Polar Sea Ice and Brine Channels
Frazil Crystal Rejection, Antarctic Bottom Water Formation, and Ice-Edge Blooms
Freshwater lakes reach maximum density at $+4.0$ degrees Celsius, so water chilled below 4 degrees floats on top and freezes easily. In 35-PSU seawater, salt pushes the density maximum below the $-1.9$-degree freezing point. Polar oceans freeze at the surface only because a permanent salinity jump (halocline) at 100 meters stops cold water from sinking to the abyss.
Unlike a calm pond that freezes into a clear sheet, a windswept polar sea begins freezing as a murky soup of needle crystals called frazil ice. Wave orbitals herd these crystals into circular slush discs with upturned rims (pancake ice). The pancakes then weld together into a wave-bending elastic skin called nilas.
When water molecules lock into the hexagonal crystal lattice of ice ($I_h$), the spacing between oxygen atoms is too tight to fit hydrated sodium or chloride ions. Freezing sea ice squeezes 70 to 90 percent of the seawater's salt out of the crystal lattice. It rejects concentrated liquid brine into internal veins and downward into the sea.
Sea ice is not a solid block, but a porous sponge riddled with thousands of branching vertical liquid brine channels per square meter. As winter air chills the upper ice to $-20$ degrees Celsius, pure water freezes onto channel walls. The trapped brine concentrates to 200 PSU—six times normal salinity—and stays liquid inside solid ice.
Living inside those freezing, hypersaline brine channels is a community of sympagic microbes led by the diatom Fragilariopsis cylindrus, which turns the bottom of pack ice golden-brown. To keep ice needles from puncturing its membrane at $-15$ degrees Celsius, the diatom secretes ice-binding proteins. Those proteins adsorb onto crystal faces and carve liquid micro-pockets.
When a plume of $-15$-degree hypersaline brine drains out the bottom of Antarctic sea ice into $-1.9$-degree seawater, it freezes a hollow tube of fresh ice around itself. Filmed under Erebus Bay in 2011, these descending brinicles touch the seafloor. There, they spread a web of black ice that freezes slow-moving sea stars and urchins.
When 100-kilometer-per-hour katabatic winds roar off Antarctica, they blow newly formed sea ice away from the coast to hold open ice factories called polynyas. Freezing new ice daily and dumping rejected salt into $-1.9$-degree shelf water, Weddell and Ross sea polynyas mint Antarctic Bottom Water. This densest water mass on Earth lines abyssal basins up to 40 degrees North.
Antarctic krill (Euphausia superba) weigh 400 million metric tons across the Southern Ocean and rely on the underside of sea ice as a winter nursery. Juveniles survive the four-month polar night by scraping algae off the ceiling of brine channels. Starving adults endure 200 winter days by molting into smaller shells and shrinking their bodies.
When polar sea ice melts back in spring across millions of square kilometers, it spreads a 20-meter lens of buoyant, low-salinity freshwater across the surface. That meltwater cap prevents deep wind mixing from dragging algae into the dark. Simultaneously, it releases iron-rich dust trapped in the ice, igniting explosive ice-edge blooms within days.
Norwegian biologist Johan Ruud confirmed at South Georgia in 1954 that Antarctic crocodile icefish (Chaenocephalus aceratus) circulate crystal-clear blood with zero hemoglobin or red blood cells. Because oxygen solubility doubles between $+30$ and $-1.9$ degrees Celsius, the family Channichthyidae deleted hemoglobin genes completely. Instead, oversized hearts pump low-viscosity plasma through wide capillaries.
Why ordinary bony fish freeze solid at $-0.8$ degrees Celsius when touching ice in $-1.9$-degree polar water, whereas Antarctic notothenioid fishes swim safely through frazil slush, was solved by Arthur DeVries in 1969. Their blood is packed with Antifreeze Glycoproteins. Christina Cheng proved in 1997 that these proteins evolved from a duplicated pancreatic trypsinogen enzyme gene.
Snow-covered sea ice reflects 85 percent of sunlight back into space ($\alpha = 0.85$), whereas dark open seawater absorbs 94 percent of solar heat ($\alpha = 0.06$). Melting one square kilometer of white Arctic ice replaces a mirror with a dark solar collector. That ice-albedo feedback warms the Arctic nearly four times faster than the global average.