Basal control of supraglacial meltwater catchments  on the Greenland Ice Sheet

Abstract. Ice surface topography controls the routing of surface meltwater generated in the ablation zones of glaciers and ice sheets. Meltwater routing is a direct source of ice mass loss as well as a primary influence on subglacial hydrology and basal sliding of the ice sheet. Although the processes that determine ice sheet topography at the largest scales are known, controls on the topographic features that influence meltwater routing at supraglacial internally drained catchment (IDC) scales (<10s of km) are less well constrained. Here we examine the effects of two processes on ice sheet surface topography: transfer of bed topography to the surface of flowing ice and thermal–fluvial erosion by supraglacial meltwater streams. We implement 2-D basal transfer functions in seven study regions of the western Greenland Ice Sheet ablation zone using recent data sets for bed elevation, ice surface elevation, and ice surface velocities. We find that ∼1–10 km scale ice surface features can be explained well by bed topography transfer in regions with different multiyear-averaged ice flow conditions. We use flow-routing algorithms to extract supraglacial stream networks from 2 to 5 m resolution digital elevation models and compare these with synthetic flow networks calculated on ice surfaces predicted by bed topography transfer. Multiple geomorphological metrics calculated for these networks suggest that bed topography can explain general ∼1–10 km supraglacial meltwater routing and that thermal–fluvial erosion thus has a lesser role in shaping ice surface topography on these scales. We then use bed topography transfer functions and flow routing to conduct a parameter study predicting how supraglacial IDC configurations and subglacial hydraulic potential would change under varying multiyear-averaged ice flow and basal sliding regimes. Predicted changes to subglacial hydraulic flow pathways directly caused by changing ice surface topography are subtle, but temporal changes in basal sliding or ice thickness have potentially significant influences on IDC spatial distribution. We suggest that changes to IDC size and number density could affect subglacial hydrology primarily by dispersing the englacial–subglacial input of surface meltwater.

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Basal control of supraglacial meltwater catchments on the Greenland Ice Sheet

10.5194/tc-2018-74 ◽

2018 ◽

Author(s):

Josh Crozier ◽

Leif Karlstrom ◽

Kang Yang

Keyword(s):

Surface Topography ◽

Transfer Functions ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Spatial Density ◽

Bed Topography ◽

Subglacial Environment ◽

Ice Surface ◽

Basal Sliding ◽

Elevation Data

Abstract. Ice surface topography controls the primary routing of surface meltwater on ablation zones of glaciers and ice sheets. Meltwater routing is important for understanding and predicting ice sheet evolution because surface melt can be both a direct source of ice mass loss and an influence on basal sliding and ice advection. Although controls on ice sheet topography at long wavelengths are well known, smaller scale features relevant for meltwater routing are not well understood. Here we examine the effects of two processes that can influence ice sheet surface topography: bed topography transfer and thermal-fluvial incision by supraglacial streams. We implement 2D bed topography and basal sliding transfer functions in seven study regions of the western Greenland Ice Sheet (GIS) ablation zone to study the influence of basal conditions on ice surface topography. Although bed elevation data quality is spatially variable, we find that ∼ 1–10 km scale ice surface features under variable ice thickness, velocity, and surface slope are well predicted by these transfer functions. We then use flow-routing algorithms to extract supraglacial stream networks from 2–5 m resolution digital elevation models, and compare these with synthetic flow networks calculated on ice surfaces predicted by bed topography transfer. Quantitative comparison of these networks reveals that bed topography can explain ∼ 1–10 km surface meltwater routing patterns without significant contributions from thermal-fluvial erosion by streams. We predict how supraglacial internally drained catchment (IDC) patterns on the GIS would change under time-varying ice flow and/or basal sliding regimes. Basal sliding variations exert a significant influence on IDC spatial distribution, and suggest a potential positive feedback between subglacial hydrologic regime to surface IDC patterning. Increased basal sliding will increase IDC spatial density (by decreasing IDC sizes) and cause more disperse meltwater input to the englacial and subglacial environment. This could result in less efficient subglacial channelization and increased basal sliding that would then further increase IDC density.

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High-resolution ice thickness and bed topography of a land-terminating section of the Greenland Ice Sheet

Earth System Science Data Discussions ◽

10.5194/essdd-7-129-2014 ◽

2014 ◽

Vol 7 (1) ◽

pp. 129-148 ◽

Cited By ~ 4

Author(s):

K. Lindbäck ◽

R. Pettersson ◽

S. H. Doyle ◽

C. Helanow ◽

P. Jansson ◽

...

Keyword(s):

Mass Balance ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Airborne Radar ◽

Ice Thickness ◽

Equilibrium Line Altitude ◽

Bed Topography ◽

Ice Surface ◽

Ice Sheet Dynamics

Abstract. We present ice thickness and bed topography maps with high spatial resolution (250 to 500 m) of a and-terminating section of the Greenland Ice Sheet derived from combined ground-based and airborne radar surveys. The data have a total area of ~12000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of the Isunnguata Sermia Glacier is over-deepened and reaches an elevation of several hundreds of meters below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The covered area is one of the most studied regions of the Greenland Ice Sheet with studies of mass balance, dynamics, and supraglacial lakes, and our combined dataset can be valuable for detailed studies of ice sheet dynamics and hydrology. The compiled datasets of ground-based and airborne radar surveys are accessible for reviewers (password protected) at doi.pangaea.de/10.1594/pangaea.830314 and will be freely available in the final revised paper.

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Bed topography and subglacial landforms in the onset region of the Northeast Greenland Ice Stream

Annals of Glaciology ◽

10.1017/aog.2020.12 ◽

2020 ◽

Vol 61 (81) ◽

pp. 143-153 ◽

Cited By ~ 3

Author(s):

Steven Franke ◽

Daniela Jansen ◽

Tobias Binder ◽

Nils Dörr ◽

Veit Helm ◽

...

Keyword(s):

Boundary Conditions ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ultra Wideband ◽

Ice Flow ◽

Detailed Model ◽

Dynamic Component ◽

Bed Topography ◽

Ice Stream ◽

Topography Model

AbstractThe Northeast Greenland Ice Stream (NEGIS) is an important dynamic component for the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The geometric boundary conditions and in particular the nature of the subglacial bed of the NEGIS are essential to understand its ice flow dynamics. We present a record of more than 8000 km of radar survey lines of multi-channel, ultra-wideband radio echo sounding data covering an area of 24 000 km2, centered on the drill site for the East Greenland Ice-core Project (EGRIP), in the upper part of the NEGIS catchment. Our data yield a new detailed model of ice-thickness distribution and basal topography in the region. The enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity, potentially over several glacial–interglacial cycles. Off-nadir reflections from the ice–bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms. Our new bed topography model will help to improve the basal boundary conditions of NEGIS prescribed for ice flow models and thus foster an improved understanding of the ice-dynamic setting.

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Derived bedrock elevations, strain rates and stresses from measured surface elevations and velocities: Jakobshavns Isbræ, Greenland

Journal of Glaciology ◽

10.1017/s0022143000017858 ◽

1995 ◽

Vol 41 (137) ◽

pp. 161-173 ◽

Cited By ~ 34

Author(s):

James L. Fastook ◽

Henry H. Brecher ◽

Terence J. Hughes

Keyword(s):

Flow Direction ◽

Greenland Ice Sheet ◽

Surface Strain ◽

Curvilinear Coordinates ◽

Surface Velocity ◽

Strain Rates ◽

Bed Topography ◽

Ice Surface ◽

Basal Sliding ◽

Grid Points

AbstractJakobshavns Isbræ (69 °10′ N, 49 °59′ W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The Jakobshavns Isbræ basin of about 10 000 km2was mapped photogrammetrically from four sets of aerial photography, two taken in July 1985 and two in July 1986. Positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation, and surface velocity vectors were computed from the positions. The two flights in 1985 yielded the best results and provided most common points (716) for velocity determinations and are therefore used in the modeling studies. The data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation. The field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates,X, Ypointing eastward and northward, and curvilinear coordinates.L, Τpointing longitudinally and transversely to the changing ice-flow direction, Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding. Our calculated ice thicknesses are in fair agreement with an ice-thickness map based on seismic sounding and supplied to us by K. Echelmeyer. Ice thicknesses were subtracted from measured ice-surface elevations to map bed topography. Our calculation shows that basal sliding is significant only in the 10–15 km before Jakobshavns Isbræ becomes afloat in Jakobshavns Isfjord.

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Assessing the summer water budget of a moulin basin in the Sermeq Avannarleq ablation region, Greenland ice sheet

Journal of Glaciology ◽

10.3189/002214311798043735 ◽

2011 ◽

Vol 57 (205) ◽

pp. 954-964 ◽

Cited By ~ 47

Author(s):

Daniel McGrath ◽

William Colgan ◽

Konrad Steffen ◽

Phillip Lauffenburger ◽

James Balog

Keyword(s):

Water Budget ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Time Lapse ◽

Balance Model ◽

Diurnal Variability ◽

Surface Uplift ◽

Ice Surface ◽

Basal Sliding

AbstractWe provide an assessment of the supraglacial water budget of a moulin basin on the western margin of the Greenland ice sheet for 15 days in August 2009. Meltwater production, the dominant input term to the 1.14 ± 0.06 km2 basin, was determined from in situ ablation measurements. The dominant water-output terms from the basin, accounting for 52% and 48% of output, respectively, were moulin discharge and drainage into crevasses. Moulin discharge exhibits large diurnal variability (0.017–0.54 m3 s−1) with a distinct late-afternoon peak at 16:45 local time. This lags peak meltwater production by ∼2.8 ± 4.2 hours. An Extreme Ice Survey time-lapse photography sequence complements the observations of moulin discharge. We infer, from in situ observations of moulin geometry, previously published borehole water heights and estimates of the temporal lag between meltwater production and observed local ice surface uplift (‘jacking’), that the transfer of surface meltwater to the englacial water table via moulins is nearly instantaneous (<30 min). We employ a simple crevasse mass-balance model to demonstrate that crevasse drainage could significantly dampen the surface meltwater fluctuations reaching the englacial system in comparison to moulin discharge. Thus, unlike crevasses, moulins propagate meltwater pulses to the englacial system that are capable of overwhelming subglacial transmission capacity, resulting in enhanced basal sliding.

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Derived bedrock elevations, strain rates and stresses from measured surface elevations and velocities: Jakobshavns Isbræ, Greenland

Journal of Glaciology ◽

10.3189/s0022143000017858 ◽

1995 ◽

Vol 41 (137) ◽

pp. 161-173 ◽

Cited By ~ 2

Author(s):

James L. Fastook ◽

Henry H. Brecher ◽

Terence J. Hughes

Keyword(s):

Flow Direction ◽

Greenland Ice Sheet ◽

Surface Strain ◽

Curvilinear Coordinates ◽

Surface Velocity ◽

Strain Rates ◽

Bed Topography ◽

Ice Surface ◽

Basal Sliding ◽

Grid Points

AbstractJakobshavns Isbræ (69 °10′ N, 49 °59′ W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The Jakobshavns Isbræ basin of about 10 000 km2 was mapped photogrammetrically from four sets of aerial photography, two taken in July 1985 and two in July 1986. Positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation, and surface velocity vectors were computed from the positions. The two flights in 1985 yielded the best results and provided most common points (716) for velocity determinations and are therefore used in the modeling studies. The data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation. The field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates, X, Y pointing eastward and northward, and curvilinear coordinates. L, Τ pointing longitudinally and transversely to the changing ice-flow direction, Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding. Our calculated ice thicknesses are in fair agreement with an ice-thickness map based on seismic sounding and supplied to us by K. Echelmeyer. Ice thicknesses were subtracted from measured ice-surface elevations to map bed topography. Our calculation shows that basal sliding is significant only in the 10–15 km before Jakobshavns Isbræ becomes afloat in Jakobshavns Isfjord.

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The meltwater feedbacks on ice dynamics, elevation versus lubrication

10.5194/egusphere-egu2020-13260 ◽

2020 ◽

Author(s):

Basile de Fleurian ◽

Petra Langebroek ◽

Paul Halas

Keyword(s):

Water Pressure ◽

Ice Sheet ◽

Drainage System ◽

Greenland Ice Sheet ◽

Ice Dynamics ◽

Ice Surface ◽

Basal Sliding ◽

Surface Melt ◽

The One ◽

Different Response

<p>In recent years, temperatures over the Greenland ice sheet have been rising leading to an increase in surface melt.&#160; Projections show that this augmentation of surface melt will continue in the future and spread to higher elevations. As it increases, melt leads to two different feedbacks on the dynamic of the Greenland ice sheet. This augmentation of melt lowers the ice surface and changes its overall geometry hence impacting the ice dynamics through ice deformation. The other feedback comes into play at the base of glaciers. Here, the increase of water availability will impact the distribution of water pressure at the base of glaciers and hence their sliding velocity. The first feedback is relatively well known and relies on our knowledge of the rheology and deformation of ice. The lubrication feedback acting at the bed of glaciers is however highly uncertain on time scales longer than a season. Here we apply the &#160;Ice &#160;Sheet &#160;System &#160;Model &#160;(ISSM) &#160;to &#160;a &#160;synthetic &#160;glacier &#160;which &#160;geometry &#160;is &#160;similar to the one of a Greenland ice sheet land terminating glacier. The dynamic contributions from ice deformation and sliding are separated to study their relative evolution. This is permitted by the use of a dynamical subglacial hydrology model that allows to link the basal sliding to the meltwater production through an appropriate friction law. The &#160;model &#160;is &#160;forced &#160;through &#160;a &#160;simple &#160;temperature &#160;distribution &#160;and &#160;a &#160;Positive &#160;Degree &#160;Day &#160;model which allows to apply a large range of different forcing scenarios. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system and their different response to the distribution of melt during the year which directly impact the sliding regime at the base of the glacier.</p>

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Influence of supraglacial lakes and ice-sheet geometry on seasonal ice-flow variability

The Cryosphere Discussions ◽

10.5194/tcd-7-1101-2013 ◽

2013 ◽

Vol 7 (2) ◽

pp. 1101-1118 ◽

Cited By ~ 4

Author(s):

I. Joughin ◽

S. B. Das ◽

G. E. Flowers ◽

M. D. Behn ◽

R. B. Alley ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Velocity ◽

Ice Flow ◽

Bed Topography ◽

Supraglacial Lakes ◽

Melt Water ◽

Flow Enhancement ◽

Lake Drainage ◽

Existing Data

Abstract. Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal melt water to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).

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Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

The Cryosphere ◽

10.5194/tc-13-3093-2019 ◽

2019 ◽

Vol 13 (11) ◽

pp. 3093-3115 ◽

Cited By ~ 4

Author(s):

Michael A. Cooper ◽

Thomas M. Jordan ◽

Dustin M. Schroeder ◽

Martin J. Siegert ◽

Christopher N. Williams ◽

...

Keyword(s):

Flow Direction ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Flow Speed ◽

Surface Velocity ◽

Ice Flow ◽

Systematic Analysis ◽

Subglacial Environment ◽

Topographic Roughness ◽

Ice Surface

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained both in its bulk properties, for example geology, the presence of sediment, and the presence of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of 2 decades of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability in along-track topography – enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow and, second, from bed-echo scattering – enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth beds, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard- or soft-bed boundary constraint for ice-sheet models.

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Dependence of century-scale projections of the Greenland ice sheet on its thermal regime

Journal of Glaciology ◽

10.3189/2013jog13j054 ◽

2013 ◽

Vol 59 (218) ◽

pp. 1024-1034 ◽

Cited By ~ 80

Author(s):

H. Seroussi ◽

M. Morlighem ◽

E. Rignot ◽

A. Khazendar ◽

E. Larour ◽

...

Keyword(s):

Steady State ◽

Thermal Regime ◽

Adaptive Mesh Refinement ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Adaptive Mesh ◽

Atmospheric Conditions ◽

Geothermal Heat ◽

Ice Flow ◽

Basal Sliding

AbstractObservations show that the Greenland ice sheet has been losing mass at an increasing rate over the past few decades, which makes it a major contributor to sea-level rise. Here we use a three-dimensional higher-order ice-flow model, adaptive mesh refinement and inverse methods to accurately reproduce the present-day ice flow of the Greenland ice sheet. We investigate the effect of the ice thermal regime on (1) basal sliding inversion and (2) projections over the next 100 years. We show that steady-state temperatures based on present-day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century-scale projections are weakly sensitive to small changes in the initial temperature field, compared with changes in atmospheric conditions or basal sliding. We conclude that although more englacial temperature measurements should be acquired to validate the models, and a better estimation of geothermal heat flux is needed, it is reasonable to use steady-state temperature profiles for short-term projections, as external forcings remain the main drivers of the changes occurring in Greenland.

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