Genesis of active sand-filled polygons in lower and central Beacon Valley, Antarctica

Abstract

Nonsorted polygons with sand-filled wedges were investigated in lower and central Beacon Valley, Antarctica (77.82ºS, 160.67ºE) using field observations coupled with a 2-m resolution Digital Elevation Model and a high-resolution aerial photograph. A gasoline-powered concrete breaker was employed to expose the sediments of four representative polygon centers and six wedges from geomorphic surfaces containing tills of two different ages. The excavated polygons ranged from 9 to 16 m in diameter (average = 12 m); the sand-filled wedges ranged from 0.2 m to 2.5 m in width (average = 0.9 m). The top of ice-bonded permafrost ranged from 12 to 62 cm in depth (average = 33 cm) in the polygon centers and from 64 to >90 cm (average = >75 cm) in wedges. One active thermal contraction fissure generally was apparent at the surface, but excavations revealed numerous inactive fissures. The wedges contain sand laminations averaging 3 mm in width when viewed in cross section. Although most of the polygons were of the sandwedge type, some contained ice veins up to 1 cm in width and could be classed as composite wedges. Three stages of polygon development were observed, including strongly developed polygons on Taylor II surfaces (ca. 117 ka), moderately developed polygons on Taylor III surfaces (ca. 200 ka), and poorly developed polygons on Taylor IVa and older (ca. >1.1 Ma) surfaces. This retrogressive development may be due to sublimation of ice-bonded bonded permafrost following thermal cracking. With the drop in ice content, the thermal coefficient of expansion is lowered, which causes a reduction in tensile stresses.