ANCIENT LANDSCAPES-ACTIVE SURFACES IN THE DRY VALLEYS, ANTARCTICA: THE ROLE OF CONTRACTION CRACKING IN SURFACE RENEWAL

Since the first explorations of the Dry Valleys by R.F. Scott, the ubiquitous polygonal patterned ground in the region has sparked interest and curiosity. Our recent measurements at 13 patterned ground sites established in the early 1960’s by R.F. Black show significant activity, and suggest steady divergence across cracks centered in the sand-wedges that delineate polygons at all study sites. To develop a more detailed understanding of the recurrent thermal expansion and contraction that leads to formation and growth of sand wedges, and to provide data for theoretical modeling of the underlying processes, we installed linear transducers at 3 sites (two in Beacon Valley and one in Victoria Valley). We measure hourly displacements across contraction cracks along with soil temperature and microclimate. Our data show dynamic contraction/expansion paralleling temperature. The annual opening and closure of the cracks ranges up to 1.6 cm. The net annual crack divergence reaches 1 mm, in good agreement with nearly four decades of data from Black’s study sites. The annual opening and closure of the cracks vary according to the ice content, the magnitude of the temperature excursions, and the polygon size. The net annual divergence, on the other hand, depends largely on the amount of fines that infill the open cracks and prevent them from fully closing while the polygons expand during the subsequent warm season. Curiously, we found the largest annual contraction/expansion across cracks in polygons developed on massive ice under a veneer of drift, but these crack show no net annual divergence; they may even be converging very slowly due, presumably due to ablational loss of ice along the crack walls. Based on our measurements of net annual divergence and polygon size, we estimate that surfaces with the most active sand wedge growth may be reworked (sand wedge growth sweeps the entire surface) on time scales as short as 2 x 10⁴ years, which is difficult to reconcile with the ancient inferred ages (up to 10⁷ years) of many surfaces in the regions. The continuing infilling growth of sand wedges across the surface leads to an inflational surface that is totally underlain by a complex of sand wedges; clasts that are too large to fall into active contraction cracks remain on the surface.