Detection of mountain permafrost by combining conventional geophysical methods and thermal monitoring in the Retezat Mountains, Romania

- Permafrost distribution was examined in Retezat Mountains, Romania.
- Thermal monitoring and geophysical investigations were used to detect permafrost.
- Permafrost is restricted to those sites where the ground cooling is very efficient.
- The observed chimney mechanism contributes to the permafrost preservation.

A combined approach consisting in thermal monitoring and geophysical investigations by means of ground penetrating radar and electrical resistivity tomography was used to examine permafrost distribution within four rock glaciers in the Retezat Mountains. The small-scale variability of permafrost presence or absence above 2000 m in Southern Carpathians is determined by local surface cover characteristics. Therefore, the permafrost existence is limited to those sites where very large boulders occur, the income of solar radiation is reduced and the cooling effect of the coarse blocks is extremely efficient. Based on the thermal data, permafrost appears likely at those sites, where a strong cooling occurs in the early winter due to ground air flow (convection, advection). During the winter, below the thick snow cover an additional cooling mechanism is likely the result of low thermal conductivity of coarse blocks at sites where internal ventilation is inhibited. At these sites the mean annual ground surface temperature (MAGST) is negative, the temperatures at the bottom of the snow cover (BTS) at the end of the winter are lower than − 3 °C and the ground freezing index (GFI) values are higher than 600 °C days. By contrast, BTS, MAGST and GFI values show a warmer pattern in the upper part of the ventilated zones, where inverse thermal behavior at the ground surface occur especially during cold seasons.