scholarly journals Challenges in predicting Greenland supraglacial lake drainages at the regional scale

2021 ◽  
Vol 15 (3) ◽  
pp. 1455-1483
Author(s):  
Kristin Poinar ◽  
Lauren C. Andrews
Keyword(s):  
Signal To Noise Ratio ◽  
Regional Scale ◽  
Ice Sheet ◽  
Strain Rates ◽  
Ice Flow ◽  
Transient Strain ◽  
Background Strain ◽  
Lake Drainage ◽  
The Relationship ◽  
Insignificant Correlation

Abstract. A leading hypothesis for the mechanism of fast supraglacial lake drainages is that transient extensional stresses briefly allow crevassing in otherwise compressional ice flow regimes. Lake water can then hydrofracture a crevasse to the base of the ice sheet, and river inputs can maintain this connection as a moulin. If future ice sheet models are to accurately represent moulins, we must understand their formation processes, timescales, and locations. Here, we use remote-sensing velocity products to constrain the relationship between strain rates and lake drainages across ∼ 1600 km2 in Pâkitsoq, western Greenland, between 2002–2019. We find significantly more extensional background strain rates at moulins associated with fast-draining lakes than at slow-draining or non-draining lake moulins. We test whether moulins in more extensional background settings drain their lakes earlier, but we find insignificant correlation. To investigate the frequency at which strain-rate transients are associated with fast lake drainage, we examined Landsat-derived strain rates over 16 and 32 d periods at moulins associated with 240 fast-lake-drainage events over 18 years. A low signal-to-noise ratio, the presence of water, and the multi-week repeat cycle obscured any resolution of the hypothesized transient strain rates. Our results support the hypothesis that transient strain rates drive fast lake drainages. However, the current generation of ice sheet velocity products, even when stacked across hundreds of fast lake drainages, cannot resolve these transients. Thus, observational progress in understanding lake drainage initiation will rely on field-based tools such as GPS networks and photogrammetry.

scholarly journals Challenges in predicting Greenland supraglacial lake drainages at the regional scale

2020 ◽  
Author(s):  
Kristin Poinar ◽  
Lauren C. Andrews
Keyword(s):  
Signal To Noise Ratio ◽  
Regional Scale ◽  
Ice Sheet ◽  
Strain Rates ◽  
Ice Flow ◽  
Transient Strain ◽  
Background Strain ◽  
Lake Drainage ◽  
The Relationship ◽  
Insignificant Correlation

Abstract. A leading hypothesis for the mechanism of fast supraglacial lake drainages is that transient extensional stresses briefly allow crevassing in otherwise-compressive ice flow regimes. Lake water can then hydrofracture the crevasse to the base of the ice sheet, and river inputs can maintain this connection as a moulin. If future ice-sheet models are to accurately represent moulins, we must understand their formation processes, timescales, and locations. Here, we use remote-sensing velocity products to constrain the relationship between strain rates and lake drainages across ~ 1600 km2 in Pâkitsoq, western Greenland, between 2002–2019. We find significantly more-extensional background strain rates at moulins associated with fast-draining lakes than at slow-draining or non-draining lake moulins. We test whether moulins in more-extensional background settings drain their lakes earlier, but find insignificant correlation. To investigate the frequency that strain-rate transients are associated with fast lake drainage, we examined Landsat-derived strain rates over 16- and 32-day periods at moulins associated with 240 fast lake drainage events over 18 years. A low signal-to-noise ratio, the presence of water, and the multi-week repeat cycle obscured any resolution of the hypothesized transient strain rates. Our results support the hypothesis that transient strain rates drive fast lake drainages. However, the current generation of ice-sheet velocity products, even when stacked across hundreds of fast lake drainages, cannot resolve these transients. Thus, observational progress in understanding lake drainage initiation will rely on field-based tools such as GPS networks and photogrammetry.

scholarly journals Winter drainage of surface lakes on the Greenland Ice Sheet from Sentinel-1 SAR imagery

2021 ◽  
Vol 15 (3) ◽  
pp. 1587-1606
Author(s):  
Corinne L. Benedek ◽  
Ian C. Willis
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Shape From Shading ◽  
Lake Area ◽  
Ice Flow ◽  
Surface Mass ◽  
Flow Acceleration ◽  
Optical Imagery ◽  
Sar Imagery ◽  
Lake Drainage

Abstract. Surface lakes on the Greenland Ice Sheet play a key role in its surface mass balance, hydrology and biogeochemistry. They often drain rapidly in the summer via hydrofracture, which delivers lake water to the ice sheet base over timescales of hours to days and then can allow meltwater to reach the base for the rest of the summer. Rapid lake drainage, therefore, influences subglacial drainage evolution; water pressures; ice flow; biogeochemical activity; and ultimately the delivery of water, sediments and nutrients to the ocean. It has generally been assumed that rapid lake drainage events are confined to the summer, as this is typically when observations are made using satellite optical imagery. Here we develop a method to quantify backscatter changes in satellite radar imagery, which we use to document the drainage of six different lakes during three winters (2014/15, 2015/16 and 2016/17) in fast-flowing parts of the Greenland Ice Sheet. Analysis of optical imagery from before and after the three winters supports the radar-based evidence for winter lake drainage events and also provides estimates of lake drainage volumes, which range between 0.000046 ± 0.000017 and 0.0200 ± 0.002817 km3. For three of the events, optical imagery allows repeat photoclinometry (shape from shading) calculations to be made showing mean vertical collapse of the lake surfaces ranging between 1.21 ± 1.61 and 7.25 ± 1.61 m and drainage volumes of 0.002 ± 0.002968 to 0.044 ± 0.009858 km3. For one of these three, time-stamped ArcticDEM strips allow for DEM differencing, which demonstrates a mean collapse depth of 2.17 ± 0.28 m across the lake area. The findings show that lake drainage can occur in the winter in the absence of active surface melt and notable ice flow acceleration, which may have important implications for subglacial hydrology and biogeochemical processes.

scholarly journals Snow Accumulation Studies on the Thule Peninsula, Greenland

1968 ◽  
Vol 7 (49) ◽  
pp. 59-76 ◽  
Author(s):  
Steven J. Mock
Keyword(s):  
Regional Scale ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Snow Accumulation ◽  
Accumulation Rates ◽  
Surface Slope ◽  
Ice Flow ◽  
Accumulation Pattern ◽  
Flow Phenomena ◽  
Independent Parameters

AbstractData from stake measurements, marker boards and pits along a 136 km trail crossing the Thule peninsula sector of the Greenland ice sheet have been used to determine both the regional and local distribution of snow accumulation, On a regional scale trend surfaces of mean annual accumulation can be adequately predicted from a model using distance from moisture source and elevation as independent parameters. A series of step- or wave-like features break the smooth profile of the ice. sheet and cause profound changes in accumulation rates on a local scale. The accumulation pattern over these features can be predicted from surface slope and departure from regional elevation. Profiles of’ surface and subsurface topography indicate a direct relationship between subsurface hills and step-like features, but cannot be quantitatively accounted for by existing ice-flow theory. Detailed accumulation studies in conjunction with a program of spirit leveling in the vicinity of Camp Century has revealed the development a shallow valley-like feature. Within this feature accumulation rates have increased indicating that it is the result of flow phenomena.

scholarly journals Snow Accumulation Studies on the Thule Peninsula, Greenland

1968 ◽  
Vol 7 (49) ◽  
pp. 59-76 ◽  
Author(s):  
Steven J. Mock
Keyword(s):  
Regional Scale ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Snow Accumulation ◽  
Accumulation Rates ◽  
Surface Slope ◽  
Ice Flow ◽  
Accumulation Pattern ◽  
Flow Phenomena ◽  
Independent Parameters

Abstract Data from stake measurements, marker boards and pits along a 136 km trail crossing the Thule peninsula sector of the Greenland ice sheet have been used to determine both the regional and local distribution of snow accumulation, On a regional scale trend surfaces of mean annual accumulation can be adequately predicted from a model using distance from moisture source and elevation as independent parameters. A series of step- or wave-like features break the smooth profile of the ice. sheet and cause profound changes in accumulation rates on a local scale. The accumulation pattern over these features can be predicted from surface slope and departure from regional elevation. Profiles of’ surface and subsurface topography indicate a direct relationship between subsurface hills and step-like features, but cannot be quantitatively accounted for by existing ice-flow theory. Detailed accumulation studies in conjunction with a program of spirit leveling in the vicinity of Camp Century has revealed the development a shallow valley-like feature. Within this feature accumulation rates have increased indicating that it is the result of flow phenomena.

Ice sheet unzipping in the mountain-piedmont region of west-central Sweden: complex late-deglaciation of the Scandinavian ice sheet

2021 ◽  
Author(s):  
Robin Blomdin ◽  
Gustaf Peterson Becher ◽  
Colby Smith ◽  
Carl Regnéll ◽  
Christian Öhrling ◽  
...  
Keyword(s):  
Regional Scale ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Spatial Density ◽  
Ice Flow ◽  
Westward Expansion ◽  
West Central ◽  
Scandinavian Ice Sheet ◽  
Elevation Model ◽  
Piedmont Region

<p>Regional-scale glacial geomorphological maps provide important empirical data for reconstructions of former ice sheets, which may serve as analogues for the behaviour of modern ice sheets under climate warming. In particular, the extensive LiDAR-derived record of former ice sheet beds, provides an outstanding archive from which to infer former ice sheet behaviour. The stacking together and analysis of, tens of thousands of individual landforms, based on their spatial coherency, provides a powerful tool to reconstruct ice flow dynamics, temporally evolving ice divide positions and the “unzipping” of ice sheets into separate masses during deglaciation. In this study, we develop a glacial geomorphological dataset focussing on the mountain-piedmont region of Jämtland in west-central Sweden. We focus on this region because it is where the last (Weichselian) ice sheet is believed to have unzipped into separate domes and was inundated by vast ice dammed lakes. Jämtland also records a complex temporal evolution of subglacial processes and was formerly mapped without the benefit of a LiDAR-based elevation model. The dataset was created by mapping in GIS and covers an area of 50 000 km<sup>2</sup> and almost 88 000 landforms, including glacial lineations, crag-and-tails, ice marginal moraines, lateral meltwater channels, eskers, and glacial lake shorelines. We use this unique dataset–in terms of spatial density and resolution–and quantitatively analyse cross-cutting relationships to establish a relative ice flow chronology. Our key findings include 1) a previously unmapped landform system, formed by the Early-to-Middle Weichselian westward expansion of a mountain centred ice sheet, and 2) a complex early Holocene deglaciation sequence with ice sheet unzipping occurring in southern and east-central Jämtland. The ice sheet split into a larger sheet retreating northward and a smaller ice sheet remaining southeast of the mountain piedmont. Our results provide new insights into the late deglaciation of the Scandinavian Ice Sheet.</p>

scholarly journals Investigating the stability of subglacial lakes with a full Stokes ice-sheet model

2008 ◽  
Vol 54 (185) ◽  
pp. 353-361 ◽  
Author(s):  
Frank Pattyn
Keyword(s):  
Ice Sheet ◽  
Drainage System ◽  
Potential Gradient ◽  
Surface Slope ◽  
Ice Flow ◽  
Subglacial Lake ◽  
Lake Drainage ◽  
Subglacial Lakes ◽  
The Impact ◽  
Subglacial Drainage

AbstractDespite the large amount of subglacial lakes present underneath the East Antarctic ice sheet and the melt processes involved, the hydrology beneath the ice sheet is poorly understood. Changes in subglacial potential gradients may lead to subglacial lake outbursts, discharging excess water through a subglacial drainage system underneath the ice sheet. Such processes can eventually lead to an increase in ice flow. In this paper, a full Stokes numerical ice-sheet model was employed which takes into account the ice flow over subglacial water bodies in hydrostatic equilibrium with the overlying ice. Sensitivity experiments were carried out for small perturbations in ice flow and basal melt rate as a function of ice thickness, general surface slope, ice viscosity and lake size, in order to investigate their influence on the subglacial potential gradient and the impact on subglacial lake drainage. Experiments clearly demonstrate that small changes in surface slope are sufficient to start and sustain episodic subglacial drainage events. Lake drainage can therefore be regarded as a common feature of the subglacial hydrological system and may influence, to a large extent, the present and future behavior of large ice sheets.

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

2013 ◽  
Vol 7 (2) ◽  
pp. 1101-1118 ◽  
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).

Diapirism and the Fold Zone controversy of the Ordovician glaciomarine Pakhuis Formation, South Africa

2017 ◽  
Vol 120 (2) ◽  
pp. 209-222
Author(s):  
H.J. Blignault ◽  
J.N. Theron
Keyword(s):  
Regional Variation ◽  
Ice Sheet ◽  
Relative Importance ◽  
Flow Data ◽  
Ice Flow ◽  
Sand Aquifer ◽  
Field Investigations ◽  
The Relationship ◽  
Best Fit

Abstract The Fold Zone controversy centers around firstly, the relationship between the direction of ice-flow and the geometry of the fold structures, and secondly the relative importance of subglacial or proglacial buckling as a folding process compared to the role of load casts or ball-and-pillow structures. The more recent recognition of diapirs associated with the Fold Zone resulted in further field investigations. Understanding the advance-retreat sequence constrains the interpretation of the glaciotectonic processes. The Sneeukop Member, the unconformity and Steenbras Member are interpreted as a continuum ice-expansion sequence without a major retreat interlude. The palaeo-ice-flow data are reinterpreted to accommodate the regional variation of ice-flow patterns. It is concluded that the Pakhuis ice sheet expansion overriding the Peninsula basin took place by means of tributary troughs converging on the main Peninsula trough thus explaining the relationship between the direction of ice-flow and fold trend variations across the basin. The folds also vary regionally in style between upright and overturned. Diapirs were recognized both as blind mushroom structures below the unconformity and piercing upwards. Chaotic breccias and lit-par-lit intermixing of tillite and deformed, attenuated banded sandstone are interpreted as blowout material presumably formed by a piercement diapir venting at the surface. Diapirism in the Pakhuis glacial context is understood as the consequence of an ice sheet expansion over a water-logged Peninsula sand aquifer. The over-pressurized sand aquifer gave rise to diapirs and associated structures. The main features of the Fold Zone are compared to modern examples as described in the literature. Push moraine and crevasse-filling examples are considered as the best-fit models to explain the genesis of the Fold Zone.

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

2013 ◽  
Vol 7 (4) ◽  
pp. 1185-1192 ◽  
Author(s):  
I. Joughin ◽  
S. B. Das ◽  
G. E. Flowers ◽  
M. D. Behn ◽  
R. B. Alley ◽  
...  
Keyword(s):  
Spatial Pattern ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Velocity ◽  
Ice Flow ◽  
Bed Topography ◽  
Supraglacial Lakes ◽  
Flow Enhancement ◽  
Lake Drainage ◽  
Existing Data

Abstract. Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal meltwater 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).

Constraints on the relationship between velocity and basal traction over the grounded regions of Greenland

2021 ◽  
Author(s):  
Nathan Maier ◽  
Florent Gimbert ◽  
Fabien Gillet-Chaulet ◽  
Adrien Gilbert
Keyword(s):  
Flow Field ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Spatial Relationship ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Warming Climate ◽  
The Relationship ◽  
Insight Into ◽  
Large Grid

<p>On glaciers and ice sheets, constraints on the bed physics which control the relationship between velocity and traction are critical for simulating ice flow. However, in Greenland the relationship between velocity and traction remains unquantified over much of the ice sheet. In this work, we determine the spatial relationship between velocity and traction in all eight drainage catchments of Greenland. The basal traction is estimated using three different methods over large grid cells to minimize biases associated with unconstrained rheologic parameters used in numerical inversions. We find that the velocity-traction relationships are consistent with our current understanding of basal physics in each catchment. We identify catchments that predominantly show Mohr-Coulomb-like behavior typical of deforming beds or significant cavitation, as well as catchments that predominantly show rate-strengthening behavior typical of Weertman-type hard-bed physics. Overall, the velocity-traction relationships suggest that the flow field and surface geometries over the grounded regions of the Greenland ice sheet are mainly dictated by Weertman-type physics. This data- and modeling based analysis provides a first constraint on the physics of basal motion over the grounded regions of Greenland and gives unique insight into future dynamics and vulnerabilities in a warming climate.</p>

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