Winter in the coastal ocean: Seasonal freezing causes seafloor expansion and contraction.

Seafloor mapping in polar regions has led to the recognition of landforms associated with subsea permafrost. Despite recent studies indicating rapid subsea permafrost degradation, information on seasonal seafloor changes remains limited. Here, we use time-lapse multibeam bathymetry (2021-2024) and bottom-water temperature data to reveal seasonal formation and degradation of small frost blisters (5 meters wide and 20 to 50 centimeters high) as ice forms within surficial sediments. These frost blisters were degraded in October 2021 at depths of ≤25 meters but were widespread in July 2023. They thawed between July and November 2023, reformed over the following winter, and were present again in August 2024. Data from moored bottom-water sensors and sediment cores indicate that the formation and degradation of frost blisters are driven by seasonal temperature changes at depth and influenced by freshened porewater in surficial sediments. This study documents previously unrecognized seasonal expansion and contraction of the seafloor driven by freeze-thaw cycles of freshened porewater.