Transitional rock glaciers at sea-level in Northern Norway

Abstract. Rock glaciers are geomorphological expressions of permafrost. Close to sea level in northernmost Norway, in the sub-Arctic Nordkinn peninsula, we have observed several rock glaciers that appear to be active now or were active in the recent past. Active rock glaciers at this elevation have never before been described in Fennoscandia, and they should be outside of the climatic limits of present-day permafrost according to models. In this study, we have investigated whether or not these rock glaciers are active under the current climate situation. We made detailed geomorphological maps of three rock glacier areas in Nordkinn (Ivarsfjorden, Store Skogfjorden, and Lille Skogfjorden), and investigated the regional ground dynamics using Synthetic Aperture Radar Interferometry (InSAR). One of the rock glaciers, namely the Ivarsfjorden rock glacier, was investigated in more detail by combining observations of vertical and horizontal changes from optical images acquired by airborne and terrestrial sensors and terrestrial laser scans (TLS). The subsurface of the same rock glacier was investigated using a combination of Electrical Resistivity Tomography (ERT) and Refraction Seismic Tomography (RST). We also measured ground surface temperatures between 2016 and 2020. We mapped the rock glaciers in the innermost parts of Store and Lille Skogfjorden as relict, while the more active ones are in the mouths of both fjords, fed by active talus in the upslopes. Several of the rock glaciers cross over both the Younger Dryas shoreline (25 m a.s.l.), and the Tapes shoreline at 13 m a.s.l. Both InSAR and optical remote sensing observations reveal low yearly movement rates (mm-cm yr−1). The ERT and RST suggest that there are no longer permafrost and ground ice in the rock glacier, while temperature observations in the front slope indicate freezing conditions also in summer. Based on the in-situ temperature measurements and the interpolated regional temperature data, we show that the MAAT of the region has raised by 2 °C since the late 19th century to about 1.5 °C in the last decade. MAATs below 0 °C 100–150 years ago suggest that the rock glaciers may have been active at the end of the Little Ice Age (LIA). These combined results indicate that the Nordkinn rock glaciers are transitioning from active to relict stages. The study shows that transitional rock glaciers are still affected by creep, rock falls, snow avalanches, etc., and are not entirely dynamically dead features. Our contrasting results concerning permafrost presence and rock glacier activity show the importance of a multi-methodical approach when investigating slope processes in the edge zones of permafrost influence.