A robotic submersible mapping the underside of the Dotson Ice Shelf in West Antarctica vanished in January 2024, halting a critical data collection effort aimed at understanding the dynamics of this rapidly changing region and its contribution to rising sea levels. The vehicle, named Ran, had previously delivered the most detailed subglacial maps ever recorded in the area, revealing previously undocumented features and challenging existing ice melt models.
The submersible was deployed as part of the International Thwaites Glacier Collaboration (ITGC), a joint US-UK research program focused on the West Antarctic Ice Sheet. Ran’s initial 27-day mission in early 2022 covered approximately 140 square kilometers beneath the Dotson Ice Shelf, utilizing multibeam sonar operating at a 50-meter distance from the ice base. The resulting data provided unprecedented insight into basal melt processes – how the ice shelf melts from below.
Mapping Reveals Complex Melt Structures
The sonar imagery revealed a complex landscape beneath the ice, characterized by flat terraces, deep melt channels, and smooth erosion zones. These formations varied significantly depending on their location and the characteristics of the surrounding water flow. In the central and eastern sections of the shelf, researchers identified stair-like terraces and plateaus with sharp vertical faces, indicative of slower-moving currents and low-turbulence melting. Conversely, the western regions exhibited smoother surfaces and signs of erosion attributed to faster, shear-driven flow.
Beyond these broad patterns, Ran detected a class of features previously unseen beneath Antarctic ice shelves: elongated, teardrop-shaped indentations ranging from 20 to 300 meters in length and up to 50 meters in depth. These features were concentrated in high-velocity outflow zones in the west. Researchers hypothesize that these teardrops are formed through boundary-layer flow rotation, potentially linked to Ekman dynamics – a phenomenon where rotating ocean currents generate asymmetric melt patterns. This hypothesis is supported by velocity and salinity measurements collected during the mission, which correlated flow behavior with observed erosion features.
The survey also identified full-thickness ice fractures extending through the base of the shelf, some of which have been visible in satellite imagery since the 1990s. These fractures appear to widen at depth and are often bordered by small-scale terraces and erosion features, suggesting persistent melt along their walls.
The findings, published in Science Advances, were corroborated by oceanographic data collected from ship-based sensors and a borehole, confirming that modified Circumpolar Deep Water – a relatively warm current – enters the cavity beneath Dotson and contributes directly to melt rates in the most affected zones.
Implications for Ice Shelf Stability and Sea Level Rise
The data collected by Ran demonstrates that melt beneath the Dotson Ice Shelf is not uniform. High melt rates, reaching up to 15 meters per year in some areas, were linked to warm water intrusion along deep sub-ice troughs and narrow channels. This focused melting is particularly pronounced along the western flank of the shelf, supporting earlier findings from satellite altimetry and ocean model simulations.
Between 1979 and 2017, the Dotson Ice Shelf contributed approximately 0.02 inches to global sea level rise, according to estimates published alongside the sonar data. Since the early 2000s, the shelf has lost nearly 390 gigatonnes of ice. These figures underscore the importance of understanding the processes driving melt in this region.
Loss of Submersible and Future Research
Ran re-entered the Dotson sub-ice cavity in January 2024 for a follow-up survey intended to assess changes since the initial 2022 mission. The vehicle was scheduled for a 24-hour dive, but no signal was received after its expected resurfacing time. Despite extensive acoustic searches, the submersible remains missing, and its final location is unknown. The British Antarctic Survey confirmed the loss and is reviewing potential causes, including equipment failure or collision with uncharted basal formations.
The loss of Ran represents a significant setback for the ITGC, pausing a broader data collection effort across the Amundsen Sea. While the original 2022 dataset remains intact and continues to inform revised estimates of basal melt and ice shelf retreat, the inability to conduct further surveys hinders ongoing research. The detailed imagery provided by Ran is crucial for refining ice melt models and improving predictions of future sea level rise. The discovery of the teardrop-shaped formations, in particular, highlights the need for more sophisticated models that account for the complex interplay between ocean currents and ice shelf geometry.
The incident underscores the challenges of conducting research in such a remote and extreme environment, but also the critical importance of continued investigation into the dynamics of West Antarctic ice shelves. The data gathered by Ran provides a valuable baseline for future studies and a clearer understanding of the processes driving ice loss in this vulnerable region.
