Core drilling in a low altitude permafrost site from temperate regions. Case study: Detunata Goală, Romanian Carpathians

Detunata Goală scree is a talus slope-rock glacier system characterized by persistent snow and ice during springtime and summer in spite of a mean annual air temperature of around +7°C. This is a porous talus made of andesitic basaltic columns affected by chimney circulation that seems to allow for a extrazonal permafrost of low altitude in a temperate climate much lower than the regional limit of alpine permafrost. In the postdoctoral project FrozenCORE electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) were applied in October 2020 in order to check permafrost presence at the end of the warm season and to determine the internal structure of the deposit. The two methods indicated contradictory results, as ERT indicated a high resistive layer in the first 10-15 m while the SRT indicated a high velocity layer at depths greater than 15 m. A borehole was drilled in June 2021 in the coldest sector of the scree and the cores recovered indicated that: 1) the talus is relatively thin, less than 13 m; 2) the deposit has a low amount of ice, several lenses were found between 3 and 10 m each of at most a few centimeters thick; 3) the scree porosity is relatively low, much smaller than at the surface. A thermistor chain was installed in the borehole at depths according to the GTN-P recommendations for future monitoring of the temperatures in the underground. Ice samples were collected from the cores for isotopic analyses in order to check if the ice from the greater depths is older than the upper one assumed to be seasonal. The drilling indicated that ERT is a better method for assessing the stratigraphy of such talus deposits.