Does Pressure Keep the Deep Ocean From Freezing? Exploring the Mariana Trench

Summary

This video investigates whether the immense pressure at the ocean floor could prevent freezing, using the Mariana Trench as a focal point. It explains how pressure, temperature, and salinity set the freezing points of freshwater and seawater, and why salty seawater tends to stay liquid even under extreme depths. The discussion highlights typical deep-ocean temperatures around 0 to 4 C and the role of circulation and convection in maintaining liquid water at great depths.

• Depth-related pressure is about 1000 atmospheres, but salt and temperature largely govern freezing behavior.
• Fresh water and saltwater respond differently to pressure, with saltwater freezing at much colder temperatures.
• Deep ocean temperatures are typically 0 to 4 C, and salinity shifts freezing points further below zero.
• Ocean circulation and convection prevent freezing, so pressure alone is not the controlling factor.

Introduction: Deep Ocean Pressure and the Freezing Question

The video analyzes whether the enormous hydrostatic pressure in the deepest ocean could prevent ice formation, using the Mariana Trench as the key example. It notes that at the trench depth, water experiences around 1000 times atmospheric pressure, but other physical factors determine freezing outcomes.

Pressure and Ice Melting: Quantitative Insights

In freshwater, high pressure can melt ice at surprisingly low temperatures: 500 atm lowers the melting point to about -4 C, and 1000 atm to about -9 C. For seawater, the presence of dissolved salts shifts these thresholds even further, meaning the same pressures require much colder temperatures for freezing to occur.

Fresh Water vs Saltwater: The Role of Salinity

On the ocean surface, saltwater freezes near -2 C, but at 500 atm the freezing point drops to around -6 C, and at trench depths to about -11 C. These shifts show that salinity substantially changes how pressure influences freezing, and the deep ocean remains liquid under typical deep-water conditions due to these salinity effects.

Actual Deep Ocean Temperatures and Circulation

Deep-ocean temperatures are generally between 0 and 4 C and rarely fall far below -2 C. The combination of salts and the heat dynamics of circulation and convection means the deep ocean stays liquid, even under very high pressure. While pressure could, in principle, impede freezing, the environmental conditions dominate the outcome.

Takeaways and Implications

The video emphasizes that the freezing behavior of seawater at depth is governed by a complex interplay of pressure, salinity, and temperature, with salinity and convection playing pivotal roles. In the Mariana Trench, the conditions keep seawater from freezing, illustrating why pressure alone cannot guarantee an ice cover on the deep ocean. This reflects the broader understanding of deep-ocean thermodynamics and marine circulation patterns.