Prolonged and profound changes of retrogressive thaw slumps (Herschel Island – Qikiqtaruk, Yukon, Canada)

Along Arctic coastlines retrogressive thaw slumps (RTS) are common thermokarst landform. They deliver a large amount of material rich in organic carbon to the nearshore zone. In the last century the number of RTS has strongly increased in the Canadian Arctic. Mainly characterized by rapidly changing topographical and internal structures (such as mud flow deposits, thaw bulbs, warm permafrost bodies or seawater-affected sediments) RTS are strongly influenced by incising gullies. We propose that due to thermal and mechanical disturbances, especially large RTS are likely to develop a polycyclic behavior. Several electrical resistivity tomography (ERT) profiles were carried out in 2011, 2012 and repeated in 2019 on the biggest RTS on Herschel Island – Qikiqtaruk, a highly active and wellmonitored study area in the Yukon, Northwest Canada. The 2D ERT transects are crossing the RTS longitudinal and transversal, reaching the undisturbed tundra on the edges. Crossing the main gully draining the slump and quasi-parallel to the shoreline, we measured seven ERT profiles in 2012 and 2019 to reveal internal changes in a 3D model. To calibrate the ERT data, we conducted frost probing to detect the unfrozen-frozen transition in the field and in the laboratory, we measured the bulk sediment resistivity versus temperature curves on samples. Thermal and topographical disturbances by gullies leading to large erosional features like RTS cause long recovery rates for disturbed permafrost. In this study, we show that ERT can be used to detect prolonged and profound thermal and mechanical disturbances in RTS. We demonstrate that these disturbances are likely to increase the susceptibility of RTS to a polycyclic behavior