Transitional rock glaciers at sea level in northern Norway

[摘要] Rock glaciers are geomorphological expressions of permafrost.Close to sea level in northernmost Norway, in the subarctic Nordkinnpeninsula, we have observed several rock glaciers that appear to be activenow or were active in the recent past. Active rock glaciers at this elevation have never before been described inFennoscandia, and they are outside the climatic limits of present-daypermafrost according to models. In this study, we have investigated whetheror not these rock glaciers are active under the current climate situation.We made detailed geomorphological maps of three rock glacier areas inNordkinn and investigated the regional ground dynamics using synthetic aperture radar interferometry (InSAR). One of the rock glaciers, namely theIvarsfjorden rock glacier, was investigated in more detail by combiningobservations of vertical and horizontal changes from optical images acquired by airborne and terrestrial sensors and terrestrial laser scans (TLSs). 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, complemented by investigations using an infrared thermal camera, and a multi-decadal climatic analysis. We mapped the rock glaciers in the innermost parts of Store and LilleSkogfjorden as relict, while the more active ones are in the mouths of bothfjords, fed by active talus in the upper slopes. Several of the rockglaciers cross over both the Younger Dryas shoreline (25 m a.s.l.) and the Early to Mid-Holocene shoreline at 13 m a.s.l. Both InSAR and optical remote sensing observations reveal low yearly movement rates (centimetres to millimetres per year). The ERT and RST suggest that there is no longer permafrost and ground ice in the rock glacier, while temperature observations on 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 mean annual air temperature (MAAT) of the region has risen 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 new rock glacier lobes may have formed at the end of the Little Ice Age (LIA). These combined results indicate that the Nordkinn rock glaciers aretransitioning from active to relict stages. The study shows thattransitional rock glaciers are still affected by creep, rock falls, snowavalanches, etc., and are not entirely dynamically dead features. Ourcontrasting results concerning permafrost presence and rock glacier activity show the importance of a multi-methodological approach when investigating slope processes in the edge zones of permafrost influence.

[学科分类] 土壤学