Slow geomorphologic evolution of rock glaciers in marginal periglacial environment of Southern Carpathians (Romania)

The development of remote sensing techniques in the last several decades now permits surveying areas that are difficult to access and allows capturing geomorphological processes that operate at low deformation rates. In marginal periglacial environments, where the kinematics of the rock glaciers are defined by slow flow, specific remote sensing techniques (e.g., optical image cross-correlation techniques or multitemporal interferometry) are preferred because they are capable of providing ground displacement accuracies on the order of a few mm. Recent multi-temporal image analysis of high-resolution optical and radar satellite imagery of the Retezat Mountains (in the southern Carpathian Mountains of Romania) rock glaciers revealed very low rates of deformation and confirmed that active rock glaciers still exist here. These findings were supported by geophysical investigations (electrical resistivity tomography and ground penetrating radar) and thermal monitoring, which confirmed the patchy distribution of permafrost within the investigated rock glaciers. Given the high depth of the active layer (between 5 and 10 m) and the limited thickness of the ground ice bodies, the permafrost in the Retezat Mountains was assumed to exist in marginal conditions. The sporadic permafrost from the alpine zone of the southern Carpathians is extremely sensitive to environmental changes and responds directly to air temperature fluctuations. As an example, geophysical investigations between 2007 and 2014 revealed significant ground ice disappearance from one of the studied rock glaciers (i.e., Pietrele) as a result of rising air temperature. Optical satellite data revealed that the disappearance of ground ice led to accelerated movement of this rock glacier after 2007. The variability of horizontal movement is strongly controlled by the ground thermal regime. The results confirm the hypothesis that permafrost is not in equilibrium with the present-day climate in the Romanian Carpathians and will probably disappear completely in the next few decades.