Viewing a single comment thread. View all comments

Chlorophilia t1_is2v3ah wrote

Reply to comment by bagonmaster in Does the salinity of ocean water increase as depth increases? by rhinotomus

No, because:

  • Any water capable of sinking into the deep ocean must be liquid (otherwise it wouldn't be dense enough to sink), so in other words, all deep water is above the freezing point (of seawater). So if all of your surrounding water is above freezing, and the sea-floor is above freezing (which is generally the case), how are you going to cool below the freezing point?
  • If you look at the phase diagram of water, you'll see that, at 0C, you'd have to reach a pressure of ~1GPa to reach the freezing point, which is equivalent to a water depth of ~100km, ~10x deeper than the deepest part of the ocean. This phase diagram is for pure water, not seawater, but it's still not possible for water to naturally freeze in the ocean through pressure changes alone.
8

GammaFork t1_is4odr2 wrote

Though you do get funky pressure effects on liquid freshwater released from deep subsurface ice shelf grounding lines and subsequently refreezing onto the base of the ice shelf as it floats up to lower pressures!

1

bagonmaster t1_is2wcno wrote

There are some faulty assumptions in there though, the biggest of which is assuming that solid water has to be less dense than water which it doesn’t. The ice that would form at 0C and 1GPa would almost certainly be denser than liquid water

−2

Chlorophilia t1_is2x37d wrote

> solid water has to be less dense than water

This is completely correct within the physical conditions that exist in the ocean. I have no idea what the density of ice is at 1GPa, but it's irrelevant, because these pressures do not exist in the ocean. The only way that you could physically generate ice in the deep ocean is by either (1) reducing the salinity, or (2) refrigeration, neither of which naturally occur in the deep ocean.

7

brunswick t1_isahr46 wrote

To add onto this about the pressures that exist in the ocean, pressure at any given depth is equal to the density of water * g * h. Let's say we have a water column consisting exclusively of pretty dense water with a density of 1029 kg/m^3. To get 1GPa, the ocean would have to be 99 km deep which is far far deeper than the deepest part of the ocean.

1

bagonmaster t1_is2xu6y wrote

The pressures in the deepest parts of the ocean are absolutely high enough to create different states of ice that would be denser than water. If there’s ice there it wouldn’t float.

It’s absolutely possible that the salinity changes at that depth or the pressure of the water above changes, the water doesn’t have to get colder to freeze.

−2

Chlorophilia t1_is2y6mm wrote

> The pressures in the deepest parts of the ocean are absolutely high enough to create different states of ice that would be denser than water.

Do you have any evidence for ice ever being observed forming in the deep ocean?

> It’s absolutely possible that the salinity changes at that depth

How? What source of freshwater are you proposing exists in the abyssal ocean?

> or the pressure of the water above changes

How?

8

GammaFork t1_is4p2rk wrote

Basal melt at the deep back of ice shelves produces freshwater at depths >1000 m locally or more. This then flows up the underside of the ice shelf and becomes locally supercooled, leading to basal refreezing. Admittedly there are entrainment effects too, but a key driver is the pressure influence on the local freezing point. Ref: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006JC003915

1

bagonmaster t1_is2yyzf wrote

A storm can change the pressure and temperature of the water above it. The salinity can change from a number of factors from organisms to changes in environment changing the solubility causing some to precipitate out.

We still know very little about the deepest parts of our ocean and until we can more easily explore them I don’t see how you could say with any certainty there’s no ice down there.

0

Chlorophilia t1_is30czg wrote

> A storm can change the pressure and temperature of the water above it.

The difference between the highest and lowest atmospheric pressure ever recorded on earth is about 25kPa. That is over 1000 times smaller than the pressure at the bottom of the average ocean depth. Even a large wave would have a larger effect on pressure than that (but still negligibly small compared the pressure at a depth of >>1km).

> I don’t see how you could say with any certainty there’s no ice down there.

Because as I've already stated, it is physically impossible to naturally form ice at depth in the ocean. There is no way to cool the water below the freezing point below the surface. Pressures >> 0.1GPa do not exist in the ocean. There is no known source of freshwater in the deep ocean, nor is there any proposed mechanism for how such a source could exist, nor is there any evidence suggesting this exists.

8

regular_modern_girl t1_is3sxqp wrote

Just to be charitable to the user who’s arguing with you, I suppose it’s maybe possible they’ve seen pictures of methane clathrate deposits on the seafloor, which does look a lot like ice and is sometimes even (erroneously) referred to as “methane ice”. I could see how someone might see photos of a methane cold seep in NatGeo or something, see people on boats topside holding samples of what looks like ice, and possibly getting confused over the text referring to it as an ice-like substance at the bottom of the ocean.

However, methane clathrate is obviously not actually ice (or at least it’s not usually classed as one of the forms of water ice and is really kind of its own thing chemically), and I might be assuming too much good faith here . In any event, this other user needs to admit that they’re probably not going to win an argument about basic physical properties of seawater with an actual expert on the physical properties of oceans.

1

TheHecubank t1_is3iw71 wrote

> We still know very little about the deepest parts of our ocean and until we can more easily explore them I don’t see how you could say with any certainty there’s no ice down there

We've been to the deepest point of the Ocean. We know what the pressure is there.

We also know what pressures are required to form the kinds of Ice you are discussing because we have made them in a lab.

The pressure difference between the two nearly 10 times greater than the pressure difference between the bottom of the ocean and the vaccum of space. It's not even close.

Edit: to help more with scale, the pressure under question (1 GPa) is roughly the pressure range we expect for the Mohorovičić discontinuity - the boundary between the Earth's crust and mantle. If Ocean water could come up with that kind of pressure, there wouldn't be an ocean floor - because the pressure would push it into the mantle.

2

TheHecubank t1_is3i46v wrote

Nothing in the ocean gets even close to the pressures required to result in high pressure variants of ice. Even the highest pressure of Challenger Deep is still an order of magnitude short of that.

The ice that forms at the point described (0 C and 1 GPa) would indeed be less that of water at the same point. It would be a mixture of Ice V and Ice VII, since that is the transition point. Neither of those forms of Ice naturally exist on Earth.

We do have a tiny amount of extremely high pressure Ice on Earth - specifically, Ice VII. It needs a much higher pressure to form than the water in the ocean can provide: thus far, we have found it in exactly one place on the planet (outside a lab) - tiny inclusions inside diamonds.

3

regular_modern_girl t1_is3mrx1 wrote

ice-Ih (the only form of water ice that occurs naturally in terrestrial conditions) is a weird solid in that it’s actually by definition significantly less dense than its liquid phase, due to peculiarities of its crystal structure.

This actually leads to a number of peculiarities when it comes to ice (the common earthly form of it, at least), such as that it floats on liquid water, it takes up more volume than liquid water, and that higher pressures actually generally melt it by lowering its freezing point (the exact opposite of how most crystalline solids work).

If we were talking about almost any substance other than water here (or talking about very different conditions than Earth) then what you’re saying here would be largely correct, but water (and especially ice-Ih) is just really weird like this.

EDIT: I forgot that ice-Ic (the cubic crystal form of ice-I) is hypothesized to occur naturally in tiny amounts in the upper atmosphere, and trace amounts of ice-VII (one of the high-pressure variants) have been found as natural inclusions in diamonds as a user below just informed me, but obviously neither of these things are really relevant to the argument at hand. Ice-Ih is still the only one we encounter in daily life, and the only one to occur in substantial quantities in nature here on Earth. The full water ice “zoo” that we’ve managed to synthesize in lab conditions up to this point consists of something like 20 or so different crystalline forms (I think depending somewhat on how exactly you distinguish some of the structures), as well as non-crystalline amorphous ice (which has a disordered molecular structure like glass, occurs in very low pressure conditions, and might actually be the most common form of water across the universe).

1