IS Hydrology Flashcards
(94 cards)
1
Q
Overview of GRIS:
A
MB - Negative = 250 gigatons per annum.
GRIS loses more than AIS, despite smaller size.
2
Q
Sources of Mass Loss:
- GRIS
A
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.
3
Q
Sources/network of Meltwater Drainage on the GRIS:
A
- low permeability of ice = surface meltwater
- self organises into complex surface drainage network. features include river, lakes, moulins.
4
Q
What did Smith et al., (2015) have to say regarding this surface meltwater drainage system?
A
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.
5
Q
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?
A
- 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.
6
Q
Example of Supraglacial Lake (Greenland)- e.g. Igneczi et al., (2016):
A
- 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.
7
Q
According to Igneczi et al., (2016) - what 3 effects (besides on ice flow) can surface-to-bed connections and efficient subglacial drainage occur:
A
- 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.
8
Q
How did Ingeczi et al., (2016) predict future supraglacial lake distribution and secondary effects?
A
- 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
9
Q
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?
A
- 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.
10
Q
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
A
- 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.
11
Q
Outline the atmospheric and surface controls on meltwater drainage and supraglacial lake location:
- climatic
- surface
- primary control
A
- 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.
12
Q
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 \_\_\_\_
A
- 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.
13
Q
What does Ingeczi et al., (2018) say regarding ‘transfer of basal variability’?
A
- 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.
14
Q
Outline the main reasons of difference between predicted/observed surface relief/depressions:
Ingeczi et al., (2018)
A
- 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
15
Q
What are the main points on pattern of supraglacial lake controls from Ingeczi et al., (2018):
- mescoscale
- relief?
- run off?
- moulin/rivers
A
- 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.
16
Q
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.
A
- 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.
17
Q
According to Ingeczi et al., (2018), what are the main controls for surface topographical undulations?
A
- 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
18
Q
Summarise AIS ice surface hydrology:
A
- 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.
19
Q
According to Kingslake et al., (2017): Summarise AIS Ice Surface Hydrology: Hints - number of systems/lakes - 3 triggers - temporal/spatial scale
A
- 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.
20
Q
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)
A
- 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.
21
Q
According to Kingslake et al., (2017): What is the primary control on surface melt? (AIS)
A
- 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.
22
Q
Detail how a southerly glacier (e.g. Shackleton, 85* S) gets water (despite low air temp):
hint - 2 reasons
A
- 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.
23
Q
What are the implications associated with enhanced melt in southerly regions of AIS/ice shelves?
- example: Larsen B
A
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
24
Q
According to Shepard et al., (2018) - what would the widespread implications of ice shelf collapse result in?
GOOD STATS on enhanced AIS melt.
A
- 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)).
25
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.
26
```
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.
27
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.
28
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)
29
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.
30
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.
31
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.
32
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.
33
```
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.
34
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.
35
What is the importance of understanding SGLs?
1. Influence on water motion beneath ice sheet.
2. Effect on ice-sheet stability and thermal regime
3. Drainage and implications on oceanic circulation
4. Archive of Quaternary climate change.
5. Paleo-biology of extreme forms
36
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?
37
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.
38
According to Le Brocq (2013) - detail the mechanism by which meltwater plumes lead to sub-shelf channels post SGL drainage:
meltwater reaches grounding line --> rises beneath shelf (less dense, warmer) --> entraining warmer ocean water --> inducing localised sub-shelf mate rates -- small channel forms.
- once small sub-ice-shelf channels --> localised melt enlarges --> meltwater plume flow increasingly focused into channel.
--> after some distance, channel becomes super cooled (decreasing pressure, less PMP, more ocean thermal driving) --> freezing = channel infill. Evident by feature disappearance on DEM after several hundred kms.
39
According to Schroeder et al., (2013) what could be the other theory to explain sub-ice-shelf channel development? - he doesnt believe ice-ocean interface explains occurance of surface features..
hint -bedrock
- found under Thwaits Glacier, using radar altimetry to look at roughness of bed.
- theory - initial notch for channel devleopment occurs same as Le Brocq (2013). (i.e. ice/ocean interface)
- organisation - effect of hard-bedrock creating corrugations and focussing water. Ice moulds around bedrock, water gets forced into, then transported downstream. Roughness of bedrock provides cavities for water to drain into.
- evidence - areas of high specularity indicate distributed system - areas of low (i.e. rough, corrugated) = channelised.
- transitions zone - between distributed and channelised around 50km from grounding line. (ponding behind bedrock ridge.
40
What are the dynamic implications of having a sub-ice-shelf channelised/distributed drainage system?
- transient
- marginal areas
- water piracy
- transient dynamic effects - periodically draining = potential seasonal speeds ups (GRIS)
- channelisation in marginal areas - slow down flow.
- water piracy --> concept that water can be captured by other systems --> speed ups/slow downs.
- paradox = active lakes occur in locations undergoing long term slow down. (odd that its so dynamic)
41
Give main points on how Bell et al., (2017) demonstrated AIS potentially stabilised by export of meltwater stored in crevasses/ponds through proglacial rivers.
- Storing water = weakens and fractures shelf (trigger)
- SO
- provided evidence of an active supraglacial drainage network on Nansen Ice shelf between 06-15.
- adaptive of varying meltwater (warmer seasons) - by providing longer and more intense waterfall terminating network.
= helpful in warmer climate scenario. - current predictions think increase melt will enhance ice-shelf disintegration.
42
```
Why do we get subglacial lakes? (what permits under ice water)
1
2
3
4
```
1. PMP
2. Insulating effect of ice
3. geothermal
4. frictional heat
43
What controls water flow beneath the sheet
- hint hydraulic
- burden pressure
- steep slope
Hydraulic gradient
- hydrological potential energy
ALSO
- burden pressure and water pressure permits uphill flow.
SO difficult to get lakes. - water can pond on topographic depressions, steep slope also squeezes confined water out, stopping ponding.
44
Outline the different subglacial lake types and conditions/controls on location:
1. basal thermal regime
- as ice/glacier moves downslope - shallower hydraulic gradients - increasing potential for lake formation.
- more on bigger ice sheets, less higher elevations,best when ice thicker (basal melt)
- upon retreat, lake may freeze, or creates slippery spot, or leave fossil lake.
45
Outline the different subglacial lake types and conditions/controls on location:
2. Ice shelf capture (90s)
- ice flow into shallow sea = floating tongue of terminus that crosses lake to other side, grounds.
- entrapping water in on a reverse bed gradient.
46
Outline the different subglacial lake types and conditions/controls on location:
3. Subglacial Volcano
- lake blister even if flat topography.
| - melting ice rapidly at location, draw down ice from surface.
47
Outline the different subglacial lake types and conditions/controls on location:
4. Frozen Base Margin
- polythermal style glaciation
- subglacial lakes behind a frozen margin used to explain formation of tunnel valleys, due to evidence of permafrost.
- can have lake ponding, bursting occasionally, creating tunnel valleys and outwash fans with coarse material.
48
How might one identify SGLs:
e. g. fricker 2007 - elevation anomalies
e. g. Siegert et al., (2016)
- radio echo sounding (RES) --> strong signal - lake
- Flat spots - prominent e.g. (sirgert and Ridley, 98)
- vertical surface elevation change by ICEsat Mercer (Fricker et al., 2007)
49
Detail some details of whats known regarding active SGLs:
2000's Breakthrough
Fricker (2007) - WAIS, radar. Found regions of difference in elevation.
- saw growth/drainge = 5 year period.
- some stationary - with cyclic fill-drain events.
- some lakes ephemeral --> due to low gradients/piracy..
e. g. Subglacial Lake Cook, EAIS (McMillan et al., 2013)
- 70m anomaly 07-08 = 260km^2 depression
- 5.2km^3 volume water
- 09-12 refilled
- 500km connected hydrological system in Wilkes Basin.
50
Outline the Mechanisms for Subglacial Lake Drainage according to:
Carter et al., (2017) -
- drainage via channels cut into sediment
--> infilling --> gets to point where has different hydraulic gradient to surrounding distributed drainage --> leakage --> accumulates toenough flow for incision.
= low pressure channel - high surrounding pressure.
= water runs out - creep by deforming sediment overcomes erosion and infills.
51
Outline the Mechanisms for Subglacial Lake Drainage according to:
Dow et al., (2016, 2018) - R-Channels
- pressure wave theory
- pulses of high Pw water transmit through system, see cyclic series of waves fitting timeline of lake drainage.
- high pressure wave passes over lake, increasing gradient and pressure to force water out by channel formation.
- little evidence
52
Outline SGL exploration in AIS:
| e.g. Mikucki et al., (2016)
3 attempts made between 2012-2013 - measure in situ properties and direct sample of envs.
- 3rd successful US.
- Lake Whillans, found to be shallow hydraulically active lake w microbial life and active biogeochemical cycling.
53
```
Since the discovery of Lake Cook, there has been no evidence of active lakes having significant volume changes. why could this be?
1.
2.
3.
4.
```
1. changes in elevation not by water
2. lakes empty during survey
3. RES survey cords offset from the location which experienced small range of elevation change.
4. surface elevation expressions are artefacts of ICESat data.
54
Give some evidence of Subglacial Lake networks or dynamics:
- Weddell Sea Embayment
- Wilkes Basin
- RES - E,Ant - 100km L and 70km W system of major incised channels (5km wide and more than 200m D), and smaller canyon structures.
- Wilkes - Geomorphic evidence - interpreted as subglacial meltwater channels formed during drainage of a large (830km^3) paleo-subglacial lake in upper parts of basin during Micocene.
55
Outline the main causes for fewer subglacial lakes beneath GRIS:
- Steeper slope = stronger hydraulic gradient
- flatter bed and cold based interior = less potential, less deep tectonically controlled troughs
- thinner (3km interior, 1km less than AIS), centre GRIS also colder. So frozen base.
- lakes tend to congregate at high geothermal.
56
Outline what Willis et al., (2015) found regarding:
| - Recharge of a subglacial lake by surface meltwater in NE GRIS
- Observed Flade Isblink ice cap
- 2001 Autumn, large subglacial lake drained into fjord, leaving ~70m deep basin collapse.
- observed over several years that supraflacial meltwater entered lake via crevasses/moulins, resulting in a late-melt season uplift of basin floor.
Concluded
- water recharged by supraglacial networks. Over scale of GRIS, potential for melt to fill 400 or more subglacial basins.
- has an effect on basal conditions, as warmer, making less viscous. Coupled with lake floods, could modulate or accelerate flow downstream.
57
Outline what Chu et al., (2016) found regarding subglacial water storage in Greenland.
- Radar reflectivity over two seasons, looking at seasonal subglacial drainage development for two glaciers in GRIS.
FINDINGS
- suggest winter storage on bedrock ridges, upon thaw seasons transition of water into topographic troughs.
- suggests the state of subglacial hydrology at onset of melt season highly impacts glacier dynamic response to surface melt in summer.
58
What did Fricker et al., (2007) find regarding subglacial hydrology affecting Ice Stream dyamics for Whillans/Mercer?
- WAIS
- 2003-06 event
- observations give clues to stability through sensitivity to basal lubrication.
- time scale for sub transport short compared to other known glacial flow drivers, suggesting it a mechanism for more rapid changes in glacial behaviour.
03-06 event may have been triggered by long-term thinning and local upstream migration of grounding line, causing lake seal to break.
59
Outline how Livingstone et al., (2016) identify paleo-SGL:
- what problems?
- geomorphic findings
- sediment little evidence
Geomorphic evidence:
- flat spots connecting channels and eskers - from LIDAR
- physical evidence of mechanism/geometry of lake drainage from last glaciation.
- shallow (10m) lenses of water perched behind ridges transverse to ice flow.
- Drained through canals in bed substrate - trending into eskers which represent depositional imprint from last outburst.
- SGL preserved on top of glacial lineaments - indicating long term re-organisation of subglacial system and coupling to ice flow.
- sediment of two flat spots - glaciolacustrine
- 51 locations within 11,000km2 where channels emanate from flat spots, of which 11 connect to another flat spot downstream.
60
```
Livingstone et al., (2016):
Conclusions;
1. geometry, sediment and lineations indicate...
2. drainage
3. transition to eskers..
4. when
5. glacial lineations switch
6. channelised mechanism
7. long term channelisation
8. transient effects and...
```
1. Given geometry of flat spots (100ms-few kms), thickness of sediment (5-10m), and subdued relief of glacially lineated topography, lakes must have existed as shallow, sediment floored water lenses.
2. R - channels - but potential for drainage through eroded substrate.
3. transition from canals to eskers downstream - indicates evolution from canals to r-channels, and a switch from sediment erosion to deposition.
4. during deglaciation 16ka BP, or during a stagnation.
5. switch from glacial lineations to SGLs a record of ice stream shutdown - initiated by developement of channelised drainage system.
6. switch to channelised - initiated by fast flow --> lowering slope --> increase potential for ponding --> switch in meltwater regime.
7. long term channelisation - capable of withdrawing water from other areas of bed, increasing basal resistance and causing IS deceleration or shutdown. -- suggested for Whillans.
8. drainage events may cause transient increases on flow, but long term hydrological feedback may do opposite. (slow/shutdown)
61
What do Livingstone et al., (2016) findings tell us about subglacial lakes and their drainage for GRIS/AIS:
- periodic drainage through channels incised into beds.
- canals --> eskers, represents depositional imprint of last high magnitude outburst.
- preservation on top of lineations - indicates long term re-organisation and coupling to flow.
62
Subglacial Lakes on MARS:
- low atmo pressure -- SO water would expand and boil away = thus lakes only on polar ice caps/subsurface ice deposits.
- first liquid found on mars.
- basal water previously thought unlikely as little heat flows from interior of Mars. Also no chance of PMP due to weak gravity. SO Brine type system
- presence of salt lowering melting point possible.
- hunt for life in isolated ecosystem.
63
Give two stats on ice-sheet mass loss in recent years:
What are the mechanisms?
92-11 AIS = ~1400 Gt Ice loss
92-11 GRIS = ~2700 Gt Ice loss.
Combined SLR = 11.2mm
Mechanisms for Mass loss:
1. Surface melting (SMB)
2. Glacier dynamics (ice flow & discharge)
64
In recent years, what dynamic changes has AIS/GRISexperienced:
changes to glacier velocity, calving rate and ice thickness.
- such changes exceed those expected by increased melt alone
65
In recent years, what dynamic changes has AIS/GRIS experienced:
- Changes in velocity (Rosenau et al., (2015)
- Linear velocity changes for over 300 observed outlet glaciers, 1999-2012
from:
Rosenau et al., (2015)
66
In recent years, what dynamic changes has AIS/GRIS experienced:
- changes in ice thickness
- pattern of thinning associated with (i) concentrated along fast flowing outlet glaciers, (ii) marine-terminating glacier thinning faster than land terminating glaciers.
67
In recent years, what dynamic changes has AIS/GRIS experienced:
- dynamic thinning
- when outlet glacier accelerates beyond velocity necessary for ice flux to balance net accumulation.
68
How much mass loss can be attributed to changes in ice dynamics?
- i.e. resulting from changes in flow, not melt.
| - currently ~50% GRIS, 90% AIS
69
In recent years, what dynamic changes has AIS/GRIS experienced:
- changes in retreat rates
- overall frontal position of glaciers between 2000-2010 widespread retreat across GRIS (Murray et al., 2015)
70
Give a simple hypothesis for future changes in GRIS dynamics:
e.g. Parizek and Alley (2004)
Main components of change:
- WARMING ( ELA, Melt season, total melt)
- SUBSTRATE TO BED CONNECTIONS (moulins, crevasses, lakes)
- FEEDBACKS (dynamic thinning, albedo)
Parizek and Alley (2004)
- linked increased melt and velocities (Zwally effect)
- enhances sensitivity of ice sheet flow in response to high melting, retreat and thinning.
- increased melt, lubrication and thus velocity.
uncertainties exist.. (e.g. basal substrate hard/soft, morphology and connectivity of subglacial drainage, inland effects?)
71
How is meltwater access to the bed important important in regard to dynamic changes of an ice sheet?
HYDROFRACTURE
- rapid (2 hours) drainage of supraglacial lake down 980m to the bed of GRIS.
- highlights potential mechanism for rapid transfer of meltwater to bed. - requires subglacial drainage networks (efficient/inefficient)
72
Subglacial drainage networks have a large effect on overhead ice dynamics, how may this be spatially variable?
e.g. Chandler et al., (2013)
relates to spatially varied channel formation:
- when/where turbulent flow to cause net channel opening, efficient channels should begin to form --> increasing efficiency and effective pressure.
- Shallow surface slopes, limited runoff and thick ice (high Pe) hinder channel development by reducing
hydropotential gradients and channel wall melt rates and increasing creep-closure rates
Chandler et al., (2013)
- dye tracing - shows distributed system can extent 41km from margin. - agrees with modelled studies that channels exist 40km from margin, until point where ice thicker but not enough discharge to maintain.
73
What is the effective pressure (Pe)?
How does efficiency of subglacial drainage affect this?
difference between ice overburden pressure and water pressure
i.e. the other control besides basal shear stress that drives basal sliding.
EFFICIENT - i.e. channels., steady state rarely fully evolves. - lower Wp, so higher effective pressure.
INEFFICIENT - reduced Ep, Increased Wp.
74
In terms of efficient/inefficient drainage configurations near the margin, how does the VPA vary depending on steady or non steady state?
- evolution of channel much smaller, rarely evolves to steady state at this point on ice sheet.
- channel cant melt quick enough to contain water so water spreads out along VPA.
- input of water and how rapidly so is important here, as water now on bed lubricates flow.
75
in terms of efficient/inefficient drainage near the ice margin, how does large or rapid pulses affect?
- transient spikes in meltwater pressure, overwhelms system.
- resulting in a gradient reversal - so water flows out from channel along VPA. (see diagram)
- VPA - linked cavities, films, canals
76
What evidence is there of VPA/pulse effects near the ice margin (drainage)?
Bore hole studies of Alpine glaciers, modelling and velocity studies of GRIS demonstrated the presence of a VPA spread of water in subglacial conditions by ice margins
77
What is the variable pressure distribution like in steady state?
- non steady state
- water pressure is lower where channels exist along VPA.
- pressure gradient is reversed, much higher in channels than on outer 'inefficient' or 'inactive' regions.
SEE DIAGRAM
78
What is important concerning inactive regions along the VPA when in a non-steady state?
inactive distributed regions:
- operate at high water pressure (low conductivity so still pressurised)
- large areas of inefficient drainage not connected to surface meltwater supply.
- evidence by borehole drilling --> remain at high water pressure (overburden) and dont fluctuate much.
shows:
large areas of bed are not connected to meltwater supply (spatial variability, remember this is to do with ice sheet changes)
- low conductivity due to style of drainage.
- large areas of bed here (margins mostly) are probably inefficient.
79
Summarise the main points regarding subglacial drainage networks near glacier/ice sheet margins:
- various channels form, generally restricted to margin.
- thicker ice and shallower gradients inhibit channel formation.
- channels become influenced by surroundings, water expands across VPA.
- influencing the basal sliding velocity by providing sufficient basal lubrication.
80
outline the temporal variations in subglacial hydrological structure at: (VPA)
- winter
- summer
- evolution of the channel occurs due to competition between creep closure and melt
WINTER
-Melt < creep closure (collapse back
to distributed network)
SUMMER
- Melt > creep closure (channelized system develops)
However - at peak meltwater, channels have sufficiently enlarged so lower water pressure, meaning water doesnt expand across the VPA. THUS associated with lower sliding velocities.
81
Outline the details from Sole et al., (2013)
| - CONTEXT/METHOD
- transect along land terminating glaciers, >115km into W GRIS, 2009-2011 (contrasting melt years)
- looking to determine if enhanced melt increased glaciers mass loss.
82
Outline the details from Sole et al., (2013)
- FINDINGS
- Conclusions
response was consistent with hydro-dynamic coupling
1. warmer years - enhanced summer flow from increased melt and longer duration of surface-bed connections.
2. resulted in reduced motion during winter - as larger SGL channels drained water - lowering basal water pressure
3. increased summer melt preconditions ice-bed-interface for reduced winter motion resulting in limited dynamic sensitivity to interannual variations in surface melt.
4. ice velocities in marginal areas consequently greater in summer than winter, due to penetration of melt and reduced friction.
THEORETICAL STUDIES - suggest that higher melt rates lead to reduced flow, due to quicker channelisation, reducing dynamics. BUT this study suggests for a period (month during melt), increased meltwater will prolong unsteady conditions in subglacial regions to accommodate larger volumes (VPA basal lubrication.)
Remember: winter motion is reduced however, may balance out melt season pulse of increase velocity. SUGGESTING inter annual stability of speed up in ablation zone is self regulating.
83
```
Outline some details from Sole et al., (2013)
- data/theory
Hints
1. May-Aug Temperature
2. Catchment runoff
3. Surface Melt
4. Lowest sites
5. Greatest change
```
1. 2010/2011 - 2.3/1.1*C warmer than 2009.
2. 2010/2011 - 92%/19% greater than 2009
3. 2010/2011 - 70%/34% greater than 2009
4. lowest sites experienced greatest velocity change - some 45-66% velocity increase.
5. greatest change during 45 day rise in catchment discharge.
84
Now we've covered what may occur on annual timescales in the ablation zone (margins), what is likely the effect of increased melt on decadal scales?
- what about land terminating areas?
Increase in drainage efficiency year-on-year:
- -> gradual net drainage of water stored in unchannelised regions of the ice sheet bed
- -> reduced basal lubrication and net ice slowdown.
- may be resilient to dynamic impacts of enhanced meltwater production than thought.
85
What is meant by the inland limit of melt induced flow variations?
e.g. Doyle et al., (2014)
- futher inland - lower meltwater supply and greater ice thickness (overburden pressure) precludes development of efficient subglacial drainage.
e. g. Doyle et al., (2014)
- 140km inland - looking at a change in velocity above the ELA (acc area). W. GRIS
- found increased penetration of water to the bed at higher elevations in warmer years.
86
Summarise what Tedstone et al., (2015) discovered when investigating decadal effects of sustained increased meltwater:
- CONTEXT
- FIGURE
- DISCUSSION
- CONCLUSION
- looked across W GRIS, increased melt since 2000.
- annual ice motion 1000 A.S.L 12% slower 2007-14 than 1985-94 --> despite 50% increase in meltwater supply.
- sustained high melt - responsible for slowdown = channelised system developed even under thick (1km) ice -- suggested to stay open for longer and larger at atmospheric pressure --> meaning water efficiently evacuated. (doesnt lubricate bed)
- proposes since 2002 - increase in suglacial drainage efficiency due to increased meltwater has reduced basal lubrication -- unclear if slowdown migrates into sheet or if it occurs at higher elevations (thicker)
87
What are the mechanisms for controlling ice flow speed variability?
H1
H1
- evolution of efficient channel network, variations in shear sediment strength.
- -> water at bed - high Wp - pore water low pressure --> water moves down gradient --> forcing grains apart --> weakening grains = deformation and speeding up.
88
What are the mechanisms for controlling ice flow speed variability?
H2
- any contradictions
H2
Meltwater induced weakening and strengthening of sediments.
- Sediment Strengthening - water driven out of sediment without having to form channels. (less lubrication)
- predictions for long term - suggest self regulation theory = continued slowdown.
- also sediment weakening - enhanced flow
89
What are the mechanisms for controlling ice flow speed variability?
H3
H3
inactive drainage systems module water pressure variations.
- potentially large areas of inactive drainage
- could late season slow down be due to changes in water pressure in this system (rather than being driven by channels)
- during summer proximal parts become activated.
- increase in water flow out of other inactive areas (increase in hydraulic gradient)
- drop in pressure during melt season --> can reproduce late-melt season drop in velocity.
90
What conceptual model has been developed for land terminating GRIS?
1. melt onset activates some inactive regions of system - expands cavities - increased sediment deformation and rapid speed up.
2. early-melt season efficiency and extent of active system increases, likely forming channels at low elevations. Meltwater supply frequent > capacity of active drainage at all elevations causes speed up.
3. Late-melt season - meltwater supply declines and water drains across VPA towards efficient drainage forms at low elevations (lubrication)
4. basal water pressure and cavity size gradually increase during post-melt season.
91
For marine terminating glaicers (30% GRIS mass loss) - what are the likely triggers of retreat?
- surface melt (atmo)
- submarine melting (oceanic)
- change in sea ice/temp (oceanic)
92
Summarise main points from Zwally et al., (2002): Surface Melt induced acceleration of GRIS
- CONTEXT
- FIGURES
- DISCUSSION
- CONCLUSION
- grounded above SL, believed to gradually respond to warming by melt @ surface. investigated relationship between summer meltwater for basal lubrication and velocity. 1996-99
- all year (96-99) - correlation for change in velocity, intensity and timing of surface melt.
- found ratio of increase in velocity to positive degree-days to be increased for warmer years.
- indication of subglacial water flow - meltwater leaves ice in subglacial streams and not surface flow.
- moulins and surface lakes in ablation zone density = 0.2 moulins per km2.
- enhanced basal sliding from meltwater may have contributed to the
1. rapid demise of Laurentide during increased summer insolation and surface ablation 10kya.
2. extensive GRIS melting during last interglacia, by causing a faster flow of ice to margins, increasing thinning rate, and more rapid inward migration of ablation zone.
93
Summarise what Rignot and Kanagaratnam (2006) found regarding GRIS acceleration:
- Context
- Discussion
- Conclusions
- widespread glacial acceleration found below 66* N 96-2000 - rapidly expanded to 70* by 2005. Increased discharge in W and E led to deficit increase from 90-220km^2 per year. --> increasingly north glacial acceleration= SLR
- peripheral thinning - 1/2 by enhanced runoff 1/2 by enhanced flow
e. g. Helheim Glacier thinned 75m 2000-05 due to 60% increased acceleration.
- 2/3rds loss due to ice dynamics, the rest due to enhanced runoff minus accumulation. Ice dynamics dominates SLR contribution from GRIS. (IMPORTANT)
- Glacier acceleration in East attributed to enhanced warming
94
Summarise what Hoffman et al., (2016) found regarding GRIS subglacial drianage evolution regulated by weakly connected regions of the bed:
- CONTEXT
- DISCUSSION
- CONCLUSION
- Numerical models of subglacial drainage and ice flow to show that limited, gradual leakage of water and lowering of water pressure in weakly connected regions of the bed can explain the dominant features in late and post melt season ice dynamics.
- results suggest a 3rd weakly connected drainage component should be included in conceptual model of subglacial hydrology.
- -> at low subglacial discharge - drainage through inefficient, dist pathways (e.g. linked cavities in bedrock) for which increasing water flux leads to increased water pressure and sliding.
- channel formation explained by critical discharge reached which dissipates heat with flow and causes a positive feedback between ice roof melting and cavity growth. --> ie. leading to formation of channels incised into above ice.
- channels evacuate large volume of water from dist system --> lowering water pressure, terminating a sliding event despite raised meltwater.
- suggest gradual evacuation of water from weakly connected, but spatially extensive areas of the bed. These areas exert the dominant control on the large-scale subglacial water pressure and basal resistance.
- current models overemphasise importance of channels.
- conceptualise weakly connected regions as discrete patches of linked cavities, similar to the distributed drainage component, but with a much lower hydraulic connectivity.
