ALBERT

Description: Subsea permafrost forms when sea level rise from deglaciation or coastal erosion results in inundation of terrestrial permafrost. The response of permafrost to flooding in these settings will be determined by both ice-rich Pleistocene deposits and the thermokarst basins that thawed out during the Holocene. Thermokarst processes lower ground ice content, create partially drained and refrozen depressions (Alases) and thaw bulbs (taliks) beneath them, warm the ground, and can thaw the ground below sea level. We hypothesize that inundated Alases offshore with relatively lower ice content and higher porewater salinities in their sediments (possibly resulting from lagoon interaction) thaw faster than Yedoma terrain. To test this hypothesis, we estimated permafrost thaw rates offshore of the Bykovsky Peninsula in Tiksi Bay, northeastern Siberia using geoelectric surveys with floating electrodes. The surveys traversed a former undrained lagoon, drained and refrozen Alas deposits, and undisturbed Yedoma terrain at varying distances from shore. A continuous Yedoma-Alas-beach-lagoon survey was also carried out to obtain an indication of pre-inundation subsurface electrical resistivity. While the estimated degradation rates of the submerged Yedoma lies in the range of similar sites, and slows with increasing distance offshore, the Alas rates were more diverse and at least twice as fast within 125 m of the coastline. The latter is possibly due to saline lagoon water that infiltrated the Alas while it was still unfrozen. The ice-bearing permafrost depths of the former lagoon were generally the deepest of the terrain units, but displayed poor correlation with distance offshore. We attribute this to heterogeneous talik thickness upon the lagoon to sea transition, as well as permafrost aggradation processes beneath the spit. Given the prevalence of thermokarst basins and lakes along parts of the Arctic coastline, their effect on subsea permafrost degradation must be similarly prevalent. Remote sensing analyses suggest that 40% of lagoons wider than 500 m originated in thermokarst basins along the pan-Arctic coast. The more rapid degradation rates shown here suggest that low-ice content conduits for fluid flow may be more common than currently thought based on thermal modelling of subsea permafrost distribution.

Description: 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.

Description: Retrogressive thaw slumps (RTS) are a common thermokarst landform along Arctic coastlines and provide a large amount of material containing organic carbon to the nearshore zone. The number of RTS has strongly increased since the last century. They are characterized by rapidly changing topographical and internal structures e.g., mud flow deposits, seawater-affected sediments or permafrost bodies and are strongly influenced by gullies. Furthermore, we hypothesize that due to thermal and mechanical disturbance, large RTS preferentially develop a polycyclic behavior. To reveal the inner structures of the RTS several electrical resistivity tomography (ERT) transects were carried out in 2011, 2012, and 2019 on the biggest RTS on Herschel Island (Qikiqtaruk, YT, Canada), a highly active and well-monitored study area. 2D ERT transects were conducted crossing the RTS longitudinal and transversal, always reaching the undisturbed tundra. Parallel to the shoreline, and crossing the main gully draining the slump, we applied 3D ERT which was first measured in 2012 and repeated in 2019. The ERT data was calibrated in the field using frost probing to detect the unfrozen-frozen transition and with bulk sediment resistivity versus temperature curves measured on samples in the laboratory. The strong thermal and topographical disturbances by gullies developing into large erosional features like RTS, lead to long recovery rates for disturbed permafrost, probably taking more than decades. In this study we demonstrate that ERT can be used to determine long-lasting thermal and mechanical disturbances. We show that they are both likely to prime the sensitivity of RTS to a polycyclic reactivation.