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One of the Episodes of BBC's documentary Frozen Planet describes an upwell phenomenon in the Antarctic Ocean. I am trying to understand the cause of seawater freezing when this upwell occurs.

Liquid sea water can remain liquid below salt water's one-atmosphere freezing temperature when that salt water is maintained under pressure at deep ocean depths. The high pressure conditions deep in the ocean apparently prevent the salt water from freezing.

According to the documentary, shifting ocean currents occasionally bring a deep mass of liquid antarctic sea water from near the bottom of the ocean all the way up to the surface very quickly. And much of that mass of liquid sea water quickly freezes due to the change in pressure. Pressure is reduces when the water moves from the bottom of the Antarctic Ocean and arrives near the surface.

If temperature remains constant but the state changes from liquid to solid, I assume this state change is caused by the pressure allowing the sea water to expand and form a crystalline lattice - ice.

Someone watching the BBC documentary with me believes the ice formed in such an upwell event is caused by the molecular movement of the water molecules slowing down with the reduction in pressure, and she argues the slowing of movement brought on by reduction in pressure causes the ice to form. She asserts the reduced molecular movement is the cause, not the physical expansion of the overall volume of the water as it rises.

My understanding is that the extreme high-pressure water simply doesn't have enough physical space to expand into a volume which will allow the molecules to form a lattice, and this has little or nothing to do with the molecular movement.

What is the correct way to understand this phenomenon?

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Can you say which episode and what time this phenomenon is mentioned. Generally the water at the sea bottom is around 4C and I can't see why an upwelling would cause instant freezing. There is a phase diagram of ice at lsbu.ac.uk/water/phase.html. At 1km depth (10MPa) the change in the freezing point is less than 1 degree. –  John Rennie Jul 9 '12 at 7:49
    
""Liquid sea water can remain liquid far below salt water's freezing temperature when that salt water is maintained under pressure at deep ocean depths. The high pressure near the bottom of the arctic ocean prevents the salt water from freezing."" Both statements are totally wrong. –  Georg Jul 9 '12 at 7:55
    
Just to echo John Rennie's comment, I've never heard of this phenomenon, but if it happens then it's something I'd be quite interested in learning more about - if anyone knows the name of the phenomenon it would be really helpful in order to look it up. –  Nathaniel Jul 9 '12 at 10:47
    
@JohnRennie the BBC Frozen Planet documentary covers this phenomenon on Disk 2, Episode 3 at around the 34:00 minute point. The camera work is impressive - they filmed underwater and showed the starfish and other crustaceans being caught in the quickly-freezing water. Here is a clip from a different phenomenon (not the upwell event). I was unable to find an online clip which describes the upwell event. –  steampowered Jul 9 '12 at 13:51
    
As requested, I've deleted my answer. can you clarify which episode I need to look at. Episode 3 is the one called "Summer". Is this the correct episode? –  John Rennie Jul 9 '12 at 17:51

1 Answer 1

The bit of The Frozen Planet that mentions this is short on detail, but I would guess they are referring to Antarctic Bottom Water; see also Weddell Sea Bottom Water. It's similar in mechanism to the brinicle you mentioned. Water cooled at the surface becomes very cold and very dense (high salinity) so it sinks and forms a layer on the sea bed of the Antarctic continental shelf. An upwelling of this water could freeze the less saline water nearer the shore. As far as I can see this has nothing to do with pressure.

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