Temperature Ice Content and

A number of borehole temperature measurements exist in mountain permafrost. Some are part of monitoring networks or research projects, others have been drilled and measured during construction or mineral prospecting, and data are seldom available for the scientific community. The most prominent scientific monitoring networks include the PACE transect of boreholes from the European Alps to the Arctic island of Spitzbergen and the PERMOS permafrost monitoring network in Switzerland (Vonder Muhll et al. 2007), which have contributed significantly to the understanding of mountain permafrost temperatures. Both networks contribute data to global monitoring organizations. The thermal response of permafrost to climate change is presented in more detail in Chap. 14.

In mountain areas, temperature does not simply increase with depth (Fig. 3.6). The subsurface temperature field is usually rather complex and governed by lateral heat fluxes, which are caused by topography and variable surface conditions (Fig. 3.7). These complex patterns can result in permafrost being induced, for instance, under a

Fig. 3.6 Schematic cross-section through the steady-state thermal field of a ridge or summit. Isotherms are shown by black lines; darker shading refers to colder temperatures. In the upper part of the section, heat flow and thermal gradient are predominantly lateral

seemingly warm sun-exposed slope from the nearby cold and shaded slope (Noetzli et al. 2007). Furthermore, recent warming has already penetrated tens of meters into the ground and can thus lead to inverted temperature profiles. As a consequence, great care must be taken in the interpretation of temperature profiles, and heat fluxes at a depth of several decameters can be positive or negative, depending on location and time (Gruber et al. 2004). The thermal profiles observed in mountain permafrost are usually either cold (i.e., colder than about 0.5°C with insignificant amounts of liquid water) or temperate. Temperature profiles in temperate permafrost have large sections (sometimes tens of meters thick) of near-isothermal conditions due to phase transition of ice contained in unconsolidated material or highly fractured rock. Areas of temperate mountain permafrost will likely increase under current atmospheric warming trends.

Glaciers and permafrost interact in many ways. Permafrost exists below the interface of cold ice and rock or sediments, and the melt of parts of a temperate glacier tongue can be followed by permafrost formation in the newly exposed material. Cold glacier tongues advancing into perennially frozen sediments can deform them into so-called push-moraines, which are landforms indicative of permafrost. Many intermediate forms of creep phenomena exist between very small debris-covered glaciers, ice-cored moraines and rock glaciers (Fig. 3.8). Ice in rock glaciers (Haeberli et al. 2006) exists in many forms, ranging from massive ice with dispersed debris to relatively homogeneous ice/rock mixtures. The origin of ice in rock glaciers is difficult to trace to either glacial or non-glacial formation, because of many shared characteristics between both ice types. Especially in the rooting zone of rock glaciers, a complex and temporally variable combination of processes such as

Fig. 3.7 Variability dominates: within short distance there is bedrock, talus slopes, several intermediate forms of fractured, thinly debris covered rock, and a rock glacier at the foot of slope. The cast shadow illustrates the variable illumination conditions

metamorphosis of debris-laden avalanche snow, ice segregation, and freezing of shallow ground water occurs. Talus slopes in permafrost areas can be cemented by interstitial ice (Fig. 3.9, left) and, as a consequence, aggrade significant amounts of material protected from erosion - but possibly released in enhanced debris flow activity if thawed during climate change.

Ice in fissures and fractures is common in bedrock permafrost (Fig. 3.9, right) and has been observed both at construction sites and in the fresh detachment scars of rock fall. The percolation of water in previously ice-filled joints can lead to fast and linear thaw of permafrost and, possibly the fast destabilization of large masses of rock. The origin of ice in fractures is unclear. Both the percolation and freezing of meteoric water and ice segregation are possible, and, at present no clear evidence pointing at one or the other process exists.

Permafrost in debris slopes and the landforms associated with it are usually of Holocene age, because their locations are subject to glacier cover and removal of unconsolidated sediments during glacial cycles. By contrast, permafrost in steep

Fig. 3.8 Different forms of ground ice that have been encountered within few hundred meters distance from each other at an elevation of about 3,000 m asl. The exposures were made by excavator during the construction of a ski run north of Gornergrat, Switzerland. a Ice-cemented coarse blocks about 4 m below the ground surface in a perennially frozen and creeping moraine. b Massive ice with visible layering that is most likely a remnant of a small glacier or perennial snow patch that has formed this moraine. c Massive ice that is partly clear and partly cloudy. The ice contains individual large clasts and parallel but undulating layers rich in fine material. d Massive ice exposed just one meter below the surface of a rock glacier. The ice contains individual large clasts, as well as areas rich in pebble-size rock. Photographs by I. Roer and O. Wild

Fig. 3.8 Different forms of ground ice that have been encountered within few hundred meters distance from each other at an elevation of about 3,000 m asl. The exposures were made by excavator during the construction of a ski run north of Gornergrat, Switzerland. a Ice-cemented coarse blocks about 4 m below the ground surface in a perennially frozen and creeping moraine. b Massive ice with visible layering that is most likely a remnant of a small glacier or perennial snow patch that has formed this moraine. c Massive ice that is partly clear and partly cloudy. The ice contains individual large clasts and parallel but undulating layers rich in fine material. d Massive ice exposed just one meter below the surface of a rock glacier. The ice contains individual large clasts, as well as areas rich in pebble-size rock. Photographs by I. Roer and O. Wild and high bedrock peaks is likely to be very old. Rock temperatures of -10°C or lower are not uncommon and, therefore, permafrost and ice in cracks and crevices of high peaks may have endured over several glacial and interglacial cycles. The age of this cold bedrock permafrost is more probably controlled by uplift and erosion than by past climate fluctuations.

Fig. 3.9 Left: Eroded debris slope and exposed ice-cemented permafrost at 2,400 m asl below the 2005 Dents Blanches rock fall (photograph by B. Rey-Bellet). Right: Ice-filled fissure in bedrock that has been exposed during construction activities just below a cable car station at Stockhorn, 3,400 m asl, Switzerland. The fine material fill of the joint is entirely on the left side and separate from the pure ice on the right that is about 20 cm thick

Fig. 3.9 Left: Eroded debris slope and exposed ice-cemented permafrost at 2,400 m asl below the 2005 Dents Blanches rock fall (photograph by B. Rey-Bellet). Right: Ice-filled fissure in bedrock that has been exposed during construction activities just below a cable car station at Stockhorn, 3,400 m asl, Switzerland. The fine material fill of the joint is entirely on the left side and separate from the pure ice on the right that is about 20 cm thick

0 0

Post a comment