Reactivation of temporarily stabilized ice-cored moraines in front of polythermal glaciers: Gravitational mass movements as the most important geomorphological agents for the redistribution of sediments (a case study from Ebbabreen and Ragnarbreen, Svalbard)

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Ice-cored moraine complexes are prominent landforms characteristic of many polythermal glacial landsystems on Svalbard, usually marking their maximum Little Ice Age extent. Despite being often stable over a period ranging from several years to 50–60 years, the abundance of dead-ice makes them potentially unstable in the future. This study aims to: (1) recognize the different processes that contribute to ice-cored moraine degradation; (2) document the spatial distribution of mass wasting sites within ice-cored moraine complexes of Ebbabreen and Ragnarbreen, Svalbard; (3) investigate the relationship between the distribution of mass-movement sites and the topographic properties of the landscape; (4) develop a model of mass-wasting mechanism in a proglacial area. We performed field-based geomorphological mapping combined with the interpretation of aerial photographs (1960, 1990, 2009), high-resolution satellite imagery (2013), UAV-based images (2014), terrestrial photographs (2011–2014), hillshade models and digital elevation models.In 2013, frontal and lateral ice-cored landforms occupied 3.33 km2 of the forelands of Ebbabreen and Ragnarbreen. The total area with an unstable ice-cored moraine condition was 0.57 km2 (127 sites). Debris falls, slides, rolls and flows were the most important processes contributing to the reworking of glacigenic sediments and altering of their properties. Four different morphological site types were identified: (1) near vertical ice-cliffs covered with debris, transformed mainly by dead-ice backwasting as well as debris falls and slides, (2) steep debris slopes with exposed ice-cores dominated by debris slides, (3) gentle sediment-mantled slopes transformed by debris flows, and (4) non-active debris-mantled areas transformed by dead-ice downwasting alone. The spatial and temporal activation of moraines' reworking was for the most part related to local geomorphic conditions (e.g., slope gradient, occurrence of streams, and meltwater channels). Gravitational mass-movements were responsible for the redistribution of debris within the moraine-complex, whereas fluvial transport by meltwater exported sediments outside the Little Ice Age moraines.