Tectonically inherited shelf bathymetry mediates ice-ocean interaction in Lützow-Holm Bay, East Antarctica

The ocean floor beneath the ice shelves of Antarctica is not a flat, featureless plain. It is a landscape carved by ancient forces and reshaped by moving ice, filled with deep trenches and shallow ridges that act as gatekeepers for the water flowing beneath the ice. This underwater topography is critical because it controls how warm water from the deep ocean reaches the base of the glaciers. When this warm water, known as modified Circumpolar Deep Water, flows into the deep channels, it melts the ice from below, causing the ice shelves to thin and the glaciers to retreat. However, for decades, scientists have struggled to see this hidden landscape clearly. Thick, persistent sea ice has blocked ships from sailing close to the glacier fronts, leaving large gaps in our maps. Without accurate maps of the depth and shape of these underwater channels, it is impossible to predict how the ocean will interact with the ice in the future, or to understand how the ice sheets behaved during past climate changes.

A team of researchers led by Masakazu Fujii and Yoshifumi Nogi from the National Institute of Polar Research in Japan has finally filled in some of these missing pieces for a specific region called Lützow-Holm Bay in East Antarctica. By taking advantage of rare summers when the sea ice broke up enough to let a ship through, they sailed close to the glacier fronts and used sound waves to map the seafloor. Their work reveals a complex network of deep troughs and shallow sills that were shaped by the underlying rock structure of the Earth's crust. They found that the depth of the channels leading to different glaciers varies significantly, creating natural barriers that can either allow or block the flow of warm water. Furthermore, the researchers discovered that the direction and location of these deep channels align with ancient cracks and boundaries in the bedrock, suggesting that the geology of the continent helped guide where the ice carved its deepest paths millions of years ago.

The researchers conducted their survey during three separate summers between 2016 and 2018, using the Japanese icebreaker Shirase to navigate the bay. They used a high-resolution single-beam acoustic system to obtain depth measurements along the ship tracks. They collected over 900,000 individual depth measurements, creating a detailed picture of the seafloor with improved positional accuracy compared to previous maps. To ensure these measurements were correct, they accounted for the speed of sound in the water, which changes with temperature and salinity. By combining their new data with older, less precise measurements taken from the sea ice in the past, they produced a comprehensive map of the bay's underwater geometry.

The new map shows a striking difference in the depth of the channels leading to the various glaciers in the bay. The main channel in front of the Shirase Glacier is deep and wide, reaching depths of over 1,000 meters and connecting directly to the deep ocean through an entrance that is approximately 740 meters deep. This deep connection means that warm water can flow relatively easily toward the base of the Shirase Glacier. In contrast, the channels leading to the Skallen, Telen, and Honnør glaciers are separated from the main deep trough by shallow underwater ridges, or sills. The entrance to the Skallen and Telen troughs is blocked by a sill about 580 meters deep, while the entrance to the Honnør trough is even shallower, at roughly 480 meters. These shallow ridges act as thresholds; they do not completely stop the warm water, but they make it harder for modified Circumpolar Deep Water to reach the glacier fronts. Whether the warm water can cross these thresholds depends on the ocean conditions at the time, but the physical shape of the seafloor sets the stage for these interactions.

The study also explored why these channels and ridges exist in their specific locations. The researchers compared the new bathymetric map with data on magnetic anomalies in the area, which reveal the structure of the ancient bedrock hidden beneath the ice and sediment. They found that the NW-SE-trending segment of the trough is spatially aligned with a major magnetic boundary in the Earth's crust. This boundary is a remnant of ancient tectonic activity when the supercontinent Gondwana broke apart. The alignment suggests that the ice did not carve these channels randomly; instead, the glaciers followed pre-existing weaknesses and fractures in the bedrock, deepening them over time. This means that the shape of the underwater landscape today is a result of both the ancient geology of the continent and the erosive power of the ice sheets that have flowed over it for thousands of years.

These findings have important implications for understanding how the Antarctic ice sheet responds to a warming climate. The depth of the sills and the alignment of the troughs determine how easily warm ocean water can reach the ice. If the water is deep enough to flow over the sills, it can accelerate melting and destabilize the ice shelves. The researchers suggest that the segmented nature of the troughs, with their varying depths and connections, creates a complex system where different glaciers may react differently to the same ocean conditions. The Shirase Glacier has a deeper and more continuous potential pathway for warm-water access, whereas shallower sills may restrict access to the Skallen, Telen, and Honnor glacier-front troughs. The actual effectiveness of these pathways depends on time-varying oceanographic conditions. This structural control helps explain why some parts of the ice sheet might retreat faster than others.

The paper does not claim to have solved the entire puzzle of ice-ocean interaction, nor does it provide a definitive prediction of future melting rates. Instead, it offers a crucial geometric framework that scientists can now use to build better computer models. By knowing exactly where the deep channels are and how deep the shallow barriers are, researchers can simulate the flow of warm water with greater accuracy. The study also highlights that the history of the Earth's crust is not just a record of the distant past; it actively shapes the present-day climate system by controlling the pathways of the ocean. The work underscores the value of rare opportunities to gather data in remote, ice-covered regions, showing that even small improvements in our knowledge of the seafloor can lead to a much clearer understanding of the forces driving change in Antarctica.