Readings Flashcards
Benn and Lehmkuhl 2000
Mass balance characteristics of glaciers in high-mountain environments complicate the relationship between ELAs and precipitation/air T
- avalanches/debris cover/topographic effects
ELA = position where over one year accumulation exactly balanced by ablation
Former ELAs = powerful method of quantifying palaeoclimates if e.g. other evidence is lacking
Maisch 1995
Knowledge of glacier ELAs = palaeoclimatic data = predict future glacier response to climatic change
Braithwaite 2002
Study of glacier mass balance 1946-1995 for 246 glaciers
Western Europe, North America and USSR
Biased towards wetter conditions
Direct stakes/snowpits method - recognised integration with geodetic/remote sensing would be more useful
Glaciers mainly gain mass (accumulation) as snow and mostly lose mass (ablation) by melting and iceberg calving
Local importance of glaciers to societies e.g. HEP or irrigation, or hazards
Hock 2005
Modelling ice/snow melt important for issues e.g. water resource management, avalanche forecasting, glacier dynamics, hydrology/hydrochemistry, climate change
Modelling of turbulent fluxes and spatial/temporal variability in albedo = major uncertainties
Typical characteristics of glacier runoff = melt-induced diurnal cyclicality and concentration of annual flow during melt season
High temporal resolution essential for predicting peak flows in glacierised/snow-covered basins
High spatial resolution needed to account for large spatial heterogeneity w.r.t. ice/snow melt due to topography
Energy balance melt models more properly describe physical processes at glacier surface than temperature-index methods but require much more data
Further research to focus on links between different energy fluxes and synoptic weather pattern, and investigate potential for operational use in melt forecasting
Jacob et al 2012
Glaciers/ice caps = important contributors to global mean SLR
Monthly GRACE method from Jan 2003-Dec 2010 for inversion of mass change over all ice-covered regions larger than 100km2
Results:
- GIC excluding Greenland and Antarctica peripheral glaciers and ice caps contributed 0.41+/-0.08mm/yr
- with G/A = 1.06+/-0.19mm/yr
Total agreed with independent estimates within error bars
Nakawo and Young 1982
Ablation under a debris layer could be estimated from meteorological variables if surface T data (estimation of thermal resistance) of layer is available
Generally good method but surface roughness (large at stagnant areas near glacier terminus) should be noted with care
Fountain and Walder 1998
Understanding water movement through glacier is fundamental to e.g. glacier dynamics/glacier-induced floods/runoff predictions
Firn temporarily stores water and smooths out variations in supply rate (accumulation zone)
In ablation zone, flux of water depends directly on rate of surface melt/rainfall = varies greatly
Describes water flow in a “nonarborescent network”, poorly connected to a well-connected aborescent channel network
Stored water may be released abruptly and catastrophically in the form of outburst floods
Episodic surging of some glaciers due to temporal changes in subglacial hydrology
Near-surface, englacial and subglacial water flow are coupled
Hydrological system components = snow, firn, surface streams; crevasses, moulins and other englacial passages; and basal channels, cavities and till
Gulley and Benn 2009
Mapped 8.25km of passage in 27 distinct englacial conduits in temperate, polythermal, cold-based and debris-covered glaciers between 2005 and 2009 using speleological techniques
In all cases = single unbranching conduit
Morphologies intimately linked to orientation of glacier’s principal stresses or presence of pre-existing lines of high hydraulic conductivity
Shreve-type englacial drainage systems do not exist - englacial conduits can only penetrate through thick ice to recharge the bed where supra glacial bodies intersect/advect through zones of acceleration
Nienow et al 1998
Used dye tracing techniques for glacier-wide changes in englacial/subglacial system of Haut Glacier d’Arolla 1990-1991
Removal of snow (high albedo and water storage) increases runoff into moulins
System of major, hydraulically efficient channels expands up glacier (retreating snowline), replacing inefficient distributed drainage system over course of season
Three factors control evolution of subglacial drainage over summer melt season (1) weather conditions (2) pattern of snowline retreat (3) distribution of moulins and crevasses
Uncrevassed glaciers with a surface layer permanently below melting point, surface melt may never reach glacier bed
Sorg et al 2012
Focus in the Tien Shan, Central Asia
Regions with little summer precipitation = glaciers important role in streamflow regimes
At first shrinking glaciers supply increased glacial runoff but ultimately will result in decrease
= ecological problems and political instability
Nienow et al 2015
Greenland Ice Sheet (GIS)
Ice motion across land-terminating region in west gIS slower in 2007-2014 compared with 1985-1994 despite 50% increase in meltwater production
i.e. these sectors of the ice sheet are more resilient to the dynamic impacts on enhanced meltwater production than previously though
Zwally et al 2002
Greenland Ice Sheet
Glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface
Floating glacier tongues and Antarctic ice shelves respond quickly. in contrast to the flow of grounded ice
Alley et al 1986
WAIS
- radar sounding = wet bed
- most ice-stream velocity arises at bed
- deformation within the till is the primary mechanism by which the ice stream moves
Bamber et al 2000
90% of AIS is drained through ice-streams
Engelhardt and Kamb (1998)
Stake method used to measure basal sliding in Ice Stream B, WAIS
Basal sliding rate is 83% of total motion
- may include contribution from shear deformation of till
Mechanism of rapid ice stream motion concentrated at/near top of till rather than spread throughout thickness of till layer
Harrison and Post (2003)
Till deformation processes dominate glacier motion in quiescence
Climate and weather affect surge initiation, termination and magnitude
Non-random geographic distribution of surge-type glaciers but poorly understood - would be useful for constraints on mechanism of surging
Distributed systems exist seasonally under parts of Variegated Glacier during quiescent phase and discrete system below surge front during surge
Hypothesis for winter initiation of surges = presence of englacial water trapped late in melt season
Kamb et al 1985
Surging-type glaciers speed up for relatively short time to flow rates as much as a hundred times the normal rate then drops back to normal flow state
1982-1983 Variegated Glacier surge caused by build up of high water pressure in basal passageway system
Sevestre and Benn 2015
Highest densities of surge-type glaciers occur within an optimal climatic envelope with T and precipitation thresholds
= intermediate conditions
Two superclusters:
1) Arctic Ring
2) High Mountain Asia
Stokes et al 2007
Sticky spots = localised patches of basal friction
4 primary causes of stickiness
1) bedrock bumps
2) till-free areas
3) areas of ‘strong’ (well-drained) till
4) freeze-on of subglacial meltwater
Act together or in isolation
Alley et al 2005
Future SL rise related to continuing build up of atmospheric GHG concentrations
Freshwater fluxes from GIS and AIS may affect oceanic circulation, contributing to climate change
Heavy concentration of population live along coastlines = substantial societal and economic impacts
- coastal erosion
- increased susceptibility to storm surges
- groundwater contamination by salt intrusion
EAIS actually likely to grow (increased accumulation if warmings don’t exceed 5’C
Warming has increased mass loss from coastal areas more than it has increased mass gain from snowfall in cold central regions
Benn et al 2007a
Calving rates increase dramatically in response to increases in velocity +/ retreat of the glacier margin
Calving and related dynamic processes poorly represented in current ice sheet models
1st order control = strain rate determining location and depth of surface crevasses
Large icebergs have a cooling effect on oceans through latent and sensible heat exchanges
Joughin and Alley 2011
Much of the grounded ice in W Antartica lies on a bed that deepens inland and extents well below SL = thinning could ultimately float much of the ice sheet’s interior
Mass loss for WAIS ranges from 100 to 200Gt/yr, dominated from draining of Amundsen Sea sector
Collapse of the marine ice sheet in West Antarctica would raise SL by more than 3m over the course of several centuries or less
Paolo and Padman 2015
West Antarctic losses increased by 70% in the last decade and earlier volume gain by EAIS ceased
In the Amundsen and Bellingshausen regions some ice shelves have lost up to 18% of their thickness in less than two decades
Shepherd et al 2001
The (grounded) Pine Island Glacier transports 69 km3 of ice each year from ~10% of WAIS
Retreat may accelerate ice discharge from WAIS interior
Nick et al 2009
Numerical ice-flow model for Helheim Glacier, one of Greenland’s largest outlet glaciers
Ice acceleration, thinning and retreat begins at calving terminus then propagates upstream through dynamic coupling along glacier - likely caused by basal lubrication through surface melt propagating to glacier bed
Tidewater outlet glaciers adjust extremely rapidly to changing boundary conditions at the calving terminus
Pattyn 2018
Shallow Ice Approximation = ice deformation is due to shearing close to bed
MISI is based on observation that since ice flux increases with ice thickness, the location of a grounding line on a bed sloping inwards is unstable
Increase in computational efficiency enabling high spatial resolution modelling, high resolution datasets of bedrock topography and surface velocity, longer time series on ice sheet changes and the improved initialisation of ice sheet models = move towards robust predictions on decadal time scales, hindcastidg and potentially reanalysis
Boulton 1974
Focus on three important subglacial erosional processes:
1) abrasion of bed by debris embedded in glacier sole
2) crushing of subglacial bedrock due to pressure fluctuation produced by glacier flow over bedrock hummock
3) removal by plucking by above
Glasser and Bennett 2004
Glacial erosion involves removal and transport of bedrock and/or sediment by glacial quarrying, glacial abrasion and glacial meltwater
All landforms of glacial erosion provide evidence for the release of subglacial meltwater and the existence of warm-based ice
Landforms dominated by glacial abrasion are created when there is no ice-bed separation
Exposure dating techniques including cosmogenic isotope dating of bedrock surface = important for increasing understand of age/chronological significance of landforms
Gordon 1981
Topographic form of areal scouring relates closely to bedrock structure (strike/dip of bedding planes, layering, joins, structural weaknesses)
Asymmetric roche moutonnée-type forms generally exist in sympathy of bedrock structure regardless of ice movement direction
Schweizer and Iken 1992
Basal ice is debris-laden so friction exists between the substratum and rock particles embedded in the basal ice
Briner et al 2003
Relative weathering of landscape surfaces has often been used to define the former location of landscape surfaces - highly weathered landscapes took a long time to form and thus must have escaped RECENT glaciation
BUT presence of pseudo-erratics on high-elevation weathered surfaces (Arctic Canada) = non erosive ice covered these
Jamieson et al 2008
Ice sheet model (GLIMMER) with erosion component, modelling evolution of landscapes under ice sheets over long time scales in hypothetical conditions
Results: bed morphology exerts a greater influence under low basal slip conditions because ice cannot respond readily to thermal instabilities
Bradwell et al 2008
Role of ice streams in landscape evolution
Ullapool area NW Scotland shows large-scale megagrooves and streamlined bedrock forms in a well-defined ~20km wide zone interpreted as signature of fast-flowing tributary that once fed a palaeo-ice stream
Elongation ratios <5:1 and >5:1 represent transition from potentially cold-based slow flow to warm-based fast flow
Rapid spatial bedroom evolution reflects an increase in subglacial erosive power that may be diagnostic of paleo-ice sheet thermal boundaries (i.e. cold to warm based)
Evans et al 2006
Reference for definitions of glacitectonite, subglacial traction till, melt-out till
Deformation, flow, sliding, lodgement and ploughing co-exist at the base of temperate glacier ice and act to mobilise, transport and deposit sediment
Till/till sequences may contain a superimposed signature of former transportation/deposition at the ice-bed interface
Glacitectonite = rock/sediment deformed by subglacial shearing/deformation but retains some of structural characteristics of parent material
Subglacial traction till = sediment deposited by a glacier sole either sliding over +/ deforming its bed, with sediment directly released from ice by pressure melting +/liberated from substrate then disaggregated and completely/largely homogenised by shearing
Melt-out till = sediment released by melting of stagnant/slowly moving debris-rich glacier ice and directly deposited without subsequent transport or deformation
Ò Cofaigh and Dowdeswell 2001
Laminated glacimarine sediments form by e.g. suspension settling from turbid overflow plumes, turbidity current deposition and contour current activity
Subglacial deformation of pre-existing sediments may also produce laminated deposits in the forms of glacitectonite/deformation till
= discrimination important for reconstruction
Boulton 1996a
Flux of glacially transported sediment is related to ice flux = inner zone of ice sheet predominantly one of erosion and outer zone of deposition
Extensive subglacial aquifers drain the glacier bed and inhibit deformation processes = reduce rates of erosion/deposition
Fine-grained sediments on glacier bed are eroded/transported more readily = contribute preferentially to till
Clark (2010)
Subglacial bedforms are emergent phenomena arising from self-organisation in the coupled flow of ice, sediment and water; patterning of bedforms due to naturally arising flow instability
Subglacial bedforms have recently been discovered beneath the Antarctic Ice Sheet
Rogen moraines should be called Subglacial Ribs because they are not actually moraines
Drumlins form by coupled interaction between ice flow and flow of soft underlying substrate (instability theory)
Scepticism of instability theory: combat by thinking about smoothing surface of dry sand on a beach - ripples will appear = analogous
Note: instability theory cannot make prediction of internal structures and sediment properties, it is a simple conceptual model with potential
Stokes et al 2013
Pro instability theory; first paper to assess if compatibility with sediments found within drumlins
Results: one would expect a diverse range of constituents depending on:
1) inheritance of sediments that pre-date drumlin formation
2) duration/variability of ice flow
3) balance between erosion and deposition at the ice-bed interface
So IS compatible
Benn 1989
Focus in N Scotland during LLS
Moraine asymmetry correlated with distribution of free faces in valleys
Within valley asymmetry of moraines is interpreted as one aspect of long-term asymmetric landscape evolution
Evans 2009
Controlled moraines = supra glacial debris concentrations that become hummocky moraine upon deicing and possess clear linearity due to inheritance of former pattern of debris-rich folia in parent ice
Gorrell and Shaw 1991
Sediment in an esker in Lanark, Ontario, Canada
Mainly coarse grained, varies from boulders with fine-sand matrix (max discharge) to diamicton (low discharge)
Small scale fining upwards sequence in fans and beads indicates deposition by large no. of flow events
Fan and bead environments are areas of ice-bed decoupling, at least during high flow conditions
Carlson et al 2005
Kettle Moraine in Wisconsin = single, ~3km wide hummocky ridge for ~90% of its 200km extent with double ridge at beginning - due to extensive underlying ice with thin debris cover between supraglacial fluvial deposits at the time of deposition
Sedimentary architecture of glacial landforms helps to explain genesis and provide better understanding of the pale-glaciology of ice sheets
Livingstone et al 2010
Brampton kame belt in Cumbria = one of largest glaciofluvial complexes within the UK (over 44km2)
Demonstrates development of complex glacier karst and process of topographic inversion:
- kettle holes
- eskers
- flat-topped hills (ice-walled lake plains)
Represents major depositional episode during advanced stages of recession of Late Devensian BIIS in Solway Lowlands = critical for constructing deglaciation style
Evans and Twigg 2002
Active temperate landsystem in Bredamerkurjökull and Fjallsjökull, Iceland
1) areas of extensive, low amplitude marginal dump, push and squeeze moraines often recording annual recession of active ice
2) incised and terraced glacifluvial forms e.g. recessional ice-contact fans and hochsandur fans/eskers/pitted outwash
3) subglacial landforms assemblages; flutes/drumlins/overridden push moraines between ice marginal glacifluvial depo-centres
Lack of supraglacial sediment in active temperate glaciers precludes the widespread development of chaotic hummocky moraine
Subtle surge signatures present = illustrates danger of employing landform-sediment associations from restricted study areas for entire glaciated terrain
Evans et al 2016
Snaefelljokull volcano-centred ice cap landsystem, W Iceland
Rapidly thinning and receding from its historical Little Ice Age maximum limit
Profound effects on generation of freshwater for surrounding communities
Provides landsystem signature for independent volcano-centred ice caps
- outer zone of ice-cored moraine
- in front = locally developed set of proglacially thrust pumice deposits
- ice-cored moraine passes proximally into bouldery drift and push moraine
- then large area of flutings/glacially abraded rock (indicative of temperate basal ice conditions)
This landsystem is an exemplar relevant specifically to Icelandic landscape but also more widely relevant to glaciered volcanic terrains globally
Carr et al 2013
Marine-terminating outlet glaciers can undergo dramatic dynamic change at annual timescales
3 primary climatic/oceanic controls on outlet glacier dynamics;
1) air T
2) ocean T
3) sea-ice concentrations
Uncertainties on controls on outlet glacier dynamics:
- spatial variation in relative importance of each factor
- contribution of glacier-specific factors to glacier dynamics
- limitations in accurate modelling
There is a danger in extrapolating rates of mass loss from a small sample of study glaciers
Arctic warming expected 4-7’C by 2100
Greenland Ie Sheet contributed 0.46mm/a to SL rise between 2000 and 2008
Marine-terminating outlet glacier = channel of fast-moving ice that drains an ice cap or ice sheet and terminates in the ocean at either a floating or grounded margin (Benn and Evans 2010)
Reverse bed slopes pose instability
Meltwater-enhanced basal sliding contributes to marine-terminating outlet glacier velocities at seasonal scales but capacity of subglacial hydrological system to evolve limits effect on inter annual behaviour
