Profili di densità e modelli profondità/densità su carote di nevato prelevate in alcuni ghiacciai della Terra Vittoria Settentrionale (Antartide)

Depth/density profiles and models from Northern Victoria Land shallow firn-cores (Antarctica)

Abstract

During the 91-92, 92-93, 93-94 and 94-95 field seasons carried out by the Italian Antarctic Program (PNRA) 11 sites have been investigated to evaluate the annual accumulation rate in the Northern Victoria Land. Fourteen shallow firn cores, 7,5 m to 21 m depth, have been drilled in different accumulation situations from the coast to the East Antarctic plateau margin, through the Transantarctic Mts. Density profiles have been performed directly in the field and simple models have been calculated to evaluate the density gradients with depth. In this work we apply a simple depth/density model at 4 cores from different geographical situations. The upper 7 m of profiles are the same for all the curves (with a gradient of 0,22 g cm-³ m-¹), but from 7 m depth downward these cores are grouped into two different density gradients. In the bottom core of the Hercules Névé (-20,5 m) the maximum density ranges from 0,58 to 0,60 g cm-³ and in the Priestley Glacier (-12 m) the maximum density is 0,54 g cm-³, both showing a gradient of 0,008 g cm-³ m-¹ . In other 2 cores, Styx GI. (-12 m) and Pilot GI. (-11,9 m), the density profiles show a gradient of 0,016 g cm-³ m-¹ with values ranging from 0,60 to 0,64 g cm-³. The density gradient of the upper part of the cores are dominated by grain packing, where load pressure increases the density up to 0,5 g cm-³ (0,55 is the maximum theoretical density for packing process). For the second deeper part of the profiles, sintering processes have been involved for the increase of density. In this situation, the temperature of the ice crystals is an important factor in grain-to-grain matter transfer. For the future drilling activities, a simple depth-density model has been applied to these 4 firn cores, to try to define the close-off depth.