The melting of Thwaites Glacier, West Antarctica's second largest marine ice stream, significantly impacts future sea-level projections. Its bed deepens upstream to >2 km below sea level, and warm, dense deep water melts its ice shelves from below, making it susceptible to runaway retreat. Satellite observations since 2011 show the glacier's trunk speeding up, thinning, and widening, with spatially variable grounding-line retreat. These changes are partly due to reduced buttressing from weakened contact with a shallow ridge at the Eastern Ice Shelf terminus and fragmentation of the Thwaites Glacier Tongue (TGT). Future retreat depends heavily on grounding-zone processes, where the glacier comes afloat. Sea-floor topography can stabilize ice sheets, and grounding-line migration affects ice-sheet stability on various timescales. Thwaites is currently grounded on prominent sea-floor ridges, making the evolution of grounding-zone processes critical. However, these processes are poorly resolved, hindering our understanding of retreat from sea-floor stabilization sites. Studying former ice-sheet grounding zones provides insights into past retreat patterns and processes. Recent sonar mapping offshore of Thwaites Glacier documented numerous seabed promontories where the ice was previously grounded, offering ideal sites to study Thwaites' past behavior. Understanding pre-satellite era changes is crucial because current retreat partly represents adjustments to past imbalance. Direct observations of grounding-line change extend back only 30–40 years for most of West Antarctica, and data on Thwaites Glacier's history over the past 10,000 years are scarce. This study focuses on an isolated sea-floor promontory ('the bump') at the southwest corner of the remnant TGT, downstream of the glacier's fastest-flowing grounded section. The bump, at ~630-670 m water depth, is deeper than the keel depths of modern icebergs, making it less likely to have been disturbed by post-retreat processes. High-resolution geophysical data were acquired using an autonomous underwater vehicle (AUV) across the bump.

Previous research highlights the significant mass loss from Thwaites Glacier and its contribution to sea-level rise. Studies using satellite radar observations have documented the glacier's acceleration, thinning, and widening since 2011, attributing these changes to reduced buttressing from ice-shelf pinning points and grounding-line retreat. The role of sea-floor topography in stabilizing ice sheets and the impact of grounding-line migration on ice-sheet stability have been extensively investigated. However, the processes operating at marine ice-stream margins are poorly understood, particularly the rates and mechanisms of glacier retreat from sites of sea-floor stabilization. Studies of former grounding zones in other Antarctic regions, such as the Antarctic Peninsula, have revealed delicate landforms that record exceptionally fast past retreat rates. However, the deglaciation in those regions was driven partly by sea-level rise from Northern Hemisphere ice sheets, making them inappropriate analogs for the near-future state of the West Antarctic Ice Sheet. While satellite monitoring has improved the temporal resolution of grounding zone changes, reconstructing pre-1990s retreat rates remains challenging.

This study utilized data collected during the NBP19-02 cruise in the Amundsen Sea Embayment. The research involved the deployment of a Kongsberg HUGIN AUV equipped with a multibeam echo-sounder and sidescan sonar. The AUV mission covered approximately 13 km² of seafloor, acquiring high-resolution (sub-meter) bathymetry and acoustic imagery. The AUV mission 009 lasted 17h 26 min. Navigation was achieved using an onboard INS (inertial navigation system) coupled with Universal Transponder Positioning (UTP) units. Data processing involved several steps: extraction of navigational and attitude data; bathymetry data conversion; ping editing; and gridding. The initial gridding produced a 1.5 m grid, and further post-processing resulted in a 0.7 m grid for improved visualization and analysis. Sidescan sonar images were slant-range corrected and processed with a pixel size of 0.05 x 0.05 m. Geomorphological mapping involved identifying and classifying glacial landforms from the gridded multibeam datasets, guided by hillshade surfaces and sidescan sonar imagery. A semi-automated approach was used to extract landform statistics, involving profile extraction, detrending, filtering, and baseline digitization for rib height and spacing analysis. A tidal model (OSU TPXO-9) was used to compare the observed rib patterns with the predicted tidal amplitudes in the region. Fast Fourier Transform (FFT) analysis investigated the dominant frequencies in the rib data and the tidal series. The study also involved analyzing the rib geometries and correlating them with tidal cycles.

Sea-floor imagery revealed that 'the bump' is a former grounding zone of Thwaites Glacier. Geomorphological mapping identified three former grounding-zone fronts, traceable as ramps marking successive steps of ice-sheet retreat. Subglacial lineations, meltwater channels, and fans indicate that Thwaites Glacier was fully grounded through its present-day ice-shelf cavity. High-frequency sidescan data revealed parallel ribs transverse to the former ice flow direction, mainly relating to the most recent grounding surface. These ribs are subtle (<20 cm in height), but widespread. Analysis of the longest series of ribs (164 individual landforms) showed a 13–15 ridge periodicity, matching present-day tides in the region. Spectral analysis revealed a dominant frequency peak at 14.9 ridges, mirroring the 14-day spring-neap tidal cycle. The amplitude and spacing of the ribs correlate strongly with daily tidal heights (R² = 0.57), suggesting that tides modulated both parameters. The researchers propose a model where tidally modulated grounding-line retreat created the ribs: grounding at low water forms a rib, grounding-line migration occurs during high water, and the grounding line settles at a new position at the next low water, forming a new rib. The rib spacing indicates a grounding-line retreat rate of 2.12–2.3 km per year over a 5.5-month period—twice the average rate observed by satellite (0.6–0.8 km per year). The daily retreat rates ranged from 4 m during neap tides to 8 m during spring tides. The study suggests that low-angle bed and ice slopes are essential for rib formation. The findings imply that even small amounts of ice-shelf thinning could instigate rapid retreat phases when the grounding line sits on such high points. The ribs likely predate the 1950s, possibly being several centuries old.

The findings address the research question by demonstrating exceptionally rapid grounding-line retreat in the past, exceeding previously documented rates. The significance lies in the identification of a mechanism (tidally modulated retreat) that explains these high rates and provides a direct analog for the glacier's current grounding zone. The results highlight the importance of considering temporal variability in grounding-line behavior, including rapid, punctuated retreat phases, which are not fully captured by long-term average rates obtained through other methods. The study suggests that bed modification by erosion during grounding may have pre-conditioned Thwaites Glacier to rapid recession, increasing its sensitivity to tidal forcing. The implication is that similar rapid retreat events are likely in the future as the grounding zone migrates off stabilizing high points.

This study provides unique high-resolution paleo-data showing exceptionally rapid grounding-line retreat at Thwaites Glacier driven by tidally modulated processes. The rapid retreat rates observed (exceeding 2 km/year) underscore the potential for sudden and significant changes in the glacier's flow dynamics. The findings highlight the importance of including tidal migration and ice-plain formation processes in ice-sheet evolution models. This new understanding allows for improved prediction of Thwaites Glacier's retreat trajectory and its impact on future sea-level rise.

Establishing the precise timing of retreat across the bump and the age of the ribs requires further investigation through direct sampling. The assumption of a monotonic retreat rate back through time may affect the accuracy of the age estimate. The study focuses on a specific location and may not be fully representative of the entire glacier's retreat pattern. The proposed mechanism for rib formation (tidally modulated retreat) may not be applicable in all areas, depending on bed and ice slope conditions.