Late Tertiary Antarctic Paleoclimate and Ice-Sheet Dynamics Inferred from Surficial Deposits in Wright Valley

ABSTRACT

Wright Valley, which extends from the edge of the East Antarctic Ice Sheet through the Transantarctic Mountains to the Ross Sea, contains widespread Neogene surficial drift deposits. ⁴⁰ Ar³⁹ Ar ages of reworked basalt clasts in these drifts afford a limiting chronology for glacial events. Peleus till (>3.8 Ma old) is stratigraphically lowest and represents expansion of an East Antarctic outlet glacier through Wright Valley. Alpine I, II, III (<3.5 Ma old), and IV (>3.7 Ma old) drifts, which flank alpine glaciers on the south valley wall, and Onyx, Wright, and Loop drifts, which record expansion of an ice lobe westward (inland) into Wright Valley, are all younger than Peleus till. Onyx, Wright, and Loop drift sheets (all Quaternary in age) postdate old alpine drifts (III and IV) and antedate young alpine drifts (I and II). Cold-desert conditions have persisted for at least 3.7 Ma and are reflected by the presence of coarse-grained tills, ventifacts, and abundant salts in the soils, and by the lack of temperate-ice landforms and glaciomarine deposits in east-central Wright Valley.

The evidence of persistent cold paleoclimate contradicts the postulated Pliocene deglaciation of East Antarctica, which requires atmospheric temperatures 20°C warmer than present to produce melting ablation surfaces on the East Antarctic Ice Sheet. Furthermore, an East Antarctic outlet glacier has not expanded in to east-central Wright Valley in at least the last 3.8 Ma, thus precluding ice-sheet overriding of the Dry Valleys region during this interval. Overall, Wright Valley data imply that the adjacent East Antarctic Ice Sheet has been stable for at least the last 3.8 Ma.