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eaglessoar t1_is266ol wrote

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

> 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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