The Blue Machine

But this rigid array can only accommodate water molecules. The salt is left out. As frazil ice crystals bump into each other and start to stick together to form bigger ice structures, they may trap pockets of salty seawater, but those pockets will only ever partially freeze. The remaining liquid gets increasingly salty as the water molecules abandon it to join the crystalline ice. These brine pockets are likely to escape back into the ocean as the ice grows, compresses and cracks. And so the formation of sea ice is not important just because it creates floating solid ice that acts as a lid on the ocean. It’s important because it’s a sorting process. Ice, largely free of salt, floats to the top of the ocean because it is less dense and therefore buoyant. And just underneath the growing sea ice, a slow stream of extra-salty seawater forms and sinks downwards, because the cold and the salt make it more dense than the rest of the seawater. In the regions where the ice grows, a conveyer of salt slumps downward during the ice’s growth season.