Mapped Submarine Landforms in Pine Island Bay, West Antarctica, Indicate Past Ice Shelf Disintegration and Grounding line Retreat

Swath bathymetry images from the inner part of Pine Island Bay reveal a well-organized subglacial drainage system carved into bedrock and the termination of a cross-shelf trough has been mapped on the outer shelf. The middle part of Pine Island Bay has, however, only been sparsely mapped due to persistent sea ice cover in the area. During the 2009/2010 austral summer the bay was virtually ice free, allowing detailed swath bathymetry mapping with the Swedish Icebreaker Oden covering 4,140 km2 of the middle part of the trough. When the ice sheet was grounded in Pine Island Trough (PIT), several common glacigenic landforms were produced including mega-scale glacial lineations (MSGL), indicating paleo-ice stream flow direction, and grounding line wedges marking the location where the ice stream's grounding line remained for a longer period. In addition, the multibeam data reveal two other landforms previously not described from this setting. The first of these are ridges oriented transverse the ice flow direction. They are on the order of 1-2 m from trough-to-peak and separated by about 60-200 m. They extend virtually across the entire width of PIT, but individual sets are separated by lineations that are spaced 50 to 500 m apart. The second feature comprises sediment mounds that terminate linear to curvilinear sets of ridges and furrows that are aligned parallel to the axis of the trough, similar to MSGL. These two feature sets are interpreted to indicate the disintegration of a former ice shelf in Pine Island Bay that extended from the paleo-ice stream in the PIT. The ridges mapped in PIT are referred to as “fishbone moraines” and the proposed formation model is that a former ice shelf in Pine Island Bay disintegrated, similarly as happened with Larsen A and B ice shelves, back to the grounding line where it breaks off, tilts landward and begins to drift seaward. With each tidal cycle the ice shelf remnant was lifted, moved seaward and then settling, squeezing sediment out to form a small ridge just behind the grounded edge of the ice shelf. The armada of icebergs from the disintegrated ice shelf moves seaward and ground in the outer trough, where they form sediment mounds, referred to as “plough moraines”. To test the fishbone moraine formation model, we examined the possible influence of spring/neap versus diurnal tides in regulating iceberg movement by comparing a tidal model with ridge spacing and height. Consistent with the tidal model, quasi-periodic fluctuations in ridge heights are observed, assuming fishbone ridges are formed daily, with higher ridges forming roughly every two weeks.