Undercutting of marine-terminating glaciers in West Greenland

Marine-terminating glaciers control most of Greenland’s ice discharge into the ocean, but little is known about the geometry of their frontal regions. Here we use side-looking, multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough. Deep glacier grounding enables contact with subsurface, warm, salty Atlantic waters (AW) which melts ice at rates of meters per day. We detect cavities undercutting the base of the calving faces at the sites of subglacial water (SGW) discharge predicted by a hydrological model. The observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of SGW transport warm AW to the ice faces. Glacier undercutting likely enhances iceberg calving, impacting ice front stability and, in turn, the glacier mass balance.

Methods
Swath bathymetry. We employ a Reson 8160 MBES system operating at 50 KHz, mounted at a 30∘ angle to provide a usable swath width of the submerged glacier faces extending from the seafloor to the sea surface. The tilting of the sonar sacrifices horizontal swath width to permit measurements from the sea surface to the seafloor. The Reson 8160 has a useful maximum depth of 3000 m in typical fjord conditions. The presence of icebergs, bergy bits, and growlers negatively affects sonar retrievals. However, sonar retrieval errors are over-come by combining scans of the same location from different look angles—available from the winding ship tracks through debris-laden fjords. The MBES is operated with a NAVCOM 3050 differential GPS unit with sub-meter position accuracy and an Applanix POSMV inertial navigation system which provides real-time vessel attitude and position. The data are acquired using the Quality Integrated Navigation System software and processed using the CARIS HIPS software. Calibration of sound speed in water is performed using Conductivity Temperature Depth (CTD) data obtained at regular intervals during the cruise and by surveying a set of topographic features on the seafloor at different look angles. Seafloor bathymetry products are generated on a 20 m grid with a vertical precision varying from 1 m in the best condition to 3–5 m when the boat changes course rapidly to avoid collision with large pieces of floating ice. Fjord surveys are conducted at a vessel speed of 5 to 6 kt and a pulse repetition frequency of 1 Hz, while face mapping requires vessel speeds of 1 kt or less.

Study area. Three major glaciers investigated include, from south to north and low to high ice discharge:
Kangilernata Sermia (69.9∘N, 50.35∘W), Store Gletscher (70.38∘N, 50.59∘W), and Rink Isbræ(71.74∘N, 51.65∘W)