HR: 0800h
AN: C21B-1089    [Abstracts]
TI: Glacial Geology of the Ohio Range, Antarctica: Constraints on Ice Sheet Elevation During the Last Glaciation
AU: * Ackert, R P
EM: rackert@fas.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University 20 Oxford St., Cambridge, MA 02138 United States
AU: Mukhopadhyay, S
EM: sujoy@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University 20 Oxford St., Cambridge, MA 02138 United States
AB: Knowledge of past ice elevations in the Antarctic interior is critical to constrain reconstructions of the West Antarctic Ice Sheet (WAIS) during the last glacial maximum and during the subsequent deglaciation. Mountains that project through the ice sheet serve as dipsticks that gauge past ice elevations that are recorded by glacial deposits and erosional features preserved on their slopes. The Ohio Range (85°S, 114°W) lies near the onset area of the Mercer Ice Stream at the boundary between the East Antarctic Ice Sheet and the WAIS. The Ohio Range forms an east-west trending escarpment rising 500 m above the adjacent surface (~1550m) of the WAIS. The granite bedrock forming the escarpment and nearby nunataks typically exhibits deep cavernous weathering with areas between pits reduced to delicate structures only several centimeters thick. Unweathered erratics occur on these weathered surfaces on nunataks and on a bedrock bench on the escarpment. In such locations, the most delicate weathering features have been broken off. The transition from such surfaces with erratics, to those lacking erratics and on which delicate weathering features are preserved, is interpreted as a trimline that marks the elevation of the WAIS during the LGM. Based on such observations over a 15 km long segment of the escarpment, the LGM trimline occurs between 1700 and 1750 m implying that the ice surface was ~150 m higher than adjacent blue ice areas in the interior of the ice sheet. Measurement of cosmogenic nuclides in the erratics and bedrock is underway in order to constrain the age of maximum ice elevation and subsequent recession. Although He is known to diffuse out of quartz, for young samples at Antarctic temperatures, He appears to be quantitatively retained. Initial results show that ³He concentrations in replicate samples are reproducible, suggesting most ³He is cosmogenic. However, the ³He measurements suggest prior exposure in many samples. The youngest ³He exposure age for an erratic, 10 ka, provides a maximum age for the last WAIS highstand consistent with previous results in Marie Byrd Land. We are in the process of measuring ¹⁰Be in quartz separates to better constrain the glacial history.
DE: 0730 Ice streams
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
SC: Cryosphere [C]
MN: Fall Meeting 2005