Heterogeneous melting near the Thwaites Glacier grounding line

Thwaites Glacier represents 15% of the ice discharge from the West Antarctic Ice Sheet and influences a wider catchment1–3. Because it is grounded below sea level4,5, Thwaites Glacier is thought to be susceptible to runaway retreat triggered at the grounding line (GL) at which the glacier reaches the ocean6,7. Recent ice-flow acceleration2,8 and retreat of the ice front8–10 and GL11,12 indicate that ice loss will continue. The relative impacts of mechanisms underlying recent retreat are however uncertain. Here we show sustained GL retreat from at least 2011 to 2020 and resolve mechanisms of ice-shelf melt at the submetre scale. Our conclusions are based on observations of the Thwaites Eastern Ice Shelf (TEIS) from an underwater vehicle, extending from the GL to 3 km oceanward and from the ice–ocean interface to the sea floor. These observations show a rough ice base above a sea floor sloping upward towards the GL and an ocean cavity in which the warmest water exceeds 2 °C above freezing. Data closest to the ice base show that enhanced melting occurs along sloped surfaces that initiate near the GL and evolve into steep-sided terraces. This pronounced melting along steep ice faces, including in crevasses, produces stratification that suppresses melt along flat interfaces. These data imply that slope-dependent melting sculpts the ice base and acts as an important response to ocean warming.

Article glacier, that would help models at large and small scales better represent melting. Therefore, how melting occurs under ice shelves and particularly at the GL, influencing ice loss, remains largely unresolved.
As part of the National Science Foundation (NSF)-Natural Environment Research Council (NERC) International Thwaites Glacier Collaboration (ITGC), a comprehensive field campaign was carried out over two austral summers, with an ice-shelf-drilling campaign in 2019-2020 to access the ocean cavity 28 and sediments below the TEIS to observe the changing system directly. We conducted detailed in situ hydrographic measurements in an area of the TEIS referred to as the 'butterfly' 12 . The ice in this region is grounded at about 500 m below sea level ( Figs. 1 and 2), typical of most of the Thwaites system outside the western trunk. We deployed the new underwater vehicle Icefin (Extended Data Fig. 1) through the borehole over five under-ice missions spanning 9-11 January 2020. The vehicle measured ocean temperature, salinity, dissolved oxygen and current velocities ( Fig. 1 and Extended Data Fig. 2), mapped the sea floor and ice base in three dimensions (Figs. 1 and 2) and imaged the ice and sea floor (Fig. 3).

Conditions under the ice shelf
The ice base deepened with distance from the GL, ranging from about 500 m to 520 m below sea level over the nearly 3 km T1 survey (Fig. 1b) and sloping downward more steeply along T2 from a minimum of 475 m depth at the GL (Fig. 1c). The sea floor (prograde) also sloped downward away from the GL (Figs. 1 and 2). The temperature, salinity and dissolved oxygen content of ocean water reflects mixing of different reservoirs, including ocean, melted glacial ice (glacial meltwater (GMW)) and subglacial water (SGW) from beneath the grounded ice. Warm water occupied much of the ocean under the TEIS, with maximum thermal driving of 2.25 °C (ocean temperatures 2.25 °C above the in situ freezing point), decreasing only slightly to 2 °C within about 5-10 m of the ice base and 400 m of the GL (Fig. 1b,c). This decrease in thermal driving resulted from pressure release, a cooling of the water from 0 °C to −0.25 °C and a freshening from 34.50 g kg −1 to 34.40 g kg −1 (Extended Data Fig. 2). Dissolved oxygen, which reflects both exchange with the atmosphere before submerging below the ice and that released from melting ice, increased in concentration over this region from 4.47 ml l −1 to 4.50 ml l −1 ; the coupled change in hydrographic conditions indicates a slight increase in GMW closer to the ice (Extended Data Fig. 3). The relatively well-mixed water column was overlain by a stratified upper layer, generally 5-10 m thick, at which the ocean cooled, freshened and increased in oxygen owing to local ice melt producing a greater admixture of GMW.
The sea floor was primarily characterized by bedform ridges oriented north-south parallel to glacier flow (Fig. 2). Sea-floor ridge-crest spacing varies by an order of magnitude from 3-25 m and tens of centimetres to 10 m heights; most ridges have 0.5-2 m heights (Extended Data Figs. [4][5][6]. Sporadic boulders and drop stones are visible through the sediment (Fig. 3 and Supplementary Video 1). Near the borehole, troughs crosscutting the ridges suggest reworking of the sediment, which could occur if the glacier was pinned near this location that coincides with the estimated 2011 GL position 12 (Fig. 2a). Upstream of the borehole, a single semi-linear feature cuts across the along-flow ridges and crests, with a sharp step in depth of 2-3 m height (Fig. 2b); this is  12 ) demonstrate notable GL retreat over the past two decades (QGIS map: Landsat 8, 15 m pixel −1 , band 8 image LC08_L1GT_003113_20200131_20200211_01_T2_B8, 31 January 2020; the red box denotes the study region). b,c, Warm water is delivered close to the ice base (upper grey regions), shown by contours of thermal driving (degrees above in situ freezing point). The ice (black line) and seabed (brown regions) elevation profiles are measured by up and down altimetry from Icefin, which compare with bathymetry from mapping and forward sonar (Fig. 2). The small circles denote the Icefin track, along two transects approaching the GL, T1 (red) and T2 (blue) shown in the lower inset (red box from a). The yellow circle in the inset and vertical line through the ice denote the location of the borehole. The T1 track is oriented 5-10° oblique to the flow direction of the glacier and T2 approximately 50° oblique to flow; Icefin reached the grounded point of the glacier at the end of T2. Triangles in b and c mark historic GL locations estimated from satellite interferometry for 2011 (white), and the furthest downstream estimate in 2016 (blue) 12 . In b, the yellow triangle denotes the potential GL wedge detected by Icefin (Fig. 2). Nearest to the GL, although temperatures are colder than the deep water, the ocean water holds more than one degree of thermal driving. The ice base transitions from rough near the GL to terraced (progressively steeper-sided step-like features) near and downstream of the borehole, suggestive of progressive melting. Crevasses also contain terraces, especially clear in c.
downstream of all estimated GL positions for 2016-2017 (ref. 12 ). We interpret this feature as the sediment wedge produced when the ice shelf was grounded at this position, approximately 1,250 m from the end of the T1 survey and 1,500 m from the furthest upstream 2017 GL location. We observe no other clear evidence of GL wedges in this region. Thus, the bathymetry suggests that the GL retreated smoothly up the prograde sea floor, with only one stable location since at least 2011. Local variations in ice-shelf basal slope (topography) influence melting through modulation of near-ice ocean-density gradients (stratification) and small-scale turbulence that control ocean heat and salt fluxes [32][33][34][35][36][37][38] . Nearest the GL, the ice base comprises a system of short-wavelength ridges (Fig. 2e) that have similar shape and 2-5-m spacing of small-amplitude (0.1-0.5 m) ridges in the sea floor ( Fig. 2b-d and Extended Data Fig. 4) that overlay broad (about 50 m) topographic undulations. Within a kilometre of the GL, the ice surface is very rough, about 30% consisting of high slopes. Relatively clear ice laden with sediment, called basal ice, is found consistently in this region and in patches downstream, interrupting white, bubble-rich meteoric ice. The fine-grained (sand to mud) debris (Fig. 3a, right and Supplementary Video 1) and interspersed angular clasts ranging in size from a few to tens of centimetres comprises strong laminated layers in the basal ice at centimetre-scale spacing. Visible melting was observed throughout the ). c, An isolated 4-m-deep channel cut into the sea floor makes two sharp turns and includes a segment that cuts perpendicular to most bedforms, suggesting that this feature formed from rerouting of subglacial water as the GL retreated (Extended Data Fig. 5). d, The bedform topography near the GL of T2 shows evidence of linear ridges striking north (Extended Data Fig. 4). e, Forward-looking sonar data of the ice base near the GL shows that the ice has the same 2-5-m-wavelength ridges as the shortest-wavelength features on the sea floor. These data together suggest that GL retreat has been largely continuous over the observable period, since at least 2011 based on remote sensing. Moreover, the similarities between the bed and ice morphology at the GL suggest that the ice-bed interactions set up slopes that are then progressively melted by the intruding seawater. Bathymetric sonar, a-d, was processed in Qimera and projected using QGIS. Forward sonar are projected using the Oculus ViewPoint software.
Article region, with grains and small drop stones steadily falling out of basal ice, adding to the turbidity of the water column (Supplementary Video 1). Small terraces and scalloped morphology carved into the ice appear within 200 m of the GL, indicating that melting rapidly erodes these sloping ice faces. The steep faces grow in vertical scale with distance from the GL, showing progressive evolution of the shape of the ice from melting the longer it is exposed to the warm ocean. The rough ice base observed at the GL erodes downstream, giving way to steep-sided, flat-roofed terraces ( Figs. 1 and 3). The walls of these features form up to 90° angles to their flat roofs and keels, rising tens of centimetres to more than 6 m in height (Fig. 1b,c) and uniformly exhibiting scalloped surface textures (Fig. 3b, right), indicative of turbulent ocean-driven melting 33 . Terraces are also observed in crevasses. Conversely, the downstream ice under the TEIS is extremely flat, with surface slopes less than 5° (Figs. 1, 4 and 5). Ice-shelf basal topography carved by melt has been observed elsewhere, such as keels and channels 29,[39][40][41][42] , including terraces at nearby Pine Island Glacier 29 associated with steep slopes along marginal and channel features that were argued to form by means of feedback between slopes and melting. We observed terraces distributed across the TEIS, in many different orientations and across a range of horizontal and vertical scales, both associated with and independent of other basal features. Our observations argue that terraces are widespread basal features of ice shelves that overlay warm ocean cavities; these are not yet represented in most ice-shelf models.

Ice-ocean interactions
In situ observations of the undisturbed ice-ocean boundary layer beneath ice shelves are inherently difficult to make through boreholes  The Icefin vehicle track is shaded by relative along-track distance from downstream (white) to upstream (black). Light-blue data denote regions with cooling and freshening in terraces and dark blue denotes the coldest/freshest data observed. a, Conditions in the near-GL water cavity show the influence of melting (freshening) close to the GL along T2 (left). Coloured stars denote close passes to the ice that also have distinct signatures of mixing and melting. Vertical profiles of thermal driving (Θ − Θ f ), absolute salinity (S A ) and dissolved oxygen (DO) binned with distance from the ice base show complex signatures that vary with location (Extended Data Fig. 3), suggesting the influence of both melting and SGW outflow (centre). Imagery near the GL (red box) shows ridged ice topography and sediment-laden clear basal ice at the GL (yellow star) (right). Scale bar, approximately 0.5 m. b, Ocean conditions in a large terrace formed in the ice base imply melting near the sidewalls (red boxes, 800 m from the GL along T2) (left). Warm, salty water (black, grey) is found along the sidewalls, whereas much fresher and more oxygenated water with low thermal driving (cold relative to in situ freezing) collects in the terrace roof (centre). Imagery of terrace sidewalls across the TEIS uniformly show scalloped surfaces reflecting turbulent melting (Extended Data Fig. 8 and Supplementary Video 1) (right). Scale bar, approximately 0.5 m. c, As in b but for a small terrace at 2,400 m downstream along T1 that contains cold, fresh and oxygen-rich water along its roof. Here the water becomes supercooled, with ice crystals forming laterally (right) across the heavily stratified interface (red box) between this 0.1 m upper boundary layer and the warm, saline and more oxygen-poor lower ocean waters. Scale bar, approximately 0.1 m. owing to contamination from the heated freshwater used to drill the access hole. Before this work, no in situ measurements existed that could constrain behaviour at the GL. To make these observations, we drove Icefin along the base of the ice to capture the boundary layer along flat interfaces, at an angle towards and then contacting the ice to measure gradients up to the ice, and straight into vertical sidewalls, in some cases measuring within about 5 cm of the interface.
Throughout the region, thermal driving was about 1.75 °C within 1 m of the ice base, providing ample heat to drive melting (Methods). Generally, the near-ice water column under the TEIS closely fits the well-defined mixing lines between GMW and a source water mass, and observations imply fully developed turbulent mixing 35,37 (Methods and Extended Data Fig. 3), although data nearest the ice reflect increased melt. Our observations show strong vertical stratification approaching

Fig. 4 | Ocean currents and ice topography contribute to variable melting in terraces and crevasses.
Here the Icefin vehicle track is shaded by relative along-track distance from downstream (white) to upstream (black) and current velocities are shaded from slowest (white) to fastest (purple). a, Horizontal and vertical trends near a corner of a wide terrace (1,900 m downstream in T1 near the borehole) show freshening and cooling water inside the terrace and slowing currents as the water feels the influence of the ice interface. The grey lines denote the bottom of the terrace. Vertical profiles of ocean-current speed (U), thermal driving (Θ − Θ f ), absolute salinity (S A ) and dissolved oxygen (DO) binned with distance from the ice base show that, although the water is warm close to the interface, the current velocity slows in the boundary layer, suggesting breaking from friction at the interface 28 . b,c, As in a for the furthest crevasse from the GL, observed along both T1 (b) and T2 (c). The panels on the right are binned with distance from the top of a step in the crevasse sidewall along T1 marked with the upper grey line. The lower grey line indicates the elevation of the bottom of the crevasse in T1. Stars in b relate to the location in the left panel. These panels show warm water with thermal driving of nearly 1.8 °C (Θ − Θ f ) reaching the crevasse walls accompanied by very slight freshening and oxygen increase that indicate melting (S A and DO) that would then rise into the crevasse.
Article flat portions of the ice-ocean interface containing GMW formed from melting along neighbouring slopes rising to the ice base (Figs. 3 and 4). Ocean currents weaken within 5 m of the ice from a background speed near 3 cm s −1 (ref. 28 ) to near zero close to the interface (Fig. 4a). By contrast, currents increased in crevasses to a measured maximum of 5.90 cm s −1 (Fig. 4b,c).
In the terraces, dissolved oxygen increases with decreasing temperature and salinity, consistent with input from melting ice. Some of the strongest stratification we observed was in a shallow terrace formed along the roof of another large terrace, at which the salinity of the boundary layer was 20 g kg −1 , or roughly one-third fresher than the surrounding ocean water. Extremely fresh layers (36-42% freshwater) in recesses along terrace roofs are not fully turbulent, as salinity and dissolved oxygen exhibit much larger signatures than temperature, suggesting a regime in which diffusive processes control heat and salt flux 43 . The thicknesses of these fresher layers are on the order of tens of centimetres and probably reflect the transition between the fully turbulent outer and viscous inner portions of the ice-ocean boundary layer 44 .
The water closest to the GL is cooler and fresher than the surrounding ocean (excluding freshwater in terrace roofs), with a dissolved oxygen signature distinct from elsewhere in the region. These data have a shallower temperature-salinity (T-S) slope of 2.05 °C (g kg −1 ) −1 than the melt mixing line (roughly 2.5 °C (g kg −1 ) −1 ) and decrease in dissolved oxygen with freshening (Extended Data Fig. 3). This admixture of fresh, oxygen-poor water suggests the presence of discharged SGW from upstream of the GL 45 . Although no SGW source is observed directly, bathymetry near the GL along T1 suggests a recent subglacial channel (Fig. 2c), and SGW outflow measured downstream varies over time 28 . Estimates of SGW concentration calculated from T-S and DO-S properties indicate maximum values of 7 ml l −1 and 24 ml l −1 , respectively. The much higher SGW estimate implied from DO-S suggests that the debris-laden basal ice prevalent near the GL is also low in oxygen (such as ref. 45 ) and originated as SGW that was accreted in the overdeepened basin further upstream (Extended Data Fig. 7).
To test the impact of melting in the region, we calculated melt rates assuming shear-driven turbulent mixing, according to local-ice base slope and using current speeds and hydrographic conditions that were averaged over regions with similar conditions (denoted in Fig. 5). We compared these to results from three autonomous phase-sensitive radio echo sounders (ApRES) and the oceanographic mooring at the borehole 28 (Fig. 5 and Methods). This approach using regionally averaged ocean conditions (Methods) yields average upward melt rates of 5 m year −1 , but melt in the region is highly variable (Figs. 5 and 6). The stratification suppresses melting along flat interfaces, whereas estimated melt rates along vertical faces approach 30 m year −1 . Although stratification suppresses melting upward (moderate vertical melt or thinning), higher lateral turbulent mixing 46 and destabilizing rising GMW 36,47,48 allows warm water to reach sloped surfaces and promote melting (high sideways melt; Figs. 5 and 6). The scalloped ice surfaces observed only on steep faces is consistent with high sideways melting ( Fig. 3b and Extended Data Fig. 8). Melting is strongest along the near-vertical walls of crevasses, at which water 1.8 °C above freezing was observed reaching within 1 m of the vertical crevasse wall (Fig. 4b).
Water cools with height in the middle of the crevasses, freshens and becomes more oxygenated, suggesting a local accumulation of meltwater exceeding 3 ml l −1 from the erosion of the crevasse walls. Currents were faster in the crevasses by up to a factor of two compared with the background 28 , with flow speeds reaching about 6 cm s −1 . These observations imply melt rates along the crevasse sidewalls of up to 43 m year −1 in one crevasse at the location of these observations (Methods), whereas melting elsewhere is more suppressed (Fig. 6; ref. 28 ).

Topographic controls on ice-shelf evolution
These results indicate that ice-ocean interactions under the TEIS are influenced by even small-scale ice topography, which would extend to other warm-based ice shelves in which low to moderate current speeds allow high levels of near-ice ocean stratification to persist. We calculate moderate average upward melting along flat surfaces at 5 m year −1 , which matches measured melt rates on similar interfaces from three long-term ApRES and mooring data 28 and are consistent with the historical estimates from ice-penetrating radar 18 . Nearest to the GL along each survey line, melt rates average 2 m year −1 but range from 1 to 10 m year −1 (Figs. 5 and 6). Our observations show that the feedback between ice slope and melting is relevant to the entire base of ice shelves, including near the GL. The varied topography of the ice base at the GL, carved as it flowed over the bed before reaching the ocean, becomes a broadly distributed network of sloped ice surfaces along which melting is promoted.
Our observations suggest that melting along sloped ice is an important factor in the total ice loss near the GL of Thwaites Glacier. In the region surveyed, 27% of the total melting occurs along slopes that are greater than 30° (Fig. 6). Because crevasses funnel water through them 28 at rates that can efficiently transfer heat and salt into the steep crevasse walls (Fig. 4), these locally high melt rates should widen both crevasses and basal rifts across the glacier, including the TGT and the TEIS, and could contribute to increased calving of the glacier 8,10 . The rough topography near the GL may enable melting to persist in this region despite low current speeds. Our work implies that basal melting of warm-based ice shelves is heterogeneous and exploits ice topography inherited from interactions with the bed and formed by crevassing. Such effects are challenging to observe, not yet captured in models of GL retreat and probably contribute to the loss of ice elsewhere along the Antarctic coast.

Online content
Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-022-05691-0.  Red curves are from T1 and blue curves are from T2. b, Sideways melting along slopes greater than 30° contributes an estimated 27% of the melting under the TEIS, whereas these slopes account for only 9% of the ice base. Upward melting along low slopes is still the most notable source of melting, in which slopes less than 30° account for 73% of melting, while representing 91% of the ice.

Icefin vehicle
The Icefin vehicle 49 is a modular, hybrid, remotely operated vehicle with autonomous capabilities that was designed for use through holes bored or melted in ice (Extended Data Fig. 1). For this work, Icefin was fitted with sensors for scientific analysis of the ice-ocean system and navigation (Extended Data Table 1). Most water-column sensors are located in the vehicle nose to provide undisturbed water flow. Forward cameras, lights and sonar provide perception for science and navigation, and bathymetric sonar maps sea-floor geometry. A high-definition camera and Doppler velocity logger (DVL) with acoustic Doppler current profiler (ADCP) point in the same sense, and altimeter in the opposite sense, in the navigation module that can be oriented down (sea-floor facing) or up (ice facing). A rear-facing camera monitors the tether/ tail. Guidance, navigation and control of Icefin allow for geolocated scientific data through the fusion of an advanced fibre-optic gyroscope inertial measurement unit, compass, DVL, altimeter and pressure sensor providing low-level motion control and high-level localization. The five thrusters provide five degrees of freedom movement, control pitch, yaw, heave and sway without protruding surfaces, and permit hovering. Icefin is rated to 1.5 km depth, weighs 120 kg in air, is 23 cm wide and 4.5 m long. At the TEIS, Icefin was deployed vertically from an articulating launch and recovery system, lowered through the borehole by means of a 3.5-km-long 4-mm-diameter Kevlar-reinforced (1,800 lb breaking strength; Linden) fibre optic attached to a strength-enforced termination at the vehicle, enabling control, communication and data retrieval. Icefin was deployed for five 6-8-h missions during 11-14 January 2020 to map environmental gradients in cross-section extending seaward from the GL, overlapping surface, airborne and ApRES surveys. The Icefin data are grouped into two composite profiles, transect 1 (T1) and transect 2 (T2); T1 consists of missions 1 (about 1.2 km south), 2 (about 1.9 km south) and 3 (about 1 km north) and T2 is mission 4 (about 1.6 km southeast). Missions 1-4 were conducted with the navigation module down and mission 5 with the navigation module up. Multipath in the clear basal ice reduced vehicle positioning accuracy in mission 5. T2 intersected the GL at 104.780° W, 75.214° S. For missions 1 and 3, the vehicle conducted survey segments alternating between pitching up towards the ice and down towards the sea floor at about 20-30° to acquire hydrographic profiles while making forward progress, similar to ocean gliders.

Post-processing of hydrographic data
Hydrographic data come from three sensors on Icefin: conductivitytemperature (C-T), pressure (P) and DO sensors. All sensors were factory calibrated before the fieldwork. C-T and DO sensors were field calibrated. Pressure measurements (1 Hz) were interpolated to match the 5-Hz C-T data to derive hydrographic variables. DO measurements (1 Hz) were not interpolated.
Data post-processing. 1. Remove background atmospheric pressure, 2. Remove outliers ±2 standard deviations from mean for C-T and DO (excluding borehole data), 3. Apply three-point weighted mean filter to C-T, P and DO, 4. Align C and T measurements with time lag (0 s lag produced the best results), 5. Remove C, T and DO data for vehicle speeds <5 cm s −1 , 6. Derive hydrographic variables (conservative temperature, absolute salinity, density and so on) using TEOS-10 (ref. 50 ), 7. Remove pressure/salinity effects on DO 51 .

Post-processing ocean-current speeds
Ocean-current speeds are derived from the onboard DVL/ADCP, which calculates the X, Y and Z vehicle velocities (major, minor and vertical axes) and retrieves water-column velocities in 2-m bins at a variable start distance from the vehicle. The minimum altitude from an interface for current profiling to occur is 10 m; gradients in vehicle pitch, roll, heading and speed dictate the distance of the first bin from the vehicle and sampling frequency (maximum 5 Hz). We subsample velocities to 1 Hz. Water-column X velocities are differenced from the vehicle velocity, resulting in an uncertainty of ≤1 cm s −1 . Vehicle Y and Z velocities are substantially lower than X, so Y and Z velocity uncertainties are likewise lower. Here we only analyse X and Y velocities.
Data post-processing: 1. Remove data in out-of-range bins (for example, below sea floor, above ice base), 2. Remove spurious data: exactly 0 m s −1 or 32,767 m s −1 , 3. Remove measurements when vehicle pitch or roll >+/−30°, 4. Convert from vehicle reference frame to geographic reference frame, 5. Apply 30 s mean filter, 6. Filter for gradients <1 standard deviation from mean vehicle speed, pitch, roll and individual bin velocity, 7. Collate bins by each up/down vehicle swoop into 1-m vertical bins, removing data >1 standard deviation of the mean for that range.

Post-processing of ice and sea-floor elevations
Ice-base and sea-floor elevations are derived from DVL and altimeter data and bathymetric sonar data. The DVL takes into account pitch, roll and heading when producing ranges and the altimeter and sonar data are corrected for these attitudes. Bathymetric sonar was processed in Qimera, in which obvious outliers were filtered or cleaned by hand. Data post-processing (DVL, altimeter): 1. Remove data >2 standard deviations of the mean gradient in interface elevation (ice base or sea floor), 2. Manually remove outliers.
After post-processing, 94% of ice-base measurements had horizontal resolution 26 ± 14 cm, with minimum and maximum spacings of 1.4 mm and 3.38 m, respectively. Ninety-three percent of sea-floor data had horizontal resolution 29 ± 20 cm and minimum and maximum spacings of 2.3 mm and 4.72 m, respectively.

Water-mass partition
We use a three-point endmember partition 35 to estimate concentrations of water masses below the shelf, which assumes that hydrographic properties (Θ, S A and DO) reflect a mixture of three water masses: 1. A source water mass (SRC) responsible for driving melt: Θ = −0. 21  This partition uses conservative tracers that only vary as a result of a physical mixture of water masses and mix under fully turbulent conditions 35,37 . Subsets of the data not in a fully turbulent mixing regime are excluded from the partition. These data are easily identified by double-diffusive characteristics-large changes in S A (and DO) relative to Θ that result from faster molecular diffusion of heat than salt 43,44 .
The source water mass (SRC, red star in Extended Data Fig. 3 and Fig. 4b) is the warmest, saltiest and most oxygen-depleted point in the T-S and DO-S space that fits the GMW mixing line on which our data lie: T-S and DO-S slopes of −2.49 °C (g kg −1 ) −1 and 0.60 ml l −1 (g kg −1 ) −1 , respectively. SRC generally resides 10 m below the ice base (outside crevasses). Warmer, saltier and more oxygen-depleted data (red data in Extended Data Fig. 3) further down in the water column exhibit a different slope in the T-S (−2.74 °C (g kg −1 ) −1 ) and DO-S (0.35 ml l −1 (g kg −1 ) −1 ) space, suggestive of mixing processes not derived from local glacial modification. Thus, SRC hydrographic properties represent the local water mass melting the ice shelf, referred to as a composite tracer 35 . SRC is a derivative mixture of modified CDW and winter water that resides around the depth of the highly variable pycnocline separating these water masses in Pine Island Bay. Relatively weak inflow of about 3 cm s −1 into our study region and at a distance of about 75 km from Pine Island Bay 28 results in a ventilation period of around one month for SRC. During this time Θ, S A and DO properties could be altered through interaction with the ice base along other sections of the TEIS, consumption by organisms or mixture with other water masses. We forgo choosing hydrographic properties for the absolute source water mass to ensure that the properties used in our three-point water-mass partition are conservative for local ice-ocean interactions.
The Θ and S A values for GMW consider the latent heat loss associated with phase change from solid to liquid freshwater. An extrapolation of the Gade or GMW mixing line 35,37 for our data to 0 g kg −1 results in an effective temperature of −86.46 °C. GMW is high in DO because of air bubbles in meteoric ice that dissolve into solution when melted. An extrapolation of the DO-S mixing line to 0 g kg −1 for our data results in a DO concentration of 25.20 ml l −1 .
The Θ and S A values for SGW represent freshwater at the pressure-depressed freezing point at GL depth of 480 m in T2 (Fig. 1c). We use in situ DO measurements from Lake Whillans, West Antarctica 45 for the SGW endmember, as active subglacial lakes exist upstream of the TEIS [53][54][55] . Thus, we expect that basal ice here should carry similarly low DO.

Three-equation melt parameterization
We estimate ice-shelf melt rates along T1 and T2 using the three-equation boundary layer (BL) parameterization for heat and salt transfer between the ocean and ice 56 , which assumes that shear-driven turbulence controls ice melt as the dominant mechanism conveying heat and salt to the viscous sublayer (VSL), beyond which molecular diffusion operates 57 . Near-ice ocean-current measurements exhibit shear (Fig. 4a), which agrees with the physics governing Ekman layers 28 and is consistent with shear-driven turbulence.
This assumption does not hold for regions with low current speeds, at which molecular processes and diffusive convection dictate heat and salt fluxes at distances greater than the typical VSL 44,58 , order several millimetres 56 . We only observed such conditions in two small regions along the roofs of two terraces with extremely fresh layers (S ≈ 20 psu) tens of centimetres thick (Fig. 3b,c) that did not hold fully turbulent mixing lines and exhibited larger salinity gradients than temperature. Other near-ice ocean data did not exhibit these thick, fresh layers but instead gradually cooled and freshened towards the ice throughout the surveyed area. This density change stabilizes the BL beneath regions of the ice base, reducing vertical shear-driven turbulent mixing 59 . Therefore, although shear-driven turbulence dominates heat and salt fluxes, turbulent fluxes are modified by stratification 47,60 . We account for stratification in our melt rates.
The three-equation BL parameterization is as follows: In equation (4), ρ w and T represent the density and temperature, respectively, of seawater outside the BL/VSL. For seawater, c p = 3,974 J kg −1 °C −1 , the friction velocity u * is the kinematic stress at the ice-ocean interface and the heat transfer coefficient Γ T describes turbulent mixing of heat across the BL. The density of ice is ρ i = 918 kg m −3 , L F = 3.34 × 10 5 J kg −1 is latent heat of fusion and ṁ is the ice-shelf melt rate (m year −1 ), in which ṁ is positive for melting. We use the quadratic stress formula to relate u * to near-ice current speeds 61 : in which C D = 2.2 × 10 −3 is a dimensionless drag coefficient assumed to be constant 27 and U is the current speed. Without a vertical-ice-column temperature profile, we vary Q I T as 0.12-0.2 of the magnitude of Q O T (refs. 42,62 ). (7) and (8) resemble equations (4) and (5), in which S is salinity outside the BL/VSL, S B is the ice-ocean interface salinity and S I is the salinity of the ice shelf (considered 0 here). The salt transfer coefficient Γ S is much smaller than Γ T , owing to slower molecular diffusion of salt than heat in the VSL 63 . We consider a range of published values for Γ S between 1 70 and 1 25 of Γ T (refs. 23,57,59 ) and choose the ratio that produces optimal agreement between melt rates derived by the heat flux and salt flux equations.
We estimated melt rates in five subregions based on ice-base characteristics and near-ice ocean conditions T, S and U (Fig. 5). We compiled integrated probability density functions of the ocean conditions in each subregion, then consider the 25th, 50th and 75th percentiles of T and S within 5 m of the ice base and the 50th, 75th and 100th percentiles of U within 10 m of the ice base for each subregion. We select higher percentiles (and therefore speeds) for U because observed ocean currents increased towards the ice (ref. 28 ; Fig. 4) before slowing from friction (ref. 28 ; Fig. 4) and because less current data were collected near the ice base. Extended Data Table 2 provides ocean properties entered into equations (4), (7) and (8). We then consider an array of ten values for the ice-ocean interface properties (T B , S B ) by varying S B from the minimum observed salinity to the 75th percentile of S for each subregion and then converted to T B using equation (1) for the ice-base mean pressure.
We estimate Γ T by dividing the published range of the thermal Stanton number (C Γ D 1/2 T = 2.18 × 10 −4 - 1.10 × 10 −3 ) from observations beneath ice shelves 24,55 by C D , yielding Γ T = 4.60 × 10 −3 -2.35 × 10 −2 , then considering that Γ S ranges between 1 70 and 1 25 of Γ T . We vary Γ T and Γ S from their minimum to maximum values based on the sine of the ice-base slope, from 0° to 90°, because slope/roughness of the ice interacts with stratification to produce variable melting 36,48,[63][64][65] , and rising buoyant GMW also destabilizes stratification along steep ice slopes. The equations by which we calculated this are as follows: We compared calculated melt rates to ApRES data at three locations 38 : T1 distance 2,250-2,857 m; region 2: T1 distance 1,810-1,904 m and T2 distance 1,480-1,608 m; and region 3: T1 distance 0-960 m. The set of conditions that produced the best fit to the observed melt rates along the flat (slope <5°) ice base were the 75th percentile of T and S, the 100th percentile of U and nearly the freshest S B and warmest T B , with heat conduction Q Q = 0.12  Tuning the melt rate uniformly over the survey region overestimates melt along portions of the ice base that are in diffusive regimes and possibly in the unmapped tops of crevasses; however, this is a small fraction of the region surveyed. Ocean properties will cool/freshen with height in the crevasses and current speeds may change compared with the lower 10% surveyed (ref. 28 ; Fig. 4). This approach however underestimates melt rates along steep ice slopes below the upper crevasse sections, because low S B (22.14 psu) and therefore high T B reduces assumed thermal forcing (T − T B ): the ApRES at the borehole measured lateral melt rates of 70 m year −1 on the terrace wall (mean slope 79°) at 1,800 m along T1, compared with 26.35 m year −1 estimated. Changing to a salinity of 34.28 psu (75th percentile for T1 subregion 3) obtains a melt rate of 45.97 m year −1 , closer to observed.
We also compared shear-driven melt rates to those from a buoyancy-driven turbulent melt-rate parameterization 36 :

Statistics
All statistics were performed in MATLAB. The above sections provide the details of these statistical analyses, with the underlying mean and standard deviation functions being native to MATLAB.

Data availability
The datasets generated and/or analysed during the present study have been submitted to https://www.usap-dc.org/ and are available at https://doi.org/10.15784/601618. The processing and figure-plotting scripts are available at https://doi.org/10.5281/zenodo.7278005. enhanced sediment deposition in this area. This is the only such feature in the survey data. f,g, Linear profiles across a possible former subglacial channel near the GL. In f, a 5-m-deep U-shaped trough in the sediment begins parallel to ice flow but then cuts perpendicular to ice flow across lineated bed features and then turns sharply, which could be consistent with a channel incised by subglacial outflow into the sediment. In g, the upstream extent of the trough is less conspicuous, suggesting either modification as the ice ungrounded or that the SGW was not routed discretely through this area. GT_003113_20200131_20200211_01_T2_B8, 31 January 2020; the red box denotes the study region. Note that the regions upstream of the present GL are grounded more than 800 m below sea level. b, As in a except coloured by the average 2019 seaward ice-flow speeds for this region from the 120 m pixel −1 ITS_LIVE remote-sensing product 66 . Note that the flow speeds increase as the ice crosses the GL and decrease near the pinning point.