Periglacial geomorphology and rates of landscape evolution in the western Dry Valleys region of Antarctica.

摘要

The East Antarctic Ice Sheet (EAIS) is an enormous physiographic feature and an important component of the global-climate system. As it plays an important role in the formation of oceanic deepwater, its history is crucial to predicting the global response to climate change. One record of EAIS evolution, purported to cover the last 15 million years (Ma), comes from the Antarctic Dry Valleys (ADV). In this 4000 km2 ice-free region, glacial deposits perched on valley walls are thought to register former fluctuations of the EAIS. Interpretations of this record hinge on the assumption that drifts, moraines, and chronostratigraphic markers have been physically stable since deposition. This dissertation critically examines the assumption of long-term geomorphic stability in the ADV.; To assess landscape stability over multiple scales, this study radiometrically dates three different landscape elements in the ADV: (1) polygonal-patterned ground (1 to 30 m domain); (2) debris-covered glaciers and associated regions of stagnant ice (10 m to 5 km domain); and (3) bedrock features formed beneath wet-based ice (0.1 to 15 km domain). Chronology for each geomorphic element is based on 40Ar/39Ar analyses of interbedded volcanic-ash deposits and 3He cosmogenic-nuclide exposure-age analyses of surface and subsurface clasts.; Results indicate that geomorphic stability in the ADV increases with increasing elevation and distance from the coast. Geomorphic elements at all scales in the interior, upland region are among the most stable on Earth and may represent close analogs for Martian features. Bedrock elements such as the Labyrinth have remained stable and unaltered during at least the last 12.23 +/- 0.37 Ma; volcanic ashfall has remained preserved in relict polygonal ground for as long as 15.01 +/- 0.26 Ma; and buried glacier ice has persisted within 50 cm of the ground surface for >8.07 +/- 0.06 Ma. The major implication of these results is that the climate and landscape of the ADV have remained unchanged since late Middle Miocene time. This requires that present EAIS was established by 12.23 +/- 0.37 Ma (and possibly by 15.01 +/- 0.26 Ma) and that future greenhouse-induced global warming is not likely to alter significantly its size, character, or its influence on oceanic deepwater circulation.