Their existence isn't intuitive. You'd expect diffusion to dilute the pools and yet they seem to last a very long time somewhere on the order of thousands of years. It's not immediately obvious to me how the physics of these pools functions because there is a persistent interface as if the brine is another liquid entirely. From another paper, "Discovery of the deep-sea NEOM Brine Pools in the Gulf of Aqaba, Red Sea,"
The CTD measurements from 1200 m depth, through the brine pool interface (1769.46 m) and 2 m below (1771.50 m) revealed that the bathyal water column above the brine pool has a stable 21.33 °C temperature, the salinity of 40 PSU, and dissolved oxygen of 180 μmol L−1.
but if you sample the pool itself,
Within 15 cm beneath the brine interface, salinity rose from 40 PSU to values higher than the limits of the conductivity probe (120 PSU). Subsequent lab analysis of the brine provided actual salinity values of 160 PSU. Dissolved oxygen values fell more than 75% at the interface, reaching 50 μmol L−1 by 20 cm submergence into the brine, and further falling to a minimum value of <10 μmol L−1 by 50 cm below the interface.
The brine is also hotter than the water just above it,
Unlike salinity and dissolved oxygen, the temperature did not abruptly change across the brine interface. Instead, the temperature gradually increases below the brine interface at a rate of 0.1 °C per 20 cm depth increase, reaching an increment of 1.0 °C above the temperature of ambient seawater by 2 m depth into the brine (Fig. 4). The temperature differential to ambient did not increase with deeper submergence. Such a modest warming increment suggests the lack of proximal hydrothermal heating of the brine.
> You'd expect diffusion to dilute the pools and yet they seem to last a very long time somewhere on the order of thousands of years.
What I've learned from things like this is that my intuition for when diffusion should work, and how fast, is just busted.
The biggest example in my mind is pyroclastic flows from a volcano. For years (mostly as a kid), I didn't understand what they were because I kept thinking roughly "if it was just gas it couldn't hold together into a coherent flow, right?" Well, as far as I can tell, yeah, they're basically just gas, but really hot and full of rocks. I still don't really get why they hold together instead of poofing out into clouds, except that it has something to do with how they're denser than the surrounding atmosphere. They're sometimes also referred to as "pyroclastic density currents", go figure. https://en.wikipedia.org/wiki/Pyroclastic_flow
Another big one that threw me for a loop as a kid was "warm/cold air masses" interacting in a way that kept their identity. "why does the warm air mass ride up on top of the cool air instead of just mixing?" I guess the answer here is that they're just too big for the mixing to happen faster than the bulk motion.
Anyway, it seems like fluids, especially fluids of very different properties, especially different density, take their time mixing, sometimes long enough to let them act like separate objects in contact for much longer than I, for one, would think. Thousands of years in the case of these brine pools (though in this case, they're being at least partially refreshed). I wish I understood it better.
There's actually an even more common example that what you've described: water heaters. The cold inlet at the bottom and the hot outlet at the top prevent mixing, and the two waters of different temperatures can maintain their difference for a surprisingly long time.
I guess what we often ascribe to diffusion (hot and cold water mixing in a glass) is helped along tremendously by convection. But convection might get more easily started in a glass if the center and the sides are different temperature. Then you get this loop. But it might then not happen in a water heater since there's no lateral temperature differences.
There is a saying: you can't step into the same river twice. And I wonder if these pools aren't the same. Like there is constant mixing, but some other force maintains the conditions of the pool. Those particular water molecules and sodium ions haven't been there for a thousand years but were gradually replaced. That's my guess, anyway.
« pyroclastic flows … basically just gas, but really hot and full of rocks »
Rocks fall to the ground (too heavy), but the eruptive cloud is still full of matter heavier/denser than air (e.g. CO2, volcanic dust and gas, forming crystal needles and “stone flakes” when cooling), and all this is thrown up miles high by thermics into a towering mushroom cloud, which eventually collapses, gaining speed on the vertical, and then flowing horizontally on hitting the relief. At least that's my understanding, assembling bits and pieces from documentaries over the years (Pompeii etc), but I have never seriously “studied” it.
These clouds (on the rise) and flows (on the way down) hold together because of density and gravity, and also because the volcano thermics have locality, perhaps not unlike a huge man-made fire (like in Hamburg or Dresden) creates its own local weather, resulting in a firestorm.
« why does the warm air mass ride up on top of the cool air instead of just mixing? » — Well, because it is lighter. Whereas a cold front moves like a wedge into warm air, throwing it up while staying close to the ground (cold = heavy), and thereby sometimes provoking frontal thunderstorms.
An example that comes to mind is layered cocktails: the syrup layer will not diffuse into the upper layer (and vice versa) on a surprisingly long timescale (at least overnight)
Feynman describes molecular interactions beautifully, and has a textbook where he explains that even the boundary between water and air operates on a model like diffusion. Hydrogen and oxygen are constantly snapping and breaking back and forth. It's all just invisible to the human eye. I imagine this is similar.
The temperature difference probably drives some process that maintains the concentration gradient. Probably something to do with geothermal heat and heat transfer rates, maybe the brine has lower heat transfer properties which makes it a self-reinforcing cycle. Or maybe the brine just oozes out of the ground somehow, maybe again something thermal going on in the rock that concentrates the solution.
Mostly it is density stratification, and the reason it seems to "defeat diffusion" is that everyday mixing is not diffusion at all.
In a glass of water, what you observe as diffusion is almost entirely convection: temperature and salinity differences drive circulation, and the circulation mixes everything in seconds to minutes. In a brine pool the denser, saltier water sits underneath lighter seawater, so the density gradient is stable and that circulation cannot start. Convection is switched off, and you are left with genuine molecular diffusion.
Salt's molecular diffusivity in water is about 1.5e-9 m^2/s. With a diffusion length around sqrt(2Dt), that is on the order of ten meters over a thousand years, and a few tens of meters over ten thousand years. So an interface a few meters thick is effectively permanent on human timescales, and no mixing means no oxygen resupply, which is why the pool below is anoxic.
The other half is that most of these pools are not closed systems. They keep getting fed: salt dissolves from exposed evaporite beds (the Jurassic salt under the Gulf of Mexico, Miocene salt in the Red Sea) or geothermally heated brine rises and settles in seafloor depressions at spreading centers. So it is less a puddle that refuses to mix, and more a puddle being topped up faster than molecular diffusion can erase it.
(Related rabbit hole: heat and salt diffuse at very different rates, roughly two orders of magnitude apart, which is the basis of double-diffusive convection and "salt fingers". Well studied since Turner's work in the 1960s.)
There are lakes underneath the ices, ponds underneath oceans, and seas beneath seas.
These brine pools are tiny salty ponds on the seabed -- David Byrne was on to something: there is water at the bottom of the ocean -- and there tiny salty ponds in Antarctica too, and lakes underneath the glaciers.
Lake Vostok underneath the Antarctic icecap is in the world top 10 by some measures, and it's one of many.
The Black Sea is two layered seas: the top layer filled in very early historical times, if you accept that it's behind the legend of the flood. It's warm and oxygenated and full of life. The older smaller Black Sea is still there underneath it, cold and dark and devoid of oxygen, but still things live in it.
There does seem to be a limit - we have found 0 life in the Dallol Geothermal vents in Ethiopia, which are simultaneous highly salty, highly acidic, and highly hot pools of water.
Anoxic brine pools are beautiful and uncanny. https://cdn.mos.cms.futurecdn.net/hB2ogu7nw4r9YZWfHoUgLi-192... https://oceanexplorer.noaa.gov/multimedia/daily-image-media-...
From the exploration vehicle Nautilus, https://www.youtube.com/watch?v=nGLtMWx28hs
Their existence isn't intuitive. You'd expect diffusion to dilute the pools and yet they seem to last a very long time somewhere on the order of thousands of years. It's not immediately obvious to me how the physics of these pools functions because there is a persistent interface as if the brine is another liquid entirely. From another paper, "Discovery of the deep-sea NEOM Brine Pools in the Gulf of Aqaba, Red Sea,"
but if you sample the pool itself,
The brine is also hotter than the water just above it,
from, https://www.nature.com/articles/s43247-022-00482-x
Nature is beautiful.
> You'd expect diffusion to dilute the pools and yet they seem to last a very long time somewhere on the order of thousands of years.
What I've learned from things like this is that my intuition for when diffusion should work, and how fast, is just busted.
The biggest example in my mind is pyroclastic flows from a volcano. For years (mostly as a kid), I didn't understand what they were because I kept thinking roughly "if it was just gas it couldn't hold together into a coherent flow, right?" Well, as far as I can tell, yeah, they're basically just gas, but really hot and full of rocks. I still don't really get why they hold together instead of poofing out into clouds, except that it has something to do with how they're denser than the surrounding atmosphere. They're sometimes also referred to as "pyroclastic density currents", go figure. https://en.wikipedia.org/wiki/Pyroclastic_flow
Another big one that threw me for a loop as a kid was "warm/cold air masses" interacting in a way that kept their identity. "why does the warm air mass ride up on top of the cool air instead of just mixing?" I guess the answer here is that they're just too big for the mixing to happen faster than the bulk motion.
Anyway, it seems like fluids, especially fluids of very different properties, especially different density, take their time mixing, sometimes long enough to let them act like separate objects in contact for much longer than I, for one, would think. Thousands of years in the case of these brine pools (though in this case, they're being at least partially refreshed). I wish I understood it better.
PS: I was half expecting this EV Nautilus brine pool video. Grim comedy of a sort. https://youtu.be/9ZYJAmAmFPw
There's actually an even more common example that what you've described: water heaters. The cold inlet at the bottom and the hot outlet at the top prevent mixing, and the two waters of different temperatures can maintain their difference for a surprisingly long time.
https://www.youtube.com/watch?v=Bm7L-2J52GU
(technology connections describing it)
I guess what we often ascribe to diffusion (hot and cold water mixing in a glass) is helped along tremendously by convection. But convection might get more easily started in a glass if the center and the sides are different temperature. Then you get this loop. But it might then not happen in a water heater since there's no lateral temperature differences.
There is a saying: you can't step into the same river twice. And I wonder if these pools aren't the same. Like there is constant mixing, but some other force maintains the conditions of the pool. Those particular water molecules and sodium ions haven't been there for a thousand years but were gradually replaced. That's my guess, anyway.
Because it is denser it sinks faster then it diffuses. You can see the same thing with LN2. https://www.youtube.com/shorts/6zv5cdB813U
« pyroclastic flows … basically just gas, but really hot and full of rocks »
Rocks fall to the ground (too heavy), but the eruptive cloud is still full of matter heavier/denser than air (e.g. CO2, volcanic dust and gas, forming crystal needles and “stone flakes” when cooling), and all this is thrown up miles high by thermics into a towering mushroom cloud, which eventually collapses, gaining speed on the vertical, and then flowing horizontally on hitting the relief. At least that's my understanding, assembling bits and pieces from documentaries over the years (Pompeii etc), but I have never seriously “studied” it.
These clouds (on the rise) and flows (on the way down) hold together because of density and gravity, and also because the volcano thermics have locality, perhaps not unlike a huge man-made fire (like in Hamburg or Dresden) creates its own local weather, resulting in a firestorm.
« why does the warm air mass ride up on top of the cool air instead of just mixing? » — Well, because it is lighter. Whereas a cold front moves like a wedge into warm air, throwing it up while staying close to the ground (cold = heavy), and thereby sometimes provoking frontal thunderstorms.
Those are indeed the standard explanations.
An example that comes to mind is layered cocktails: the syrup layer will not diffuse into the upper layer (and vice versa) on a surprisingly long timescale (at least overnight)
The ratio of volume to surface area changes with scale.
Feynman describes molecular interactions beautifully, and has a textbook where he explains that even the boundary between water and air operates on a model like diffusion. Hydrogen and oxygen are constantly snapping and breaking back and forth. It's all just invisible to the human eye. I imagine this is similar.
The temperature difference probably drives some process that maintains the concentration gradient. Probably something to do with geothermal heat and heat transfer rates, maybe the brine has lower heat transfer properties which makes it a self-reinforcing cycle. Or maybe the brine just oozes out of the ground somehow, maybe again something thermal going on in the rock that concentrates the solution.
The brine definitely comes out of the ground. It comes from seawater interacting with buried salt beds. Some details in the article.
Mostly it is density stratification, and the reason it seems to "defeat diffusion" is that everyday mixing is not diffusion at all.
In a glass of water, what you observe as diffusion is almost entirely convection: temperature and salinity differences drive circulation, and the circulation mixes everything in seconds to minutes. In a brine pool the denser, saltier water sits underneath lighter seawater, so the density gradient is stable and that circulation cannot start. Convection is switched off, and you are left with genuine molecular diffusion.
Salt's molecular diffusivity in water is about 1.5e-9 m^2/s. With a diffusion length around sqrt(2Dt), that is on the order of ten meters over a thousand years, and a few tens of meters over ten thousand years. So an interface a few meters thick is effectively permanent on human timescales, and no mixing means no oxygen resupply, which is why the pool below is anoxic.
The other half is that most of these pools are not closed systems. They keep getting fed: salt dissolves from exposed evaporite beds (the Jurassic salt under the Gulf of Mexico, Miocene salt in the Red Sea) or geothermally heated brine rises and settles in seafloor depressions at spreading centers. So it is less a puddle that refuses to mix, and more a puddle being topped up faster than molecular diffusion can erase it.
Background: https://en.wikipedia.org/wiki/Brine_pool and a paper on the NEOM pools in the Gulf of Aqaba: https://www.nature.com/articles/s43247-022-00482-x
(Related rabbit hole: heat and salt diffuse at very different rates, roughly two orders of magnitude apart, which is the basis of double-diffusive convection and "salt fingers". Well studied since Turner's work in the 1960s.)
There are lakes underneath the ices, ponds underneath oceans, and seas beneath seas.
These brine pools are tiny salty ponds on the seabed -- David Byrne was on to something: there is water at the bottom of the ocean -- and there tiny salty ponds in Antarctica too, and lakes underneath the glaciers.
Lake Vostok underneath the Antarctic icecap is in the world top 10 by some measures, and it's one of many.
The Black Sea is two layered seas: the top layer filled in very early historical times, if you accept that it's behind the legend of the flood. It's warm and oxygenated and full of life. The older smaller Black Sea is still there underneath it, cold and dark and devoid of oxygen, but still things live in it.
“Metals like iron, potassium, manganese, copper, and molybdenum are known to accumulate in deep-sea brine pools”
Time to reassess the reflecting pool saga
I’m always amazed at when we find life in places where the conditions aren’t very suitable, but I’m never surprised. Life is everywhere
There does seem to be a limit - we have found 0 life in the Dallol Geothermal vents in Ethiopia, which are simultaneous highly salty, highly acidic, and highly hot pools of water.
Extremely interesting page, thank you, am reading through it.
It's amazing what can live where. There are microbes a mile down and a mile up, in near boiling water and in subzero conditions.
That was interesting