A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is Ͻ1.0 m thick and rests with sharp planar contacts on stagnant glacier ice reportedly of Miocene age, older than 8.1 Ma. The age of the ice is based on 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice.Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patternedground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feed-*back associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondaryice lenses contrast markedly with glacial ice by lying on a ␦D versus ␦ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs.The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains Ͼ25% striated clasts in the Ͼ16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarsegrained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ؎ 0.08 Ma and 0.18 ؎ 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimat...

A major obstacle in understanding the evolution of Cenozoic climate has been the lack of well dated terrestrial evidence from high-latitude, glaciated regions. Here, we report the discovery of exceptionally well preserved fossils of lacustrine and terrestrial organisms from the McMurdo Dry Valleys sector of the Transantarctic Mountains for which we have established a precise radiometric chronology. The fossils, which include diatoms, palynomorphs, mosses, ostracodes, and insects, represent the last vestige of a tundra community that inhabited the mountains before stepped cooling that first brought a full polar climate to Antarctica. Paleoecological analyses, 40 Ar/ 39 Ar analyses of associated ash fall, and climate inferences from glaciological modeling together suggest that mean summer temperatures in the region cooled by at least 8°C between 14.07 ؎ 0.05 Ma and 13.85 ؎ 0.03 Ma. These results provide novel constraints for the timing and amplitude of middle-Miocene cooling in Antarctica and reveal the ecological legacy of this global climate transition.climate change ͉ tundra biota ͉ Dry Valleys ͉ diatoms ͉ ostracods

A 50؉-km-long network of bedrock channels and scoured terrain occupies the ice-free portion of a major trough that crosses the Transantarctic Mountains in southern Victoria Land. The channels, collectively termed the Labyrinth, emerge from beneath the margin of the East Antarctic Ice Sheet (Wright Upper Glacier) and are incised into a 300-m-thick sill of Ferrar Dolerite at the head of Wright Valley. Upper-and intermediate-elevation erosion surfaces of the Labyrinth exhibit striations and molding characteristic of glacial erosion. Channels and canyons on the lower surface are as much as 600 m wide and 250 m deep, have longitudinal profiles with many reverse gradients, and contain potholes Ͼ35 m deep at tributary junctions. These characteristics are most consistent with incision from fast-flowing subglacial meltwater; estimated discharge is on the order of 1.6-2.2 ؋ 10 6 m 3 s ؊1. Our 40 Ar/ 39 Ar analyses of volcanic tephra from the Labyrinth show that the channels are relict, that major channel incision predates 12.4 Ma, and that the last major subglacial flood occurred sometime between 14.4 Ma and 12.4 Ma. The most plausible origin for the Labyrinth is erosion associated with episodic drainage of subglacial lakes in East Antarctica. One compelling possibility is that discharge of large volumes of subglacial meltwater to the Southern Ocean, and to the Ross Sea in particular, may have coincided with, and contributed to, oscillations in regional and/or global climate during the middle Miocene.

No abstract

No abstract

No abstract

We present a glacial record from the western Olympus Range, East Antarctica, that documents a permanent shift in the thermal regime of local glaciers, from wet-to coldbased regimes, more than 13.94 m.y. ago. This glacial record provides the fi rst terrestrial evidence linking middle Miocene global climate cooling to a permanent reorganization of the Antarctic cryosphere and to subsequent growth of the polar East Antarctic Ice Sheet. The composite stratigraphic record constructed from fi eld mapping and analyses of 281 soil excavations shows a classic wetbased till (Circe till, including an extensive melt-out facies), overlain by a weathered colluvial deposit (Electra colluvium), and then a series of stacked tills deposited from cold-based ice (Dido drift). Chronologic control comes from 40 Ar/ 39 Ar analyses of concentrated ash-fall deposits interbedded within glacial deposits. The shift from wetto cold-based glaciation refl ects a drop in mean annual temperature of 25-30 °C and is shown to precede one or more major episodes of ice-sheet expansion across the region, the youngest of which occurred between 13.62 and 12.44 Ma. One implication is that atmospheric cooling, following a relatively warm mid-Miocene climatic optimum ca. 17 to 15 Ma, may have led to, and thus triggered, maximum ice-sheet overriding.

No abstract