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Chlorophilia t1_is1o5bf wrote

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

> As the name suggests this circulation is driven almost entirely by changes in temperature and salinity

This is one of these 'facts' that are often repeated, but are actually false, and it's simple to explain why. Aside from mixing within the ocean interior, all processes that affect the density of water (e.g. evaporation, precipitation, and sea-ice formation) occur at the surface. As a result, whilst it's possible to form dense waters at the surface of the ocean (which can sink), there is no process that can reduce the density of the resulting deep waters, and thereby bring them back to the surface. In other words, it is energetically impossible for the 'thermohaline circulation' to be driven by changes in temperature and salinity.

In actual fact, the more-accurately-named meridional overturning circulation is chiefly driven by a combination of winds in the Southern Ocean, and interior mixing (e.g. Gnanadesikan (1999), Johnson et al. (2007)). The formation of dense water masses is a necessary condition to have strong overturning, but it isn't the driver.

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jellyfixh t1_is1t12y wrote

Thank you for the correction, I had never heard of the mechanical input to the MOC. So it seems density drives the initial formation but mechanical energy is needed to have the water circulate back to surface?

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Chlorophilia t1_is1ugnv wrote

Yes exactly. As you completely correctly wrote, the parts of the ocean with the right conditions to create very dense water masses (e.g. the marginal seas of the North Atlantic and Southern Ocean) are the parts of the ocean where deep water formation occurs. You can't have an overturning circulation without deep water formation. But this isn't an energy source, because no energy input is required for dense waters to sink below lighter waters. The problem is that, in order to have vigorous overturning, these dense waters have to somehow be returned to the surface (and at a significant rate). There's no process in the deep-ocean that adds (non-negligible) freshwater or heat, so the deep waters can't rise buoyantly. As a result, the only way water can return to the surface is by being dragged up by wind-induced upwelling, or (probably to a lesser extent) mechanically mixed up, probably mainly due to tides. What exactly are the key processes that determine the strength of the AMOC is an active research question, and the freshening of the North Atlantic is absolutely capable of reducing the AMOC strength (because if you're generating deep water at a lower rate, you're also going to upwell less deep water), but the point is that deep water formation isn't a driver (or energy source) of the overturning.

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Bear_Wills t1_is1xafc wrote

>There's no process in the deep-ocean that adds (non-negligible) freshwater or heat, so the deep waters can't rise buoyantly.

Freshwater makes sense, but do geothermal vents in the deep-ocean not add non-negligible heat? (Apologies if that is a silly question, just found the conversation very interesting as someone with little knowledge in this area, but that part stood out to me)

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Chlorophilia t1_is1yzjg wrote

This is a good point! Geothermal vents are too localised to be significant, but there is a geothermal heat flux everywhere on the ocean floor (due to heat escaping from the Earth's interior). However, this heat flux is on the order of 0.1W/m^2. By contrast, the heat flux at the ocean surface from the sun is of order 100W/m^2. So for the heat budget of the ocean as a whole, the geothermal heat flux is negligible. Locally, at the ocean floor, it has been argued that the geothermal heat flux could be non-negligible. However, this is not routinely incorporated into ocean or climate models (I will admit that I didn't even know this had been properly looked into before your comment made me look it up!) and, whilst it's possible that it could have some second-order effect, it's orders of magnitude too small to drive the kind of large-scale overturning we see in the modern Atlantic.

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phantasmagorical_owl t1_is28geh wrote

I can't speak for other ocean or climate models, bit geothermal heating is included in NASA's ECCO ocean models and state estimates. Its magnitude is small relative to ocean surface heat fluxes but geothermal heating does help maintain a more realistic deep ocean state by reducing the drift in deep temperatures.

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Chlorophilia t1_is28uwa wrote

> I can't speak for other ocean or climate models, bit geothermal heating is included in NASA's ECCO ocean models and state estimates.

That's good to know! I was not aware of this.

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Aeellron t1_is24aot wrote

Man the facts about the sun just never cease to amaze.

Orders of magnitude more heat introduced than geothermal vents.

As soon as you think about it it makes sense.

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Shaetane t1_is2zdnv wrote

We really aint nothing without our resident well-distanced, well-temperatured, star

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salsashark99 t1_is39c04 wrote

What is the range of salinity of ocean water?

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Chlorophilia t1_is3aa0f wrote

Most of the ocean is between 32-36g/L (so quite a tight range!). You can get more extreme values in some marginal seas, and of course places like lagoons and estuaries.

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Swiss_cake_raul t1_is3xvlq wrote

I actually knew this fact already but reading it written as g/L instead of ppt just made me realize it's the same ratio of salt:water as my sourdough recipe!

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phantasmagorical_owl t1_is299zb wrote

Wouldn't one expect local Ekman pumping (wind induced upwelling) and internal tidal mixing to be indifferent to the rate of remote deep water formation? The properties of the upwelled deep waters certainly vary based on what their surface properties when they descended.

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Chlorophilia t1_is2agyq wrote

The processes of deep water formation, and return pathways to the surface, are closely coupled because of conservation of volume. Unless you have an enormous expansion of abyssal water masses, Ekman suction + tidal mixing cannot be independent of deep water formation rates. The point is that, regardless of the buoyancy forcing taking place in locations of deep water formation, they're fundamentally limited by the amount of water that you're (mechanically) returning to the surface. You can create as much dense water in the North Atlantic as you want but, if there's no return pathway to the surface, you're not going to generate deep water at a significant rate.

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phantasmagorical_owl t1_is2l9ad wrote

Ekman suction and tidal mixing are independent of deep water formation rates. Deep water formation by surface buoyancy forcing at high latitudes is not the only way that abyssal waters can be renewed. A thought experiment: an isothermal isohaline ocean is subjected to ekman pumping from surface wind stress in one region. Upwelled water would subsequently redistribute away. Conservation of volume dictates that the upwelled waters are replaced, and of course they are by surrounding waters at depth, possibly many depths. An overturning circulation will become established, with waters away from the upwelling site "sinking" to replace the upwelled waters, although the sinking is more akin to falling, as there is a decrease in the volume of waters below. Possibly the sinking would occur uniformly over the entire non-upwelling basin, or it might be confined to an area around to upwelling, but the resulting overturning circulation would look different than our current MOC.

The rate of surface water transformation depends only on local buoyancy forcing and initial seawater properties, it doesn't know about remote upwelling rates. However, if upwelling ceases, the abyss would eventually fill up with dense transformed water and the rising isopycnal of that "deep water" would eventually limit the depth to which the newly formed dense water sinks. So, in that sense, the rate of deep water formation does eventually depend on there being upwelling or tidal mixing elsewhere.

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Chlorophilia t1_is2mosd wrote

> Ekman suction and tidal mixing are independent of deep water formation rates.

I think you're misunderstanding what I'm saying, because I'm not disagreeing with you - I'm not saying that Ekman suction and tidal mixing are a function of deep water formation rates. I'm saying that deep water formation rates are (to first-order) a function of Ekman suction and diapycnal mixing. As you say, at equilibrium, the rate of deep water formation is limited by the available return pathways. If upwelling ceases, it is not possible to maintain deep water formation.

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ReynAetherwindt t1_is32r7a wrote

I don't mean to be obtuse but "deep water formation" sounds like the result of a flood, like, "That there's some deep water, and it weren't there before."

What the heck does it actually mean?

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Chlorophilia t1_is3561q wrote

It's a good question. The uppermost layer of the ocean is called the 'mixed layer'. As the name suggests, it's a well-mixed layer where the properties are set (over short timescales) by the atmospheric conditions above and, because of weaker stratification at higher latitudes, it tends to be shallow at low latitudes and deep at high latitudes, particularly in the winter. When we talk about a water mass being formed, this usually refers to water leaving the mixed layer, and thereby no longer having its properties directly forced by the atmosphere. This can either occur through a time-mean vertical velocity, or horizontal currents (if the mixed layer profile is sloped). Deep water formation specifically refers to the formation of a water mass that is deep (where "deep" usually means "below the thermocline").

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TheProfessorO t1_is2x0b0 wrote

Eddy flow over the bottom produces larger vertical velocities than the mean wind driven upwelling

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Chlorophilia t1_is2ym6p wrote

Eddy velocities are by definition zero in the Eulerian time-mean, so that in itself isn't going to result in a time-mean vertical transport. Eddies in the Southern Ocean actually counteract Ekman suction but I'm not sure on what basis you're arguing that eddies are responsible for most upwelling in the Southern Ocean? Can you provide a study supporting this?

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TheProfessorO t1_is38c7h wrote

I was not talking about any ocean in particular. The eddy vertical velocity is proportional to the eddy horizontal velocity dotted with the topographic gradient. So the average eddy vertical velocity can be nonzero when the mean eddy horizontal velocity is zero. The importance of this term for mesoscale ocean dynamics was shown by Tom Rossby and a student in the late 80s.

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DonArgueWithMe t1_is26xdb wrote

Wouldn't underwater heat vents, volcanoes and such be methods of adding energy back to the water and cycling that back up?

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Chlorophilia t1_is277gq wrote

Technically yes, but these heat sources are extremely small compared to the heating at the surface of the ocean, see this response.

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eaglessoar t1_is266ol wrote

> no process that can reduce the density of the resulting deep waters

so if i had some salty water below and some fresh water flowed over it that fresh water would not become salty at all?

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Chlorophilia t1_is28nvb wrote

It depends on how thick the layers of fresh and salty water are, and whether there's anything mixing them together. The molecular diffusivity of salt in water is about 10^-9 m^(2)/s, which means that the distance salt can diffuse is roughly sqrt(10^(-9)t), where t is the time in seconds. So, for instance, if you had a 100m thick layer of freshwater lying on top of a 100m thick layer of saltwater, it would take well over 100,000 years for molecular diffusion to fully mix them.

In practice, in the ocean, there is mechanical forcing that causes mixing (e.g. the winds, tides, turbulence, etc). A typical value for the real, effective vertical diffusivity in the ocean (taking into account mechanical mixing) is 10^-5 m^(2)/s but, even then, it would take several decades to mix these layers together. And 100m is pretty small compared to the thickness of deep-ocean water masses.

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eaglessoar t1_is2948e wrote

Wow I would've thought it was much faster! Thanks for the reply

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AlkaliActivated t1_is2oc2c wrote

You can use this to do science demonstrations in classrooms: A golf ball will float on saturated salt water, but sink in fresh water. You can partially fill a container with saturated salt water, then carefully fill the remainder with tap water, and the golf ball will float at the boundary between the two for a while. IIRC it can last a few weeks.

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regular_modern_girl t1_is2xubu wrote

>As a result, whilst it’s possible to form dense waters at the surface of the ocean (which can sink), there is no process that can reduce the density of the resulting deep waters, and thereby bring them back to the surface.

Is this why those mini-brine lakes form on the seafloor in some areas? Like will the densest, most saline waters end up all coalescing together deep in the ocean until the salt content is so concentrated that it approaches saturation? Or are those brine pools the result of something geological instead? (I know that that they’re associated with methane cold seeps)

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Chartarum t1_is35oyz wrote

Wouldn't underwater volcanic activity and thermal vents be able heat up at least some deep water and lower its density causing it to rise? At least on a local scale?

Or is the amount of heat/energy released in such areas and processes just too small to matter in any significant way?

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Chlorophilia t1_is36q3w wrote

Locally yes, and these plumes can be biologically and geochemically important. But if we're thinking about the large-scale overturning circulation of the ocean, these heat sources are negligibly small (not to mention the fact that there's no direct correlation between where these vents are and where upwelling of deep waters occurs).

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darkest_irish_lass t1_is2m8un wrote

Thank you for this excellent explanation! Do you know what happens at thermal vents and undersea volcanoes? Will the heated water rise or just 'flow' away from the volcano?

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Chlorophilia t1_is2n0mc wrote

Yes, it will rise is a buoyant plume, until it has lost enough heat to surrounding water through mixing to reach a neutral density, e.g. see this acoustic image from a survey of a hydrothermal vent field.

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bagonmaster t1_is2stci wrote

Wouldn’t the water eventually getting cold enough or under the right pressure conditions to freeze make it less dense so it would float up?

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Chlorophilia t1_is2v3ah wrote

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.
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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!

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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

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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.

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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.

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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.

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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?

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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

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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.

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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.

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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.

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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.

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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.

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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).

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jaxdraw t1_is34791 wrote

Are tidal forces completely absent at depth? I would have assumed that water, being non compressible, would be impacted throughout based on tidal forces.

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Chlorophilia t1_is34fm8 wrote

Tidal forces are absolutely present at depth, and they're actually the source of much of the mechanical energy that drives mixing in the ocean interior.

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Kandiru t1_is354d8 wrote

Presumably geothermal vents can heat up water which can send it rising?

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Chlorophilia t1_is35fnb wrote

Yes, but geothermal vents are highly localised and have a negligible contribution to the whole-ocean heat budget. Plumes associated with hydrothermal vents are important biologically and geochemically, but not for ocean physics as a whole.

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HeIsSparticus t1_is3hcae wrote

Is it possible that there is any osmotic forces at play that encourage mixing / movement of neighboring bodies of water of differing salinity? No idea if this would be the case.

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