How does salt leave seawater when sea ice freezes?

When seawater freezes to form sea ice, the salt within the water is expelled as brine. As the ice crystals form a matrix, they trap pockets of this liquid brine and air.12

The process of desalination—or the removal of salt from the ice—occurs through several mechanisms:

• Brine Expulsion and Migration: Salt is rejected from the ice structure as it freezes. The degree of this rejection is higher when the rate of freezing is slower. Over time, brine pockets can migrate through the ice due to temperature gradients.12
• Gravity Drainage: This is the most significant process for salt removal. Brine moves out of the ice through a network of cells and channels within the porous ice structure.2
• Flushing: In the Arctic, this process is further enhanced during the summer months when snow and ice meltwater percolate into the ice, effectively washing out the remaining brine.2

As a result of these processes, the average salinity of sea ice decreases significantly as it ages, eventually allowing multiyear ice to be melted for drinking water.21