Rapidly forming submarine craters and massive ice outcrops along the Arctic shelf edge: by-products of subsea permafrost degradation

Substantial morphological changes are rapidly occurring along the Canadian Arctic shelf edge (Paull et al., 2022, PNAS). During a 2022 IBRV Araon cruise, autonomous underwater vehicle mapping surveys identified several new craters that formed between 2019 and 2022. Five multibeam bathymetric mapping surveys, each partially covering a 15 km2 study area between 120 and 200 mwd have now been conducted over a 12-year time period. These repeat surveys reveal 65 new depressions developed averaging 6.5 m deep and reaching up to 30 m deep. Remotely operated vehicle investigations also discovered outcrops of massive ice exposed on the flanks of the newest craters. This ice is not believed to be relic permafrost formed during Pleistocene sea-level low-stands because the host sediments were deposited in a submarine setting. The low porewater salinity and light isotopic compositions in the meltwater of ice samples from sediment cores indicate brackish waters reflecting a meteoric source are discharging and freezing in this area. The ascending brackish groundwater is likely derived from melting relict permafrost under the shelf. The ~ -1.4°C bottom water temperatures provide conditions appropriate for freezing brackish porewaters within the near seafloor sediments. Conditions appropriate for the melting of ice also exist nearby where ice is in contact with seawater or warmed by ascending groundwater. Small variations in either temperature or salinity, over time, can shift equilibrium conditions of ice formation and degradation, which leads to repetitive freezing and thawing of ascending brackish groundwater and the development of wide-spread ice layers in the near seafloor sediments. These conditions have produced a dramatic submarine thermokarst morphology riddled with multi-aged depressions captured in the repeat mapping surveys. These findings suggest that the distribution of submarine permafrost ice should be reassessed as it may include extensive areas where ice formed during the Holocene where groundwaters discharge at sub-zero temperatures, in addition to relict Pleistocene permafrost.