Ice Fingers of Death!

Y'know those times when you're doing something simple, like washing your dog, staring at waves or snorkeling on the Maine coast, and you suddenly find yourself captivated and slightly bewildered by the impossibly intricate nature of fluid dynamics?

Maybe it's just me.

Fluids--that is, liquids and gases--are amazing things, with movements so ridiculously complex that some scientists spend their whole lives trying to figure them out, while the rest of us take them completely for granted. The fluid nature of water, air, gasoline, or even crowds of people, not only allows us to use things like pipes and boats and airplanes, but it also allows us to do little things, like breathe. Or exist.

But, much as I am eager to dive into the world of fluid dynamics and the way that gasses and liquids move around us and each other, I really just want to explore one concept that I've just learned about: the brinicle.

Now, Sir David Attenborough (as always) gives an informative and poetic basic description of what's going on here, but after watching this clip, I wanted to know more. What exactly is going on here?

So let's start with the basics. We all know that water freezes, and that fresh water, at 1 atmosphere of pressure, freezes at 32˚F/0˚C/273.15 K. Seawater, however, has a slightly lower freezing temperature, due to the inclusion of salt, which interferes with the ability of water molecules to crystallize together (there's a good explanation and doohickey to explain how that works here). With a greater concentration of salt molecules in the water, the freezing point drops, so saltier water can get below 0˚ without freezing. Because the saltier water has all those salt ions dissolved in it, it becomes denser as well, so salty water sinks below less-salty water.

Water also happens to be one of the only things that is denser as a liquid than it is as a solid. That means that solid water will float on the surface of liquid water, because of the shape of the lattice that frozen water molecules form. That lattice doesn't allow for things like pesky salt ions to get in the way, so ice, even from seawater, is fresher than the water it came from. (And since ice forms on the top of water and not on the bottom, animals like frogs and turtles can hang out in the mud at the bottom of a pond all winter without becoming frogcubes and turtlecicles.)

Taking that into the real world, when it gets cold enough for ice to start forming in the ocean, all the water molecules start connecting up and pushing out the salt ions. Those salt ions collect in liquid water at the surface, creating pools of increasingly salty water that sit on top of the ice. Eventually that super-salty water, or brine, finds its way down through the ice in little tunnels called brine channels. If conditions are right, those channels will all end up in the same spot, creating a plume of sinking brine similar to a column of rising smoke. Since it's also heavier than the seawater under the ice, when it gets to the bottom of the sheet of ice, it keeps sinking.

But here's the neat thing: since that brine has been traveling through an ice sheet, it's just as cold as the ice, but the salt has kept it from freezing. So when it runs into the water below, that relatively fresher water freezes, creating a tube of crystals around the outflowing brine. As you see in the video, this tube grows and grows as the brine travels out of it, kind of like a stream of water building its own hose: a briny icicle, or brinicle.

If the ice is forming at the right rate, the water isn't moving around too much under the ice, and the sheet is in shallow enough water, you can get a brinicle that extends all the way to the seafloor. when it hits the bottom, it does the same thing that any other dense fluid would do: roll downhill. Even if a full brinicle didn't form, a pool of brine would collect in a low spot at the seafloor, if only until conditions changed and it dissipated. You can see this downhill travel happening in the video, except in this case, it becomes an icy stream of death!

Brinicle formation a pretty amazing phenomenon, and the conditions required to form one as big as this one are pretty unique. How fortunate that the cameramen from the BBC were able to catch it, but also, you really gotta give them mad props for diving in this climate. Setting up a time-lapse movie can be challenging on land, so imagine having to do it under an ice sheet in (literally) freezing-cold water, hauling heavy equipment a good distance from your diving hole while Weddell seals frequently zip by, knocking down your cameras and the brinicles you're trying to film. Mad props to these guys:

Some day when I have no use for a wife, kids, or dogs, I'm going to be a filmmaker traveling around the world collecting natural history footage for the BBC and hanging out with chipper Scotsmen:


For more info, check out these resources:
The BBC's article about filming brinicles
Wikipedia's info on brinicles
Modeling brinicle formation in a lab
How salt affects the formation of ice
How zombie frogs survive winter

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