Unless they're at absolute zero, they move.

September 18, 2023 5:02 AM

In 6th grade my crappy science teacher told us that water molecules at the bottom of the ocean don't move. This makes no sense, and he gave no context or explanation that would make it makes sense. What factoid was he getting at?

I'm guessing there's some similar statement like "water molecules under high pressure tend not to move relative to each other" or something like that. So does anyone have a guess at what "fact" he was mangling?

posted by Tehhund to Science & Nature (6 answers total)

As a one-time school science teacher, I can only guess.

Thermohaline circulation is the main source of current in the deep ocean, but you're still talking about several cm per second. Compared to surface currents, that's very slow.

But if they were talking about molecules, perhaps they were just explaining (badly) how lower temperature means less energy and so the molecules move slower relative to each other. Not sure why you'd use the ocean floor to explain that though.
posted by pipeski at 5:33 AM on September 18, 2023 [3 favorites]

Best answer: Very charitably: Water in the ocean has a surprisingly long residence time, on the order of thousands of years.

Water in the abyssal depths would seem to have an even longer residence time. In some sense it "stays put" pretty well.

This may have been what they were getting at. The molecules move, there are currents swishing around. But the water there is pretty stationary insofar as it stays there a very long time (on human scales).
posted by SaltySalticid at 6:13 AM on September 18, 2023 [7 favorites]

Response by poster: Do you have any suggestions for where I could read more about this?
posted by Tehhund at 6:34 AM on September 18, 2023 [1 favorite]

The Wikipedia page on the water cycle has a nice table of residence times, it gives 3,200 years as a mean residence time for the oceans as a whole.

The water in the oceans is mostly "deep ocean", i.e. only the first 200m or so is considered "shallow" and mixes or evaporates easily. So the residence time of 3k years must be mostly driven by the deep ocean. At the same time, it would seem water down at 3km deep would have slower turnover than water in the middle, but ocean circulation is complex, and I haven't been able to find any good references for the residence time of water right at the bottom.

Here's a nice set of lecture notes that discusses broadly how the ocean mixes, how water is different in different places, and gives residence time of water and various compounds. It does not address specifically the idea that deep water has longer residence time than shallow or medium depth waters, I'll update if I can find a better source on that claim.
posted by SaltySalticid at 8:08 AM on September 18, 2023 [2 favorites]

You might also read up on thermohaline circulation, which explains the extremely slow water cycle in our oceans.

The Gulf Stream is a (small) part of this cycle, and this topic is likely to be increasingly in the news in the coming years, especially if and when the GS collapses, wreaking havoc on European weather.
posted by intermod at 9:28 AM on September 18, 2023 [2 favorites]

I also think of "heavy water". Some Hydrogen atoms have a Neutron or even sometimes two in addition to the standard Proton/Electron simplest element sort of thing. The Isotopes do occur naturally, they're around here and there. Water (H2O) can have one or both of the Hydrogen atoms being Isotopes. That water is "heavy" and does sink down to the bottom of the ocean and pretty much stays there. Collecting in little dips on the ocean floor. But it's probably the thermal and deep dark pits thing. They'll probably have higher concentrations of "heavy water" than the rest of the ocean.
posted by zengargoyle at 11:00 AM on September 21, 2023