Shallow Core Drilling and Bore-Hole Measurements in the Permafrost of an Active Rock Glacier near the Grubengletscher, Wallis, Swiss Alps

Abstract

On the active rock glacier Gruben near the Grubengletscher Wallis, Swiss Alps, shallow core drilling to a depth of 700 cm and geophysical bore-hole investigation was carried out to collect data about the rock-glacier permafrost and to gain experience for future deep drilling.Despite strong thaw disturbance of the recovered cores, the material below the permafrost table consists of frozen silty sand with some gravels. Ice is present in the pore space, as coatings around rock particles and in the form of ice lenses and layers up to about 60 cm thick. The content of Na and Ca in the rock-glacier ice is much higher than in ice from the nearby Grubengletscher, but the interpretation of this observation is difficult because of possible contamination from the drilling procedure. Former glacier ice has not been found.The interpretation of a set of radioactive bore-hole logs (gamma, neutron-neutron, and gamma-gamma) run in the hole immediately after drilling shows that the ice content of the frozen sediments is about 90% by volume at the permafrost table and probably asymptotically reaches an average value some meters below (50 to 70%). This is in good agreement with observations in arctic regions and also with the observed P-wave velocity values from seismic refraction. Fine material (clay?) seems to be present at the permafrost table.The mean annual temperature near the permafrost table is estimated to be −0.5 to −0.6°C. Besides air temperature (altitude) and solar radiation (exposure), snowcover thickness during winter seems to be the main factor influencing the ground temperatures. Based on geothermal calculations and estimations it is suggested that the rock-glacier permafrost is not in equilibrium with the present-day surface temperature but with a surface temperature from the last centuries, the “Little Ice Age,” which was some 0.5 to 1.0°C colder. Furthermore, it must be assumed from the thermal history since about 1850 that quite a thick layer at the base of the rock-glacier permafrost is very near the pressure melting point. The actual permafrost thickness is estimated to be around 50 m.The results presented here confirm that alpine rock glaciers are creep phenomena in discontinuous permafrost. Future deep drilling could furnish important information about the long-term behavior of perennially frozen Quaternary sediments under low stresses.