Dry sedimentation processes in the high-elevation McMurdo Dry Valleys, Antarctica: A case study in University Valley

<p>The hyper-arid, cryotic, wind-dominated conditions in the high-elevation McMurdo Dry Valleys of Antarctica are among Earth’s most extreme environments and represent the closest terrestrial analog to the surface of Mars. These unique conditions result in complex surface processes that occur in the overall absence of liquid water. However, since water is typically believed to be required for these processes to occur, the mechanisms responsible for how these processes can persist in this environment are poorly understood. Previous studies that focused on individual processes of sedimentation in the Dry Valleys leave questions regarding the role of water in dry cryotic sedimentation as well as the rates at which these processes occur. This thesis addresses these questions by combining Optically Stimulated Luminescence (OSL) dating, meteoric Beryllium-10 (10Be) measurements, soil geochemistry analysis, and petrographic microscopy analysis on ice-cemented permafrost cores taken from University Valley, one of the high-elevation Dry Valleys, where the availability and effects of liquid water are minimal. These analyses were used to explore four main sedimentation processes that occur in the Dry Valleys: chemical weathering, fine particle translocation, eolian transport, and physical weathering. Analyzed together, findings from these analyses comprehensively describe the complex processes involved in dry cryotic sedimentation and determine the roles of different phases of water in this environment. Sediments in University Valley have accumulated at a rate of approximately 2.1 mm/ka for the last 200 ka, as dated by OSL, from erosion of the valley walls and deposition of windblown dust. Sediment accumulation is influenced by topography of the valley floor, depth of the ice table, aspect of the valley walls, wind direction, and mechanical breakdown of rocks due to solar heating. While persistent winds constantly remobilize fine particles and dust in the upper few cm of the dry ground, sediment grains that are sand-sized or larger do not undergo significant remobilization, and sediments in the ice-cemented ground are unaffected by remobilization and translocation processes. Rare clay bridges seen in thin section show that small, infrequent, transient surface wetting events have occurred over the last 200 ka. High anion concentrations associated with high surface meteoric 10Be measurements and clay bridges indicate that the source of these wetting events is the melting of wind-blown snow from coastal regions. Patterns in meteoric Be measurements show that these small transient wetting events are not sufficient to translocate fine particles through the soil profile, which suggests that the role of liquid water as a transporting agent is negligible in this environment. Chemical weathering in University Valley appears to be controlled by two main components: dolerite content of the sediments, and exposure to the atmosphere at the ground surface where condensation of water vapor onto grain surfaces readily leaches ions from dolerite grains under the oxidizing conditions of the Dry Valleys. In the absence of liquid water, chemical processes that occur in this environment rely on water vapor. Together, these results indicate that surfaces in University Valley are remarkably young and sedimentologically active. Because University Valley represents one of the closest terrestrial analogs to the surface of Mars, findings from this thesis may be applicable to understanding the timescales and the processes that control anhydrous sedimentation on the surface of Mars.</p>