INVESTIGATING CLIMATIC CHANGE IN THE DRY VALLEYS OF EASTERN ANTARCTICA WITH TEPHRACHRONOLOGY AND COSMOGENIC NUCLIDE SURFACE EXPOSURE DATING

The present-day western Dry Valleys climate is characterised by mean annual temperatures of -30°C to -35°C and precipitation of less than 10 mm/yr. Cold-based glaciers, polygonal patterned ground, desert deflation pavements, and ventifacts typify this environment. Past climatic changes are recorded by surficial deposits and landforms documenting multiple alpine glacial advances. Dating these advances is central to understanding factors forcing climatic change in the Dry Valleys. Tephras erupted by volcanoes along the eastern Ross Sea embayment are interbedded with the surficial deposits and yield ⁴⁰Ar/³⁹Ar ages of about 4.3 to 15.0 Ma. The earliest ages are associated with deposits of warm-based glaciation and point to a major climatic shift between 10 to 15 Ma. After this time, alpine glaciation in the Dry Valleys became entirely cold-based and progressively more limited in extent. Dating of erratics deposited by small cold-based cirque glaciers with cosmogenic ³He is also in progress. Based upon relative weathering criteria and stratigraphic position, these advances reflect the latest period of major climate change to affect the western Dry Valleys. We are also investigating patterned ground formation with cosmogenic ³He depth profiles sampled through periglacial polygons developed in till overlying relic glacial ice. Preliminary results show the till to be a lag deposit produced by 10 to 15 m of sublimation. Patterned ground processes have been active over long time periods as shown by ³He exposure ages ranging from 1.14 Ma to 0.17 Ma. Convective churning of the till is absent as indicated by the progressive decrease of ³He concentrations from the surface downward.