Re: Distribution of salinity in the ocean?

Hi Max:

Salinity changes with depth, but the way it changes depends on where on the globe one is. Salinity at the surface can be increased by the loss of water to ice or evaporation, or decreased by the input of fresh waters, such as from rivers. Salinity at depth is pretty much fixed, because there's no way that water can be "lost", nor any way to "add" salt. As an example, here's a generalized diagram that shows salinity with depth in the Pacific Ocean:

As you can see, at higher latitudes (i.e. to the north) the surface waters are less salty (from freshwater input), while at lower latitudes (i.e. towards the equator) surface waters are more saline (due to evaporation). Salinity in the deep waters is about the same.

Now, about your question about pressure and salinity: At depth, differences in salinity are basically caused by the mixing of different water masses, so I'm afraid you've technically lost your arguement!
Although pressure might change the solubility of water slightly (so that the saturation of salt is lower), deep seawater isn't anywhere near the saturation point for salt- therefore, pressure has no effect on salinity.
To put that in other words, water can hold a lot of salt before it becomes saturated- over 100 parts per thousand. In the ocean, salinity is seldom greater than 40 parts per thousand, especially in deep waters. So, while pressure might lower the maximum amount of salt that can be dissolved in water slightly (say to only 90 parts per thousand), there's never so much salt in the water that it will matter.

You might also be interested in a couple of other answers that relate to salinity that I've posted previously:

Hope that helps!
Rob Campbell, MAD Scientist