Oceanography & Marine Resources20 Concepts & Facts

Sea Ice Desalination: Brine Rejection, Salt Exclusion and Freezing

Open ocean seawater maintains an average salinity of approximately 35 practical salinity units, or parts per thousand, reflecting substantial concentrations of dissolved sodium, chloride, magnesium, and sulfate ions. Because dissolved salts induce colligative freezing point depression, standard seawater freezes not at 0 degrees Celsius, but at roughly minus 1.8 degrees Celsius. When polar seawater reaches this thermal threshold, phase transition produces solid sea ice that exhibits a remarkably lower salinity than the surrounding ocean. This compositional divergence occurs because the thermodynamic structure of solid water cannot accommodate foreign solutes. As water molecules freeze, strong directional hydrogen bonding locks them into a rigid, highly organized hexagonal crystal lattice. The ionic radii of dissolved ions like sodium and chloride are physically incompatible with this crystalline framework, leading to natural chemical exclusion during phase change.

Although the microscopic ice lattice itself is pure freshwater ice, newly formed sea ice is not completely free of salt due to dynamic physical entrapment during rapid freezing. In turbulent polar seas, initial crystallization produces minute, suspended disk-like crystals known as frazil ice, which coalesce into grease ice and elastic nilas sheets. During this rapid congelation, pockets of hyper-saline liquid become mechanically trapped within intercellular boundaries between growing ice platelets, forming concentrated brine pockets and micro-tubular brine channels. Consequently, newly formed first-year sea ice exhibits a bulk salinity between 4 and 10 parts per thousand, retaining only a fraction of oceanic salts. As temperatures drop further, the volume of pure ice within brine pockets increases, forcing the remaining liquid brine into an increasingly concentrated, dense solution that resists freezing.

Desalination continues systematically through post-formational drainage mechanisms that progressively purify the sea ice pack over time. Dense liquid brine drains downward under gravity through vertical drainage networks into the underlying ocean, a process known as gravity drainage. In polar summers, surface warmth melts top layers of snow and ice, generating fresh meltwater that percolates downward through the porous ice matrix in a process termed gravity flushing, which flushes out residual salt solutions. Surviving multiple summer seasons, multi-year pack ice attains a salinity below 1 to 2 parts per thousand, rendering melted surface ice potable for polar explorers. The cold, dense, hyper-saline brine rejected during winter freezing sinks rapidly off continental shelves in the Weddell and Ross Seas, driving the production of Antarctic Bottom Water and energizing global thermohaline ocean conveyor circulation.

Reviewed by the Master10 Editorial Board for accuracy, clarity and competitive-exam relevance.Editorial Policy

Key Concepts & Self-Assessment20 Key Facts

Review key Sea Ice Desalination & Brine Rejection Mechanisms exam facts and rate your mastery to track revision.

Progress: 0/20 Rated 0 Mastered 0 Review Later

Sea ice contains significantly less salt than seawater due to crystal lattice exclusion, brine pocket drainage, and gravity flushing.

Standard open seawater with 35 parts per thousand salinity freezes at approximately minus 1.8 degrees Celsius due to freezing point depression.

Water molecules form a hexagonal crystalline lattice during freezing that physically excludes sodium and chloride ions from the crystal matrix.

Frazil ice consists of microscopic, disk-shaped crystals of pure freshwater ice that form spontaneously in turbulent sub-cooled surface water.

Newly formed first-year sea ice retains an initial bulk salinity of 4 to 10 parts per thousand trapped within intercellular brine pockets.

Multi-year sea ice that has survived multiple summer melt cycles exhibits an extremely low salinity typically under 1 to 2 parts per thousand.

Brine rejection refers to the continuous physical expulsion of concentrated, dense saline liquid from freezing ice into the underlying water column.

Brine channels are micro-tubular vertical networks measuring 0.1 to 1.0 millimeters in diameter through which concentrated salts drain downward.

Gravity drainage occurs because heavy, highly concentrated brine within vertical channels is denser than underlying seawater and sinks under gravity.

#10

Gravity flushing happens during polar summers when freshwater melt percolates down through sea ice, washing away residual trapped brine pockets.

#11

Norwegian explorer Fridtjof Nansen verified during the 1893 to 1896 Fram expedition that multi-year Arctic pack ice melts into potable fresh drinking water.

#12

The hyper-saline brine rejected during Antarctic sea ice formation increases water density, triggering downwelling that forms Antarctic Bottom Water.

#13

Brine rejection drives deep-water ventilation and powers the global thermohaline conveyor belt connecting polar and equatorial ocean basins.

#14

Brinicles, colloquially known as underwater ice stalactites or icicles of death, form when sinking supercooled rejected brine freezes surrounding seawater.

#15

Colligative properties dictate that increasing solute concentration depresses freezing points while increasing osmotic pressure and boiling points.

#16

The eutectic temperature for seawater brine is minus 21.1 degrees Celsius, below which hydrated sodium chloride crystals precipitate as hydrohalite.

#17

First-year sea ice appears opaque and milky due to trapped air and brine pockets, whereas multi-year ice appears dense and translucent blue.

#18

Microscopic brine channels host specialized psychrophilic microbial communities, including ice algae, bacteria, and microscopic crustaceans.

#19

The progressive desalination of multi-year ice increases its mechanical strength, making older pack ice harder and more resistant to crushing.

#20

Global warming trends that reduce multi-year Arctic ice coverage weaken deep oceanic overturning by diminishing dense brine formation.

Subject Specialist Commentary

Analytical perspective & practical exam advice from the Master10 academic board

Educator's Insight

The desalination of sea ice is a classic demonstration of phase equilibrium and crystal chemistry. The key concept to understand is that ice crystals themselves are entirely pure water: dissolved salt ions cannot fit into the tight hexagonal ice lattice. Salt in sea ice is merely liquid brine mechanically trapped in interstitial micro-tubular channels between crystals.

In competitive examinations, candidates frequently err by assuming polar sea ice freezes from freshwater snowfall rather than ocean water, or that all sea ice has uniform salinity. Distinguish carefully between first-year ice (salinity 4 to 10 ppt) and multi-year ice (salinity below 2 ppt, which is potable). Remember the critical oceanographic link: brine rejection produces cold, hyper-dense water that forms Antarctic Bottom Water, driving thermohaline circulation. Recall this sequence using the mnemonic 'SLICK': Solidification exclusion, Lattice purity, Intercellular brine, Channel drainage, and Kinetic conveyor driving.

Sea Ice Desalination: Brine Rejection, Salt Exclusion and Freezing

Key Concepts & Self-Assessment20 Key Facts

Subject Specialist Commentary

Related Knowledge Topics to Discover

Looking for more GK practice?