Ice Sheet Motion Research Articles

Tidally Modulated Glacial Slip and Tremor at Helheim Glacier, Greenland

AbstractNumerical modeling of ice sheet motion and hence projections of global sea level rise require information about the evolving subglacial environment, which unfortunately remains largely unknown due to its difficulty of access. Here we advance such subglacial observations by reporting multi‐year observations of seismic tremor likely associated with glacier sliding at Helheim Glacier. This association is confirmed by correlation analysis between tremor power and multiple environmental forcings on different timescales. Variations of the observed tremor power indicate that different factors affect glacial sliding on different timescales. Effective pressure may control glacial sliding on long (seasonal/annual) timescales, while tidal forcing modulates the sliding rate and tremor power on short (hourly/daily) timescales. Polarization results suggest that the tremor source comes from an upstream subglacial ridge. This observation provides insights on how different factors should be included in ice sheet modeling and how their timescales of variability play an essential role.

A parallel implementation of the confined–unconfined aquifer system model for subglacial hydrology: design, verification, and performance analysis (CUAS-MPI v0.1.0)

Abstract. The subglacial hydrological system affects (i) the motion of ice sheets through sliding, (ii) the location of lakes at the ice margin, and (iii) the ocean circulation by freshwater discharge directly at the grounding line or (iv) via rivers flowing over land. For modeling this hydrology system, a previously developed porous-media concept called the confined–unconfined aquifer system (CUAS) is used. To allow for realistic simulations at the ice sheet scale, we developed CUAS-MPI, an MPI-parallel C/C++ implementation of CUAS (MPI: Message Passing Interface), which employs the Portable, Extensible Toolkit for Scientific Computation (PETSc) infrastructure for handling grids and equation systems. We validate the accuracy of the numerical results by comparing them with a set of analytical solutions to the model equations, which involve two types of boundary conditions. We then investigate the scaling behavior of CUAS-MPI and show that CUAS-MPI scales up to 3840 MPI processes running a realistic Greenland setup on the Lichtenberg HPC system. Our measurements also show that CUAS-MPI reaches a throughput comparable to that of ice sheet simulations, e.g., the Ice-sheet and Sea-level System Model (ISSM). Lastly, we discuss opportunities for ice sheet modeling, explore future coupling possibilities of CUAS-MPI with other simulations, and consider throughput bottlenecks and limits of further scaling.

Complex motion of Greenland Ice Sheet outlet glaciers with basal temperate ice.

Uncertainty associated with ice sheet motion plagues sea level rise predictions. Much of this uncertainty arises from imperfect representations of physical processes including basal slip and internal ice deformation, with ice sheet models largely incapable of reproducing borehole-based observations. Here, we model isolated three-dimensional domains from fast-moving (Sermeq Kujalleq/Store Glacier) and slow-moving (Isunnguata Sermia) ice sheet settings in Greenland. By incorporating realistic geostatistically simulated topography, we show that a spatially highly variable layer of temperate ice (much softer ice at the pressure-melting point) forms naturally in both settings, alongside ice motion patterns which diverge substantially from those obtained using smoothly varying BedMachine topography. Temperate ice is vertically extensive (>100meters) in deep troughs but thins notably (<5meters) over bedrock highs, with basal slip rates reaching >90 or<5% of surface velocity dependent on topography and temperate layer thickness. Developing parameterizations of the net effect of this complex motion can improve the realism of predictive ice sheet models.

Changes in Antarctic Ice Sheet Motion Derived From Satellite Radar Interferometry Between 1995 and 2022

AbstractIce motion and boundaries are critical information for ice sheet models that project ice evolution in a warming climate. We present four historical, continent‐wide, maps of Antarctic‐wide ice motion and boundaries for the time period 1995–2022. The results reveal no change in the interior region of East Antarctica, iceberg detachments at ice shelf fronts, and widespread glacier speedup that propagates 100 km's inland in West Antarctica and the Antarctic Peninsula. Speedup affects the entire drainage of the Amundsen Sea Embayment sector; the entire west coast of the Antarctic Peninsula down to GeorgeVI Ice Shelf; the east coast down to Larsen C Ice Shelf; Getz Ice Shelf, Hull and Land glaciers in West Antarctica; Matusevitch, Ninnis, Mertz and Denman glaciers, glaciers in Porpoise and Vincennes Bay; and Robert, Wilma and Rayner glaciers in Enderby Land, in East Antarctica. We attribute the observed glacier changes to increased melting by warmer ocean waters.

Supraglacial River Forcing of Subglacial Water Storage and Diurnal Ice Sheet Motion

AbstractSurface melting impacts ice sheet sliding by supplying water to the bed, but subglacial processes driving ice accelerations are complex. We examine linkages between surface runoff, transient subglacial water storage, and short‐term ice motion from 168 consecutive hourly measurements of meltwater discharge (moulin input) and GPS‐derived ice surface motion for Rio Behar, a ∼60 km2 moulin‐terminating supraglacial river catchment on the southwest Greenland Ice Sheet. Short‐term accelerations in ice speed correlate strongly with lag‐corrected measures of supraglacial river discharge (r = 0.9, τ = 0.7, p &lt; 0.01). Though our 7 days record cannot address seasonal‐scale forcing, diurnal ice accelerations align with normalized differenced supraglacial and proglacial discharge, a proxy for subglacial storage change, better than GPS‐derived ice surface uplift. These observations counter theoretical steady state basal sliding laws and suggest that moulin and proglacially induced fluctuations in subglacial water storage, rather than absolute subglacial water storage, drive short‐term ice accelerations.

Basal roughness of the East Antarctic Ice Sheet in relation to flow speed and basal thermal state

AbstractBasal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.

Differential interferometric synthetic aperture radar for tide modelling in Antarctic ice-shelf grounding zones

Abstract. Differential interferometric synthetic aperture radar (DInSAR) is an essential tool for detecting ice-sheet motion near Antarctica's oceanic margin. These space-borne measurements have been used extensively in the past to map the location and retreat of ice-shelf grounding lines as an indicator for the onset of marine ice-sheet instability and to calculate the mass balance of ice sheets and individual catchments. The main difficulty in interpreting DInSAR is that images originate from a combination of several SAR images and do not indicate instantaneous ice deflection at the times of satellite data acquisitions. Here, we combine the sub-centimetre accuracy and spatial benefits of DInSAR with the temporal benefits of tide models to infer the spatio-temporal dynamics of ice–ocean interaction during the times of satellite overpasses. We demonstrate the potential of this synergy with TerraSAR-X data from the almost-stagnant southern McMurdo Ice Shelf (SMIS). We then validate our algorithm with GPS data from the fast-flowing Darwin Glacier, draining the Antarctic Plateau through the Transantarctic Mountains into the Ross Sea. We are able to reconstruct DInSAR-derived vertical displacements to 7 mm mean absolute residual error and generally improve traditional tide-model output by up to 39 % from 10.8 to 6.7 cm RMSE against GPS data from areas where ice is in local hydrostatic equilibrium with the ocean and by up to 74 % from 21.4 to 5.6 cm RMSE against GPS data in feature-rich coastal areas where tide models have not been applicable before. Numerical modelling then reveals Young's modulus of E=1.0±0.56 GPa and an ice viscosity of ν=10±3.65 TPa s when finite-element simulations of tidal flexure are matched to 16 d of tiltmeter data, supporting the hypothesis that strain-dependent anisotropy may significantly decrease effective viscosity compared to isotropic polycrystalline ice on large spatial scales. Applications of our method include the following: refining coarsely gridded tide models to resolve small-scale features at the spatial resolution and vertical accuracy of SAR imagery, separating elastic and viscoelastic contributions in the satellite-derived flexure measurement, and gaining information about large-scale ice heterogeneity in Antarctic ice-shelf grounding zones, the missing key to improving current ice-sheet flow models. The reconstruction of the individual components forming DInSAR images has the potential to become a standard remote-sensing method in polar tide modelling. Unlocking the algorithm's full potential to answer multi-disciplinary research questions is desired and demands collaboration within the scientific community.

Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained both in its bulk properties, for example geology, the presence of sediment, and the presence of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of 2 decades of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability in along-track topography – enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow and, second, from bed-echo scattering – enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth beds, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard- or soft-bed boundary constraint for ice-sheet models.

A constraint upon the basal water distribution and thermal state of the Greenland Ice Sheet from radar bed echoes

Abstract. There is widespread, but often indirect, evidence that a significant fraction of the bed beneath the Greenland Ice Sheet is thawed (at or above the pressure melting point for ice). This includes the beds of major outlet glaciers and their tributaries and a large area around the NorthGRIP borehole in the ice-sheet interior. The ice-sheet-scale distribution of basal water is, however, poorly constrained by existing observations. In principle, airborne radio-echo sounding (RES) enables the detection of basal water from bed-echo reflectivity, but unambiguous mapping is limited by uncertainty in signal attenuation within the ice. Here we introduce a new, RES diagnostic for basal water that is associated with wet–dry transitions in bed material: bed-echo reflectivity variability. This technique acts as a form of edge detector and is a sufficient, but not necessary, criteria for basal water. However, the technique has the advantage of being attenuation insensitive and suited to combined analysis of over a decade of Operation IceBridge survey data.The basal water predictions are compared with existing analyses of the basal thermal state (frozen and thawed beds) and geothermal heat flux. In addition to the outlet glaciers, we demonstrate widespread water storage in the northern and eastern interior. Notably, we observe a quasilinear corridor of basal water extending from NorthGRIP to Petermann Glacier that spatially correlates with elevated heat flux predicted by a recent magnetic model. Finally, with a general aim to stimulate regional- and process-specific investigations, the basal water predictions are compared with bed topography, subglacial flow paths and ice-sheet motion. The basal water distribution, and its relationship with the thermal state, provides a new constraint for numerical models.

Ocean Tide Influences on the Antarctic and Greenland Ice Sheets

AbstractOcean tides are the main source of high‐frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide‐induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite‐based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice‐ocean models and analytical glacier models that quantify the effect of ocean tides on lower‐frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice‐ocean interface, and higher‐resolution models with improved representation of tidal energy sinks.

Southernmost evidence of large European Ice Sheet-derived freshwater discharges during the Heinrich Stadials of the Last Glacial Period (Galician Interior Basin, Northwest Iberian Continental Margin)

Reconstruction of circum-Atlantic ice-sheet motion and instabilities is crucial to understanding the mechanisms that triggered and/or enhanced abrupt climate changes. Using enviromagnetic and geochemical data, we provide a continuous and well-dated record of the evolution of glacial/interglacial sedimentation on the Northwest Iberian Margin during the last glacial period, covering the last six Heinrich Stadials. The record shows European sediments that were related to meltwater pre-events during the initial stages of HS1, HS2, and HS4 that corroborate the Channel River depositional history. The record also includes IRD from the Laurentide Ice Sheet and the European Ice Sheet during the final stages of these stadials, i.e., Heinrich Events. Therefore, this study provides insight into one of the potential forcing mechanisms for Heinrich Events and, by inference, for Heinrich Stadials.

Study of Subsidence detection by DinSAR and evaluation of some factors to the outcome

Study of Subsidence detection by DinSAR and evaluation of some factors to the outcome

Greenland ice sheet motion insensitive to exceptional meltwater forcing

Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt-induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt-induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ~3.9 σ above the 1958-2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt-induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.

Micromorphological evidence of liquefaction, injection and sediment deposition during basal sliding of glaciers

The sliding of an ice mass over its bed represents one of the main mechanisms for the forward motion of glaciers and ice sheets, facilitated by the periodic introduction of meltwater along the ice-bed interface or regelation of the overriding ice. In the geological record, however, physical evidence of this process having occurred beneath former ice masses is apparently limited. This paper presents the results of a detailed micromorphological study of thinly stratified subglacial tills exposed at two sites: (i) Galmis in Switzerland and (ii) Plumpe Farm, near Gretna in SW Scotland. The stratification within these tills comprises alternating layers of massive to weakly foliated diamicton and variably deformed (folded, faulted) laminated silt and clay. Microtextural and microstructural evidence is interpreted in terms of repeated phases of basal sliding as the ice overrode a soft-sediment bed. Elevated meltwater contents/pressures encountered immediately prior to, and during basal sliding promoted localised liquefaction within the bed. Decoupling of the ice mass from its bed enabled the injection of the liquefied diamicton along the ice-bed interface and/or into the laminated sediments immediately adjacent to this boundary. The laminated silts and clays record the settling out of fines (clay, silt) from meltwater trapped along the ice-bed interface after an individual phase of basal sliding has ceased. Injection of till into the locally water saturated silts and clays resulted in partial liquefaction and incomplete mixing of these fine-grained sediments with the diamicton. Density contrasts between the two liquefied sediments led to the development of a complex ‘vinaigrette-like’ texture comprising rounded to irregular till pebbles within a matrix of variably homogenised silty clay. Recoupling of the ice with its bed led to localised folding and thrusting within the laminated silts and clays, hydrofracturing and injection of a network of sand-filled veins, and the imposition of a variably developed clast microfabric in the diamicton layers. Analysis of the clast microfabrics indicates that the intensity of these fabrics is highly variable reflecting the variation in the intensity of deformation imposed by the overriding ice.

Data assimilation and prognostic whole ice sheet modelling with the variationally derived, higher order, open source, and fully parallel ice sheet model VarGlaS

Abstract. We introduce a novel, higher order, finite element ice sheet model called VarGlaS (Variational Glacier Simulator), which is built on the finite element framework FEniCS. Contrary to standard procedure in ice sheet modelling, VarGlaS formulates ice sheet motion as the minimization of an energy functional, conferring advantages such as a consistent platform for making numerical approximations, a coherent relationship between motion and heat generation, and implicit boundary treatment. VarGlaS also solves the equations of enthalpy rather than temperature, avoiding the solution of a contact problem. Rather than include a lengthy model spin-up procedure, VarGlaS possesses an automated framework for model inversion. These capabilities are brought to bear on several benchmark problems in ice sheet modelling, as well as a 500 yr simulation of the Greenland ice sheet at high resolution. VarGlaS performs well in benchmarking experiments and, given a constant climate and a 100 yr relaxation period, predicts a mass evolution of the Greenland ice sheet that matches present-day observations of mass loss. VarGlaS predicts a thinning in the interior and thickening of the margins of the ice sheet.

Seasonal changes in ice sheet motion due to melt water lubrication

A numerical model is used to calculate how the motion of an idealized ice-sheet margin is affected by the subglacial drainage of melt water from its surface. The model describes the evolution of the drainage system and its coupling with ice flow through a sliding law that depends on the effective pressure. The results predict ice acceleration during early summer when the inefficient drainage system is temporarily overwhelmed. The growth of a more efficient drainage system leads to a subsequent slowdown of the ice very close to the margin, but high water pressure and ice velocity are maintained through much of the summer further inland. Annual mean ice velocity increases with the total quantity of melt water except close to the margin, where it is almost insensitive to the amount of melting. Short-term variability of melt water input leads to rapid changes in ice velocity that result in a slight increase in the mean velocity relative to a smoother input. Linked-cavity and poroelastic models for the distributed drainage system are compared, and their relative merits discussed. Two different sliding laws are considered, and the need for a holistic description of hydraulically controlled sliding is highlighted.

Short‐term variability in Greenland Ice Sheet motion forced by time‐varying meltwater drainage: Implications for the relationship between subglacial drainage system behavior and ice velocity

High resolution measurements of ice motion along a ∼120 km transect in a land‐terminating section of the GrIS reveal short‐term velocity variations (&lt;1 day), which are forced by rapid variations in meltwater input to the subglacial drainage system from the ice sheet surface. The seasonal changes in ice velocity at low elevations (&lt;1000 m) are dominated by events lasting from 1 day to 1 week, although daily cycles are largely absent at higher elevations, reflecting different patterns of meltwater input. Using a simple model of subglacial conduit behavior we show that the seasonal record of ice velocity can be understood in terms of a time‐varying water input to a channelized subglacial drainage system. Our investigation substantiates arguments that variability in theduration and rate, rather than absolute volume, of meltwater delivery to the subglacial drainage system are important controls on seasonal patterns of subglacial water pressure, and therefore ice velocity. We suggest that interpretations of hydro‐dynamic behavior in land‐terminating sections of the GrIS margin which rely on steady state drainage theories are unsuitable for making predictions about the effect of increased summer ablation on future rates of ice motion.

Ice flow in Greenland for the International Polar Year 2008–2009

A digital representation of ice surface velocity is essential for a variety of glaciological, geologic and geophysical analyses and modeling. Here, we present a new, reference, comprehensive, high‐resolution, digital mosaic of ice motion in Greenland assembled from satellite radar interferometry data acquired during the International Polar Year 2008 to 2009 by the Envisat Advanced Synthetic‐Aperture Radar (ASAR), the Advanced Land Observation System (ALOS)'s Phase‐Array L‐band SAR (PALSAR) and the RADARSAT‐1 SAR that covers 99% of the ice sheet in area. The best mapping performance is obtained using ALOS PALSAR data due to higher levels of temporal coherence at the L‐band frequency; but C‐band frequency SAR data are less affected by the ionosphere. The ice motion map reveals various flow regimes, ranging from patterned enhanced flow into a few large glaciers in the cold, low precipitation areas of north Greenland; to diffuse, enhanced flow into numerous, narrow, fast‐moving glaciers in the warmer, high precipitation sectors of northwest and southeast Greenland. We find that the 100 fastest glaciers (v&gt; 800 m/yr) drain 66% of the ice sheet in area, marine‐terminating glaciers drain 88% of Greenland, and basal‐sliding motion dominates internal deformation over more than 50% of the ice sheet. This view of ice sheet motion provides significant new constraints on ice flow modeling.

A numerical model for meltwater channel evolution in glaciers

Abstract. Meltwater channels form an integral part of the hydrological system of a glacier. Better understanding of how meltwater channels develop and evolve is required to fully comprehend supraglacial and englacial meltwater drainage. Incision of supraglacial stream channels and subsequent roof closure by ice deformation has been proposed in recent literature as a possible englacial conduit formation process. Field evidence for supraglacial stream incision has been found in Svalbard and Nepal. In Iceland, where volcanic activity provides meltwater with temperatures above 0 °C, rapid enlargement of supraglacial channels has been observed. Supraglacial channels provide meltwater through englacial passages to the subglacial hydrological systems of big ice sheets, which in turn affects ice sheet motion and their contribution to eustatic sea level change. By coupling, for the first time, a numerical ice dynamic model to a hydraulic model which includes heat transfer, we investigate the evolution of meltwater channels and their incision behaviour. We present results for different, constant meltwater fluxes, different channel slopes, different meltwater temperatures, different melt rate distributions in the channel as well as temporal variations in meltwater flux. The key parameters governing incision rate and depth are channel slope, meltwater temperature loss to the ice and meltwater flux. Channel width and geometry are controlled by melt rate distribution along the channel wall. Calculated Nusselt numbers suggest that turbulent mixing is the main heat transfer mechanism in the meltwater channels studied.

Quantifying ice sheet flow characteristics

Advances have been made in describing ice sheet motion, but in situ rheology (characteristics that affect the flow) of the ice has been hard to measure in the field.Gillet‐Chaulet et al. show that they can measure ice rheology and strain rates in situ using a phase‐sensitive radar. They used the technique on the Greenland ice sheet to quantify the rheology there. The researchers were able to achieve sufficient resolution to measure a flow phenomenon known as the Raymond effect, in which the ice sheet shows horizontal variations of the vertical strain rate pattern, sometimes creating anticlines in radar‐detected stratigraphic layers that are known as Raymond arches. This effect is due to a highly viscous plug of nearly stagnant ice under an ice ridge. The study is, the researchers believe, the first direct confirmation of the Raymond effect. Their results suggest that laboratory ice studies do not capture the full range of ice flow that exists in nature, so additional field studies are needed. (Geophysical Research Letters, doi:10.1029/2011GL049843, 2011)