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

Nonsorted polygons with sand-filled wedges were investigated in Beacon Valley, Antarctica (77.82°S, 160.67°E) using field observations coupled with 2-m resolution aerial photography. A gasoline-powered concrete breaker was employed to expose the sediments of four polygon centres and six wedges from geomorphic surfaces containing tills of two different ages. The excavated polygons ranged from 9 to 16 m in diameter; the sand-filled wedges ranged from 0.2 to 2.5 m in width. The top of ice-bonded permafrost ranged from 12 to 62 cm in depth in the polygon centres and from 64 to >90 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 sand-wedge 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 well-developed polygons on Taylor II surfaces (ca. 117 ka), moderately developed polygons on Taylor III surfaces (ca. 200 ka) and poorly expressed polygons on Taylor IVa and older (ca. >1.1 Ma) surfaces. This retrogressive development may be due to sublimation of ice-bonded permafrost following thermal cracking. With the drop in ice content, the thermal coefficient of expansion of the permafrost may be lowered, which would result in a reduction in tensile stresses.