scholarly journals Intercomparison of subglacial sediment-deformation models: application to the Late Weichselian western Barents margin

2000 ◽  
Vol 30 ◽  
pp. 187-196 ◽  
Author(s):  
Daniel Howell ◽  
Martin J. Siegert
Keyword(s):  
Sediment Transport ◽  
Normal Load ◽  
Water Pressure ◽  
Ice Sheet ◽  
Model Parameters ◽  
Sediment Layer ◽  
Ice Stream ◽  
Sediment Deformation ◽  
Late Weichselian ◽  
Deformation Models

AbstractNumerical experiments, where a simple ice-sheet model was coupled with sediment-deformation models, were performed to investigate the transport of glacigenic material to the western Barents Shelf during the Late Weichselian. The ice-sheet model, and its environmental inputs, has been matched previously with a series of geological datasets relating to the maximum extent of the ice sheet (Howell and others, http://www.ggg.qub.ac.uk [rp05/1999]). Additional geological data on the volumes of sediment delivered to the Bear Island fan (Barents continental margin) are available for comparison. The experiments indicate the sensitivity of sediment transport and deposition to variations in (a) the ice-stream model and (b) a variety of model parameters. Two ice-stream models were used: (1) a height-above-buoyancy model, in which basal velocity is controlled by basal driving stress and a buoyancy-induced reduction in the normal load beneath a marine-based ice sheet; and (2) a modified version of the method presented by Alley (1990) in which basal velocity is related to pore-water pressure, sediment thickness, and driving basal stress. The results of the two different models were then compared. An extensive set of sensitivity tests was carried out to determine sediment-transport response to changes in the model’s parameters. Results indicate that, using physically realistic parameters for deforming subglacial sediment, both models reproduce the volume of Late Weichselian sediment measured on the Bear Island fan. Results from both models are sensitive to (1) cohesion of the sediment and (2) the thickness of deforming sediment beneath the ice sheet. The two models exhibited different degrees of sensitivity to the sediment parameters, with the height-above-buoyancy model proving to be less sensitive to variations in the thickness of the deforming sediment layer than the model proposed by Alley (1990). The differences between the two models examined here highlight the need for a comprehensive comparison of all the methodologies for calculating basal-ice motion currently in use.

scholarly journals Groundwater flow beneath Late Weichselian glacier ice in Nordfjord, Norway

2007 ◽  
Vol 53 (180) ◽  
pp. 84-90 ◽  
Author(s):  
Carolyn A. Moeller ◽  
D.M. Mickelson ◽  
M.P. Anderson ◽  
C. Winguth
Keyword(s):  
Groundwater Flow ◽  
Flow Model ◽  
Water Pressure ◽  
Ice Sheet ◽  
Sheet Thickness ◽  
Overburden Pressure ◽  
Glacier Ice ◽  
Late Weichselian ◽  
Southern Norway ◽  
Basal Melting

AbstractBasal water pressure and water flow patterns are significant factors in controlling the behavior of an ice sheet, because they influence ice-sheet thickness, stability and extent. Water produced by basal melting may infiltrate the subsurface, or occur as sheet or channelized flow at the ice/bed interface. We examine subglacial groundwater conditions along a flowline of the Scandinavian ice sheet through Nordfjord, in the western fjords region of southern Norway, using a steady-state, twodimensional groundwater-flow model. Meltwater input to the groundwater model is calculated by a two-dimensional, time-dependent, thermomechanically coupled ice-flow model oriented along the same flowline. Model results show that the subglacial sediments could not have transmitted all the meltwater out of the fjord during times of ice advance and when the ice sheet was at its maximum position at the edge of the continental shelf. In order for pore-water pressures to remain below the overburden pressure of the overlying ice, other paths of subglacial drainage are necessary to remove excess water. During times of retreat, the subglacial aquifer is incapable of transmitting all the meltwater that was probably generated. Pulses of meltwater reaching the bed could explain nonclimatically driven margin readvances during the overall retreat phase.

scholarly journals Genesis of the glaciotectonic thrust-fault complex at Halk Hoved, southern Denmark.

2012 ◽  
Vol 60 ◽  
pp. 61-80
Author(s):  
Tillie M. Madsen
Keyword(s):  
Pore Water Pressure ◽  
Water Pressure ◽  
Ice Sheet ◽  
Thrust Fault ◽  
Complex Deformation ◽  
North East ◽  
Macro Scale ◽  
Scandinavian Ice Sheet ◽  
Late Weichselian ◽  
Southern Fringe

The coastal cliff of Halk Hoved, southern Jutland, Denmark, is a major glaciotectonic complex formed by proglacial deformation of the North-East (NE) advance from the Scandinavian Ice Sheet in Late Weichselian. We describe and interpret the pre-, syn- and post-tectonic sedimentary successions and macro-scale architecture of this complex. Initially, the Lillebælt Till Formation (unit 1) and the overlying glaciofluvial sediments (unit 2) were deposited during the Warthe glaciation in Late Saalian. During the NE advance towards the Main Stationary Line (MSL) in Late Weichselian, these sediments were pushed along a décollement surface whereby a thrust-fault complex was formed. In a cross section the complex extends for more than 900 m and consists of eighteen c. 15–20 m thick thrust sheets stacked by piggyback thrusting. Accumulated displacement amounts to at least 235 m along thrust faults dipping at 30–40° towards N-NE, resulting in at least 24% glaciotectonic shortening of the complex. Deformation was presumably facilitated by elevated pore-water pressure in the Lillebælt Till Formation. As the compressive stress exceeded the shear strength of the weakened till, failure occurred, and a décollement horizon formed along the lithological boundary between the Lillebælt Till Formation and the underlying aquifer. During deformation, piggyback basins formed wherein sediments of hyperconcentrated flow (unit 3) and glaciolacustrine diamicton (unit 4) were deposited. The whole thrust-fault complex and the intervening sediments were truncated subglacially as the NE advance finally overrode the complex. Following the retreat of the NE advance, a succession of glaciofluvial sediments (unit 5) and finally the East Jylland Till Formation (unit 6) were deposited during the advance of the Young Baltic Ice Sheet. The Halk Hoved thrust-fault complex is a prominent example of glaciotectonism at the southern fringe of the Scandinavian Ice Sheet.

scholarly journals Temporal variations in the flow of a large Antarctic ice-stream controlled by tidally induced changes in the subglacial water system

2015 ◽  
Vol 9 (2) ◽  
pp. 2397-2429 ◽  
Author(s):  
S. H. R. Rosier ◽  
G. H. Gudmundsson ◽  
J. A. M. Green
Keyword(s):  
Water Pressure ◽  
Water System ◽  
Surface Velocity ◽  
Decay Length ◽  
Model Parameters ◽  
Effective Pressure ◽  
Tidal Period ◽  
Ice Stream ◽  
Highly Nonlinear ◽  
Basal Sliding

Abstract. Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77 day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modeling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidal subglacial water pressure variations. Furthermore, the subglacial water system must be highly conductive and at low effective pressure, and the relationship between sliding velocity and effective pressure highly nonlinear in order for the model results to match GPS measurements. Hydrological and basal sliding model parameters that produced a best fit to observations were a mean effective pressure N of 105 kPa, subglacial drainage system conductivity K of 7 × 109 m2d-1, with sliding law exponents m = 3 and q =10. Coupled model results show the presence of tides result in a ~ 12% increase in mean surface velocity. Observations of tidally-induced variations in flow of ice-streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

scholarly journals Unravelling the complex nature of the Upper Weichselian till section at Gdynia Babie Doły, northern Poland

Geologos ◽  
2016 ◽  
Vol 22 (1) ◽  
pp. 15-32 ◽  
Author(s):  
Piotr Paweł Woźniak ◽  
Piotr Czubla
Keyword(s):  
Ice Sheet ◽  
Complex Nature ◽  
Main Role ◽  
Ice Flow ◽  
The North ◽  
Ice Stream ◽  
Ice Sheet Dynamics ◽  
Late Weichselian ◽  
Till Fabric ◽  
The Baltic

AbstractThe complexity of glacial sequences may increase when these formed underneath ice sheets despite subsequent changes in their extent that are accompanied by alterations in the direction of the ice flow. Our aim was to determine whether or not changes in ice sheet dynamics during the Late Weichselian are also recorded in sediments formed north of the area of its fluctuating margin (i.e., where the ice sheet prevailed independent of such fluctuations). It is shown that in these areas such a record could have occurred, as documented by results of till studies at Babie Doły. The examination was carried out using several analyses: lithofacies properties of sediments, petrographic till composition (fine gravel fraction, indicator erratics), till matrix CaCO3content, till fabric, as well as orientation of striae on the top surfaces of large clasts. In parallel, datings of sub- and supra-till sediments using the TL method were carried out. The basal till at Babie Doły represents almost the entire Upper Weichselian, but it can be divided into subunits whose features indicate different ice flow directions and debris supply. The lower subunit developed as a result of the palaeo-ice stream along the main axis of the Baltic Sea (from the north), expanding to areas adjacent to the depression of the Gulf of Gdańsk. The upper subunit developed when the influence of the palaeo-ice stream in the study area decreased, the main role having been taken over by the ice flowing from the northwest. The till analysed also shows considerable lateral variation, indicative of the mosaic nature of subglacial sedimentation. We consider the diversity of permeability of deposits over which the ice sheet extended to be the prime factor that determined such a situation.

scholarly journals Upstream flow effects revealed in the EastGRIP ice core using a Monte Carlo inversion of a two-dimensional ice-flow model

2021 ◽  
Author(s):  
Tamara Annina Gerber ◽  
Christine Schøtt Hvidberg ◽  
Sune Olander Rasmussen ◽  
Steven Franke ◽  
Giulia Sinnl ◽  
...  
Keyword(s):  
Monte Carlo ◽  
Ice Core ◽  
Ice Sheet ◽  
Source Area ◽  
Model Parameters ◽  
Two Dimensional ◽  
Ice Flow ◽  
Ice Stream ◽  
Flow Effects ◽  
Upstream Flow

Abstract. The Northeast Greenland Ice Stream (NEGIS) is the largest active ice stream on the Greenland Ice Sheet (GrIS) and a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice core is drilled in the upstream part of the NEGIS, termed the East Greenland Ice-Core Project (EastGRIP). Upstream flow effects introduce non-climatic bias in ice cores and are particularly strong at EastGRIP due to high ice-flow velocities and the location inside an ice stream on the eastern flank of the GrIS. Understanding and ultimately correcting for such effects requires information on the source area and the local atmospheric conditions at the time of ice deposition. We use a two-dimensional Dansgaard–Johnsen model to simulate ice flow along three approximated flow lines between the summit of the ice sheet (GRIP) and EastGRIP. Model parameters are determined using a Monte Carlo inversion by minimizing the misfit between modelled isochrones and isochrones observed in radio-echo-sounding (RES) images. We calculate backward-in-time particle trajectories to determine the source area of ice found in the EastGRIP ice core and present estimates of surface elevation and past accumulation rates at the deposition site. Our results indicate that increased accumulation in the upstream area is predominantly responsible for the constant annual layer thicknesses observed in the upper part of the ice column at EastGRIP. Inverted model parameters suggest that the imprint of basal melting and sliding is present in large segments along the flow profiles and that most internal ice deformation happens in the lower half of the ice column. The results of this study act as a basis for applying upstream corrections to a variety of ice-core measurements, and the model parameters are useful constraints for more sophisticated modelling approaches in the future.

10Be exposure age constraints on the Late Weichselian ice-sheet geometry and dynamics in inter-ice-stream areas, western Svalbard

Boreas ◽  
2012 ◽  
Vol 42 (1) ◽  
pp. 43-56 ◽  
Author(s):  
Jon Y. Landvik ◽  
Edward J. Brook ◽  
Lyn Gualtieri ◽  
Henriette Linge ◽  
Grant Raisbeck ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Exposure Age ◽  
Ice Stream ◽  
Late Weichselian ◽  
Age Constraints

scholarly journals A confined–unconfined aquifer model for subglacial hydrology and its application to the North East Greenland Ice Stream

2017 ◽  
Author(s):  
Sebastian Beyer ◽  
Thomas Kleiner ◽  
Vadym Aizinger ◽  
Martin Rückamp ◽  
Angelika Humbert
Keyword(s):  
Computational Cost ◽  
Ice Sheet ◽  
Unconfined Aquifer ◽  
Ocean Modeling ◽  
Model Parameters ◽  
Efficient System ◽  
East Greenland ◽  
North East ◽  
The North ◽  
Ice Stream

Abstract. Subglacial hydrology plays an important role in the ice sheet dynamics as it determines the sliding velocity of ice sheets and also drives freshwater into the ocean. Modeling subglacial water has been a challenge for decades, and only recently new approaches have been developed such as representing subglacial channels and thin water sheets by separate layers of variable permeability. We extend this concept by modeling a confined and unconfined aquifer system (CUAS) in a single layer. The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We also performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms governing the opening and closure of channels. Furthermore, we applied the model to the North East Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500 km downstream from the ice divide. Using the effective pressure from the hydrology model in the Ice Sheet System Model (ISSM) shows considerable improvements of modeled velocities in the coastal region.

scholarly journals Upstream flow effects revealed in the EastGRIP ice-core using a Monte Carlo inversion of a 2D ice-flow model

2021 ◽  
Author(s):  
Tamara Annina Gerber ◽  
Christine Hvidberg ◽  
Aslak Grinsted ◽  
Daniela Jansen ◽  
Steven Franke ◽  
...  
Keyword(s):  
Ice Core ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Source Area ◽  
Model Parameters ◽  
Ice Flow ◽  
East Greenland ◽  
Ice Stream ◽  
Flow Effects ◽  
Upstream Flow

<p>The North East Greenland ice-stream (NEGIS) is the largest active ice-stream on the Greenland ice-sheet and is a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice-core is drilled in the upstream part of the NEGIS, termed the East Greenland ice-core project (EastGRIP). Upstream flow effects introduce non-climatic bias in ice-cores and are particularly strong at EastGRIP due to high ice-flow velocities and the location in an ice-stream on the eastern flank of the Greenland ice-sheet. Understanding and ultimately correcting for such effects requires information on the source area and the local atmospheric conditions at the time of ice deposition. We use a two-dimensional Dansgaard-Johnsen model to simulate ice-flow along three approximated flow-lines between the summit of the ice-sheet and EastGRIP. Model parameters are determined using a Monte Carlo inversion by minimizing the misfit between modeled isochrones and isochrones observed in radio-echo-sounding images. We calculate backward-in-time particle trajectories to determine the source area of ice found in the EastGRIP core today and present estimates of surface elevation and past accumulation-rates at the deposition site. The thinning function and accumulated strain obtained from the modeled velocity field provide useful information on the deformation history in the EastGRIP ice. Our results indicate that increased accumulation in the upstream area is predominantly responsible for the constant annual layer thickness observed in the upper part of the ice column at EastGRIP. Inverted model parameters suggest that the imprint of basal melting and sliding is present in large parts along the flow profiles and that most internal ice deformation happens close to the bedrock. The results of this study can act as a basis for applying upstream corrections to a variety of ice-core measurements, and the model parameters can be useful constraints for more sophisticated modeling approaches in the future. </p>

scholarly journals A confined–unconfined aquifer model for subglacial hydrology and its application to the Northeast Greenland Ice Stream

2018 ◽  
Vol 12 (12) ◽  
pp. 3931-3947 ◽  
Author(s):  
Sebastian Beyer ◽  
Thomas Kleiner ◽  
Vadym Aizinger ◽  
Martin Rückamp ◽  
Angelika Humbert
Keyword(s):  
Computational Cost ◽  
Ice Sheet ◽  
Coastal Region ◽  
Single Layer ◽  
Unconfined Aquifer ◽  
Model Parameters ◽  
Efficient System ◽  
Aquifer System ◽  
Ice Stream ◽  
Ice Sheet Dynamics

Abstract. Subglacial hydrology plays an important role in ice sheet dynamics as it determines the sliding velocity. It also drives freshwater into the ocean, leading to undercutting of calving fronts by plumes. Modeling subglacial water has been a challenge for decades. Only recently have new approaches been developed such as representing subglacial channels and thin water sheets by separate layers of variable hydraulic conductivity. We extend this concept by modeling a confined–unconfined aquifer system (CUAS) in a single layer of an equivalent porous medium (EPM). The advantage of this formulation is that it prevents unphysical values of pressure at reasonable computational cost. We performed sensitivity tests to investigate the effect of different model parameters. The strongest influence of model parameters was detected in terms of governing the opening and closure of the system. Furthermore, we applied the model to the Northeast Greenland Ice Stream, where an efficient system independent of seasonal input was identified about 500 km downstream from the ice divide. Using the effective pressure from the hydrology model, the Ice Sheet System Model (ISSM) showed considerable improvements in modeled velocities in the coastal region.

scholarly journals Differentiation of subglacial conditions on soft and hard bed settings and implications for ice sheet dynamics: a case study from north-central Poland

2020 ◽  
Vol 109 (8) ◽  
pp. 2699-2717
Author(s):  
Robert J. Sokołowski ◽  
Wojciech Wysota
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
North Central ◽  
Central Poland ◽  
Ice Stream ◽  
Basal Sliding ◽  
Ice Sheet Dynamics ◽  
Two Phases ◽  
Form Type ◽  
Basal Till

Abstract We reconstruct patterns of subglacial processes on a hard bedrock and a soft bed under the southern sector of Scandinavian Ice Sheet (SIS) occurring in the basal till of the Late Saalian Glaciation at the Wapienno, Barcin and Młodocin sites (north-central Poland). Based on detailed sedimentological studies, two phases of SIS transgression were recognised. In the initial phase of the transgression, the SIS advanced onto a frozen substrate (continuous permafrost). The low permeability of the substratum led to a high subglacial water pressure (SWP) and increased basal sliding. The local increase of SWP led to the development of different types of structures and sediments. On a hard bedrock, with low SWP, abrasion predominated and linear structures were developing, while in the case of high SWP, the ice was decoupled from the hard substrate, pressurised liquefied sediment flowed, and structures of the p-form and s-form type developed. On a soft bed, the ice-bed contact was of a mosaic type and the ice movement had an ice-stream character. The ice-stream developed towards the east in the marginal zone of the SIS and used a W-E oriented valley filled by the Wapienno Formation fluvial complex. During a later phase, the ice movement was slower and did not have a stream character. Its direction changed to SE. The deposition of the main part of the diamicton occurred mainly as a result of the lodgement process.

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