SUBMARINE PERMAFROST DYNAMICS ALONG THE ARCTIC SHELF EDGE

Exploration in the Canadian Beaufort Sea, offshore of the Tuktoyaktuk Peninsula, has revealed a remarkable a zone of rugose morphology at the shelf edge and upper slope. This morphology is especially common in 100 to 200 m water depths where seafloor features include topographic mounds, pockmarks, slope parallel ridges, and slide scars. This area occurs at the seaward edge of a sub-sea ~600 m thick relict permafrost zone and geothermal modelling suggests that the lower 100 m of the permafrost zone has decomposed during the Holocene. Sediment cores show escaping brackish waters with pore water chloride content indicating widespread down core freshening, especially near the shelf edge on the upper slope. Bottom waters corresponding with this band have a mean annual temperature of less than -1.4°C, cold enough to freeze escaping brackish pore waters. Positive relief mound features are up to 10 m high circular to oval shaped and ~50 m in diameter, occurring at a density of ~6 per km². Pore and lense ice has been observed in sediment cores and we interpret these features as offshore pingos. Intermixed are circular topographic depressions up to 20 m deep. Detailed investigations utilizing a mapping Autonomous Underwater Vehicle (AUV) to provide 1-m grid bathymetric and Chirp profiles, and Remotely Operated Vehicle observations, were made to provide insights as to the origin and age of these features. AUV surveys of one 8 km² area first conducted in 2013 were repeated in 2017. Repeat mapping shows significant changes within this 4-year period. Multiple circular or elongated depressions have developed which are up to 10 m deep and 100 m long. A corresponding volume of newly accreted material around the depressions equal to the missing volume was not detected. No evidence for high methane concentrations were found within this survey area as pore waters sulfate gradients indicate the sulfate-methane transition zone is >8 m below seafloor in most sediment cores and no chemosynthetic seep fauna or authigenic carbonates were seen. We attribute the concentrated band of features to be related to the on-going degradation of relict permafrost, the expulsion of brackish waters, and formation of ground ice within the near seafloor sediments. These observations have significant geohazard implications, which may be characteristic of Arctic settings.