Mega retrogressive thaw slumps (MRTS, >10(6) m(3)) are a major threat to Arctic infrastructure, alter regional biogeochemistry, and impact Arctic carbon budgets. However, processes initiating and reactivating MRTS are insufficiently understood. We hypothesize that MRTS preferentially develop a polycyclic behavior because the material is thermally and mechanically prepared for subsequent generation failure. In contrast to remote sensing, geophysical reconnaissance reveals the inner structure and relative thermal state of MRTS decameters beneath slump surfaces, potentially controlling polycyclicity. Based on their life cycle development, five (M)RTS were studied on Herschel Island, an MRTS hotspot on the Canadian Beaufort coast. We combine >2 km of electrical resistivity tomography (ERT), 500 m of ground-penetrating radar (GPR) and annual monitoring of headwall retreat from 2004 to 2013 to reveal the thermal state, internal structure, and volume loss of slumps. ERT data were calibrated with unfrozen-frozen transitions from frost probing of active layer thickness and shallow boreholes. In initial stage MRTS, ERT displays surficial thermal perturbations a few meters deep, coincident with recent mud pool and mud flow development. In early stage polycyclic MRTS, ERT shows decameter deep-reaching thermal perturbations persisting even 300 years after the last activation. In peak-stage polycyclic MRTS, 3D-ERT highlights actively extending deep-reaching thermal perturbations caused by gully incisions, mud slides and mud flows. GPR and headwall monitoring reveal structural disturbance by historical mud flows, ice-rich permafrost, and a decadal quantification of headwall retreat and slump floor erosion. We show that geophysical signatures identify long-lasting thermal and mechanical disturbances in MRTS predefining their susceptibility to polycyclic reactivation.

Permafrost soils contain c. 1980 Pg carbon (C; Schuur et al., 2015), more than twice the size of the atmospheric C pool. Thawing permafrost, subsequent changes in hydrological conditions and resulting microbial decomposition of previously frozen organicCis one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate (Schuur et al., 2008; Hugelius et al., 2012; Hope& Schaefer, 2016): such changes are now occurring at a dramatic pace over large regions of the Northern Hemisphere.

The Canadian Arctic is characterized by a high variation in landform types and there are complex interactions between land, water and the atmosphere which dramatically affect the distribution of biota. Biodiversity depends upon the intensity, predictability and scale of these interactions. Observations, as well as predictions of large-scale climate models which include ocean circulation, reveal an anomalous cooling of northeastern Canada in recent decades, in contrast to the overall significant increase in average annual temperature in the Northern Hemisphere. Predictions from models are necessary to forecast the change in the treeline in the 21st century which may lead to a major loss of tundra. The rate of change in vegetation in response to climate change is poorly understood. The treeline in central Canada, for example, is showing infilling with trees, and in some locations, northerly movement of the boundary. The presence of sea ice in Hudson Bay and other coastal areas is a major factor affecting interactions between the marine and terrestrial ecosystems. Loss of ice and therefore hunting of seals by polar bears will reduce bear and arctic fox populations within the region. in turn, this is likely to have significant effects on their herbivorous prey populations and forage plants, Further, the undersurface of sea ice is a major site for the growth of algae and marine invertebrates which in turn act as food for the marine food web. A rise in sea-level may flood coastal saltmarsh communities leading to changes in plant assemblages and a decline in foraging by geese and other consumers. The anomalous cooling in the eastern Arctic, primarily in late winter and early spring, has interrupted northern migration of breeding populations of geese and ducks and led to increased damage to vegetation in southern arctic saltmarshes as a result of foraging. It is likely that there has been a significant loss of invertebrates in those areas where the vegetation has been destroyed, Warming will have major effects on permafrost distribution and on ground-ice resulting in a major destabilization of slopes and slumping of soil, and disruption of tundra plant communities. Disruption of peat and moss surfaces lead to loss of insulation, an increase in active-layer depth and changes in drainage and plant assemblages. Increases of UV-B radiation will strongly affect vulnerable populations of both plants and animals, The indigenous peoples will face major changes in life style, edibility of food and health standards, if there is a significant warming trend. The great need is for information which is sensitive to the changes and will assist in developing an understanding of the complex interactions of the arctic biota, human populations and the physical environment.