[ACG38-P19] グリーンランド北西部Bowdoin氷河における末端位置の変動と海底地形の関係
キーワード：氷河、グリーンランド
The Greenland ice sheet(GrIS) is losing their mass due to reasons such as increasing surface melting and ice discharge from marine-terminating outlet(calving) glaciers. Especially in the southern area of the ice GrIS, mass loss from carving glacier were reported after 2000. This trend is also reported in the northern area of the GrIS in recent years. Bowdoin Glacier is a marine-terminating outlet glacier in northwestern Greenland (77°41’N, 68°35’W), the width of the terminal is about 3 km. The ice front had kept at the same position for 20 years, but the front has retreated rapidly since 2008. A hypothesis why this retreat occerd after 2008 is that the surface elevation of the glacier declined and therefore the buoyancy acting on the front of the glacier approached the gravity, as a result, the carving was promoted and a rapid recession occurred. However, the verification of this hypothesis is not sufficient, because there are few observations at the site in the northern area of the GrIS. In this study, in order to clarify the details of the bed topography and how much surface elevation was up to Flotation level (surface elevation at which gravity and buoyancy balance equilibrium to ice), we measured the bed topography of the fjord in front of the glacier and the surface elevation of the glacier. Then we discussed the flotation level by using, on-site observation and satellite data analysis on the Bowdoin glacier.
We measured the bed topography at the fjord in front of the Bowdoin glacier on July 29, 2016 by using a boat with a sonar and a transducer. The ice thickness at the glacier was measured using ice radar and the surface elevation of the glacier was measured by GPS kinematic survey at the same point. Surface elevation from 2001 to 2012 was also acquired from DEM (digital elevation model) created from images taken by optical sensors of artificial satellites Terra and ALOS. Then, we calculated the elevation of the bed topography under the glacier by subtracting the ice thickness from the surface elevation of the glacier.As a result, the depth in front of the glacier were 280 m in the west side and 180 m in the east side at maximum. The surface elevation of the glacier had declined at 2.8 m a-1 on average from 2001 to 2012. Comparing the surface elevation with the Flotation level, the glacial surface elevation in 2001 was more than 10 m higher than the Flotation level. However, the surface elevation was 0 - 10 m below the flotation level in the west side of the glacier in 2007. Therefore, the Bowdoin glacier has, reached a condition to float on water in a wide area on the west side of the glacier, and it might cause to greatly retreat the ice front.
We measured the bed topography at the fjord in front of the Bowdoin glacier on July 29, 2016 by using a boat with a sonar and a transducer. The ice thickness at the glacier was measured using ice radar and the surface elevation of the glacier was measured by GPS kinematic survey at the same point. Surface elevation from 2001 to 2012 was also acquired from DEM (digital elevation model) created from images taken by optical sensors of artificial satellites Terra and ALOS. Then, we calculated the elevation of the bed topography under the glacier by subtracting the ice thickness from the surface elevation of the glacier.As a result, the depth in front of the glacier were 280 m in the west side and 180 m in the east side at maximum. The surface elevation of the glacier had declined at 2.8 m a-1 on average from 2001 to 2012. Comparing the surface elevation with the Flotation level, the glacial surface elevation in 2001 was more than 10 m higher than the Flotation level. However, the surface elevation was 0 - 10 m below the flotation level in the west side of the glacier in 2007. Therefore, the Bowdoin glacier has, reached a condition to float on water in a wide area on the west side of the glacier, and it might cause to greatly retreat the ice front.