Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica

Abstract

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...

Fossil genes and microbes in the oldest ice on Earth

“…1 A). These ice chronologies are supported by 3 He and 21 Ne cosmogenic dating of surface boulders (5) and laser-fusion 40 Ar/ 39 Ar radiometric dating of surface ash-fall deposits (6,7) [supporting information (SI) Text]. Analyses of the ice crystal structure and stable-isotope composition (␦ 18 O, ␦D) indicate that the ice samples have remained frozen since transformation from snow to glacier ice (e.g., refs.…”

“…Ice samples were obtained by removing Ϸ20-75 cm of thick debris and the top Ϸ15 cm of the buried glacier and cutting out a square block Ϸ20 cm on a side (6). The 0°C isotherm reaches a maximum depth of Ϸ15 cm in the region; the ice samples we analyzed were from well below this depth (35-cm depth minimum).…”

Polygon pattern geomorphometry on Svalbard (Norway) and western Utopia Planitia (Mars) using high-resolution stereo remote-sensing data

“…In contrast to the Adventdalen high-centered polygons, where the trough expansion is related to ice-wedge degradation, it is suggested that the enlargement of polygon troughs on present-day Mars occurred by progressive sublimation of interstitial ice exposed after initial thermal contraction cracking (Mangold, 2005;Levy et al, 2010). Examples of such highcentered sublimation polygons are known from Beacon Valley, Antarctica (Marchant et al, 2002). These high-centered polygons are formed on sediments which cover massive ice bodies (e.g., a stagnant glacier) by thermal contraction and the absence of a liquid phase.…”

“…The UP4 polygonal field could be an expression of the flattening and recent surface stabilization by permanent insolation and continuous but slow ground-ice sublimation on the south-facing scalloped depression slope (Ulrich et al, 2010). In summary, from the comparison of polygon geomorphometry alone it seems problematic to classify the Martian polygons as icewedge, sand-wedge, or sublimation polygons, not least because the Adventdalen polygons also show close similarities to sand-wedge or sublimation polygons in Antarctica (e.g., Péwé, 1959;Marchant et al, 2002;Sletten et al, 2003;Bockheim et al, 2009;Levy et al, 2010). If we consider the evolution of the UP polygons in the geomorphological context of the scalloped depression formation, it becomes obvious, however, that sand or even composite wedges are likely to be formed and that all the different polygonal fields represent different evolutionary stages within the landscape formation process (Fig.…”

“…If local conditions favor the occasional support of moisture, composite wedges can develop (Murton, 1996). Special types of sand-wedge polygons described from the Antarctic Dry Valleys are high-centered sublimation polygons (Marchant et al, 2002;Marchant and Head, 2007). However, sand-wedge polygons can hardly be distinguished from icewedge polygons in the plan view (Black, 1976).…”