Bed Morphology and Ice Thickness, Bering Glacier, Alaska

Bering Glacier is the longest and largest glacier in continental North America, with a length of ~190 km and an area of ~5,200 sq. km. During the Pleistocene, it was significantly larger and longer, extending at least another 75 km seaward of its present terminus position. Data produced by five complimentary geophysical investigations confirm the presence of a complex ~125-km-long fiord system that underlies much of the southern part of Bering Glacier and also extends proglacially under its ice-marginal lake, its coastal foreland, and onto the Gulf of Alaska continental shelf. Bering Trough, the seaward end of the fiord system is a large u-shaped erosional depression cut into the seafloor of the Gulf of Alaska, with bed depths approaching 500 m below sea level. It formed during episodes of Pleistocene erosion during multiple advances and retreats. Although the details of the proglacial fiord are significant, this abstract focuses on the subglacial portion of this fiord system and the ice that fills it. Interpretation of the results of USGS ice-surface ice-penetrating radar surveys conducted in 1991 and 1992, and an airborne monopulse radar survey conducted by NASA in October 2008, confirm that the southern third of Bering Glacier, specifically Bering's piedmont lobe and it principal eastern tributary, the Central Valley Reach, currently fill the northern ~50 km of the fiord system. The morphology of the northern part of the fiord system is complex, with local relief of several hundred meters. At least two channels extending more than 300 m below sea level underlie the terminus and parts of the eastern Bering Lobe. These include an eastern channel, located adjacent to the Grindle Hills, with maximum depths between 320 and 347 m below sea level and a western channel, located adjacent to the Central Medial Moraine Band (CMMB), with maximum depths reaching between 361 and 388 m below sea level. At least two other channels reach 200 m or more below sea level. Intervening topographic highs extend above sea level. Limited data suggest that at least one channel extends under the CMMB and that at least two channels underlie the central part of the Steller Lobe, the western part of the piedmont lobe, with a maximum measured depth of 369 m below sea level. Measured thicknesses of the ice that fills the fiord range from <100 m to nearly a kilometer. Comparison of surface elevations derived with a 1950s National Elevation Dataset (NED), 2-arc-second (about 60 meters) grid spacing digital elevation model (DEM) and an early 21st century ASTER 2 (30 meters) grid spacing DEM show that the glacier has thinned as much as 150 m in the approximate half-century between the creation dates of the DEMs. At individual locations, this represents ice loss of less than 5% to more than 20% of the 1950s glacier thickness.