediment? unconstrained floods?
How has theory developed in terms of channel development and connectivity of SGL beneath AIS?
- Hint 2 - ice marginal channel development (Le Brocq 2013) - for ice shelves
Le Brocq (2013) –> modelled approach to predict outflow underneath Filchner-Ronne ice shelf.
- surface depressions on DEM (surface indication), generated by massive channel at base (250 W, 300 H) found by radar.
- observed surface features on ICE SHELF (depressions) show correspondence with channels beneath.
- Sub-ice-shelf channels aligned with locations where the outflow of subglacial meltwater predicted by the model. - extension of subglacial meltwater channel
- agreement of modelled outflow and sub-surface channels - suggests channels formed by meltwater plumes.
