Weird question.

[Gawdzilla Sama]

What would be the freezing point of sea water at 10,000 feet below the surface of the ocean?

[Xamonas Chegwé]

Unless the entire ocean above it was frozen, this would never happen! Water is at its most dense at 4ºC. Colder water rises to the surface and freezes there.

Here is the phase diagram for pure water.



Freezing point varies very little with pressure but does slightly decrease - so the water would freeze at a slightly lower temperature - you would need to calculate the absolute pressure of the water to work out by exactly how much.

[Gawdzilla Sama]

Huh?

[Xamonas Chegwé]

Huh?

In layman's terms - about twelfty.

[Rum]

Unless the entire ocean above it was frozen, this would never happen! Water is at its most dense at 4ºC. Colder water rises to the surface and freezes there.

Here is the phase diagram for pure water.



Freezing point varies very little with pressure but does slightly decrease - so the water would freeze at a slightly lower temperature - you would need to calculate the absolute pressure of the water to work out by exactly how much.

Boy, you knocked that up pretty damn quick!

[Gawdzilla Sama]

Huh?

In layman's terms - about twelfty.

Thanks.

[mistermack]

I think the answer is about minus 20 deg c.

The water would freeze from the surface first, and the ice would keep getting thicker, till it reached the bottom. As it freezes, the ice contains more water, and less salt, than the brine. So the sea would be getting more and more salty, as the thickness of the ice increased. When the saltiness reaches saturation point, the sea can't get any more salty. The freezing point of saturated brine is about minus 21 degrees. So it's likely that by the time the ice reached the bottom, it would have to be minus 21 to freeze.

[Gawdzilla Sama]

I think the answer is about minus 20 deg c.

The water would freeze from the surface first, and the ice would keep getting thicker, till it reached the bottom. As it freezes, the ice contains more water, and less salt, than the brine. So the sea would be getting more and more salty, as the thickness of the ice increased. When the saltiness reaches saturation point, the sea can't get any more salty. The freezing point of saturated brine is about minus 21 degrees. So it's likely that by the time the ice reached the bottom, it would have to be minus 21 to freeze.

Thank you!

[Xamonas Chegwé]

Good point. The average ocean depth is about 12,400 feet, so, once the lower ice level reached 10,000 feet, 4/5 of the water in the ocean would be frozen and the salt concentration would be 5 times normal ( ~17.5%) This is about 2/3 saturated. So you could probably assume a freezing point of around -14ºC (assuming that the change in freezing point of brine changes linearly with saturation.)

There are a few flaws in this simplification though.

Firstly, the ocean would freeze faster at the poles and slower at the equator.

Secondly, pressure differences would also have an effect on the freezing point, lowering it slightly.

Thirdly, the reflective effect of the covering ice would insulate the lower ocean from surface temperatures and rising, geothermal heat would keep the bulk of it liquid. In order for the sea to freeze to as deep as 10,000 feet, the air and core temperatures would need to be very cold indeed - ie. you would have to take away the sun and allow the Earth to cool for many million years!

[Gawdzilla Sama]

K.I.S.S.

[Xamonas Chegwé]

K.I.S.S.

OK, just read the last sentence...

"In order for the sea to freeze to as deep as 10,000 feet, the air and core temperatures would need to be very cold indeed - ie. you would have to take away the sun and allow the Earth to cool for many million years!"

[Gawdzilla Sama]

K.I.S.S.

OK, just read the last sentence...

"In order for the sea to freeze to as deep as 10,000 feet, the air and core temperatures would need to be very cold indeed - ie. you would have to take away the sun and allow the Earth to cool for many million years!"

Snowball Earth?

[Xamonas Chegwé]

K.I.S.S.

OK, just read the last sentence...

"In order for the sea to freeze to as deep as 10,000 feet, the air and core temperatures would need to be very cold indeed - ie. you would have to take away the sun and allow the Earth to cool for many million years!"

Snowball Earth?

Snowball Earth was not a complete freeze - only the surface of the ocean - and probably not all of that.

[Gawdzilla Sama]

Snowball Earth Done Right This Time?

[Mysturji]

In layman's terms, the freezing point of sea water at 10,000 feet below the surface of the ocean would be precicely 10,000 feet below the surface of the ocean.