Thermokarst primes subsea permafrost degradation and coastal change

• Main Authors: Angelopoulos, Michael, Overduin, Pier Paul, Jenrich, Maren, Nitze, Ingmar, Günther, Frank, Strauss, Jens, Krautblatter, Michael, Grigoriev, Mikhail, Grosse, Guido
• Format: Conference Object
• Language: unknown
• Published: AGU 2020
• Subjects: Arctic; Talik; The Spit; Tiksi; Ice; permafrost; Thermokarst; Tiksi Bay; Siberia
• Online Access: https://epic.awi.de/id/eprint/53793/; https://agu.confex.com/agu/fm20/meetingapp.cgi/Paper/732165; https://hdl.handle.net/10013/epic.0164bc98-adee-4293-953e-7d592618083c

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.