Miocene and Pliocene paleoclimate of the Dry Valleys region, Southern Victoria land: a geomorphological approach

The Dry Valleys region is a hyper-arid, cold polar desert. Modem climate varies systematically with increasing elevation and distance from the coast. We distinguish three microclimate zones on the basis of varying precipitation, wind direction, relative humidity, temperature, and soil-moisture content. Zone 1 represents coastal, Zone 2 intermediate, and Zone 3 far-western areas of the Dry Valleys region. Soil-moisture content and relative humidity are the key parameters that control the area1 distribution of solifluction terraces, gelifluction lobes, polygonal ground, scree slopes, and soil development in Zones 1,2 and 3. The coastal Zone I shows active solifluction terraces, gelifluction lobes, levees, streams, debris flows, mudflows, and subxerous soils. The intermediate Zone 2 contains little evidence for modem downslope movement; here active gelifluction lobes, debris flows, and streams are largely restricted to north-facing slopes with high moisture content. The inland Zone 3 lacks evidence for significant modem downslope movement. There are no active solifluction terraces, stream channels, debris flows, or levees in Zone 3. Instead, Zone 3 shows Mioceneand Pliocene-age sand wedges, avalanche cones, and desert pavements. The mid-Miocene landsurface of Zone 3 is preserved to a remarkable degree. The antiquity and longevity of paleoforms in Zone 3 can be readily demonstrated by the topographic position of dated ashfall deposits. Our chronology comes from laser-fusion 40Ar/39Ar analyses of in-situ ashfall deposits that rest at, or just below, the ground surface in Zones 2 and 3 (Marchant et al., 1993a,b,c, 1995). The lack of gelifluction lobes, solifluction terraces, rills, levees, and stream channels on in-situ Mioceneand Pliocene-age deposits in Zone 3 indicates that here mean-annual air temperature, soil moisture content, and relative humidity did not reach levels that now occur in Zones 1 and 2. The present mean-annual air temperature and relative humidity of Zones 1 and 2 are about 17”C/ 75% and 27”C/ 45%. respectively. The implication is that climatic warming of the magnitude necessary for East Antarctic Ice Sheet deglaciation predicted by some glaciological models (e.g., about 20°C above present values according to Huybrechts, 1993) and growth of vascular vegetation in the Transantarctic Mountains (e.g., Webb and Harwood, 1993) could not have occurred during Pliocene time. In addition, the preservation of Miocene-age ashfall deposits, avalanche cones, and delicate desert pavements strongly suggest that no wet-based, erosive glaciers advanced into the far western Dry Valleys region above 1200 m elevation during late Pliocene time. Overall, our paleoclimate record from the Dry Valleys region implies an enduring East Antarctic Ice Sheet since Middle Miocene time. This makes it difficult to ascribe large-scale Pliocene sea-level fluctuations to ice-volume variations on the East Antarctic craton. * Present address: Department of Barth Sciences, Boston University, 675 Commonwealth Avenue, Boston, MA 022 IS, USA 0377-8398/96/$15.00