Ice Shelf Grounding Zone Research Articles

Direct observations of melting, freezing, and ocean circulation in an ice shelf basal crevasse.

Ocean conditions near the grounding zones of Antarctica's ice shelves play a key role in controlling the outflow and mass balance of the ice sheet. However, ocean observations in these regions are largely absent. Here, we present a detailed spatial survey collected with an underwater vehicle in a basal crevasse located in the ocean cavity at the Ross Ice Shelf grounding zone. The observations depict fine-scale variability in ocean forcing that drives asymmetric melting along the lower crevasse sidewalls and freezing in the upper reaches of the crevasse. Freshwater release from melting at depth and salt rejection from freezing above drives an overturning circulation. This vertical circulation pattern overlays a dominant throughflow jet, which funnels water parallel to the coastline, orthogonal to the direction of tidal currents. Importantly, these data reveal that basal crevasses influence ocean circulation and mixing at ice shelf grounding zones to an extent previously unknown.

Repeated Tidally Induced Hydrofracture of a Supraglacial Lake at the Amery Ice Shelf Grounding Zone

AbstractSurface melting and lakes are common to Antarctic ice shelves, and their existence and drainages have been invoked as a precursor for ice shelf collapse. Here, we present satellite observations over 2014–2020 of repeated, rapid drainages of a supraglacial lake at the grounding zone of Amery Ice Shelf, East Antarctica. Post‐drainage imagery in 2018 reveals lake bottom features characteristic of rapid, vertical lake drainage. Observed lake volumes indicate drainages are not associated with a threshold meltwater volume. Instead, drainages typically coincide with periods of high daily tidal amplitude, suggesting hydrofracture is assisted by tidally forced ice flexure inherent to the ice shelf grounding zone. Combined with observations of widespread grounding zone lake drainages on Amery, these findings indicate ice shelf meltwater accumulation may be inhibited by grounding zone drainage events, thus representing a potential stabilizing mechanism despite enhanced melting common to these regions.

Tidal Pressurization of the Ocean Cavity Near an Antarctic Ice Shelf Grounding Line

AbstractMass loss from the Antarctic ice sheet is sensitive to conditions in ice shelf grounding zones, the transition between grounded and floating ice. To observe tidal dynamics in the grounding zone, we moored an ocean pressure sensor to Ross Ice Shelf, recording data for 54 days. In this region the ice shelf is brought out of hydrostatic equilibrium by the flexural rigidity of ice, yet we found that tidal pressure variations at a constant geopotential surface were similar within and outside of the grounding zone. This implies that the grounding zone ocean cavity was overpressurized at high tide and underpressurized at low tide by up to 10 kPa with respect to glaciostatic pressure at the ice shelf base. Phase lags between ocean pressure and vertical ice shelf motion were tens of minutes for diurnal and semidiurnal tides, an effect that has not been incorporated into ocean models of tidal currents below ice shelves. These tidal pressure variations may affect the production and export of meltwater in the subglacial environment and may increase basal crevasse heights in the grounding zone by several meters, according to linear elastic fracture mechanics. We find anomalously high tidal energy loss at the K1 constituent in the grounding zone and hypothesize that this could be explained by seawater injection into the subglacial environment at high tide or internal tide generation through interactions with topography. These observations lay the foundation for improved representation of the grounding zone and its tidal dynamics in ocean circulation models of sub–ice shelf cavities.

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.

Ocean Stratification and Low Melt Rates at the Ross Ice Shelf Grounding Zone

AbstractOcean‐driven melting of ice shelves is a primary mechanism for ice loss from Antarctica. However, due to the difficulty in accessing the sub‐ice shelf ocean cavity, the relationship between ice shelf melting and ocean conditions is poorly understood, particularly near the grounding zone, where the ice transitions from grounded to floating. We present the first borehole oceanographic observations from the grounding zone of the Ross Ice Shelf, Antarctica's largest ice shelf by area. Contrary to predictions that tidal currents near grounding zones mix the water column, we found that Ross Ice Shelf waters were vertically stratified. Current velocities at middepth in the ocean cavity did not change significantly over measurement periods at two different parts of the tidal cycle. The observed stratification resulted in low melt rates near this portion of the grounding zone, inferred from phase‐sensitive radar observations. These melt rates were generally <10 cm/year, which is lower than average for the Ross Ice Shelf (~20 cm/year). Melt rates may be higher at portions of the grounding zone that experience higher subglacial discharge or stronger tidal mixing. Stratification in the cavity at the borehole site was prone to diffusive convection as a result of ice shelf melting. Since diffusive convection influences vertical heat and salt fluxes differently than shear‐driven turbulence, this process may affect ice shelf melting and merits further consideration in ocean models of sub‐ice shelf circulation.

Glacier change along West Antarctica's Marie Byrd Land Sector and links to inter-decadal atmosphere–ocean variability

Abstract. Over the past 20 years satellite remote sensing has captured significant downwasting of glaciers that drain the West Antarctic Ice Sheet into the ocean, particularly across the Amundsen Sea Sector. Along the neighbouring Marie Byrd Land Sector, situated west of Thwaites Glacier to Ross Ice Shelf, glaciological change has been only sparsely monitored. Here, we use optical satellite imagery to track grounding-line migration along the Marie Byrd Land Sector between 2003 and 2015, and compare observed changes with ICESat and CryoSat-2-derived surface elevation and thickness change records. During the observational period, 33 % of the grounding line underwent retreat, with no significant advance recorded over the remainder of the ∼ 2200 km long coastline. The greatest retreat rates were observed along the 650 km-long Getz Ice Shelf, further west of which only minor retreat occurred. The relative glaciological stability west of Getz Ice Shelf can be attributed to a divergence of the Antarctic Circumpolar Current from the continental-shelf break at 135∘ W, coincident with a transition in the morphology of the continental shelf. Along Getz Ice Shelf, grounding-line retreat reduced by 68 % during the CryoSat-2 era relative to earlier observations. Climate reanalysis data imply that wind-driven upwelling of Circumpolar Deep Water would have been reduced during this later period, suggesting that the observed slowdown was a response to reduced oceanic forcing. However, lack of comprehensive oceanographic and bathymetric information proximal to Getz Ice Shelf's grounding zone make it difficult to assess the role of intrinsic glacier dynamics, or more complex ice-sheet–ocean interactions, in moderating this slowdown. Collectively, our findings underscore the importance of spatial and inter-decadal variability in atmosphere and ocean interactions in moderating glaciological change around Antarctica.

Unraveling InSAR Observed Antarctic Ice-Shelf Flexure Using 2-D Elastic and Viscoelastic Modeling

Ice-shelf grounding zones link the Antarctic ice-sheets to the ocean. Differential interferometric synthetic aperture radar (DInSAR) is commonly used to monitor grounding-line locations, but also contains information on grounding-zone ice thickness, ice properties and tidal conditions beneath the ice shelf. Here, we combine in-situ data with numerical modelling of ice-shelf flexure to investigate 2-D controls on the tidal bending pattern on the Southern McMurdo Ice Shelf. We validate our results with 9 double-differential TerraSAR-X interferograms. It is necessary to make adjustments to the tidal forcing to directly compare observations with model output and we find that when these adjustments are small (< 1.5 cm) a viscoelastic model matches better, while an elastic model is more robust overall. Within landward embayments, where lateral stresses from surrounding protrusions damp the flexural response, a 2-D model captures behaviour that is missed in simple 1-D models. We conclude that improvements in current tide models are required to allow for the full exploitation of DInSAR in grounding-zone glaciology.

Climate and surface mass balance of coastal West Antarctica resolved by regional climate modelling

ABSTRACTWest Antarctic climate and surface mass balance (SMB) records are sparse. To fill this gap, regional atmospheric climate modelling is useful, providing that such models are employed at sufficiently high horizontal resolution and coupled with a snow model. Here we present the results of a high-resolution (5.5 km) regional atmospheric climate model (RACMO2) simulation of coastal West Antarctica for the period 1979–2015. We evaluate the results with available in situ weather observations, remote-sensing estimates of surface melt, and SMB estimates derived from radar and firn cores. Moreover, results are compared with those from a lower-resolution version, to assess the added value of the resolution. The high-resolution model resolves small-scale climate variability invoked by topography, such as the relatively warm conditions over ice-shelf grounding zones, and local wind speed accelerations. Surface melt and SMB are well reproduced by RACMO2. This dataset will prove useful for picking ice core locations, converting elevation changes to mass changes, for driving ocean, ice-sheet and coupled models, and for attributing changes in the West Antarctic Ice Sheet and shelves to changes in atmospheric forcing.

Channelized Melting Drives Thinning Under a Rapidly Melting Antarctic Ice Shelf

AbstractIce shelves play a vital role in regulating loss of grounded ice and in supplying freshwater to coastal seas. However, melt variability within ice shelves is poorly constrained and may be instrumental in driving ice shelf imbalance and collapse. High‐resolution altimetry measurements from 2010 to 2016 show that Dotson Ice Shelf (DIS), West Antarctica, thins in response to basal melting focused along a single 5 km‐wide and 60 km‐long channel extending from the ice shelf's grounding zone to its calving front. If focused thinning continues at present rates, the channel will melt through, and the ice shelf collapse, within 40–50 years, almost two centuries before collapse is projected from the average thinning rate. Our findings provide evidence of basal melt‐driven sub‐ice shelf channel formation and its potential for accelerating the weakening of ice shelves.

On the interpretation of ice-shelf flexure measurements

ABSTRACTTidal flexure in ice shelf grounding zones has been used extensively in the past to determine grounding line position and ice properties. Although the rheology of ice is viscoelastic at tidal loading frequencies, most modelling studies have assumed some form of linear elastic beam approximation to match observed flexure profiles. Here we use density, radar and DInSAR measurements in combination with full-Stokes viscoelastic modelling to investigate a range of additional controls on the flexure of the Southern McMurdo Ice Shelf. We find that inclusion of observed basal crevasses and density dependent ice stiffness can greatly alter the flexure profile and yet fitting a simple elastic beam model to that profile will still produce an excellent fit. Estimates of the effective Young's modulus derived by fitting flexure profiles are shown to vary by over 200% depending on whether these factors are included, even when the local thickness is well constrained. Conversely, estimates of the grounding line position are relatively insensitive to these considerations for the case of a steep bed slope in our study region. By fitting tidal amplitudes only, and ignoring phase information, elastic beam theory can provide a good fit to observations in a wide variety of situations. This should, however, not be taken as an indication that the underlying rheological assumptions are correct.

Grounding-zone ice thickness from InSAR: Inverse modelling of tidal elastic bending

AbstractIce-thickness measurements in Antarctic ice-shelf grounding zones are necessary for calculating the mass balance of individual catchments, but remain poorly constrained for most of the continent. We describe a new inverse modelling optimization approach to estimate ice thickness in the grounding zone of Antarctic outlet glaciers and ice shelves using spatial patterns of tide-induced flexure derived from differential interferometric synthetic aperture radar (InSAR). We demonstrate that the illposedness of the inverse formulation of the elastic-plate equations for bending can be controlled by regularization. In one dimension, the model recreates smooth, synthesized profiles of ice thickness from flexure information to within 1–2%. We test the method in two dimensions and validate it in the grounding zone of Beardmore Glacier, a major outlet glacier in the Transantarctic Mountains, using interferograms created from TerraSAR-X satellite imagery acquired in 2012. We compare our results with historic and modern ice-thickness data (radio-echo sounding from 1967 and ground-penetrating radar from 2010). We match both longitudinal and transverse thickness transects to within 50 m root-mean-square error using an effective Young’s modulus of 1.4 GPa. The highest accuracy is achieved close to the grounded ice boundary, where current estimates of thickness based on surface elevation measurements contain a systematic bias towards thicker ice.

Correction to “Ocean tides in the Weddell Sea: New observations on the Filchner-Ronne and Larsen C ice shelves and model validation”

Ocean tides under the large Weddell Sea ice shelves are among the least well observed on Earth. Here we present new, spatially extensive observations of the vertical tidal motion of the Filchner-Ronne and Larsen C ice shelves using Global Positioning System (GPS) data spanning a few weeks to years. We pay particular attention to the major tidal constituents (M2, S2, O1, K1) as well as important GRACE aliasing periods (K2 and S1). We compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves. The root-sum-square errors are 7-8 cm (CATS2008a and TPXO7.2) and 11-12 cm (GOT4.7 and FES2004) with the energetic M2 (RMSE = 4-8 cm) and S2 (4-5 cm) generally dominating these statistics. The FES2004 K1 is particularly inaccurate near the Larsen C Ice Shelf, with errors approaching 20 cm, meaning that GRACE Release 4 estimates of mass change in the northern Antarctic Peninsula will be biased. We find tidal energy at 3, 4, 5, 6 and, weakly, at 7 cycles per day at all of our sites. The largest amplitudes within these bands are at M4, MO3 and SP3 and approach 30 mm, although significant spatial variations exist. We show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones. Comparing model estimates with our M4, MS4 and MN4 values shows that models do not accurately represent these terms.

Ocean tides in the Weddell Sea: New observations on the Filchner-Ronne and Larsen C ice shelves and model validation

[1] Ocean tides under the large Weddell Sea ice shelves are among the least well observed on Earth. Here we present new, spatially extensive observations of the vertical tidal motion of the Filchner-Ronne and Larsen C ice shelves using Global Positioning System (GPS) data spanning a few weeks to years. We pay particular attention to the major tidal constituents (M2, S2, O1, K1) as well as important GRACE aliasing periods (K2 and S1). We compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves. The root-sum-square errors are 7–8 cm (CATS2008a and TPXO7.2) and 11–12 cm (GOT4.7 and FES2004) with the energetic M2 (RMSE = 4–8 cm) and S2 (4–5 cm) generally dominating these statistics. The FES2004 K1 is particularly inaccurate near the Larsen C Ice Shelf, with errors approaching 20 cm, meaning that GRACE Release 4 estimates of mass change in the northern Antarctic Peninsula will be biased. We find tidal energy at 3, 4, 5, 6 and, weakly, at 7 cycles per day at all of our sites. The largest amplitudes within these bands are at M4, MO3 and SP3 and approach 30 mm, although significant spatial variations exist. We show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones. Comparing model estimates with our M4, MS4 and MN4 values shows that models do not accurately represent these terms.

Nonlinear interaction between ocean tides and the Larsen C Ice Shelf system

[1] We report on three GPS records of flow of the Larsen C Ice Shelf, spanning 2 months to 2 years. Variations in speed are evident at periods from a few hours to ∼182 days, including semi-diurnal, diurnal and ∼14.76 days. At fortnightly periods the ice shelf varies by ±10% from its long-term speed but at diurnal timescales it is up to ±100%. A nonlinear relationship between ocean tides and velocity is required to explain the observed variations in flow. As an initial examination we model flow as a non-linear function of basal shear stress including tidal perturbations in the ice shelf grounding zone and show that the major features of the observations are reproduced, notably the long-period signal largely absent from the vertical tidal signal. Alternative explanations are discussed. These observations demonstrate that the ice shelf system is highly sensitive to relatively modest changes in forcing at its boundaries.

Ice shelf grounding zone structure from ICESat laser altimetry

We present a technique for investigating the grounding zone (GZ) of Antarctic ice shelves using laser altimetry from the Ice, Cloud and land Elevation Satellite (ICESat). Most surface height variability in the GZ is easily resolved by the ICESat laser's ∼65 m footprint and ∼172 m along‐track spacing. Comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide‐forced ice flexure, providing GZ location and width information for each track. Using ICESat data in the Institute Ice Stream region of southern Ronne Ice Shelf, we demonstrate that the location of the GZ based on feature identification in satellite imagery or digital elevation models may be in error by several km. Our results show that ICESat will contribute significantly to improving knowledge of GZ structure and to studies requiring accurate GZ locations, e.g., ice mass balance calculations and ice‐sheet/ocean modeling.

On the representation of ice-shelf grounding zones in SAR interferograms

AbstractWe investigate limitations of the one-dimensional elastic-beam model to detect grounding line and thickness of an ice shelf from a differential interferogram. Spatial limitations due to grounding-line curvature and variable ice thickness are analyzed by comparison with two-dimensional plate flexure. Temporal limitations from the tide-dependent shift of the grounding line are analyzed by superpositions of four tidal flexure profiles representing differential interferograms. (i) At scales greater than one ice thickness, seaward protrusions of the grounding line are well represented by the elastic-beam model, while landward embayments of the same scale produce significant misplacements &gt;10% of the ice thickness. (ii) For reasonable spatial variations of shelf thickness, the elastic-beam model gives reliable estimates of grounding-line position and unfractured mean ice thickness near the grounding line. (iii) For about 20% of superpositions of four tidal flexure profiles, the resulting grounding-line misplacements exceed the physical tidal shift of the grounding line by factors &gt;2. For differential tide levels &lt;10% of a 1 m tide dynamics, a physical shift of the grounding line of 0.3 km per metre of tide can lever misplacements of &gt;2 km. Examples of real interferometric profiles from West Antarctic ice shelves corroborate our results.

Ice shelf grounding zone features of western Prydz Bay, Antarctica: sedimentary processes from seismic and sidescan images

Several grounding zone wedges were left on the floor and flanks of Prydz Channel in western Prydz Bay by the Lambert Glacier during the last glacial cycle. Seismic profiles indicate that vertical accretion at the glacier bed was the most important depositional process in forming the wedges, rather than progradation by sediment gravity flows. Sidescan sonographs reveal extensive development of flutes on the sea floor inshore from the wedges, indicating deformable bed conditions beneath the ice. The region inshore of the east Prydz Channel wedge features extensive dune fields formed by currents flowing towards the grounding zone. This orientation is consistent with models of circulation beneath ice shelves in which melting at the grounding line generates plumes of fresher water that rise along the base of the ice shelf, entraining sea water into a circulation cell. The Lambert Deep is surrounded by a large composite ridge of glacial sediments. Internal reflectors suggest formation mostly by subglacial accretion. The sea floor in the Lambert Deep lacks dune fields and shows evidence of interspersed subglacial cavities and grounded ice beneath the glacier. The absence of bedforms reflects sea floor topography that would have inhibited the formation of energetic melt water-driven circulation.