Ice-stream Velocity Research Articles

Satellite and ground based estimates for ice surface velocities in the part of central Dronning Maud Land, East Antarctica: Implications for ice flux calculations

The ice sheet and glaciers of Antarctica and Greenland represent the largest sources of freshwater on planet Earth. The understanding and quantification of their dynamic properties such as albedo, precipitation, ice mass movement, and ice elevation changes are critical for the improved climate and mass balance models. The present study utilizes space-borne optical and synthetic aperture radar (SAR) imagery to measure the ice surface velocity at high spatial resolution for a part of the central Dronning Maud Land (cDML), East Antarctica. The datasets from Landsat-8 and Sentinel-1 SAR satellite are used for ice stream velocity estimation using feature-offset tracking and differential interferometric SAR (DInSAR) methods. The derived velocity products are validated with ground based stakes network at annual time scale. The fundamental ice flow laws are used to estimate the ice outflux or discharge for selected ice stream drainage basins of cDML at fluxgate locations. The ice stream basin has been delineated using combination of elevation, slope and continental scale velocity maps. The ice influx for study area is estimated using ECMWF fifth generation reanalysis (ERA5) and Regional Atmospheric Climate Model (RACMO) v2.3 model outputs. The estimated influx and outflux are in the ranges of 0.18–4.167 Gt/y and 0.201 to 1.278 Gt/y respectively, indicating net positive mass balance for the selected area.

Thermal controls on ice stream shear margins

AbstractIce stream discharge responds to a balance between gravity, basal friction and lateral drag. Appreciable viscous heating occurs in shear margins between ice streams and adjacent slow-moving ice ridges, altering the temperature-dependent viscosity distribution that connects lateral drag to marginal strain rates and ice stream velocity. Warmer ice deforms more easily and accommodates faster flow, whereas cold ice supplied from ice ridges drives advective cooling that counteracts viscous heating. Here, we present a two-dimensional (three velocity component), steady-state model designed to explore the thermal controls on ice stream shear margins. We validate our treatment through comparison with observed velocities for Bindschadler Ice Stream and verify that calculated temperatures are consistent with results from previous studies. Sweeping through a parameter range that encompasses conditions representative of ice streams in Antarctica, we show that modeled steady-state velocity has a modest response to different choices in forcing up until temperate zones develop in the shear margins. When temperate zones are present, velocity is much more sensitive to changes in forcing. We identify key scalings for the emergence of temperate conditions in our idealized treatment that can be used to identify where thermo-mechanical feedbacks influence the evolution of the ice sheet.

Slipping on till

Ice Sheets How do glaciers flow over the ground underlying them? We know that friction, ice stream velocity, and water pressure at the ice bed all matter, but we still do not know how to represent the process over both hard beds (which are solid rock) and soft ones (composed of unconsolidated erosion products called till). Zoet and Iverson present experimental results describing how glacial ice moves over watersaturated till (see the Perspective by Minchew and Joughin). These observations should help to solve the long-standing problem of constructing a generalized slip law that combines the processes of hard-bedded sliding and bed deformation. Science , this issue p. [76][1]; see also p. [29][2] [1]: /lookup/doi/10.1126/science.aaz1183 [2]: /lookup/doi/10.1126/science.abb3566

Evaluation of ice-stream model sensitivities for parameter estimation

Large-ensemble perturbed-parameter forward ice-flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice-flow parameters. Inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling. Because ice flow depends on many such parameters with their associated uncertainties, which may interact in nonlinear ways, full uncertainty assessment for parameter estimates is challenging. With rising computational power, it is increasingly practicable to explore co-dependencies and sensitivities. Here, we use a well-characterized higher-order flowline model configured for a well-lubricated (“shelfy”) ice stream to run large ensembles, perturbing the magnitude and spatial pattern of basal drag, basal topography, and input flux. We find for steady state that ice-stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end, but with greater local correlation during transients. The modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local. Perturbations of input ice fluxes interact with the others in interesting ways. These insights point to the value of inversions informed by concentrated observations during forced transients such as lake-drainage events, accumulation-rate fluctuations or ice-shelf losses, and to the care needed when interpreting local results of some inversions for basal-drag parameters.

Subglacial processes on an Antarctic ice stream bed. 2: Can modelled ice dynamics explain the morphology of mega-scale glacial lineations?

ABSTRACTMega-scale glacial lineations (MSGLs) are highly elongate subglacial bedforms associated with ice streaming. However, the link between MSGLs and rapid ice flow is largely qualitative, and there have been few attempts to quantitatively link their formation to ice flow characteristics (e.g. ice velocity, thickness, basal shear stress). We take measurements of MSGLs from a palaeo-ice stream that once occupied Marguerite Trough, Antarctic Peninsula and explore a range of possible correlations with ice dynamics generated from an ensemble of numerical modelling experiments that reproduce the deglaciation of the ice stream. Our results confirm that high mean ice velocities and a weak bed correlate with longer MSGLs. Furthermore, the height of MSGLs are low (2–3 m) where modelled basal shear stress is low, but their height tends to be higher and more variable where basal shear stress is larger. The mean density of MSGLs decreases as ice flux increases. Our analysis further suggests that the length of MSGLs is a function of basal ice velocity and time. Although our data/model correlations confirm the importance of ice velocity in MSGL formation, a significant challenge remains if we are to employ MSGLs as a quantifiable measure of past ice stream velocity.

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

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 modelling 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 tidally induced subglacial water pressure variations, transmitted through a highly conductive drainage system at low effective pressure. Furthermore, the basal sliding law requires a water pressure exponent that is strongly nonlinear with q = 10 and a nonlinear basal shear exponent of m = 3. Coupled model results show that sub-ice shelf tides result in a ∼12 % increase in mean horizontal velocity of the adjoining ice stream. 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.

Ice-shelf flexure and tidal forcing of Bindschadler Ice Stream, West Antarctica

Viscoelastic models of ice-shelf flexure and ice-stream velocity perturbations are combined into a single efficient flowline model to study tidal forcing of grounded ice. The magnitude and timing of ice-stream response to tidally driven changes in hydrostatic pressure and/or basal drag are found to depend significantly on bed rheology, with only a perfectly plastic bed allowing instantaneous velocity response at the grounding line. The model can reasonably reproduce GPS observations near the grounding zone of Bindschadler Ice Stream (formerly Ice Stream D) on semidiurnal time scales; however, other forcings such as tidally driven ice-shelf slope transverse to the flowline and flexurally driven till deformation must also be considered if diurnal motion is to be matched.

Modeling Antarctic tides in response to ice shelf thinning and retreat

Tides play an important role in ice sheet dynamics by modulating ice stream velocity, fracturing, and moving ice shelves and mixing water beneath them. Any changes in ice shelf extent or thickness will alter the tidal dynamics through modification of water column thickness and coastal topography but these will in turn feed back onto the overall ice shelf stability. Here, we show that removal or reduction in extent and/or thickness of the Ross and Ronne‐Filchner ice shelves would have a significant impact on the tides around Antarctica. The Ronne‐Filchner appears particularly vulnerable, with an increase in M2 amplitude of over 0.5 m beneath much of the ice shelf potentially leading to tidally induced feedbacks on ice shelf/sheet dynamics. These results highlight the importance of understanding tidal feedbacks on ice shelves/streams due to their influence on ice sheet dynamics.

The influence of subglacial hydrology on the flow of Kamb Ice Stream, West Antarctica

AbstractIce streams on the Siple Coast, West Antarctica, have a complex history of flow because their basal motion is governed by time‐varying basal conditions. Although the mechanical interaction between ice and till is well established, very little is known about the potential effect of regionally scaled water transport in a basal water system, which has only recently become apparent. To investigate the combined effect of hydrological and mechanical processes, we developed the Hydrology, Ice and Till model, in which ice flow is coupled to a Coulomb‐plastic till layer and a basal water system consisting of discrete conduits. When the model is applied to Kamb Ice Stream (KIS), results confirm that it is capable of oscillating between fast and stagnant modes of flow. We show that when subglacial conduits are disregarded or do not extend to the grounding line, the oscillatory behavior of the ice stream is governed by the basal thermal regime. When conduits extend to the grounding line, the modelled ice stream oscillation period is increased, peak speeds are reduced, and oscillations may ultimately cease if the volume of water supplied is sufficiently high. Three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line. Modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years, although varying according to hydrological activity. Our results indicate that KIS could reactivate this century, given its hydrological setting and ~170 years of stagnation.

21st-Century Evolution of Greenland Outlet Glacier Velocities

Earlier observations on several of Greenland's outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200+) of Greenland's major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf-terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below proposed upper bounds.

Links between ocean temperature and iceberg discharge during Heinrich events

The last glacial period was punctuated by several periods of massive iceberg discharge from the Northern Hemisphere ice sheets. Numerical simulations indicate that these discharge events are linked to an interplay between ice-sheet accumulation, marine ice-shelf stability and periodically oscillating surface ocean temperature. Palaeoclimate records have revealed the presence of millennial-scale climate oscillations throughout the last glacial period1. Six periods of extreme cooling in the Northern Hemisphere—known as Heinrich events—were marked by an enhanced discharge of icebergs into the North Atlantic Ocean2,3, increasing the deposition of ice-rafted debris2. Increased sliding at the base of ice sheets as a result of basal warming has been proposed to explain the iceberg pulses4,5,6, but recent observations7,8 suggest that iceberg discharge is related to a strong coupling between ice sheets, ice shelves and ocean conditions. Here we use a conceptual numerical model to simulate the effect of ocean temperature on ice-shelf width, as well as the impact of the resultant changes in ice-shelf geometry on ice-stream velocities. Our results demonstrate that ocean temperature oscillations affect the basal melting of the ice shelf and will generate periodic pulses of iceberg discharge in an ice sheet with a fringing shelf. We also find that the irregular occurrence of Heinrich events seen in the palaeoclimate records can be simulated by periodic ocean forcing combined with varying accumulation rates of the ice sheet. Our model simulations support a link between millennial-scale ocean temperature variability and Heinrich events during the last glacial period.

Spatiotemporal dynamics of ice streams due to a triple-valued sliding law

We show that a triple-valued sliding law can be heuristically motivated by the transverse spatial structure of an ice-stream velocity field using a simple one-dimensional model. We then demonstrate that such a sliding law can lead to some interesting stream-like patterns and time-oscillatory solutions. We find a generation of rapid stream-like solutions within a slow ice-sheet flow, separated by narrow internal boundary layers (shear margins), and analyse numerical simulations in two horizontal dimensions over a homogeneous bed and including longitudinal shear stresses. Different qualitative behaviours are obtained by changing a single physical parameter, a mass source magnitude, leading to changes from a slow creeping flow to a relaxation oscillation of the stream pattern, and to steady ice-stream-like solution. We show that the adjustment of the ice-flow shear margins to changes in the driving stress in the one-dimensional approximation is governed by a form of the Ginzburg–Landau equation and use stability analysis to understand this adjustment. In the model analysed here, the width scale of the stream is not set spontaneously by the ice flow dynamics, but rather, it is related to the mass source intensity and spatial distribution.

Subglacial bedforms reveal complex basal regime in a zone of paleo–ice stream convergence, Amundsen Sea embayment, West Antarctica

The flow of ice streams, which account for most discharge from large ice sheets, is controlled by processes operating at the ice stream bed. Data from modern ice stream beds are difficult to obtain, but where ice advanced onto continental shelves during glacial periods, extensive areas of the former bed can be imaged using modern swath sonar tools. We present new multibeam swath bathymetry data analyzed alongside sparse preexisting data from the Amundsen Sea embayment. The compilation is the most extensive, continuous area of multi-beam data coverage yet obtained on the inner continental shelf of Antarctica. The data reveal streamlined subglacial bedforms that define a zone of paleo–ice stream convergence, but, in contrast to previous models, do not show a simple downflow progression of bedform types along paleo–ice stream troughs. We interpret high spatial variability of bedforms as indicating a complex mechanical and hydrodynamic regime at the former ice stream beds, consistent with observations from some modern ice streams. We conclude that care must be taken when using bedforms to infer paleo–ice stream velocities.

Equilibrium dynamics of ice streams: a Bayesian statistical analysis

Studies of the behaviors of glaciers, ice sheets, and ice streams rely heavily on both observations and physical models. Data acquired via remote sensing provide critical information on geometry and movement of ice over large sections of Antarctica and Greenland. However, uncertainties are present in both the observations and the models. Hence, there is a need for combining these information sources in a fashion that incorporates uncertainty and quantifies its impact on conclusions. We present a hierarchical Bayesian approach to modeling ice-stream velocities incorporating physical models and observations regarding velocity, ice thickness, and surface elevation from the North East Ice Stream in Greenland. The Bayesian model leads to interesting issues in model assessment and computation.

Ice flow modulated by tides at up to annual periods at Rutford Ice Stream, West Antarctica

For over 2‐years we have collected GPS data ∼40 km upstream of the grounding line on Rutford Ice Stream, West Antarctica. Here we examine deviations from mean downstream flow. Although the record is incomplete during winter, there is clear modulation of flow at semi‐diurnal, diurnal, two‐weekly, semi‐annual, and annual ocean tidal frequencies. This is the first observation of ice stream flow variations over such a long time‐period, and such a wide range of frequencies. The ice stream flows fastest at equinoxes when there are two semi‐diurnal tides of equal magnitude, and slowest at solstices when one of the semi‐diurnal tides has lower amplitude. The sensitivity of the downstream flow is greatest to the long‐period forcing, which suggests that ice stream velocity may be affected by future changes in sea level. If so, this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge.

Ice stream D flow speed is strongly modulated by the tide beneath the Ross Ice Shelf

The flow velocity of ice stream D, West Antarctica has been measured to vary by a factor of three over the course of a day. These fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream D. The diurnal velocity fluctations appear to be driven by the tide beneath the Ross Ice Shelf. These results suggest that there is significant, and heretofore poorly understood, influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow.

Changes in west Antarctic ice stream velocities: Observation and analysis

We have produced a map of velocity covering much of the Siple Coast ice streams. The map confirms earlier estimates of deceleration on Whillans Ice Stream. Comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams. Force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams. We have used a theoretical analysis to examine the controls on fast flow. This analysis suggests that ice plains (very wide ice streams) are inherently unstable. This instability may be responsible for the current deceleration on the Ice Plain of Whillans Ice Stream and the shutdown of Ice Stream C 150 years ago. Thinning‐induced reductions in driving stress may also explain some of the observed deceleration, particularly in upstream areas. The active portions of Ice Stream C coincide well with the areas where we estimate that melt should be taking place. Current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain. Uncertainty remains about the basal conditions on Ice Stream D, while the basal resistance on Ice Stream E is large enough to ensure basal melting.

Basal mechanics of Ice Stream B, west Antarctica: 2. Undrained plastic bed model

Based on the results of our studies of the physical conditions beneath Ice Stream B, we formulate a new analytical ice stream model, the undrained plastic bed model (henceforth the UPB model). Mathematically, the UPB model is represented by a non‐linear system of four coupled equations which express the relationships among ice sliding velocity, till strength, water storage in till, and basal melt rate. We examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant (less than hundreds of years). Temporal variability is introduced into the UPB model only by the direct dependence of till void ratio changes (ė = ∂e/∂t) on the basal melting rate mr. Since till strength τb{e} and ice stream velocity Ub{τb} change as long as till void ratio varies, the first condition for ice stream stability is that of constant till water storage ė = 0. The second condition for ice stream stability arises from the feedback between ice stream velocity, till strength, and the basal melting rate which depends on shear heating mr{ Ub τb}. This is the “weak till” condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress τb < (n + 1)−1 τd. The salient feature of the UPB model is its ability to produce two thermo mechanically controlled equilibrium states, one with a strong bed and slow ice velocities (“ice sheet” mode) and one with a weak bed and fast ice velocities (“ice‐stream” mode). This bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in West Antarctica. Basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes. The UPB model demonstrates that ice streams may be prone to thermally triggered instabilities, during which small perturbations in the basal thermal energy balance grow, leading to generation or elimination of the basal conditions which cause ice streaming.

A continuum mixture model of ice stream thermomechanics in the Laurentide Ice Sheet 2. Application to the Hudson Strait Ice Stream

Episodic exportations of ice‐rafted debris to the North Atlantic in the late Pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the Laurentide Ice Sheet. Much of this efflux of ice may originate from an ice stream issuing from Hudson Strait and tapping into core regions of the Laurentide in Hudson Bay, Labrador, and the Foxe Basin. Applying the continuum mixture theory outlined by Marshall and Clarke [this issue], we model the thermomechanical evolution of the Hudson Strait Ice Stream in a three‐dimensional finite difference model of the Laurentide Ice Sheet. Our simulations focus on internal dynamics of the ice stream. Under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed. Modeled surge durations vary from 105 to 3260 years, while surge periodicities range from 585 to 22,410 years. With pervasively warm or cold internal temperature distributions in the ice, ice streams can also establish modes of permanent activity or inactivity under thermal regulation. Our most vigorous ice streams produce peak values of approximately 0.03 Sv of freshwater flux to the North Atlantic from basal meltwater and iceberg production. Associated ice stream velocities in this maximum case approach 6700 m yr−1. The total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0.04m in the most tranquil surge and almost 0.6m in the most extreme case. These velocities and sea level impacts axe an order of magnitude less than those predicted by MacAyeal [1993a,b], and only our most exuberant streams approach the iceberg flux estimates of Dowdeswell et al. [1995]. We propose that the sediment load of icebergs emanating from Hudson Strait in a surge event may exceed expectations from contemporary icebergs.

The marginal shear stress of Ice Stream B, West Antarctica

AbstractTo ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis ofc-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994) and the corresponding strain-rate enhancement factors differ greatly (E≈ 1–2 vs 10–12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ∼1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.