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

2021 ◽  
Vol 15 (7) ◽  
pp. 3119-3128
Author(s):  
Ole Zeising ◽  
Angelika Humbert
Keyword(s):  
Heat Budget ◽  
Water System ◽  
Hydrological Regime ◽  
Ice Streams ◽  
Geothermal Heat ◽  
Ice Stream ◽  
Drill Site ◽  
Phase Sensitive ◽  
Basal Melting

Abstract. The accelerated ice flow of ice streams that reach far into the interior of the ice sheets is associated with lubrication of the ice sheet base by basal meltwater. However, the amount of basal melting under the large ice streams – such as the Northeast Greenland Ice Stream (NEGIS) – is 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 relative importance 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 findings 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 centre of NEGIS. Within 2 subsequent years, we found basal melt rates of 0.19±0.04 m a−1 that are based on analysis of repeated phase-sensitive radar measurements. In order to quantify the contribution of processes that contribute to melting, we carried out an assessment of the energy balance at the interface and found the subglacial water system to play a key role in facilitating such high melt rates.

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 Using ice-flow models to evaluate potential sites of million year-old ice in Antarctica

2013 ◽  
Vol 9 (3) ◽  
pp. 2859-2887 ◽  
Author(s):  
B. Van Liefferinge ◽  
F. Pattyn
Keyword(s):  
Ice Sheet ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Flow Models ◽  
Drill Site ◽  
Slow Moving ◽  
Basal Melting ◽  
The Antarctic ◽  
Suitable Potential

Abstract. Finding suitable potential sites for an undisturbed record of million-year old ice in Antarctica requires a slow-moving ice sheet (preferably an ice divide) and basal conditions that are not disturbed by large topographic variations. Furthermore, ice should be thick and cold basal conditions should prevail, since basal melting would destroy the bottom layers. However, thick ice (needed to resolve the signal at sufficient high resolution) increases basal temperatures, which is a conflicting condition in view of finding a suitable drill site. In addition, slow moving areas in the center of ice sheets are also low-accumulation areas, and low accumulation reduces potential cooling of the ice through vertical advection. While boundary conditions such as ice thickness and accumulation rates are relatively well constraint, the major uncertainty in determining basal conditions resides in the geothermal heat flow (GHF) underneath the ice sheet. We explore uncertainties in existing GHF datasets and their effect on basal temperatures of the Antarctic ice sheet and propose an updated method based on Pattyn (2010) to improve existing GHF datasets in agreement with known basal temperatures and their gradients to reduce this uncertainty. Both complementary methods lead to a better comprehension of basal temperature sensitivity and a characterization of potential ice coring sites within these uncertainties.

Identifying Antarctic Ice Sheet Tipping Points

2020 ◽  
Author(s):  
Bradley Reed ◽  
Mattias Green ◽  
Hilmar Gudmundsson ◽  
Adrian Jenkins
Keyword(s):  
Ice Sheet ◽  
Fold Increase ◽  
Tipping Points ◽  
Ice Streams ◽  
Antarctic Ice Sheet ◽  
Ice Shelves ◽  
Ice Stream ◽  
The Stability ◽  
Basal Melting ◽  
Gradual Variation

<p>Warmer atmospheric and oceanic temperatures have led to a six-fold increase in mass loss from Antarctica in the last four decades. It is difficult to predict how the ice sheet will respond to future warming because it is subject to positive feedback mechanisms, which could lead to destabilisation. Observational and modelling work has shown that ice streams in West Antarctica may be undergoing unstable and possibly irreversible retreat due to increased basal melting beneath their ice shelves. Being able to identify and predict stability thresholds in ice streams draining the Antarctic Ice Sheet could help establish early warning indicators of near-future abrupt changes in sea level. <br> <br>Here, we use the shallow-ice flow model Úa to investigate the stability of an idealised ice stream from the third Marine Ice Sheet Model Intercomparison Project (MISMIP+). Initial results show that a gradual variation in ice viscosity, which corresponds to a change in temperature, causes the ice stream to undergo hysteresis across an overdeepened bed. This hysteresis means there are two tipping points, one for an advance phase and one for a retreat phase, both of which lie off the retrograde sloping bedrock. Beyond these tipping points, changes in ice stream grounding line position are unstable and irreversible. This behaviour is also apparent in wider ice streams although there is a change to the onset of instability and the location of tipping points. Further studies will investigate the additional effects of basal melting on these tipping points.</p>

scholarly journals Active subglacial lakes and channelized water flow beneath the Kamb Ice Stream

2016 ◽  
Vol 10 (6) ◽  
pp. 2971-2980 ◽  
Author(s):  
Byeong-Hoon Kim ◽  
Choon-Ki Lee ◽  
Ki-Weon Seo ◽  
Won Sang Lee ◽  
Ted Scambos
Keyword(s):  
Water System ◽  
Topographic Feature ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ross Ice Shelf ◽  
Subglacial Lake ◽  
Ice Stream ◽  
Grounding Line ◽  
Subglacial Lakes ◽  
Kamb Ice Stream

Abstract. We identify two previously unknown subglacial lakes beneath the stagnated trunk of the Kamb Ice Stream (KIS). Rapid fill-drain hydrologic events over several months are inferred from surface height changes measured by CryoSat-2 altimetry and indicate that the lakes are probably connected by a subglacial drainage network, whose structure is inferred from the regional hydraulic potential and probably links the lakes. The sequential fill-drain behavior of the subglacial lakes and concurrent rapid thinning in a channel-like topographic feature near the grounding line implies that the subglacial water repeatedly flows from the region above the trunk to the KIS grounding line and out beneath the Ross Ice Shelf. Ice shelf elevation near the hypothesized outlet is observed to decrease slowly during the study period. Our finding supports a previously published conceptual model of the KIS shutdown stemming from a transition from distributed flow to well-drained channelized flow of subglacial water. However, a water-piracy hypothesis in which the KIS subglacial water system is being starved by drainage in adjacent ice streams is also supported by the fact that the degree of KIS trunk subglacial lake activity is relatively weaker than those of the upstream lakes.

scholarly journals Bed reflectivity beneath inactive ice streams in West Antarctica

2003 ◽  
Vol 36 ◽  
pp. 287-291 ◽  
Author(s):  
Ginny A. Catania ◽  
Howard B. Conway ◽  
Anthony M. Gades ◽  
Charles F. Raymond ◽  
Hermann Engelhardt
Keyword(s):  
Water System ◽  
West Antarctica ◽  
Ice Streams ◽  
Weak Measurements ◽  
Ice Stream ◽  
Spatial Changes ◽  
Geometric Spreading ◽  
Radio Echo Sounding ◽  
Measured Power ◽  
Echo Sounding

AbstractRadio-echo sounding (RES) techniques are used to examine spatial changes in bed reflectivity across relict ice streams inWest Antarctica. Measurements from adjacent interstream ridges are used to correct the measured power returned from the bed for attenuation and losses due to geometric spreading, scattering and absorption. RES measurements near boreholes drilled on Ice Stream C (ISC) indicate high coefficients of bed reflectivity (R > 0.1) in locations where the bed was thawed and boreholes connected to the basal water system, and low reflectivity coefficients (R < 0.02) at locations that were frozen and not connected. Intermediate values of bed reflectivity were measured at locations where the connection to the basal water system was weak. Measurements across four relict margins show that bed reflectivity usually jumps from low to high values several kilometers inside the outermost buried crevasses. We interpret this to be a transition from frozen to thawed basal conditions and discuss implications of these observations.

scholarly journals A full-Stokes ice flow model for the vicinity of Dome Fuji, Antarctica, with induced anisotropy and fabric evolution

2009 ◽  
Vol 3 (1) ◽  
pp. 1-31 ◽  
Author(s):  
H. Seddik ◽  
R. Greve ◽  
T. Zwinger ◽  
L. Placidi
Keyword(s):  
Flow Model ◽  
Stokes Equations ◽  
Melting Rate ◽  
Induced Anisotropy ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Climate Conditions ◽  
Anisotropic Flow ◽  
Drill Site ◽  
Basal Melting

Abstract. A three-dimensional, thermo-mechanically coupled ice flow model with induced aniso-tropy has been applied to a ~200×200 km domain around the Dome Fuji drill site, Antarctica. The model ("Elmer/Ice") is based on the open-source multi-physics package Elmer (http://www.csc.fi/elmer/) and solves the full-Stokes equations. Flow-induced anisotropy in ice is accounted for by an implementation of the Continuum-mechanical, Anisotropic Flow model, based on an anisotropic Flow Enhancement factor ("CAFFE model"). Steady-state simulations for present-day climate conditions are conducted. The main findings are: (i) the flow regime at Dome Fuji is a complex superposition of vertical compression, horizontal extension and bed-parallel shear; (ii) for a geothermal heat flux of 60 mW m−2 the basal temperature at Dome Fuji reaches the pressure melting point and the basal melting rate is ~1 mm a−1; (iii) the fabric shows a weak single maximum at Dome Fuji, which increases the age of the ice compared to an isotropic scenario; (iv) as a consequence of spatially variable basal melting conditions, and contrary to intuition, the basal age is smaller where the ice is thicker and larger where the ice is thinner. The latter result is of great relevance for the consideration of a future drill site in the area.

scholarly journals Distribution of basal melting and freezing beneath tributaries of Ice Stream C: implication for the Holocene decay of the West Antarctic ice sheet

2003 ◽  
Vol 36 ◽  
pp. 273-282 ◽  
Author(s):  
Stefan W. Vogel ◽  
Slawek Tulaczyk ◽  
Ian R. Joughin
Keyword(s):  
Ice Sheet ◽  
Ice Streams ◽  
The West ◽  
Antarctic Ice Sheet ◽  
West Antarctic Ice Sheet ◽  
The Holocene ◽  
West Antarctic ◽  
Basal Ice ◽  
Ice Stream ◽  
Basal Melting

AbstractIce-stream tributaries connect the relatively slow-moving interior of the West Antarctic ice sheet (WAIS) with the fast-flowing Siple Coast ice streams. Basal water underneath these ice streams reduces basal resistance and enables the fast motion of the ice. Basal melting being the only source for this water, it is important to include the distribution of basal melting and freezing into numerical models assessing the stability of the WAIS. However, it is very difficult to constrain its distribution from existing field observations. Past borehole observations confirmed the presence of a wet bed at Byrd Station in the WAIS interior and at different locations within Siple Coast ice streams. However, the recent discovery of a 12–25m thick sediment-laden bubble-free basal ice layer at the UpC boreholes indicates that basal freezing is either currently occurring or had occurred upstream during the last glacial–interglacialcycle.We use a flowline model of ice thermodynamics to assess and quantify the spatial and temporal distribution of basal melting and freezing beneath Ice Stream C tributaries, taking into account the geothermal flux, shear heating and heat conduction away from the bed. Under the assumption that the ice was moving over a weak bed (τb =1–10 kPa) our model is able to reproduce a layer of frozen-on ice similar in thickness to the UpC “sticky spot” basal ice layer. Increased basal melting in the early Holocene possibly could have initiated the Holocene decay of the WAIS, whereas increased freezing rates over the past few thousand years could have decreased the amount of basal water in the system, resulting in a strengthening of the bed. This is consistent with current force-budget calculations for ice-stream tributaries and with observed stoppages and slow-downs of ice streams.

scholarly journals Sensitivity of the Lambert-Amery glacial system to geothermal heat flux

2016 ◽  
Vol 57 (73) ◽  
pp. 56-68 ◽  
Author(s):  
M. L. Pittard ◽  
J. L. Roberts ◽  
B. K. Galton-Fenzi ◽  
C. S. Watson
Keyword(s):  
Heat Flux ◽  
Ice Sheet ◽  
Ice Streams ◽  
Control Solution ◽  
Geothermal Heat ◽  
Local Flow ◽  
Thermal Boundary Conditions ◽  
Grounding Line ◽  
The Difference ◽  
Basal Melting

ABSTRACTGeothermal heat flux (GHF) is one of the key thermal boundary conditions for ice-sheet models. We assess the sensitivity of the Lambert-Amery glacial system in East Antarctica to four different GHF datasets using a regional ice-sheet model. A control solution of the regional model is initialised by minimising the misfit to observations through an optimisation process. The Lambert-Amery glacial system simulation contains temperate ice up to 150 m thick and has an average basal melt of 1.3 mm a−1, with maximum basal melting of 504 mm a−1. The simulations which use a relatively high GHF compared to the control solution increase the volume and area of temperate ice, which causes higher surface velocities at higher elevations, which leads to the advance of the grounding line. The grounding line advance leads to changes in the local flow configuration, which dominates the changes within the glacial system. To investigate the difference in spatial patterns within the geothermal datasets, they were scaled to have the same median value. These scaled GHF simulations showed that the ice flow was most sensitive to the spatial variation in the underlying GHF near the ice divides and on the edges of the ice streams.

scholarly journals A full Stokes ice flow model for the vicinity of Dome Fuji, Antarctica, with induced anisotropy and fabric evolution

2011 ◽  
Vol 5 (2) ◽  
pp. 495-508 ◽  
Author(s):  
H. Seddik ◽  
R. Greve ◽  
T. Zwinger ◽  
L. Placidi
Keyword(s):  
Flow Model ◽  
Stokes Equations ◽  
Melting Rate ◽  
Induced Anisotropy ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Climate Conditions ◽  
Anisotropic Flow ◽  
Drill Site ◽  
Basal Melting

Abstract. A three-dimensional, thermo-mechanically coupled ice flow model with induced anisotropy has been applied to a ~200 × 200 km domain around the Dome Fuji drill site, Antarctica. The model ("Elmer/Ice") is based on the open-source multi-physics package Elmer (http://www.csc.fi/elmer/) and solves the full Stokes equations. Flow-induced anisotropy in ice is accounted for by an implementation of the Continuum-mechanical, Anisotropic Flow model, based on an anisotropic Flow Enhancement factor ("CAFFE model"). Steady-state simulations for present-day climate conditions are conducted. The main findings are: (i) the flow regime at Dome Fuji is a complex superposition of vertical compression, horizontal extension and bed-parallel shear; (ii) for an assumed geothermal heat flux of 60 mW m−2 the basal temperature at Dome Fuji reaches the pressure melting point and the basal melting rate is ~0.35 mm a−1; (iii) in agreement with observational data, the fabric shows a strong single maximum at Dome Fuji, and the age of the ice is decreased compared to an isotropic scenario; (iv) as a consequence of spatially variable basal melting conditions, the basal age tends to be smaller where the ice is thicker and larger where the ice is thinner. The latter result is of great relevance for the consideration of a future drill site in the area.

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.

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