Spring-water temperature suggests widespread occurrence of Alpine permafrost in pseudo-relict rock glaciers

Abstract. Runoff originating from ground ice contained in landforms like rock glaciers and talus slopes represents an important water supply for the lowlands. Pseudo-relict rock glaciers host patchy permafrost, but appear to be visually relict, and therefore can be misinterpreted by using standard classification approaches. Permafrost content, spatial distribution and frequency of this type of rock glaciers are poorly known. Therefore, identifying pseudo-relict rock glaciers that might still host permafrost, and potentially ice, is crucial for understanding their hydrological role in a climate change context.

This work analyses rock-glacier spring-water temperature in a 795 km² catchment in the Eastern Italian Alps to understand how many rock glaciers classified as relict could have spring-water temperature comparable to intact rock glaciers, as possible evidence of their pseudo-relict nature. Spring-water temperature, often used as auxiliary to other approaches for specific sites, was used for a preliminary estimate of the permafrost presence in 50 rock glaciers classified as relict. In addition, we present electrical resistivity tomography (ERT) results on two relict rock glaciers with opposing spring-water temperature and surface characteristics to constrain spring-water temperature results at local scale.

The results show that about 50 % of rock glaciers classified as relict might be pseudo-relict, thus potentially containing permafrost. Both supposedly relict rock glaciers investigated by geophysics contain frozen sediments. The majority of cold springs are mainly associated with rock glaciers with blocky and sparsely vegetated surface, but geophysics suggest that permafrost may also exist in rock glaciers below 2000 m a.s.l., entirely covered by vegetation and with spring-water temperature up to 3.7 °C. We estimate that pseudo-relict rock glaciers might contain a significant portion (20 %) of all the ice stored in the rock glaciers in the study area. These results highlight the relevance of pseudo-relict rock glaciers in periglacial environments. Even if not a conclusive method, spring-water-temperature analyses can be used to preliminarily distinguish between relict and pseudo-relict rock glaciers in wide regions.