Basal melt at the deep back of ice shelves produces freshwater at depths >1000 m locally or more. This then flows up the underside of the ice shelf and becomes locally supercooled, leading to basal refreezing. Admittedly there are entrainment effects too, but a key driver is the pressure influence on the local freezing point. Ref: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006JC003915