Below is a short summary and detailed review of this video written by FutureFactual:
Does Pressure Keep the Deep Ocean from Freezing? Ice, Salinity, and the Mariana Trench
The video investigates whether the enormous pressures at the bottom of the ocean prevent freezing. It explains that at the Mariana Trench’s depths (about 11 km) the pressure is around 1000 atmospheres, which shifts freezing points for freshwater and seawater in surprising ways. For freshwater, 500 atm can melt ice down to about -4 °C, and 1000 atm could push that to roughly -9 °C. Saltwater behaves differently: surface seawater freezes near -2 °C, at 500 atm it shifts to about -6 °C, and at trench depths to around -11 °C. Despite these theoretical shifts, real deep-ocean temperatures hover around 0–4 °C due to circulation and salinity effects, so deep water rarely freezes. In short, pressure could, in principle, affect freezing, but ocean physiology keeps the water liquid much of the time.
Deep-ocean pressures and the freezing point
The deepest parts of the ocean sit around 11 kilometers below the surface, where pressures reach about 1000 atmospheres. This pressure can influence phase changes, especially for ice. For freshwater, 500 atmospheres can melt ice down to about -4 °C, and 1000 atmospheres would push that melting point to roughly -9 °C.
"Pressure can melt ice." - MinutePhysics
Freshwater versus seawater under pressure
Sea water’s freezing point shifts with salinity. Surface seawater freezes near -2 °C, but at 500 atmospheres that freezing point drops to about -6 °C, and at the trench bottom it could be around -11 °C. The video emphasizes that salt changes the game, compared with freshwater under the same pressure.
"Salt shifts freezing points in seawater, so the bottom can stay liquid despite low temperatures." - MinutePhysics
Why the deep ocean stays liquid in practice
Even with these pressure-driven shifts, the ocean’s actual deep-water temperatures stay around 0–4 °C due to circulation and salinity patterns, and because saltwater tends to remain a liquid under these conditions. The video notes that while pressure could theoretically prevent freezing, real-world ocean dynamics keep deep water from freezing.
"In the real world, circulation and salinity keep deep water around 0 to 4 °C." - MinutePhysics