Mapping Pine Island Glacier's Sub-ice Cavity with Airborne Gravimetry

Pine Island Glacier, located on Antarctica’s Amundsen Sea coast, is among the main contributors to mass loss from the West Antarctic Ice Sheet. Over the past several decades, Pine Island Glacier has experienced rapid thinning, acceleration, and grounding line retreat. The circulation of relatively warm and dense circum polar deep water beneath the floating ice shelf in front of Pine Island Glacier is seen as the main cause for melt rates at the base of the ice shelf that exceed known melting beneath Antarctica’s largest ice shelves by two orders of magnitude. The interplay between basal melting beneath the floating ice shelf and the thinning and acceleration of grounded ice upstream remains poorly understood. Measurements from an autonomous underwater vehicle have shown that the circumpolar deep water flows directly into the sub-ice cavity below the Pine Island Glacier ice shelf (Jenkins et al. 2010). The precise shape and depth of the cavity, however, remain poorly understood. During 2009, NASA’s Operation IceBridge collected airborne gravity, ice-penetrating radar and laser altimetry data on a 5 km grid of flight lines over the floating ice shelf in front of Pine Island Glacier and the grounding line. The load carrying capability of NASA’s DC-8 long-range aircraft allowed us to acquire a comprehensive suite of airborne geophysical data that constrains the geometry of the sub-ice cavity for input into coupled ocean circulation and ice sheet models. Forward modeling and inversion of airborne gravity data confirmed the existence of a ridge previously discovered by the autonomous underwater vehicle. Preliminary analysis of the gravity data reveals a deep, sinuous channel that reaches all the way from the edge of the ice shelf to the grounding line, allowing warm water to reach the grounding line and cause basal melting of the ice shelf. The downward slope of the subglacial topography makes the thinning, acceleration, and grounding line retreat a potentially unstable process that could result in further acceleration of the process. A detailed understanding of the ocean ice interactions and the processes in the sub-ice cavity near the grounding line is critical for predicting the future behavior of Pine Island Glacier and West Antarctica’s contributions to sea-level rise.