GROUND TEMPERATURE REGIMES OF A DOLOMITIC AREA CHARACTERIZED BY CRYOGENIC LANDFORMS

The studied area corresponds to the so-called Basin of Vauz, located between 1840 m and 2250 m of elevation. It lies between the Pordoi Pass (Dolomites) and the village of Arabba (BL). There are several landforms related to frost action and solifluction in this area, such as patterned ground, terracettes and lobes. This research aims at i) studying the processes involved in the slow movement of the ground and in the development of the landforms of the area, and ii) understanding the role of water circulation inside the slope. Analogous phenomena are present in many areas of the Dolomites, especially on the passes and in several locations where pelitic and volcano-clastic formations crop out (i.e. S. Cassiano Fm. and Wengen Fm.). Indeed, the movements usually develop within the detrital deposits generated from the degradation of the two previously mentioned geological formations. The particle sizes of debris involved in the movements are between coarse sand and fine silt, with a predominance of medium sand and coarse silt (from 4mm to 0,002mm). Measurements of the thermal state of the soil were recently undertaken in the study area, using thermometric probes (PB-5001-1M5; accuracy ±0.2°C) connected to specific data loggers (TGP-4520 TINYTAG PLUS 2). They allow to store hourly minimum, maximum and average temperature. Measurements were collected at 1874 m, 1935 m, 2190 m and 2365 m of altitude, at sites exposed to the south and with an inclination of respectively 23 °, 14 °, 11 ° and 36 °. In order to analyze the trend of temperature with depth, in one of the monitoring sites (at 1935 m) three thermometers were placed at different depth (5 cm, 25 cm and 50 cm from the ground level). Data on air temperatures and snow thickness are provided by the weather stations of Arabba and Passo Pordoi - Belvedere (data supplied by the ARPAV Meteorological Centre of Arabba and Meteotrentino). Data series between November 2009 and September 2010 have been already collected, while a second set of data spanning from autumn 2010 to spring 2011 are going to be collected. The first data show that in autumn 2009, in absence of snow cover, soil temperatures fluctuated in connection the daily air temperature cycles but the ground temperature remained above 0°C. After the first snowfall, this connection weakened and the ground temperatures developed independently from the air temperature variations. The soil temperatures at the various monitoring sites reached stable values above 0°C during winter, until the snow cover disappeared. Thermometers placed at different depths show an attenuation of the thermal wave with increasing depth. The thermal cycles are visible at a depth of 5 cm, while at 25 cm depth they are more attenuated (only when snow cover is absent, e.g. autumn and late spring). Moreover, higher temperatures were recorded at 50 cm than at 5 cm and at 25 cm in autumn and in winter, the opposite in spring and in summer. During winter, with the presence of snow cover, generally an almost stable value of temperature is reached at every depth without temperature oscillations. The collected temperatures indicate the absence of freeze conditions during the full year at all the monitoring sites. Actually, soil temperatures never fell below 0°C, even if air temperatures reached values well below 0° C (- 15°, -18° C). The lowest temperatures are not recorded at the highest monitoring sites, indicating that not only the altitude but also the steepness and the exposure of the slopes exert a strong control on the thermal regime of the soil. The results obtained so far suggest that the thermal regime of the ground controls in a complicated way the solifluction phenomena affecting the slopes of this area.