Seawater freezes at about −1.8°C, but sea ice itself is nearly fresh — it can only hold a bulk salinity of roughly 5–8 psu, far below the ~34 psu of the water it grows from. The salt that doesn't fit in the ice lattice is expelled downward into the water column as hyper-saline brine. This is brine rejection, and it is the single largest source of dense water on the planet: cold, salty, heavy water sinks off Antarctica's coastal polynyas (Weddell Sea, Ross Sea) to form Antarctic Bottom Water, which then spreads along the seafloor and fills the deep ocean worldwide.
Ice growth (Stefan's law): dh/dt = k(T_f − T_air) / h
→ thin/open water grows FAST, thick ice insulates and grows slower
Salt flux from rejection: F_salt = ρ_ice (S_sw − S_ice) · dh/dt
Density anomaly of brine: Δρ ≈ ρ₀ · β_S · ΔS (β_S ≈ 7.8×10⁻⁴ psu⁻¹)
Plume sinking speed: w ∝ √(g·Δρ/ρ₀)
- Air temperature — colder air steepens T_f − T_air, accelerating ice growth and brine rejection.
- Coastal polynya — wind or currents keep a patch of water ice-free; with no insulating ice cap, that patch loses heat and grows new ice (and rejects brine) many times faster than the surrounding pack — real coastal polynyas are the dominant source of the world's densest water.
- Ocean turbulence — mixing disperses the sinking brine plumes laterally instead of letting them fall as a coherent, fast plume.
- The cyan particles are rejected brine sinking under its own excess density; once at the seafloor they spread outward exactly as Antarctic Bottom Water does, ventilating the abyssal ocean over centuries.