We present a comprehensive ice‐penetrating radar survey of a subglacial embayment and adjacent peninsula along the grounding zone of Whillans Ice Stream, West Antarctica. Through basal waveform and ...reflectivity analysis, we identify four distinct basal interfaces: (1) an ice‐water‐saturated till interface inland of grounding; (2) a complex interface in the grounding zone with variations in reflectivity and waveforms caused by reflections from fluting, sediment deposits, and crevasses; (3) an interface of anomalously low‐reflectivity downstream of grounding in unambiguously floating areas of the embayment due to basal roughness and entrained debris; and (4) a high‐reflectivity ice‐seawater interface that occurs immediately seaward of grounding at the subglacial peninsula and several kilometers seaward of grounding in the embayment, occurring after basal debris and grounding zone flutes have melted off the ice bottom. Sediment deposition via basal debris melt‐out occurs in both locations. The higher basal melt rate at the peninsula contributes to greater grounding line stability by enabling faster construction of a stabilizing sediment wedge. In the embayment, the low slopes of the ice bottom and bed prevent development of a strong thermohaline circulation leading to a lower basal melt rate and less rapid sediment deposition. Thus, grounding lines in subglacial embayments are more likely to lack stabilizing sediment deposits and are more prone to external forcing, whether from the ocean, the subglacial water system, or large‐scale ice dynamics. Our conclusions indicate that subglacial peninsulas and embayments should be treated differently in ice sheet‐ocean models if these models are to accurately simulate grounding line response to external forcing.
Key Points
Radar basal reflectivity in a grounding zone embayment is lower than that expected from an ice‐seawater interface
Radar amplitude and waveform modeling suggests that basal reflectivity is reduced by entrained sediment and basal roughness
A reflectivity increase downflow of the grounding line suggests that sediment deposition in grounding zones occurs via basal debris melt‐out
Ocean‐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.
Plain Language Summary
Ice shelf melting is an important player in ice loss from the Antarctic Ice Sheet, affecting sea level rise. Ice shelf melting is controlled by ocean properties and processes, but sparse observations of the sub‐ice shelf ocean cavity limit our understanding of these controls and thus limit our ability to predict sea level rise. This study presents rare ocean observations deep below the largest ice shelf by area, the Ross Ice Shelf, far from the open ocean. The observed ocean setting is surprisingly quiescent, and waters are cold, around −2 °C. This study also presents new, highly localized ice shelf melting measurements at the site that show that these ocean conditions lead to slow ice shelf melting of only centimeters per year. These observations reveal the ways in which the Ross Ice Shelf contrasts with rapidly melting ice shelves affected by warmer seawater elsewhere in West Antarctica. Thus, they adds nuance to our scientific understanding of ice‐ocean interactions around the Antarctic continent.
Key Points
The ocean cavity near the grounding zone of the Ross Ice Shelf is vertically stratified with a boundary layer freshened by ice melting
A tidally mixed zone and tidal currents are absent from this 10‐m‐thick ocean cavity in the flexure zone
The observed stratification and low current velocities result in low melt rates (7 cm/year)
In the last 2 decades, Pine Island Glacier (PIG) experienced marked speedup, thinning, and grounding-line retreat, likely due to marine ice-sheet instability and ice-shelf basal melt. To better ...understand these processes, we combined 2008–2010 and 2012–2014 GPS records with dynamic firn model output to constrain local surface and basal mass balance for PIG. We used GPS interferometric reflectometry to precisely measure absolute surface elevation (zsurf) and Lagrangian surface elevation change (Dzsurf∕ Dt). Observed surface elevation relative to a firn layer tracer for the initial surface (zsurf − zsurf0′) is consistent with model estimates of surface mass balance (SMB, primarily snow accumulation). A relatively abrupt ∼ 0.2–0.3 m surface elevation decrease, likely due to surface melt and increased compaction rates, is observed during a period of warm atmospheric temperatures from December 2012 to January 2013. Observed Dzsurf∕ Dt trends (−1 to −4 m yr−1) for the PIG shelf sites are all highly linear. Corresponding basal melt rate estimates range from ∼ 10 to 40 m yr−1, in good agreement with those derived from ice-bottom acoustic ranging, phase-sensitive ice-penetrating radar, and high-resolution stereo digital elevation model (DEM) records. The GPS and DEM records document higher melt rates within and near features associated with longitudinal extension (i.e., transverse surface depressions, rifts). Basal melt rates for the 2012–2014 period show limited temporal variability despite large changes in ocean temperature recorded by moorings in Pine Island Bay. Our results demonstrate the value of long-term GPS records for ice-shelf mass balance studies, with implications for the sensitivity of ice–ocean interaction at PIG.
Calving of glacial ice into the ocean from the Greenland Ice Sheet is an important component of global sea level rise. The calving process itself is relatively poorly observed, understood, and ...modeled; as such, it represents a bottleneck in improving future global sea level estimates in climate models. We organized a pilot project to observe the calving process at Helheim Glacier in East Greenland in an effort to better understand it. During an intensive one-week survey, we deployed a suite of instrumentation including a terrestrial radar interferometer, GPS receivers, seismometers, tsunameters, and an automated weather station. This effort captured a calving process and measured various glaciological, oceanographic, and atmospheric parameters before, during, and after the event. One outcome of our observations is evidence that the calving process actually consists of a number of discrete events, spread out over time, in this instance over at least two days. This time span has implications for models of the process. Realistic projections of future global sea level will depend on accurate parametrization of calving, which will require more sustained observations.
Seismic measurements on Thwaites Glacier show a spatially variable bed character, with implications for ice-sheet stability. The West Antarctic Ice Sheet is losing mass rapidly through outlet ...glaciers and ice streams in the Amundsen Sea Embayment, including Thwaites Glacier, where limited observations and modeling suggest that ice-flow rates depend on bed properties. Here we characterize bed properties of Thwaites Glacier based on amplitude analysis of reflection-seismic data from a ∼40-km-long profile collected in the approximate flow direction and two ∼10-km-long profiles transverse to flow. The upstream portion of the seismic profile reveals a ∼12-km long sedimentary basin with ice-flow-aligned bedforms capped by a continuous till layer that is likely soft and deforming (porosity ∼0.4–0.45), with several locations where water has pooled at the bed. Downstream of the sedimentary basin, the bed rises by ∼400 m over ∼25 km into subglacial highlands. Our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography (∼200 m amplitude at ∼2- to 5-km wavelength) resembling crag-and-tails. Till on the stoss sides (facing upglacier) of topographic highs is more consolidated (porosity ∼0.3–0.35 or lower), whereas the lee sides (facing downglacier) and flat regions exhibit porosity similar to the till of the upstream sedimentary basin. Modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of Thwaites Glacier, resulting in more-realistic models of future grounding-line retreat. Our findings highlight the need for more geophysical constraints on bed properties for important outlets in Antarctica and Greenland.
•P-wave reflectivities show bed character varies with subglacial morphology.•Relatively flat, ∼12-km-long basin is “soft” bedded, capped by dilatant till.•“Hard” beds on stoss, “soft” beds on lee sides of topography in subglacial highlands.•Localized water layers are scattered beneath upper Thwaites Glacier.
Swath radar technology enables three‐dimensional mapping of modern glacier beds over large areas at resolutions that are higher than those typically used in ice‐flow models. These data may enable new ...understanding of processes at the ice‐bed interface. Here, we use two densely surveyed swath‐mapped topographies (<50 m2 resolution) of Thwaites Glacier to investigate the sensitivity of inferred basal friction proxies to bed roughness magnitude and orientation. Our work suggests that along‐flow roughness influences inferred friction more than transverse‐flow roughness, which agrees with analytic form‐drag sliding theory. Using our model results, we calculate the slip length (the ratio of internal shear to basal slip). We find excellent agreement between the numerically derived slip lengths and slip lengths predicted by analytic form‐drag sliding theory, which suggests that unresolved short wavelength bed roughness may control sliding in the Thwaites interior.
Plain Language Summary
Ice‐sheet model simulations used to predict sea‐level rise require estimates of the slipperiness at the ice‐sheet base. The slipperiness is typically inferred from observations of the ice‐sheet surface; however, these inferences depend critically on how well the selected model domain resolves bumps, hills, and valleys that make up the landscape beneath the ice sheet. Over large regions, these small‐scale features are not well mapped, but new ice‐penetrating radar technology is making this more possible. Using a unique high‐resolution map of the landscape beneath a large glacier in Antarctica, we unravel how the size of bumps and hills beneath the ice affect the parameterization of the resistance field used in ice‐sheet models to simulate flow. We find that the hills, valleys, and bumps that create roughness in the landscape beneath the ice sheet influence the inferred resistance field below the spatial resolution of models and observations. We also find that bumps that block the flow of the glacier affect the inferred resistance/slipperiness of the glacier bed more than bumps that align with the flow direction.
Key Points
We infer high‐resolution basal resistance on Thwaites Glacier using a 3D, full‐stress model with 3D radar swath‐mapped basal topographies
Drag due to material properties and flow around obstacles remain entangled below the spatial resolutions of standard models
High‐wavenumber (short wavelength) basal roughness parallel to ice flow has the largest effect on the model inferred basal flow resistance
The geometry of ice-sheet internal layers is frequently interpreted as an indicator of present and past ice-sheet flow dynamics. One of the primary goals of radio-echo sounding is to accurately ...reproduce that layer geometry. Internal layers show a loss in reflection amplitude as a function of increasing dip angle. We posit that this energy loss occurs via several mechanisms: destructive interference in trace stacking, energy dispersion through synthetic aperture radar (SAR) processing and off-nadir ray path losses. Adjacent traces collected over a dipping horizon contain reflection arrivals which are not in phase. Stacking these traces results in destructive interference. When the phase shift between adjacent traces exceeds one-half wavelength, SAR processing, which otherwise coherently combines data from dipping reflectors, disperses the energy, reducing image quality further. Along with amplitude loss from destructive stacking and SAR dispersion, imaging reflectors from off-nadir angles results in additional travel time and thus additional englacial attenuation relative to horizontal reflectors at similar depths. When selecting radar frequency, spatial sample rate and stacking interval for a given survey, the geometry of the imaging target must be considered. Based on our analysis, we make survey design recommendations for these parameters.
Ice streams are bounded by abrupt transitions in speed called shear margins. Some shear margins are fixed by subglacial topography, but others are thought to be self‐organizing, evolving by thermal ...feedback to ice viscosity and basal drag which govern the stress balance of ice sheets. Resistive stresses (and properties governing shear‐margin formation) manifest nonuniquely at the surface, motivating the use of subsurface observations to constrain ice sheet models. In this study, we use ice‐penetrating radar data to evaluate three 3‐D thermomechanical models of the Northeast Greenland Ice Stream, focusing on model reproductions of ice temperature (a primary control on viscosity) and subsurface velocity. Data/model agreement indicates elevated temperatures in the Northeast Greenland Ice Stream margins, with depth‐averaged temperatures between 2 °C and 6 °C warmer in the southeast margin compared to ice in streaming flow, driven by vertical heat transport rather than shear heating. This work highlights complexity in ice divergence across stagnant/streaming transitions.
Plain Language Summary
Ice sheet models used to project future sea level rise are calibrated using modern observations of ice flow at the ice sheet surface. However, the subsurface ice and rock properties that ultimately control the patterns of ice flow cannot be uniquely determined using observations of the surface alone. In this study, we use the structural and electromagnetic characteristics of the Greenland Ice Sheet (determined from ice‐penetrating radar data) to evaluate the subsurface performance of three different ice‐flow models of the Northeast Greenland Ice Stream. We show that fast flow in Northeast Greenland is, in part, controlled by softer, warmer ice, and that correctly modeling heat transport at the boundaries of ice streams is critical for realistic projections of their future behavior. Ultimately, we provide insight into a sensitive region of Greenland together with a new approach to geophysical data use in model evaluation, with the goal of reducing the range of plausible models projecting the future of the Greenland and Antarctic Ice Sheets.
Key Points
Thermal softening of ice is present in the Northeast Greenland Ice Stream (NEGIS) shear margins, despite low strain rates
Vertical advection of heat dominates the shear‐margin temperature structure here, validated by radar reflectivity and isochron geometry
Radar data can be used to constrain ice temperature and subsurface velocity to evaluate ice sheet model spin‐up and inversions
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