Isochrones and isotherms beneath migrating ice divides

Trilateration and single line surveys have been made to about 900 km inland of Casey, Wilkes Land, to measure surface elevation, ice thickness, horizontal velocity, and other parameters. On the large scale the velocity U increases smoothly from 8 m a−1, 800 km inland, to 280 m a−1 inland of the fast outlet streams. This increase in velocity is associated with a corresponding increase in the large-scale smoothed (over about 30 ice thicknesses) basal shear stress τb from 0.4 to 1.5 bar. The mean shear strain-rate through the ice sheet U/Z = kτb4 , where Z is the ice thickness (range 4 500 to 1 700 m). At scales of one to several ice thicknesses large variations occur in surface slope and ice thickness without proportionally large velocity variations, because of the effect of the longitudinal stress. Detailed measurements made over a 30 km section indicated that the surface longitudinal strain-rate gradient varied from -1.7 to +1.3×l0−6 a−1 m−1 along with variations in surface slope of from -3.5 to +1.5%. A multilayer model, based on the solution of the biharmonic equation for the stream function, was used in a study of the ice flow associated with these surface undulations. Given the bedrock topography and large-scale flow parameters, the model closely predicted the measured surface profile when the variation of the surface accumulation rate over an undulation was also considered.

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Is there 1.5 million-year old ice near Dome C, Antarctica?

10.5194/tc-2017-69 ◽

2017 ◽

Cited By ~ 2

Author(s):

Frédéric Parrenin ◽

Marie G. P. Cavitte ◽

Donald D. Blankenship ◽

Jérome Chappellaz ◽

Hubertus Fischer ◽

...

Keyword(s):

Accumulation Rate ◽

Ice Core ◽

Ice Sheets ◽

Atmospheric Composition ◽

Regional Environment ◽

Ice Flow ◽

True Nature ◽

Flow Modelling ◽

North East ◽

Geothermal Flux

Abstract. Ice sheets provide exceptional archives of past changes in polar climate, regional environment and global atmospheric composition. The oldest dated deep ice core drilled in Antarctica has been retrieved at EPICA Dome C (EDC), reaching ~ 800,000 years. Obtaining an older paleoclimatic record from Antarctica is one of the greatest challenges of the ice core community. Here, we use internal isochrones, identified from airborne radar coupled to ice-flow modelling to estimate the age of basal ice along transects in the Dome C area. Three glaciological properties are inverted from isochrones: surface accumulation rate; geothermal flux; and the exponent of the Lliboutry velocity profile. We find that old ice (> 1 Myr, 1 million years) likely exists in two regions: one ~ 40 km south-west of Dome C along the ice divide to Vostok, close to a secondary dome that we name "Little Dome C" (LDC); and a second region named "North Patch" (NP) located 10–30 km north-east of Dome C, in a region where the geothermal flux is apparently relatively low. Our work demonstrates the value of combining radar observations with ice flow modelling to accurately represent the true nature of ice flow, and the formation of ice-sheet architecture, in the centre of large ice sheets.

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Large-Scale Numerical Modelling of the Antarctic Ice Sheet

Annals of Glaciology ◽

10.1017/s0260305500002500 ◽

1982 ◽

Vol 3 ◽

pp. 42-49 ◽

Cited By ~ 54

Author(s):

W.F. Budd ◽

I.N. Smith

Keyword(s):

Sea Level ◽

Large Scale ◽

Accumulation Rate ◽

Ice Sheet ◽

Ice Thickness ◽

Ice Flow ◽

Antarctic Ice Sheet ◽

Flow Parameters ◽

The Antarctic ◽

Thinning Rate

A large-scale dynamic numerical model of the Antarctic ice sheet has been developed to study its present state of ice flow and mass balance as well as its response to long-term changes of climate or sea-level.The flow of ice over a two-dimensional grid is determined from the ice thickness, the basal shear stress, the bedrock depth, and ice flow parameters derived from velocities of existing ice sheets. The change in ice thickness with time is governed by the continuity equation involving the ice flux divergence and the ice accumulation or ablation. At the ice sheet seaward boundary, a floating criterion and floating ice thinning rate apply. Bedrock depression with a time-delayed response dependent on the history of the ice load is also included.A 61 × 61 point grid with 100 km spacing has been used to represent the ice-sheet surface, bedrock, and accumulation rate. The model has been used to simul a te the growth of the present ice sheet and i ts reaction to changes of sea-level, bedrock depression, accumulation rate, ice flow parameters, and the iceshelf thinning rate.Preliminary results suggest that the present ice sheet is not in equilibrium but rather is still adjusting to changes of these parameters.

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Is there 1.5-million-year-old ice near Dome C, Antarctica?

The Cryosphere ◽

10.5194/tc-11-2427-2017 ◽

2017 ◽

Vol 11 (6) ◽

pp. 2427-2437 ◽

Cited By ~ 16

Author(s):

Frédéric Parrenin ◽

Marie G. P. Cavitte ◽

Donald D. Blankenship ◽

Jérôme Chappellaz ◽

Hubertus Fischer ◽

...

Keyword(s):

Accumulation Rate ◽

Ice Core ◽

Ice Sheets ◽

Atmospheric Composition ◽

Regional Environment ◽

Ice Flow ◽

True Nature ◽

Flow Modelling ◽

North East ◽

Geothermal Flux

Abstract. Ice sheets provide exceptional archives of past changes in polar climate, regional environment and global atmospheric composition. The oldest dated deep ice core drilled in Antarctica has been retrieved at EPICA Dome C (EDC), reaching ∼ 800 000 years. Obtaining an older paleoclimatic record from Antarctica is one of the greatest challenges of the ice core community. Here, we use internal isochrones, identified from airborne radar coupled to ice-flow modelling to estimate the age of basal ice along transects in the Dome C area. Three glaciological properties are inferred from isochrones: surface accumulation rate, geothermal flux and the exponent of the Lliboutry velocity profile. We find that old ice (> 1.5 Myr, 1.5 million years) likely exists in two regions: one ∼ 40 km south-west of Dome C along the ice divide to Vostok, close to a secondary dome that we name Little Dome C (LDC), and a second region named North Patch (NP) located 10–30 km north-east of Dome C, in a region where the geothermal flux is apparently relatively low. Our work demonstrates the value of combining radar observations with ice flow modelling to accurately represent the true nature of ice flow, and understand the formation of ice-sheet architecture, in the centre of large ice sheets.

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Large-Scale Numerical Modelling of the Antarctic Ice Sheet

Annals of Glaciology ◽

10.3189/s0260305500002500 ◽

1982 ◽

Vol 3 ◽

pp. 42-49 ◽

Cited By ~ 4

Author(s):

W.F. Budd ◽

I.N. Smith

Keyword(s):

Sea Level ◽

Large Scale ◽

Accumulation Rate ◽

Ice Sheet ◽

Ice Thickness ◽

Ice Flow ◽

Antarctic Ice Sheet ◽

Flow Parameters ◽

The Antarctic ◽

Thinning Rate

A large-scale dynamic numerical model of the Antarctic ice sheet has been developed to study its present state of ice flow and mass balance as well as its response to long-term changes of climate or sea-level.The flow of ice over a two-dimensional grid is determined from the ice thickness, the basal shear stress, the bedrock depth, and ice flow parameters derived from velocities of existing ice sheets. The change in ice thickness with time is governed by the continuity equation involving the ice flux divergence and the ice accumulation or ablation. At the ice sheet seaward boundary, a floating criterion and floating ice thinning rate apply. Bedrock depression with a time-delayed response dependent on the history of the ice load is also included.A 61 × 61 point grid with 100 km spacing has been used to represent the ice-sheet surface, bedrock, and accumulation rate. The model has been used to simul a te the growth of the present ice sheet and i ts reaction to changes of sea-level, bedrock depression, accumulation rate, ice flow parameters, and the iceshelf thinning rate.Preliminary results suggest that the present ice sheet is not in equilibrium but rather is still adjusting to changes of these parameters.

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Ice Flow Along AN Iagp Flow Line, Wilkes Land, Antarctica (Abstract only)

Annals of Glaciology ◽

10.3189/s0260305500003189 ◽

1982 ◽

Vol 3 ◽

pp. 346-346

Author(s):

N.W. Young ◽

D. SheehY ◽

T. Hamley

Keyword(s):

Strain Rate ◽

Large Scale ◽

Flow Line ◽

Accumulation Rate ◽

Surface Profile ◽

Ice Thickness ◽

Shear Strain Rate ◽

Surface Slope ◽

Ice Flow ◽

Wilkes Land

Trilateration and single line surveys have been made to about 900 km inland of Casey, Wilkes Land, to measure surface elevation, ice thickness, horizontal velocity, and other parameters. On the large scale the velocity U increases smoothly from 8 m a−1, 800 km inland, to 280 m a−1 inland of the fast outlet streams. This increase in velocity is associated with a corresponding increase in the large-scale smoothed (over about 30 ice thicknesses) basal shear stress τb from 0.4 to 1.5 bar. The mean shear strain-rate through the ice sheet U/Z = kτb4, where Z is the ice thickness (range 4 500 to 1 700 m).At scales of one to several ice thicknesses large variations occur in surface slope and ice thickness without proportionally large velocity variations, because of the effect of the longitudinal stress. Detailed measurements made over a 30 km section indicated that the surface longitudinal strain-rate gradient varied from -1.7 to +1.3×l0−6 a−1 m−1 along with variations in surface slope of from -3.5 to +1.5%.A multilayer model, based on the solution of the biharmonic equation for the stream function, was used in a study of the ice flow associated with these surface undulations. Given the bedrock topography and large-scale flow parameters, the model closely predicted the measured surface profile when the variation of the surface accumulation rate over an undulation was also considered.

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Dilation of subglacial sediment governs incipient surge motion in glaciers with deformable beds

Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rspa.2020.0033 ◽

2020 ◽

Vol 476 (2238) ◽

pp. 20200033 ◽

Cited By ~ 1

Author(s):

B. M. Minchew ◽

C. R. Meyer

Keyword(s):

Pore Water ◽

Flow Model ◽

Pore Water Pressure ◽

Water Pressure ◽

Slip Rate ◽

Effective Pressure ◽

Ice Flow ◽

History Of ◽

Glacier Surges ◽

Geographical Clustering

Glacier surges are quasi-periodic episodes of rapid ice flow that arise from increases in slip rate at the ice–bed interface. The mechanisms that trigger and sustain surges are not well understood. Here, we develop a new model of incipient surge motion for glaciers underlain by sediments to explore how surges may arise from slip instabilities within a thin layer of saturated, deforming subglacial till. Our model represents the evolution of internal friction, porosity and pore water pressure within the till as functions of the rate and history of shear deformation, and couples the till mechanics to a simple ice-flow model. Changes in pore water pressure govern incipient surge motion, with less permeable till facilitating surging because dilation-driven reductions in pore water pressure slow the rate at which till tends towards a new steady state, thereby allowing time for the glacier to thin dynamically. The reduction of overburden (and thus effective) pressure at the bed caused by dynamic thinning of the glacier sustains surge acceleration in our model. The need for changes in both the hydromechanical properties of the till and the thickness of the glacier creates restrictive conditions for surge motion that are consistent with the rarity of surge-type glaciers and their geographical clustering.

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Ice thickness estimates of Lemon Creek Glacier, Alaska, from active-source seismic imaging

Journal of Glaciology ◽

10.1017/jog.2021.32 ◽

2021 ◽

pp. 1-9

Author(s):

Stephen A. Veitch ◽

Marianne Karplus ◽

Galen Kaip ◽

Lucia F. Gonzalez ◽

Jason M. Amundson ◽

...

Keyword(s):

Ice Thickness ◽

Equilibrium Line Altitude ◽

Seismic Line ◽

Ice Flow ◽

Southeast Alaska ◽

Velocity Modeling ◽

Active Source ◽

Valley Glacier ◽

Subglacial Topography ◽

The Impact

Abstract Lemon Creek Glacier, a temperate valley glacier in the Juneau Icefield of Southeast Alaska, is the site of long running (>60 years) glaciological studies. However, the most recent published estimates of its thickness and subglacial topography come from two ~50 years old sources that are not in agreement and do not account for the effects of years of negative mass balance. We collected a 1-km long active-source seismic line on the upper section of the glacier parallel and near to the centerline of the glacier, roughly straddling the equilibrium-line altitude. We used these data to perform joint reflection-refraction velocity modeling and reflection imaging of the glacier bed. We find that this upper section of Lemon Creek Glacier is as much as 150 m (~65%) thicker than previously suggested with a large overdeepening in an area previously believed to have a uniform thickness. Our results lead us to reinterpret the impact of basal motion on ice flow and have a significant impact on expectations of subglacial hydrology. We suggest that further efforts to develop a whole-glacier model of subglacial topography are necessary to support studies that require accurate models of ice thickness and subglacial topography.

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The Age-Depth Profile in the Upper Part of a Steady-State Ice Sheet

Journal of Glaciology ◽

10.3189/s0022143000009345 ◽

1989 ◽

Vol 35 (121) ◽

pp. 406-417 ◽

Cited By ~ 1

Author(s):

Niels Reeh

Keyword(s):

Steady State ◽

Layer Thickness ◽

Flow Line ◽

Numerical Models ◽

Accumulation Rate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ice Thickness ◽

Western Slope ◽

Simple Models

AbstractSimple analytical models are developed in order to study how up-stream variations in accumulation rate and ice thickness, and horizontal convergence/ divergence of the flow influence the age and annual layer-thickness profiles in a steady-state ice sheet. Generally, a decrease/increase of the accumulation rate and an increase/decrease of the ice thickness in the up-stream direction (i.e. opposite to the flow direction) results in older/younger ice at a given depth in the ice sheet than would result if the up-stream accumulation rate and ice thickness were constant along the flow line.Convergence/divergence of the up-stream flow will decrease/increase the effect of the accumulation-rate and ice-thickness gradients, whereas convergence/divergence has no influence at all on the age and layer-thickness profiles if the up-stream accumulation rate and ice thickness are constant along the flow line.A modified column-flow model, i.e. a model for which the strain-rate profile (or, equivalently, the horizontal velocity profile) is constant down to the depth corresponding to the Holocene/Wisconsinan transition 10 750 year BP., seems to work well for dating the ice back to 10 000–11 000 year B P. at sites in the slope regions of the Greenland ice sheet. For example, the model predicts the experimentally determined age profile at Dye 3 on the south Greenland ice sheet with a relative root-mean-square error of only 3% back to c. 10 700 year B.P. As illustrated by the Milcent location on the western slope of the central Greenland ice sheet, neglecting up-stream accumulation-rate and ice-thickness gradients, may lead to dating errors as large as 3000–000 years for c. 10 000 year old ice.However, even if these gradients are taken into account, the simple model fails to give acceptable ages for 10 000 year old ice at locations on slightly sloping ice ridges with strongly divergent flow, as for example the Camp Century location. The main reason for this failure is that the site of origin of the ice cannot be determined accurately enough by the simple models, if the flow is strongly divergent.With this exception, the simple models are well suited for dating the ice at locations where the available data or the required accuracy do not justify application of elaborate numerical models. The formulae derived for the age-depth profiles can easily be worked out on a pocket calculator, and in many cases will be a sensible alternative to using numerical flow models.

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Hydrological changes in the Rzecin peatland (Puszcza Notecka, Poland) induced by anthropogenic factors: Implications for mire development and carbon sequestration

The Holocene ◽

10.1177/0959683616670468 ◽

2016 ◽

Vol 27 (5) ◽

pp. 651-664 ◽

Cited By ~ 11

Author(s):

Krystyna Milecka ◽

Grzegorz Kowalewski ◽

Barbara Fiałkiewicz-Kozieł ◽

Mariusz Gałka ◽

Mariusz Lamentowicz ◽

...

Keyword(s):

Carbon Sequestration ◽

Plant Communities ◽

Human Activity ◽

Accumulation Rate ◽

Carbon Accumulation ◽

Anthropogenic Factors ◽

Poor Fen ◽

History Of ◽

Carbon Accumulation Rates

Wetlands are very vulnerable ecosystems and sensitive to changes in the ground water table. For the last few thousand years, hydrological balance has also been influenced by human activity. To improve their cropping features, drainage activity and fertilizing were applied. The drainage process led to an abrupt change of environment, the replacement of plant communities and the entire ecosystem. The problem of carbon sequestration is very important nowadays. A higher accumulation rate is related to higher carbon accumulation, but the intensity of carbon sequestration depends on the type of mire, habitat, and climatic zone. The main aim of this article was an examination of the changes in poor-fen ecosystem during the last 200 years in relation to natural and anthropogenic factors, using paleoecological methods (pollen and macrofossils). The second aim was a detailed investigation of the sedimentary record to aid our understanding of carbon sequestration in the poor fen of temperate zone. This case study shows that fens in temperate zones, in comparison with boreal ones, show higher carbon accumulation rates which have been especially intensive over the last few decades. To reconstruct vegetation changes, detailed palynological and macrofossil analyses were done. A 200-year history of the mire revealed that it was influenced by human activity to much degree. However, despite the nearby settlement and building of the drainage ditch, the precious species and plant communities still occur.

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