LATE CENOZOIC ANTARCTIC PALEOCLIMATE RECONSTRUCTED FROM VOLCANIC ASHES IN THE DRY-VALLEYS REGION OF SOUTHERN VICTORIA-LAND

We report the discovery of numerous in situ Miocene and Pliocene airfa ll volcanic ashes that occur within the hyperarid Dry Valleys region o f the Transantarctic Mountains in southern Victoria Land, Antarctica. Ashes that occur above 1000 m elevation rest at the ground surface, co vered only by a thin ventifact pavement 1 to 2 cm thick, The ash depos its are loose and unconsolidated and show no signs of chemical weather ing. Laser-fusion Ar-40/Ar-39 analyses of volcanic crystals and glass shards indicate that the ashes range from 4.33 Ma to 15.15 Ma in age. The Arena Valley ash (4.33 +/- 0.07 Ma) rests on the surface of a well -developed desert pavement and ultraxerous soil profile at 1410 m elev ation, Lack of geomorphic evidence of liquid water on surficial sedime nts coeval and older than the Arena Valley ash, together with the pris tine condition of volcanic crystals and lack of authigenic clay format ion, indicates a cold desert at and since 4.33 Ma. The Beacon Valley a sh (10.66 +/- 0.29 Ma), the Koenig Valley ash (13.65 +/- 0.06 Ma), and the Nibelungen Valley ash (15.15 +/- 0.02 Ma) fill the upper half of relict sand-wedge troughs that form only in cold-desert conditions. Th e lack of authigenic clay-sized minerals in these ash deposits, along with preservation of sharp lateral contacts with surrounding sand-and- gravel deposits, suggests that frozen conditions (without rain or well -developed active layers during summer months) have persisted in Beaco n, Koenig, and Nibelungen Valleys since ash deposition. Ash-avalanche deposits that rest on rectilinear slopes contain matrix ash dated to 7 .42 +/- 0.31 Ma in upper Arena Valley and 11.28 +/- 0.05 Ma in lower A rena Valley. Little slope development has occurred since emplacement o f these ash-avalanche deposits, Such slope stability is consistent wit h cold-desert conditions well below 0 degrees C. Taken together, these ash deposits point to persistent polar conditions similar to the pres ent at elevations above 1000 m in the western Dry Valleys region durin g at least the last 15.0 m.y. This conclusion contradicts the view tha t, during part of the Pliocene epoch, East Antarctica was largely free of glacier ice and that scrub vegetation (Nothofagus, Southern Beech) survived along the Transantarctic Mountain front in the Dry Valleys r egion and to at least lat 86 degrees S (Webb and Harwood, 1993). Inste ad, it supports marine and geomorphological evidence that calls for a stable Antarctic cryosphere, much the same as today, since middle Mioc ene time.