Subglacial Water Research Articles

Subcritical crack propagation in glacial quarrying during subglacial water pressure variation

AbstractGlacial quarrying stems from the fracturing of subglacial bedrock. Much evidence shows that subcritical crack propagation of bedrock is closely related to subglacial water pressure fluctuations. Here we employ a model that assesses the impact of subglacial water pressure fluctuation on cavity length and subcritical crack propagation, while analyzing the effect of a pre-existing crack location using a phase-field model (PFM). Our results indicate that the cavity length is reduced during diurnal fluctuations in water pressure. There are two patterns of subcritical crack propagation on the corner of the step. The first stems from a rapid drop in water pressure. The second occurs after the water pressure recovers from the fluctuation to the initial steady state. This pattern is a consequence of enhanced stress concentration on the step since the modeled cavity length exceeds its steady value and has higher efficiency in promoting subcritical crack propagation. Additionally, based on the PFM results, we speculate that the subcritical crack initiation and propagation happen on a broader scale, including the ice-bed contact region and its adjacent region. Our findings imply that the duration of subcritical crack propagation is short and typically ceases once the cavity length adjusts to reduced water pressure levels.

Automatic Detection of Subglacial Water Bodies in the AGAP Region, East Antarctica, Based on Short-Time Fourier Transform

Subglacial water bodies are critical components in analyzing the instability of the Antarctic ice sheet. Their detection and identification normally rely on geophysical and remote sensing methods such as airborne radar echo sounding (RES), ground seismic, and satellite/airborne altimetry and gravity surveys. In particular, RES surveys are able to detect basal terrain with a relatively high accuracy that can assist with the mapping of subglacial hydrology systems. Traditional RES processing methods for the identification of subglacial water bodies mostly rely on their brightness in radargrams and hydraulic flatness. In this study, we propose an automatic method with the main objective to differentiate the basal materials by quantitatively evaluating the shape of the A-scope waveform near the basal interface in RES radargrams, which has been seldom investigated. We develop an automatic algorithm mainly based on the traditional short-time Fourier transform (STFT) to quantify the shape of the A-scope waveform in radargrams. Specifically, with an appropriate window width applied on the main peak of each A-scope waveform in the RES radargram, STFT shows distinct and contrasting frequency responses at the ice-water interface and ice-rock interface, which is largely dependent upon their different reflection characteristics at the basal interface. We apply this method on 882 RES radargrams collected in the Antarctic’s Gamburtsev Province (AGAP) in East Antarctica. There are 8822 identified A-scopes with the calculated detection value larger than the set threshold, out of the overall 1,515,065 valid A-scopes in these 882 RES radargrams. Although these identified A-scopes only takes 0.58% of the overall A-scope population, they show exceptionally continuous distribution to represent the subglacial water bodies. Through a comprehensive comparison with existing inventories of subglacial lakes, we successfully verify the validity and advantages of our method in identifying subglacial water bodies using the detection probability for other basal materials of theoretically the highest along-track resolution. The frequency signature obtained by the proposed joint time–frequency analysis provides a new corridor of investigation that can be further expanded to multivariable deep learning approaches for subglacial and englacial material characterization, as well as subglacial hydrology mapping.

Shear dilation of subglacial till results in time-dependent sliding laws

The dynamics of glacial sliding over water-saturated tills are poorly constrained and difficult to capture realistically in large-scale models. Experiments characterize till as a plastic material with a pressure-dependent yield stress, but the subglacial water pressure may fluctuate on annual to daily timescales, leading to transient adjustment of the till. We construct a continuum two-phase model of coupled fluid and solid deformation, describing the movement of water through the pore space of a till that is itself dilating and deforming. By forcing the model with time-dependent effective pressure at the ice–till interface, we infer the resulting relationships between basal traction, solid fraction and rate of deformation. We find that shear dilation introduces internal pressure variations and transient dilatant strengthening emerges, leading to hysteretic behaviour in low-permeability materials. The result is a time-dependent effective sliding law, with permeability-dependent lag between changes in effective pressure and the slidingspeed. This deviation from traditional steady-state sliding laws may play an important role in a wide range of transient ice-sheet phenomena, from glacier surges to the tidal response of ice streams.

Topographical analysis of a candidate subglacial water region in Ultimi Scopuli, Mars

Radar signals of the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument at the base of the South Polar Layered Deposits (SPLD) in the region known as Ultimi Scopuli, on Mars, have been interpreted as reflections from putative subglacial standing bodies of liquid water. The interpretation of the MARSIS radar signals is not unique and different explanations have been proposed for the anomalous bright radar reflection observed in Ultimi Scopuli, including the presence of CO2 deposits within the ice, or clays and metal-bearing minerals. We analysed the surface topography of Ultimi Scopuli with Digital Elevations Models (DEMs) derived from MOLA (Mars Orbiter Laser Altimeter), photogrammetric processing of the High-Resolution Stereo Camera (HRSC) images and SHARAD radargrams. In each case, we found an anomalous flat and smooth area in correspondence to the potential subglacial water zone, indicating a condition of hydrostatic equilibrium of the ice, similar to the surface observations found above large terrestrial subglacial lakes. We found that the surface topography, in addition to MARSIS radar sounder signals, are compatible with the presence of subglacial standing bodies of liquid water at the base of Ultimi Scopuli.

Subglacial hydrology modulates basal sliding response of the Antarctic ice sheet to climate forcing

Abstract. Major uncertainties in the response of ice sheets to environmental forcing are due to subglacial processes. These processes pertain to the type of sliding or friction law as well as the spatial and temporal evolution of the effective pressure at the base of ice sheets. We evaluate the classic Weertman–Budd sliding law for different power exponents (viscous to near plastic) and for different representations of effective pressure at the base of the ice sheet, commonly used for hard and soft beds. The sensitivity of the above slip laws is evaluated for the Antarctic ice sheet in two types of experiments: (i) the ABUMIP experiments in which ice shelves are instantaneously removed, leading to rapid grounding-line retreat and ice sheet collapse, and (ii) the ISMIP6 experiments with realistic ocean and atmosphere forcings for different Representative Concentration Pathway (RCP) scenarios. Results confirm earlier work that the power in the sliding law is the most determining factor in the sensitivity of the ice sheet to climatic forcing, where a higher power in the sliding law leads to increased mass loss for a given forcing. Here we show that spatial and temporal changes in water pressure or water flux at the base modulate basal sliding for a given power, especially for high-end scenarios, such as ABUMIP. In particular, subglacial models depending on subglacial water pressure decrease effective pressure significantly near the grounding line, leading to an increased sensitivity to climatic forcing for a given power in the sliding law. This dependency is, however, less clear under realistic forcing scenarios (ISMIP6).

Different characteristics of two surges in Weigeledangxiong Glacier, northeastern Tibetan Plateau

Glacier surge is a special form of glacier displacement caused by the instability of the glacial dynamic system. It is a quasi-periodic oscillation behavior, which affects the estimation of the overall change of glaciers in the region and potentially threatens the infrastructure and human life in the downstream regions. Most glaciers experience a mass loss with rising air temperatures in recent decades, but little attention has been paid to the influence of climate change on glacial surges. This study identified two surges, triggered in 1992 and 2015 in Weigeledangxiong Glacier, Ányêmaqên Mountains, northeastern Tibetan Plateau, using multi-source remote sensing data (Landsat images, Sentinel-2 images, topographic map, shuttle radar topography mission digital elevation model [SRTM DEM], and the elevation change database). The 1992 surge accelerated abruptly with the maximum velocity of 350 ± 9 m a−1, and a large volume of ice transported downward, causing a sudden advance of 392 ± 42 m from 1992 to 1994, and clear thickening of the ice tongue. The recent surge is still in the active phase, exhibiting a gentler process of slower advance speed and lower peak velocity, as well as a smaller expansion zone than the previous one. These phenomena may be associated with the reduced glacier basal resistance and energy caused by rising temperatures in recent decades. Higher temperatures may cause the discharge of subglacial water through a more developed drainage system, leading to a longer active phase duration. Similar phenomena may exist widely in the Tibetan Plateau and its surrounding areas. Meanwhile, the frontal position of Weigeledangxiong Glacier advancing in the recent surge is not expected to threaten roads near the ice tongue.

Tunnel valley formation beneath deglaciating mid-latitude ice sheets: Observations and modelling

The geological record of landforms and sediments produced beneath deglaciating ice sheets offers insights into inaccessible glacial processes. Large subglacial valleys formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. Obtaining a better understanding of these features may help with the parameterisation of basal melt rates and the interplay between basal hydrology and ice dynamics in numerical models of past, present, and future ice-sheet configurations. However, the mechanisms and timescales over which tunnel valleys form remain poorly constrained. Here, we present a series of numerical modelling experiments, informed by new observations from high-resolution 3D seismic data (6.25 m bin size, ∼4 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate subglacial water routing, seasonal water discharges, and the rates at which tunnel valleys are eroded beneath deglaciating ice sheets. Networks of smaller or abandoned channels, pervasive slump deposits, and subglacial landforms are imaged inside and at the base of larger tunnel valleys, indicating that these tunnel valleys were carved through the action of migrating smaller channels within tens of kilometres of the ice margin and were later widened by ice-contact erosion. Our model results imply that the drainage of extensive surface meltwater to the ice-sheet bed is the dominant mechanism responsible for tunnel valley formation; this process can drive rapid incision of networks of regularly spaced subglacial tunnel valleys beneath the fringes of retreating ice sheets within hundreds to thousands of years during deglaciation. Combined, our observations and modelling results identify how tunnel valleys form beneath deglaciating mid-latitude ice sheets and have implications for how the subglacial hydrological systems of contemporary ice sheets may respond to sustained climate warming.

Surface topographic impact of subglacial water beneath the south polar ice cap of Mars

Bright radar reflections observed in the Ultimi Scopuli region of Mars’ south polar layered deposits1,2,3 by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument have been interpreted as the signature of areas of subglacial water beneath it. However, other studies put forward alternative explanations, which do not imply the presence of liquid water4,5,6. Here we shed light on the issue by looking at the surface topography of the region. On Earth, reduced or absent basal friction, and consequent ice velocity changes, cause a distinct topographic signature over subglacial lakes7. Using Mars Orbiter Laser Altimeter data8, we identify and characterize an anomaly in the surface topography of the south polar layered deposits overlying the area of the putative lakes, similar to those found above terrestrial subglacial lakes of similar size. Ice flow model results suggest that comparable topographic anomalies form within 0.5–1.5 Myr with locally elevated geothermal heating9 or 2–5 Myr without elevated geothermal heating2. These findings offer independent support for the presence of basal water beneath Ultimi Scopuli and suggest that surface topography could supplement radar returns to help identify other potential subglacial water bodies.

Ground-penetrating radar survey of subsurface features at the margin of ice sheet, East Antarctica

The subglacial environment of East Antarctica remains largely unexplored. In this study, ground-penetrating radar (GPR) data of the ice-sheet margin in East Antarctica were collected and analysed, an average velocity of 0.18 m/ns was obtained through ice density conversion and common midpoint (CMP) survey. The thickness, internal layers, and bed elevation were used to determine the spatial characteristics of snow/firn in the survey area. The relative amplitudes of the reflections along the bedrock interface were computed to determine the potential presence of subglacial water in depressions, which is the most likely source of strong reflections at basal interfaces. Because the subglacial topography is associated with the dynamic characteristics of the ice sheet, the two-parameter roughness index was used to assess the regional roughness of the bedrock beneath the snow/firn and discuss the implications of changes at the bottom. This research provides insights into the physical characteristics of ice-sheet margins. The methodology proposed in this study should be applied in the future to quantify the physical parameters required for ice-sheet studies.

Persistent, extensive channelized drainage modeled beneath Thwaites Glacier, West Antarctica

Abstract. Subglacial hydrology is a leading control on basal friction and the dynamics of glaciers and ice sheets. At low discharge, subglacial water flows through high-pressure, sheet-like systems that lead to low effective pressures. However, at high discharge, subglacial water melts the overlying ice into localized channels that efficiently remove water from the bed, thereby increasing effective pressure and basal friction. Recent observations suggest channelized subglacial flow exists beneath Thwaites Glacier, yet it remains unclear if stable channelization is feasible in West Antarctica, where surface melting is nonexistent and water at the bed is limited. Here, we use the MPAS-Albany Land Ice model to run a suite of over 130 subglacial hydrology simulations of Thwaites Glacier across a wide range of physical parameter choices to assess the likelihood of channelization. We then narrow our range of viable simulations by comparing modeled water thicknesses to previously observed radar specularity content, which indicates flat, spatially extensive water bodies at the bed. In all of our data-compatible simulations, stable channels reliably form within 100–200 km of the grounding line and reach individual discharge rates of 35–110 m3 s−1 at the ice–ocean boundary. While only one to two channels typically form across the 200 km width of the glacier in our simulations, their high efficiency drains water across the entire lateral extent of the glacier. We posit the large catchment size of Thwaites Glacier, its funnel-like geometry, and high basal melt rates together accumulate enough water to form stable channels. No simulations resembled observed specularity content when channelization was disabled. Our results suggest channelized subglacial hydrology has two consequences for Thwaites Glacier dynamics: (i) amplifying submarine melting of the terminus and ice shelf while (ii) simultaneously raising effective pressure within 100 km of the grounding line and increasing basal friction. The distribution of effective pressure implied from our modeling differs from parameterizations typically used in large-scale ice sheet models, suggesting the development of more process-based parameterizations may be necessary.

Controls on Greenland moulin geometry and evolution from the Moulin Shape model

Abstract. Nearly all meltwater from glaciers and ice sheets is routed englacially through moulins. Therefore, the geometry and evolution of moulins has the potential to influence subglacial water pressure variations, ice motion, and the runoff hydrograph delivered to the ocean. We develop the Moulin Shape (MouSh) model, a time-evolving model of moulin geometry. MouSh models ice deformation around a moulin using both viscous and elastic rheologies and melting within the moulin through heat dissipation from turbulent water flow, both above and below the water line. We force MouSh with idealized and realistic surface melt inputs. Our results show that, under realistic surface melt inputs, variations in surface melt change the geometry of a moulin by approximately 10 % daily and over 100 % seasonally. These size variations cause observable differences in moulin water storage capacity and moulin water levels compared to a static, cylindrical moulin. Our results suggest that moulins are important storage reservoirs for meltwater, with storage capacity and water levels varying over multiple timescales. Implementing realistic moulin geometry within subglacial hydrologic models may therefore improve the representation of subglacial pressures, especially over seasonal periods or in regions where overburden pressures are high.

A newly discovered subglacial lake in East Antarctica likely hosts a valuable sedimentary record of ice and climate change

The Princess Elizabeth Land sector of the East Antarctic Ice Sheet is a significant reservoir of grounded ice and is adjacent to regions that experienced great change during Quaternary glacial cycles and Pliocene warm episodes. The existence of an extensive subglacial water system in Princess Elizabeth Land (to date only inferred from satellite imagery) bears the potential to significantly impact the thermal and kinematic conditions of the overlying ice sheet. We confirm the existence of a major subglacial lake, herein referred to as Lake Snow Eagle (LSE), for the first time using recently acquired aerogeophysical data. We systematically investigated LSE’s geological characteristics and bathymetry from two-dimensional geophysical inversion models. The inversion results suggest that LSE is located along a compressional geologic boundary, which provides reference for future characterization of the geologic and tectonic context of this region. We estimate LSE to be ~42 km in length and 370 km2 in area, making it one of the largest subglacial lakes in Antarctica. Additionally, the airborne ice-penetrating radar observations and geophysical inversions reveal a layer of unconsolidated water-saturated sediment around and at the bottom of LSE, which—given the ultralow rates of sedimentation expected in such environments—may archive valuable records of paleoenvironmental changes and the early history of East Antarctic Ice Sheet evolution in Princess Elizabeth Land.

A dynamic saline groundwater system mapped beneath an Antarctic ice stream

Antarctica's fast-flowing ice streams drain the ice sheet, with their velocity modulated by subglacial water systems. Current knowledge of these water systems is limited to the shallow portions near the ice-bed interface, but hypothesized deeper groundwater could also influence ice streaming. Here, we use magnetotelluric and passive seismic data from Whillans Ice Stream, West Antarctica, to provide the first observations of deep sub-ice stream groundwater. Our data reveal a volume of groundwater within a >1-kilometer-thick sedimentary basin that is more than an order of magnitude larger than the known subglacial system. A vertical salinity gradient indicates exchange between paleo seawater at depth and contemporary basal meltwater above. Our results provide new constraints for subglacial water systems that affect ice streaming and subglacial biogeochemical processes.

Formation of murtoos by repeated flooding of ribbed bedforms along subglacial meltwater corridors

Fluctuations in meltwater discharge below modern glaciers and ice sheets due to diurnal, seasonal and long-term temperature variations are modulated by complex interactions between subglacial drainage, basal processes and bedform development. The bed of palaeo-ice sheets contains a variety of bedforms recording these modulations and provides an open window into the subglacial environment. Through the morphometric analysis of natural and experimental bedforms, respectively mapped along Scandinavian meltwater corridors and produced in a physical model simulating transitory subglacial water flow, we observe a morphological and genetic bedform continuum corresponding to the progressive transformation of ribbed bedforms into murtoos. Two alternating drainage configurations, related to repeated subglacial flooding events, are involved in this transformation: (i) significant meltwater discharge, high hydraulic connectivity and ice-bed decoupling during flooding events lead to hydraulic alteration of ribbed bedforms by erosion, sediment deposition and channel incision, while (ii) limited meltwater flow, low hydraulic connectivity and ice-bed recoupling that follow flooding events lead to their deformational reshaping into murtoos. The degree of transformation of ribbed bedforms into murtoos can be quantified by combining two dimensionless morphometric parameters (circularity and sinuosity) and provides a convenient proxy to constrain magnitudes, durations and/or frequencies of subglacial floods in palaeo-meltwater corridors.

Subglacial Water Flow Over an Antarctic Palaeo‐Ice Stream Bed

AbstractThe subglacial hydrological system exerts a critical control on the dynamic behavior of the overlying ice because its configuration affects the degree of basal lubrication between the ice and the bed. Yet, this component of the glaciological system is notoriously hard to access and observe, particularly over timescales longer than the satellite era. In Antarctica, abundant evidence for past subglacial water flow over former ice‐sheet beds exists around the peripheries of the ice sheet including networks of huge channels carved into bedrock (now submarine) on the Pacific margin of West Antarctica. Here, we combine detailed bathymetric investigations of a channel system in Marguerite Trough, a major palaeo‐ice stream bed, with numerical hydrological modeling to explore subglacial water accumulation, routing and potential for erosion over decadal‐centennial timescales. Detailed channel morphologies from remotely operated vehicle surveys indicate multiple stages of localized incision, and the occurrence of potholes, some gigantic in scale, suggests incision by turbulent water carrying a significant bedload. Further, the modeling indicates that subglacial water is available during deglaciation and was likely released in episodic drainage events, from subglacial lakes, varying in magnitude over time. Our observations support previous assertions that these huge bedrock channel systems were incised over multiple glacial cycles through episodic subglacial lake drainage events; however, here we present a viable pattern for subglacial drainage at times when the ice sheet existed over the continental shelf and was capable of continuing to erode the bedrock substrate.

3D Interpretation of a Broadband Magnetotelluric Data Set Collected in the South of the Chinese Zhongshan Station at Prydz Bay, East Antarctica

Antarctica is covered by a thick ice sheet, and the application of geophysical methods is necessary to image the subglacial structures for studying the hydrologic systems and tectonic deformations in the Antarctic continent. The magnetotelluric (MT) method is one of the best approaches to obtain the subglacial electrical resistivities. However, only a very small volume of data has been collected so far in Antarctica using this method. In this paper, we report on a broadband MT profile collected at 10 sites in the south of the Chinese Zhongshan Station at Prydz Bay, and a 3D resistivity model was constructed by inversion of these data. This 3D model shows two low resistivity zones at a depth shallower than 6 km. They are consistent with the low-velocity zones in the previous shear-wave model and can be interpreted as the result of interconnected fluids associated with a downward migration of subglacial water. In addition, a distinct eastward dipping low resistivity zoneis present in the crust, which extends from the top surface to the lower crust. Since its location coincides with the highly positive magnetization named Amery Lineament, it is proposed that this eastward dipping low resistivity zoneextending in the entire crust probably reveals the geometry structure of the Amery Lineament at depth. Besides, it can be inferred from this new 3D resistivity model that the Amery Lineament is at least a crustal-scale structure, which probably outcrops on the land surface but was covered by the ice sheets in the study area.

Polarimetric Airborne Radar Sounding as an Approach to Characterizing Subglacial Röthlisberger Channels

Conventional airborne radar sounding techniques are well suited to the detection and characterization of flat-lying, specular subglacial water bodies. However, topographically positive, diffusively scattering Rthlisberger (R-)channels are more difficult to image, while also exerting substantial control on basal friction and ice dynamics. As subglacial R-channels share geometrical similarities with targets of interest in polarimetric ground-penetrating radar studies (i.e., cylindrical pipes), in this study we investigate whether similar concepts can be adapted to detect and characterize R-channels. While closed form analytical solutions exist for the scattering widths of perfect electrically conducting (PEC) and dielectric circular cylinders, the insight they provide for the polarimetric response of half-cylinder R-channels is limited. As such, a series of modeling experiments have been performed to characterize the scattering widths of half cylinders. Our results demonstrate that scattering from subglacial R-channels depends on numerous factors including the polarization and frequency of the incident radar wave, the size of the R-channel, and the relative orientation of the R-channel to a pair of orthogonally oriented, linearly polarized radar antennas. The results imply that patterns in the like-polarized echo powers across the signal bandwidth may be useful in inferring R-channel existence and possibly estimate R-channel size. However, as differences in scattering width are small and decrease with increasing misalignment between the acquisition system antennas and R-channel strike, achieving a high degree of radiometric resolution and stability should be a driving factor in polarimetric radar system design.

A Novel Approach to the Detection and Imaging of Candidate Martian Subglacial Water Bodies by Radar Sounder Data

Research based on Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data detected unusual radar bright basal reflections located at about 1.5 km depth in a Mars region denoted as Ultimi Scopuli. These reflections were interpreted as a signature of subglacial liquid water even though this interpretation is still being debated in the literature. In this article, we propose a novel approach to the detection and imaging of candidate subglacial liquid water from radar sounding data. The approach combines the radar echo power traces with a suitable digital elevation model to provide a bidimensional representation of the surface. Even if the imaging method reconstructs a representation of the surface, we prove that it can be used to identify subsurface bright reflections in icy regions. Imaging is feasible even if the basal interface is not directly included in the processed data for image generation. To support this experimental evidence, we show that a relationship exists between the value of the reflected echo power originating from the englacial layers and the basal-to-surface-echo-power ratio. The observed relationship holds on both Ultimi Scopuli radar sounding data acquired on Mars and Lake Vostok data acquired on Earth. Our results show that the 2-D imaging provides an alternative way for locating candidate subglacial liquid water bodies on Mars over large areas also where the basal interface is not directly measured. The proposed approach complements previous research for further evaluation of the actual presence of liquid water on Mars.

A Semiautomatic Method for Predicting Subglacial Dry and Wet Zones Through Identifying Dry–Wet Transitions

In the past decades, radio-echo sounding (RES) data have been used to predict basal dry-wet distributions in glaciated regions through manual inspection of the records. Extending such work, we propose a semi-automatic method for predicting such distributions. The method improves previous work in two ways: (1) subglacial water bodies are taken as reference to correct the thresholds of dry and wet beds identification at a regional scale; and (2) five distinct features are defined and used to automatically identify the dry-wet transition, allowing a classification model based on a support vector machine. To demonstrate its effectiveness, the method is applied to airborne RES data collected in recent years over Princess Elizabeth Land in East Antarctica. A comparative analysis of the new vs previous method was carried out in the Ridge B region of the East Antarctica and at the Thwaites Glacier region of West Antarctica. The results show the method can obtain more accurate subglacial dry-wet distribution results with larger coverage and has the potential to determine dry-wet transitions at a continental scale if applied to the full set of known Antarctic RES data.

Water flow through sediments and at the ice-sediment interface beneath Sermeq Kujalleq (Store Glacier), Greenland

AbstractSubglacial hydrology modulates basal motion but remains poorly constrained, particularly for soft-bedded Greenlandic outlet glaciers. Here, we report detailed measurements of the response of subglacial water pressure to the connection and drainage of adjacent water-filled boreholes drilled through kilometre-thick ice on Sermeq Kujalleq (Store Glacier). These measurements provide evidence for gap opening at the ice-sediment interface, Darcian flow through the sediment layer, and the forcing of water pressure in hydraulically-isolated cavities by stress transfer. We observed a small pressure drop followed by a large pressure rise in response to the connection of an adjacent borehole, consistent with the propagation of a flexural wave within the ice and underlying deformable sediment. We interpret the delayed pressure rise as evidence of no pre-existing conduit and the progressive decrease in hydraulic transmissivity as the closure of a narrow (< 1.5 mm) gap opened at the ice-sediment interface, and a reversion to Darcian flow through the sediment layer with a hydraulic conductivity of ≤ 10−6m s−1. We suggest that gap opening at the ice-sediment interface deserves further attention as it will occur naturally in response to the rapid pressurisation of water at the bed.