scholarly journals The role of subglacial hydrology in ice streams with elevated geothermal heat flux

2020 ◽  
Vol 66 (256) ◽  
pp. 303-312
Author(s):  
Silje Smith-Johnsen ◽  
Basile de Fleurian ◽  
Kerim H. Nisancioglu
Keyword(s):  
Heat Flux ◽  
Ice Sheet ◽  
Complex Surface ◽  
Surface Velocity ◽  
Ice Streams ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Ice Stream ◽  
The Impact

AbstractThe spatial distribution of geothermal heat flux (GHF) under ice sheets is largely unknown. Nonetheless, it is an important boundary condition in ice-sheet models, and suggested to control part of the complex surface velocity patterns observed in some regions. Here we investigate the effect of including subglacial hydrology when modelling ice streams with elevated GHF. We use an idealised ice stream geometry and a thermomechanical ice flow model coupled to subglacial hydrology in the Ice Sheet System Model (ISSM). Our results show that the dynamic response of the ice stream to elevated GHF is greatly enhanced when including the interactive subglacial hydrology. On the other hand, the impact of GHF on ice temperature is reduced when subglacial hydrology is included. In conclusion, the sensitivity of ice stream dynamics to GHF is likely to be underestimated in studies neglecting subglacial hydrology.

scholarly journals Indication of high basal melting at EastGRIP drill site on the Northeast Greenland Ice Stream

2021 ◽  
Author(s):  
Ole Zeising ◽  
Angelika Humbert
Keyword(s):  
Heat Flux ◽  
Heat Budget ◽  
Ice Sheet ◽  
Water System ◽  
Hydrological Regime ◽  
Ice Streams ◽  
Geothermal Heat ◽  
Ice Stream ◽  
Drill Site ◽  
Basal Melting

Abstract. The accelerated ice flow of ice streams that reach far into the interior of the ice sheet, is associate with lubrication of the ice sheet base by basal melt water. However, the amount of basal melting under the large ice streams – such as the Northeast Greenland Ice Stream (NEGIS) – are largely unknown. In-situ measurements of basal melt rates are important from various perspectives as they indicate the heat budget, the hydrological regime and the role of sliding in glacier motion. The few previous estimates of basal melt rates in the NEGIS region were 0.1 m a−1 and more, based on radiostratigraphy methods. These finding raised the question of the heat source, since even an increased geothermal heat flux could not deliver the necessary amount of heat. Here, we present basal melt rates at the recent deep drill site EastGRIP, located in the center of NEGIS. Within two subsequent years, we found basal melt rates of (0.16–0.22) ± 0.01 m a−1, that are based on analysis of repeated phase-sensitive radar measurements. In order to quantify the contribution of processes that cause a heat flux into the ice, we carried out an assessment of the energy sources and found the subglacial water system to play a key role in facilitating such high melt rates.

scholarly journals Of isbræ and ice streams

2003 ◽  
Vol 36 ◽  
pp. 66-72 ◽  
Author(s):  
Martin Truffer ◽  
Keith A. Echelmeyer
Keyword(s):  
Ice Sheet ◽  
Shear Zones ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Bed Topography ◽  
Ice Stream ◽  
Fast Ice ◽  
Fast Flow ◽  
End Members

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

scholarly journals Thermomechanical modelling of the Scandinavian ice sheet: implications for ice-stream formation

1999 ◽  
Vol 28 ◽  
pp. 83-89 ◽  
Author(s):  
A. J. Payne ◽  
D.J. Baldwin
Keyword(s):  
Three Dimensional ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Low Viscosity ◽  
Ice Stream ◽  
Scandinavian Ice Sheet ◽  
The Baltic ◽  
Stream Formation ◽  
Initial Results

AbstractThis work attempts to explain the fan-like landform assemblages observed in satellite images of the area covered by the former Scandinavian ice sheet (SIS). These assemblages have been interpreted as evidence of large ice streams within the SIS. If this interpretation is correct, then it calls into doubt current theories on the formation of ice streams. These theories regard soft sediment and topographic troughs as being the key determinants of ice-stream location. Neither can be used to explain the existence of ice streams on the flat, hard-rock area of the Baltic Shield. Initial results from a three-dimensional, thermomechanical ice-sheet model indicate that interactions between ice flow, form and temperature can create patterns similar to those mentioned above. The model uses a realistic, 20 km resolution gridded topography and a simple parameterization of accumulation and ablation. It produces patterns of maximum ice-sheet extent, which are similar to those reconstructed from the area’s glacial geomorphology. Flow in the maximum, equilibrium ice sheet is dominated by wedges of warm, low-viscosity, fast-flowing ice. These are separated by areas of cold, slow-flowing ice. This patterning appears to develop spontaneously as the modelled ice sheet grows.

scholarly journals Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration?

2014 ◽  
Vol 2 (2) ◽  
pp. 911-933 ◽  
Author(s):  
N. F. Glasser ◽  
S. J. A. Jennings ◽  
M. J. Hambrey ◽  
B. Hubbard
Keyword(s):  
Flow Line ◽  
Ice Sheet ◽  
Surface Structures ◽  
Surface Velocity ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Ice Surface ◽  
Ice Stream ◽  
The Antarctic

Abstract. Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr-1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

Investigating basal thaw as a potential driver of ice flow acceleration in Antarctica

2020 ◽  
Author(s):  
Eliza Dawson ◽  
Dustin Schroeder ◽  
Winnie Chu ◽  
Elisa Mantelli ◽  
Helene Seroussi
Keyword(s):  
Numerical Models ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Flow Acceleration ◽  
Critical Areas ◽  
Potential Impact ◽  
Basal Melting ◽  
The Impact ◽  
Selection Of

<p>Glacial thermal processes exert a fundamental control on ice flow, governing viscosity and frozen-to-thawed transitions at the ice-bed interface. Across Antarctica, frozen bed regions characterized by numerical models and geophysical observations, can also reduce ice flow by increasing basal traction. Some frozen bed regions can separate or confine fast-flowing glaciers and ice streams. Others separate inland catchments with thawed beds from the grounding zone of marine ice-sheet sectors. If regions with frozen bed experienced thawing, such a transition may lead to ice-sheet acceleration, reconfiguration, or retreat. To investigate the potential impact of such a thermal transition, we use the JPL/UCI Ice Sheet System Model (ISSM) to identify vulnerable regions across Antarctica that are close to the basal melting point. We assess the impact of thawing these regions by quantifying resulting volume changes and surface expressions. This allows us to identify the areas of the ice sheet where the thermal regime at the ice-bed interface has the largest potential impact on ice-sheet stability and sea-level contribution. We also examine the potential basal temperature and thaw-propagation thresholds governing this process. We then compare the ISSM results to a selection of ice-penetrating radar sounding observations to refine our constraints of the configuration, distribution, and extent of these thermally critical areas.</p>

Recent developments in modeling ice sheet deformation

2020 ◽  
Author(s):  
Felicity McCormack ◽  
Roland Warner ◽  
Adam Treverrow ◽  
Helene Seroussi
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Vertical Shear ◽  
Shear Stresses ◽  
Polar Ice ◽  
Ice Flow ◽  
Polar Ice Sheets ◽  
Basal Shear ◽  
The Impact

<p>Viscous deformation is the main process controlling ice flow in ice shelves and in slow-moving regions of polar ice sheets where ice is frozen to the bed. However, the role of deformation in flow in ice streams and fast-flowing regions is typically poorly represented in ice sheet models due to a major limitation in the current standard flow relation used in most large-scale ice sheet models – the Glen flow relation – which does not capture the steady-state flow of anisotropic ice that prevails in polar ice sheets. Here, we highlight recent advances in modeling deformation in the Ice Sheet System Model using the ESTAR (empirical, scalar, tertiary, anisotropic regime) flow relation – a new description of deformation that takes into account the impact of different types of stresses on the deformation rate. We contrast the influence of the ESTAR and Glen flow relations on the role of deformation in the dynamics of Thwaites Glacier, West Antarctica, using diagnostic simulations. We find key differences in: (1) the slow-flowing interior of the catchment where the unenhanced Glen flow relation simulates unphysical basal sliding; (2) over the floating Thwaites Glacier Tongue where the ESTAR flow relation outperforms the Glen flow relation in accounting for tertiary creep and the spatial differences in deformation rates inherent to ice anisotropy; and (3) in the grounded region within 80km of the grounding line where the ESTAR flow relation locally predicts up to three times more vertical shear deformation than the unenhanced Glen flow relation, from a combination of enhanced vertical shear flow and differences in the distribution of basal shear stresses. More broadly on grounded ice, the membrane stresses are found to play a key role in the patterns in basal shear stresses and the balance between basal shear stresses and gravitational forces simulated by each of the ESTAR and Glen flow relations. Our results have implications for the suitability of ice flow relations used to constrain uncertainty in reconstructions and projections of global sea levels, warranting further investigation into using the ESTAR flow relation in transient simulations of glacier and ice sheet dynamics. We conclude by discussing how geophysical data might be used to provide insight into the relationship between ice flow processes as captured by the ESTAR flow relation and ice fabric anisotropy.</p>

Relating polarimetric radar measurements of ice fabric to ice-flow enhancement of Rutford Ice Stream

2020 ◽  
Author(s):  
Thomas Jordan ◽  
Alex Brisbourne ◽  
Carlos Martin ◽  
Rebecca Schlegel ◽  
Dustin Schroeder ◽  
...  
Keyword(s):  
Polarimetric Radar ◽  
Ice Streams ◽  
Ice Flow ◽  
Near Surface ◽  
Flow Models ◽  
Relative Contribution ◽  
Ice Stream ◽  
Glacier Ice ◽  
Flow Enhancement ◽  
The Impact

<p>Lateral shear margins provide resistance to ice flow within ice streams and play an important role in the overall dynamics of ice sheets. The strength and location of shear margins are known to be influenced by both subglacial factors (e.g. bed roughness, meltwater availability) and ice rheology (ice temperature, ice fabric, and damage). Assessing the relative contribution of these factors upon ice-stream flow is complex but can be aided by geophysical measurements (e.g. radar-sounding and seismic imaging) of the ice-stream subsurface. There are, however, ongoing challenges in obtaining geophysical information in an appropriate form to be incorporated into ice-flow models. This is true of ice fabric, and its direction-dependent effect upon ice viscosity is typically neglected in models of ice streams.</p><p>Here we develop a framework to relate ice fabric measurements from polarimetric radar sounding to ice-flow enhancement within ice streams. First, we extend a `polarimetric coherence’ radar method to automate the extraction of ice fabric using quad-polarized data.  Second, using a previously developed anisotropic flow-law formulation, we relate the radar fabric measurements to direction-dependent enhancement factors of glacier ice. We demonstrate the approach using a radar ground survey, collected by the British Antarctic Survey, which traverses between the centre and shear margin of Rutford Ice Stream. The data indicate that a vertical girdle fabric is present in the near-surface of the ice stream (approximately the top 300 m) which azimuthally rotates and strengthens toward the shear margin. We then assess the effect that the girdle fabric has upon shear and compression and the impact upon ice-flow models of Rutford Ice Stream.</p><p> </p><p> </p><p> </p>

scholarly journals The Dynamics of the Antarctic Ice Sheet

1989 ◽  
Vol 12 ◽  
pp. 16-22 ◽  
Author(s):  
W.F. Budd ◽  
D. Jenssen
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Accumulation Rate ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Surface Velocity ◽  
Geothermal Heat ◽  
Antarctic Ice Sheet ◽  
The Past ◽  
The Antarctic

A three-dimensional dynamic, thermodynamic ice-sheet model has been developed to simulate the past, present, and future behaviour of the Antarctic ice sheet. The present ice velocities depend on the deep ice temperatures which in turn depend on the past changes of the ice sheet, including surface temperature, accumulation rate, and ice thickness. The basal temperatures are also strongly dependent on the geothermal heat flux. The model has therefore been used to study the effect on the basal temperatures, of changes to the geothermal heat flux, as well as the past changes of surface temperature and accumulation rate based on results obtained from the Vostok deep ice core. The model is also used to compute the distribution of surface velocity required to balance the present accumulation rate and the dynamics velocity based on the stress, temperature, and flow properties of ice, for the internal deformation, plus a component due to ice sliding. These velocities are compared to observed surface velocities in East Antarctica to assess the state of balance and the performance of the dynamics formulation.

scholarly journals The impact of subglacial hydrology on the force balance of a physically modelled ice stream

2013 ◽  
Vol 54 (63) ◽  
pp. 333-342 ◽  
Author(s):  
Benjamin M. Wagman ◽  
Ginny A. Catania
Keyword(s):  
Water Distribution ◽  
Water Discharge ◽  
Water Layer ◽  
Flow Speed ◽  
Force Balance ◽  
Ice Streams ◽  
Basal Resistance ◽  
Ice Flow ◽  
Ice Stream ◽  
The Impact

AbstractWe use a physical model to investigate how changes in subglacial hydrology affect ice motion of Antarctic ice streams. Ice streams are modelled using silicone polymer placed over a thin water layer to mimic ice flow dominated by basal sliding. The model ice-stream force balance is calculated and compared directly to the observed force balance of Whillans Ice Stream (WIS). Dynamic similarity between the model and WIS is achieved when their force balances are equivalent. The WIS force balance has evolved over time owing to increased basal resistance. We test two hypotheses: (1) the subglacial water distribution influences the ice-flow speed and thus the force balance; (2) shear margins are locations where transitions in water layer thickness occur. We find that the velocity and force balance are sensitive to pulsed water discharge events and changes in lubrication that result in sticky spots, and that model shear margins tend to overlie water lubrication boundaries. We conclude that local changes in basal lubrication near margins (possibly as a result of the presence of sticky spots or subglacial lakes) influence the stability of ice-stream margin position and may be responsible for large and rapid shifts in margin location.

scholarly journals Where to find 1.5 million yr old ice for the IPICS "Oldest-Ice" ice core

2013 ◽  
Vol 9 (6) ◽  
pp. 2489-2505 ◽  
Author(s):  
H. Fischer ◽  
J. Severinghaus ◽  
E. Brook ◽  
E. Wolff ◽  
M. Albert ◽  
...  
Keyword(s):  
Heat Flux ◽  
Ice Core ◽  
Ice Sheet ◽  
Ice Thickness ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Quaternary Climate ◽  
Drill Site ◽  
Heat Flow Model

Abstract. The recovery of a 1.5 million yr long ice core from Antarctica represents a keystone of our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we tackle the question of where such ice may still be found in the Antarctic ice sheet. We can show that such old ice is most likely to exist in the plateau area of the East Antarctic ice sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle position on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, however, we strongly suggest significantly reduced ice thickness to avoid bottom melting. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than but close to about 2500 m would be required to find 1.5 Myr old ice (i.e., more than 700 m less than at the current EPICA Dome C drill site). Within this constraint, the resolution of an Oldest-Ice record and the distance of such old ice to the bedrock should be maximized to avoid ice flow disturbances, for example, by finding locations with minimum geothermal heat flux. As the geothermal heat flux is largely unknown for the EAIS, this parameter has to be carefully determined beforehand. In addition, detailed bedrock topography and ice flow history has to be reconstructed for candidates of an Oldest-Ice ice coring site. Finally, we argue strongly for rapid access drilling before any full, deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.

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