https://www.youtube.com/watch?v=5dt1se6AG_I
Sediment-core isotopes from the Arctic Ocean and Nordic Seas indicate that, at least twice in the last ~150,000 years, a roughly kilometer-scale shelf-ice lid sealed the basin.
Under that ice, the enclosed ocean became essentially fresh — a paleoclimate state reconstructed by Alfred Wegener Institute researchers from the absence of seawater tracers such as thorium-230.