PARALYZED LANDSCAPES OF SOUTHERN VICTORIA LAND: CLIMATE AND ICE-SHEET STABILITY SINCE THE MIDDLE MIOCENE

Surficial deposits in the Dry Valleys region of Antarctica provide records of Neogene climate change, East Antarctic Ice Sheet evolution, and alpine glaciation. Chronological control comes from 75 laser-fusion 40Ar/39Ar analyses of ashfall deposits and numerous cosmogenic-nuclide analyses of boulders on moraines, drift, and bedrock. Temperate glaciation associated with deposition of Sirius Group deposits in the western Dry Valleys pre-dates major scarp retreat in Beacon Valley and the Asgard Range at > ~ 15 Ma. Wet-based alpine glaciers, perhaps associated with the last phase of Sirius Group deposition, occupied the western Dry Valleys region sometime prior to ~ 14.8 Ma. Major expansion(s) of East Antarctic ice overtopped the Dry Valleys region during Miocene time; the last of these expansions occurred sometime between 12.6 Ma and 14.8 Ma. Moss and grasses that predate the last glacial overriding are well preserved in ancient ice-marginal deposits in the Olympus Range.

Since ~ 12.6 Ma, outlet glaciers have retreated predominantly via sublimation. Dated moraines pinpoint the ice-surface elevation of Taylor Glacier along the walls and tributaries of Taylor Valley at ~ 1.5 Ma, 3.0 Ma, 7.1 Ma, and Ma, 10.1 Ma. Likewise, dated moraines in the western Asgard Range and in the Wilkniss Mountains delineate Wright Upper Glacier and Ferrar Glacier at ~ 13.0 Ma and ~ 4.0 Ma, respectively. Dated relict ice from Taylor Glacier at least 8.6 Ma is buried beneath sublimation till and is unaffected by glacial meltwater on the floor of central Beacon Valley. Similarly, debris-covered alpine glaciers at the head of Beacon Valley contain ice from between ~ 12,000 a to at least ~ 7.1 Ma. Cold-based alpine glaciers throughout the western Dry Valleys have oscillated within 1 to 2 km of mapped Late Miocene/ Early Pliocene limits.

A record of Neogene climate evolution is consistent with mapped ice volume fluctuations showing persistent hyper-arid cold-desert conditions since ~ 13 Ma in the western Dry Valleys region. The record is derived from the analyses of soils developed on exposed ashfall deposits, the areal distribution of geomorphic features associated with ancient meltwater, and from the geomorphic stability of periglacial and slope deposits