Basal Crevasses Research Articles

Theoretical stability of ice shelf basal crevasses with a vertical temperature profile

Abstract Basal crevasses threaten the stability of ice shelves through the potential to form rifts and calve icebergs. Furthermore, it is important to determine the dependence of crevasse stability on temperature due to large vertical temperature variations on ice shelves. In this work, considering the vertical temperature profile through ice viscosity, we compare (1) the theoretical crack depths and (2) the threshold stress causing the transition from basal crevasses to full thickness fractures in several fracture theories. In the Zero Stress approximation, the depth-integrated force at the crevassed and non-crevassed location are unbalanced, violating the volume-integrated Stokes equation. By incorporating a Horizontal Force Balance (HFB) argument, recent work showed analytically that the threshold stress for rift initiation is only half of that predicted by the Zero Stress approximation. We generalize the HFB theory to show that while the temperature profile influences crack depths, the threshold rifting stress is insensitive to temperature. We compare with observations and find that HFB best matches observed rifts. Using HFB instead of Zero Stress for cracks in an ice-sheet model would substantially enlarge the predicted fracture depth, reduce the threshold rifting stress and potentially increase the projected rate of ice shelf mass loss.

An analysis of the interaction between surface and basal crevasses in ice shelves

Abstract. The prescription of a simple and robust parameterization for calving is one of the most significant open problems in ice sheet modelling. One common approach to the modelling of crevasse propagation in calving in ice shelves is to view crevasse growth as an example of linear elastic fracture mechanics. Prior work has, however, focused on highly idealized crack geometries, with a single fracture incised into a parallel-sided slab of ice. In this paper, we study how fractures growing from opposite sides of such an ice slab interact with each other, focusing on different simple crack arrangements: we consider either perfectly aligned cracks or periodic arrays of laterally offset cracks. We visualize the dynamics of crack growth using simple tools from dynamical systems theory and find that aligned cracks tend to impede each other's growth due to the torques generated by normal stresses on the crack faces, while periodically offset cracks facilitate simultaneous growth of bottom and top cracks. For periodic cracks, the presence of multiple cracks on one side of the ice slab, however, also generates torques that slow crack growth, with widely spaced cracks favouring calving at lower extensional stresses than closely spaced cracks.

Evaluating and locating a suitable bedrock drilling site near zhongshan station with airborne and ground-based observations

Understanding the interaction between the Antarctic biogeochemical cycle and global environmental change requires a comprehensive investigation of the ice sheet and underlying bedrock. An effective approach to investigating subglacial characteristics is through bedrock drilling, making the identification of a suitable drilling site a crucial prerequisite. In this study, data from airborne ice-penetrating radar and an Uncrewed Aerial Vehicle (UAV) remote sensing were used in combination to obtain real-time and high-resolution information about the surface, ice thickness, and subglacial conditions in a local area near Zhongshan Station, and further to identify a drilling site that is both safe and optimal for the successful acquisition of bedrock samples beneath the East Antarctic Ice Sheet. By analyzing the airborne ice-penetrating radar data from the 35th Chinese National Antarctic Research Expedition (CHINARE) (2018–2019), a potential drilling site was selected based on ice thickness (≤200 m) and a dry basal condition with no channels or basal crevasses, to avoid drilling fluid leakage. UAV data confirmed stable surface topography, ice flow, and the absence of surface crevasses. The optimal site was identified as located close to the western flank of the upstream part of the Dålk Glacier. In 2019, CHINARE successfully retrieved a valuable bedrock core sample from the borehole at this location. This study provides a feasible and effective methodology for locating a bedrock drilling site by using data from airborne ice-penetrating radar and UAV remote sensing.

Environment‐Modulated Glacial Seismicity Near Dålk Glacier in East Antarctica Revealed by Deep Clustering

AbstractOver the past decades, seismic monitoring has been increasingly used to track glacial activities associated with ice loss. Many seismological studies focus on West Antarctica, whereas glacial seismicity in East Antarctica is much less studied. Here, we apply unsupervised deep learning to a dense nodal seismic array near Dålk Glacier, East Antarctica, operating from 6 December 2019 to 2 January 2020. An autoencoder is used to automatically extract event features, which are then input into a Gaussian mixture model for clustering. We divide the data into 50 clusters and merge them according to their temporal and spectral characteristics. The results reveal five main types of seismic signals: two groups with monochromatic and high frequencies, two groups with broadband frequency and short duration, and a group with mainly low frequency and long duration. By comparing the environmental conditions (wind, temperature and tides), we infer that the two monochromatic groups are wind‐induced vibrations of the near‐station flag markers and topography; the two broadband groups are likely thermal contractions on the blue ice surface and stick‐slip events at the ice base; and the low‐frequency events are water‐filled basal crevassing and iceberg calving. In particular, we observe one type of low‐frequency event preceded by high‐frequency onset, which is likely basal crevassing near the grounding line of Dålk Glacier and predominantly occurred during rising tides. Our findings show that deep clustering is effective in identifying a wide range of glacial seismic events and can contribute to the rapid growth of passive glacier seismic monitoring.

Controls on calving at a large Greenland tidewater glacier: stress regime, self-organised criticality and the crevasse-depth calving law

Abstract We investigate the physical basis of the crevasse-depth (CD) calving law by analysing relationships between glaciological stresses and calving behaviour at Sermeq Kujalleq (Store Glacier), Greenland. Our observations and model simulations show that the glacier has a stable position defined by a compressive arch between lateral pinning points. Ice advance beyond the arch results in calving back to the stable position; conversely, if melt-undercutting forces the ice front behind the stable position, it readvances because ice velocities exceed subaqueous melt rates. This behaviour is typical of self-organising criticality, in which the stable ice-front position acts as an attractor between unstable super-critical and sub-critical regimes. This perspective provides strong support for a ‘position-law’ approach to modelling calving at Sermeq Kujalleq, because any calving ‘rate’ is simply a by-product of how quickly ice is delivered to the critical point. The CD calving law predicts ice-front position from the penetration of surface and basal crevasse fields, and accurately simulates super-critical calving back to the compressive arch and melt-driven calving into the sub-critical zone. The CD calving law reflects the glaciological controls on calving at Sermeq Kujalleq and exhibits considerable skill in simulating its mean position and seasonal fluctuations.

The role of fresh water in driving ice shelf crevassing, rifting and calving

Meltwater is widely implicated in the disintegration of ice shelves and the lower density of fresh meltwater relative to salty seawater can reconcile observations and models of ice shelf rifting. Advanced numerical models predict that a seawater-filled basal crevasse will break through a shelf to form a rift only where extensional stresses equal those at the seaward shelf edge. However, rifts open in ice shelf interiors where resistance to flow at the sides and base of the ice shelf, or buttressing, is estimated to significantly reduce extensional stresses compared to those at the shelf edge. The stress in a shelf is often quantified in terms of a buttressing number and rifts are seen to open in areas where the estimated buttressing number is between 0.2 and 0.3. Numerical models assuming fresh meltwater-filled basal crevasses predict rift opening for buttressing numbers up to about 0.2. Published estimates indicate that about a quarter of the area of the Antarctic ice shelves are characterized by buttressing numbers less than 0.2. The present results indicate that those areas are directly vulnerable to rifting in the presence of abundant meltwater and that calving of those areas could drive further rifting. The numerical results suggest that rifting can be understood in terms of a horizontal force balance, justifying derivation of a simple analytic relation between the density of water infilling a rift and the maximum buttressing number allowing rifting. This analytic relation should hold for any spacing of crevasses or any vertical variations in ice rheology. Enforcing a horizontal force balance also allows derivation of an analytic relation between the heights of closely spaced ice shelf crevasses and buttressing numbers.

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.

Crevasse refreezing and signatures of retreat observed at Kamb Ice Stream grounding zone

Ice streams flowing into Ross Ice Shelf are presently responsible for around 10% of the mass flux from West Antarctica, with the noteworthy exception of Kamb Ice Stream, which stagnated in the late 1800s. The subsequent reduction in ice supply led to grounding-line retreat at the coastal margin where Kamb transitions into the floating Ross Ice Shelf. Grounding-line migration is linked to broader changes in ice-sheet mass balance and sea level, but our understanding of related ice, ocean and seafloor interactions is limited by the difficulty in accessing these remote regions. Here we report in situ observations from an underwater vehicle deployed at Kamb that show how fine-scale variability in ice and ocean structure combine to influence a diversity of ice–ocean interactions. We found a stratified water column within a tenth of a degree of freezing at the ice base and mapped basal crevasses with supercooled water and active marine ice formation. At the seafloor, we interpret parallel ridges as crevasse impressions left as the ice lifted off during grounding-line retreat. These observations from a recently ungrounded sub-shelf environment illuminate both the geomorphological signatures of past grounding-line retreat and the fine-scale sensitivity of ongoing ice–ocean interactions to ice topography.

Basal hydrofractures near sticky patches

Basal crevasses are macroscopic structural discontinuities at the base of ice sheets and glaciers that arise by fracture. Motivated by observations and by the mechanics of elastic fracture, we hypothesise that spatial variations in basal stress (in the presence of basal water pressure) can promote and localise basal crevassing. We quantify this process in the theoretical context of linear elastic fracture mechanics. We develop a model evaluating the effect of shear-stress variation on the growth of basal crevasses. Our results indicate that sticky patches promote the propagation of basal crevasses, increase their length of propagation into the ice and, under some conditions, give them curved trajectories that incline upstream. A detailed exploration of the parameter space is conducted to gain a better understanding of the conditions under which sticky-patch-induced basal crevassing is expected beneath ice sheets and glaciers.

Basal Crevasse Formation on Byrd Glacier, East Antarctica, as Proxy for Past Subglacial Flooding Events

AbstractLinear elastic fracture mechanics suggests that short‐lived flow accelerations, such as the one initiated by a flooding event beneath Byrd Glacier in 2006, can form abnormally large basal crevasses at the grounding line. Airborne radar measurements acquired in 2011 reveal hundreds of basal crevasses ranging in height from 40–335 m. Particle tracking results show that formation of the largest basal crevasse occurred at the grounding line during the 2006 flooding event. Very large basal crevasses form distinctive surface depressions directly overhead, which are observed along the Byrd Glacier flowline to the terminus of the Ross Ice Shelf. By using these surface depressions as proxies for abnormally large basal crevasses, we create a timeline of past subglacial flooding events on Byrd Glacier. Understanding the frequency of flooding events and their effect on glacier dynamics will help inform models of ice sheet stability and subglacial hydrology.

Flexural Control of Basal Crevasse Opening Under Ice Shelves

AbstractClassical analyses of basal crevasse opening do not account for the free surface of a floating ice layer. We describe a high‐resolution numerical treatment of the opening of a single crevasse in a finite thickness elastic layer floating on an inviscid substrate. For low extensional stress (less than about half of the expected maximum for a freely floating shelf) the resulting crevasse height and width match previous studies. For larger magnitude applied extensional stresses, the new results predict basal crevasse widths an order of magnitude greater than the classical solution. An analysis using the thin‐layer approximation shows that the greatly increased predicted width of basal crevasse opening results from layer bending. Given that the height and width of basal crevasses are non‐linear functions of the stress experienced by an ice shelf, the new model results may enable better estimation of buttressing stresses for different parts of ice shelves.

Radio-echo sounding and waveform modeling reveal abundant marine ice in former rifts and basal crevasses within Crary Ice Rise, Antarctica

AbstractCrary Ice Rise formed after the Ross Ice Shelf re-grounded ~1 kyr BP. We present new ice-penetrating radar data from two systems operating at center frequencies of 7 and 750 MHz that confirm the ice rise is composed of a former ice shelf buried by subsequent accumulation. Stacks of englacial diffraction hyperbolas are present almost everywhere across the central ice rise and extend up to ~350 m above the bed. In many cases, bed reflections beneath the diffraction hyperbolas are obscured for distances up to 1 km. Waveform modeling indicates that the diffraction hyperbolas are likely caused by marine ice deposits in former basal crevasses and rifts. The in-filling of rifts and basal crevasses may have strengthened the connection between the ice rise and the surrounding ice shelf, which could have influenced local and regional ice dynamics. Three internal reflection horizons mark the upper limit of disturbed ice and diffraction hyperbolas in different sections of the ice rise, indicating at least three stages of flow stabilization across the ice rise. A surface lineation visible in MODIS imagery corresponds spatially to deepening and strong deformation of these layers, consistent with the characteristics of former grounding lines observed elsewhere in Antarctica.

The cooling signature of basal crevasses in a hard-bedded region of the Greenland Ice Sheet

Abstract. Temperature sensors installed in a grid of nine full-depth boreholes drilled in the southwestern ablation zone of the Greenland Ice Sheet recorded cooling in discrete sections of ice over time within the lowest third of the ice column in most boreholes. Rates of temperature change outpace cooling expected from vertical conduction alone. Additionally, observed temperature profiles deviate significantly from the site-average thermal profile that is shaped by all thermomechanical processes upstream. These deviations imply recent, localized changes to the basal thermal state in the boreholes. Although numerous heat sources exist to add energy and warm ice as it moves from the central divide towards the margin such as strain heat from internal deformation, latent heat from refreezing meltwater, and the conduction of geothermal heat across the ice–bedrock interface, identifying heat sinks proves more difficult. After eliminating possible mechanisms that could cause cooling, we find that the observed cooling is a manifestation of previous warming in near-basal ice. Thermal decay after latent heat is released from freezing water in basal crevasses is the most likely mechanism resulting in the transient evolution of temperature and the vertical thermal structure observed at our site. We argue basal crevasses are a viable englacial heat source in the basal ice of Greenland's ablation zone and may have a controlling influence on the temperature structure of the near-basal ice.

Evidence of ice streaming and ice tongue shutdown in western Latvia: revealed from the mapping of crevasse-squeeze ridges

Glacial geomorphological mapping of western Latvia using a 1-m-resolution digital elevation model generated from airborne LiDAR data has revealed two sets of mega-scale glacial lineations (MSGLs), one of which is superimposed by crevasse-squeeze ridges (CSRs). CSRs occur as a dense ridge network with a dominant orientation of ridges perpendicular to the ice flow direction. The landform assemblage is interpreted as evidence for two separate phases of fast ice flow with different ice flow directions during the overall deglaciation of the Fennoscandian Ice Sheet (FIS). The first fast ice flow phase occurred from the northwest by the Usma Ice Lobe that extended in the Eastern Kursa Upland. The second fast ice flow occurred from the north by the Venta Ice Tongue in a narrow flow corridor limited mainly to the Kursa Lowland. Active ice streaming caused ice crevassing perpendicular to the ice flow direction and formation of CSRs by squeezing of subglacial till into basal crevasses. A good preservation of the CSRs and general lack of recessional moraines suggest widespread stagnation and ice mass melting after the shutdown of the Venta Ice Tongue followed by the formation of the Venta-Usma ice-dammed lake and glaciolacustrine deposition in the lowest areas of lowland. Our data provide the first evidence of CSRs in the south-eastern terrestrial sector of the FIS suggesting the dynamic ice streaming or surging behaviour of the ice lobes and tongues in this region during deglaciation.

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.

A Decade of Variability on Jakobshavn Isbrae: Ocean Temperatures Pace Speed Through Influence on Mélange Rigidity.

The speed of Greenland's fastest glacier, Jakobshavn Isbrae, has varied substantially since its speedup in the late 1990s. Here we present observations of surface velocity, mélange rigidity, and surface elevation to examine its behaviour over the last decade. Consistent with earlier results, we find a pronounced cycle of summer speedup and thinning followed by winter slowdown and thickening. There were extended periods of rigid mélange in the winters of 2016-17 and 2017-18, concurrent with terminus advances ~6 km farther than in the several winters prior. These terminus advances to shallower depths caused slowdowns, leading to substantial thickening, as has been noted elsewhere. The extended periods of rigid mélange coincide well with a period of cooler waters in Disko Bay. Thus, along with the relative timing of the seasonal slowdown, our results suggest that the ocean's dominant influence on Jakobshavn Isbrae is through its effect on winter mélange rigidity, rather than summer submarine melting. The elevation time series also reveals that in summers when the area upstream of the terminus approaches flotation, large surface depressions can form, which eventually become the detachment points for major calving events. It appears that as elevations near flotation, basal crevasses can form, which initiates a necking process that forms the depressions. The elevation data also show that steep cliffs often evolve into short floating extensions, rather than collapsing catastrophically due to brittle failure. Finally, summer 2019 speeds were slightly faster than the prior two summers, leaving it unclear whether the slowdown is ending.

Spatial and temporal variations in basal melting at Nivlisen ice shelf, East Antarctica, derived from phase-sensitive radars

Abstract. Thinning rates of ice shelves vary widely around Antarctica, and basal melting is a major component of ice shelf mass loss. In this study, we present records of basal melting at a unique spatial and temporal resolution for East Antarctica, derived from autonomous phase-sensitive radars. These records show spatial and temporal variations of basal melting in 2017 and 2018 at Nivlisen, an ice shelf in central Dronning Maud Land. The annually averaged basal melt rates are in general moderate (∼0.8 m yr−1). Radar profiling of the ice shelf shows variable ice thickness from smooth beds to basal crevasses and channels. The highest basal melt rates (3.9 m yr−1) were observed close to a grounded feature near the ice shelf front. Daily time-varying measurements reveal a seasonal melt signal 4 km from the ice shelf front, at an ice draft of 130 m, where the highest daily basal melt rates occurred in summer (up to 5.6 m yr−1). In comparison with wind, air temperatures, and sea ice cover from reanalysis and satellite data, the seasonality in basal melt rates indicates that summer-warmed ocean surface water was pushed by wind beneath the ice shelf front. We observed a different melt regime 35 km into the ice shelf cavity, at an ice draft of 280 m, with considerably lower basal melt rates (annual average of 0.4 m yr−1) and no seasonality. We conclude that warm deep-ocean water at present has a limited effect on the basal melting of Nivlisen. On the other hand, a warming in surface waters, as a result of diminishing sea ice cover, has the potential to increase basal melting near the ice shelf front. Continuous in situ monitoring of Antarctic ice shelves is needed to understand the complex mechanisms involved in ice shelf–ocean interactions.

Evolution of ice shelf rifts: Implications for formation mechanics and morphological controls

Ice shelf rifts are predecessors to iceberg calving events. Observations of their morphology over time and interaction with their surroundings are presented. Newly-available Landsat-derived annual velocity data and laser altimetry from NASA's ICESat, ICESat-2 and Operation IceBridge missions were used to characterize several ice shelf rift systems. Actively propagating ice shelf rifts exhibit uplift on both sides, with an offset in height between the two rift walls, suggesting an asymmetric buoyancy force resulting from the rifting process itself. It is observed that the direction of the asymmetry is the same for rifts within a single ice shelf, but not between ice shelves. This finding suggests a rheological or basal properties influence on rift formation and buoyancy asymmetry. It is shown that this asymmetry can be accounted for by modeling ice shelf rifts as subvertical faults. With time and inactivity, rift walls eventually relax and become rounded off. Measurements of widening suggest that rift infilling is active on many active rifts. Active rifts experience higher spreading rates than do dormant rifts. Lastly, two candidate processes are identified for rift nucleation (subvertical basal crevassing and localized melt). It is found that more than any other ice shelf in this study, rifting in Pine Island Glacier is driven by ocean heat variations and is showing signs of increasing destabilization.

Holocene Formation of Henry Ice Rise, West Antarctica, Inferred From Ice‐Penetrating Radar

Ice rises are regions of grounded ice embedded within floating ice shelves. The deformation of ice past them increases the back stress generated by the ice shelf, slowing the flow of the ice sheet. We present ground‐based ice‐penetrating radar data from Henry Ice Rise in the Ronne Ice Shelf, West Antarctica, that indicates regrounding during the Holocene. Relic crevasses and melt synclines are observed upstream of the present‐day grounding line. We conclude that these features formed during a previous flow configuration, from which the grounding line has since advanced to its current position. In agreement with previous work, our observations can be explained if initial grounding of the ice shelf occurred on a bathymetric high, forming an ice rumple that migrated upstream and temporarily ungrounded over the topographic high. The grounding line then advanced, preserving relic basal crevasses in the newly grounded ice. Using a simple ice‐flow model, we simulate the burial of these crevasses. While accounting for uncertainty in accumulation, firn density, radar‐derived depth, ice‐thickening history, initial crevasse height, and glacial isostatic adjustment, we estimate a burial time of 6 ± 2 kyr before present for the oldest relic crevasses, indicating that the ice rise formed at approximately this time. This potentially increased the buttressing generated by the Ronne Ice Shelf, causing thickening and advance of the ice sheet. By dating the formation and providing details of ice‐rise formation, these new results can provide useful constraints on both large‐scale ice‐sheet models and models of ice‐rumple and ice‐rise formation.

Interrogating a Surging Glacier With Seismic Interferometry

AbstractSeismic interferometry can be spectacularly sensitive to temporal changes within the Earth. Zhan (2019, https://doi.org/10.1029/2019GL082411) analyzed 12 years of data from seismic stations situated near Bering Glacier, Alaska, that spanned a 2008–2011 surge. Interferometrically isolated glacier‐crossing surface waves revealed an anisotropic seismic wave speed decrease during the surge. 1‐D and 3‐D modeling suggests pressurized englacial water flow being stored and guided by a network of basal crevasses, thereby sustaining high water pressure at the bed and supporting the high surge velocities. This result is demonstrative of the potential for seismic interferometry and other cryoseismological methods to continuously monitor and reveal changes within dynamic glacial‐hydrological systems.