Long Summary
The ocean's deepest regions, such as the Mariana Trench, experience immense pressure due to the weight of approximately 11 kilometers of water above, generating nearly 1000 times atmospheric pressure. This high pressure can lower the freezing point of water, with fresh water melting down to around -9 °C under such conditions.
However, the ocean water in these depths is saline, which changes the freezing dynamics. Saltwater freezes at temperatures below 0 °C; at the surface, seawater begins to freeze around -2 °C, and this freezing point decreases further under pressure—to about -11 °C at the bottom of the Mariana Trench. This means pressure does contribute to lowering the freezing temperature of seawater.
Despite these lower freezing points, the actual ocean temperatures in deep regions remain just above or slightly below freezing, typically between 0 to -2 °C. This temperature range is maintained due to complex factors such as ocean circulation patterns, salinity distribution, and convection processes. Additionally, because saltwater tends to freeze into freshwater ice, these physical and chemical factors complicate straightforward freezing.
Therefore, while pressure theoretically helps keep the deep ocean from freezing by lowering the ice formation temperature, the actual lack of freezing is primarily due to salinity and oceanic processes. The deep ocean remains liquid not just because of pressure but because saltwater naturally freezes at much lower temperatures than freshwater, and the deep ocean temperature does not usually reach those extreme lows.
