Rock glacier dynamics in Southern Carpathian Mountains from high-resolution optical and multi-temporal SAR satellite imagery

Abstract

The dynamics of rock glaciers in marginal periglacial environments are poorly understood, especially in Eastern Europe where the enhanced continentality produces a distinct pattern of periglacial phenomena. Multi-temporal image analysis of high-resolution optical and radar satellite imagery of the Southern Carpathian Mountains, Romania revealed the small dynamic nature and a slow geomorphologic evolution of rock glaciers over a 46-yr period of record (1968–2014). Nine rock glaciers located in glacial cirques and troughs within the central area of Retezat Mountain were included in this study. Overall, the estimated displacement rates are extremely low (i.e., a few cm/year) compared with other active rock glaciers from all over the world. Despite their relative attenuated activity, it appears that Judele, Valea Rea, and Pietrele are still active rock glaciers, but in an evident disequilibrium/imbalance with the actual climate. These findings document the lowest altitude and easternmost active rock glaciers at this latitude from Europe. Quantitative investigations were concentrated on Pietrele rock glacier, subject of recent field campaigns. Optical data analysis indicated a slight acceleration of the horizontal velocities at the surface of Pietrele rock glacier in recent years. This acceleration appears to be caused by an increase in the temperature of permafrost, resulting from an evident warming of external air temperature. Radar data analysis suggested seasonal variability of surface motion, with higher deformation in autumn, whereas in early summer or spring the deformation is negligible. Overall, the results obtained with cross-correlation analysis, interferometric synthetic aperture radar (InSAR) coherence analysis, and Small BAseline Subset (SBAS) multitemporal interferometry, are consistent, displaying similar deformation patterns with the highest creep rates located in the southern and western portions of the glacier. These findings are supported by thermal and geophysical measurements, which suggest the probable presence of permafrost within these areas. The observed displacements were interpreted as permafrost creep, and their very low velocity rates suggest the deforming frozen layers are very thin. These results provide (i) baseline information and decadal-scale trends and a (ii) strategy for future monitoring of the health and integrity of the rock glacier environment in the Carpathian Mountains.