BBC Focus Magazine

[scott fairbrass]

why is the ocean salt water and not fresh?

[M Paul Lloyd]

Ah yes, now then I think I know this one........... but I could be wrong?

Anyway I believe the seas are saline because of things like deep ocean thermal vents and such like underwater geological systems which over geological time have released a great many chemicals such as Sodium and Chlorine and these react with each other to create salt.
Thats why you get saline, or even acidic lakes inside volcanoic calderas but fresh water seas like the Caspian which is a lot less saline than the wider oceans and the Great Lakes in North America which are fresh...... or at least fresh in the sense that they are not saline, but they are incredibly polluted.

I think its all a fair bit more complicated than that but thats the basis of the process.

[scott fairbrass]

prob guner make me sen sound a bit more stupid here but here it goes. When water is evaporated from the sea is the rain that falls fresh or salty?

[M Paul Lloyd]

As far as I am aware all rain (on Earth) is fresh water as the evaporation process leaves the salt behind.

[Colm]

As far as I am aware all rain (on Earth) is fresh water as the evaporation process leaves the salt behind. ;)

Well yes, otherwise we'd all be in trouble... Since most rain comes from the sea!

[M Paul Lloyd]

Glad you agree Colm.

[M Paul Lloyd]

Rainwater is salt free, but when it hits the ground it immediately starts to pick up various salts. Where underground water finds its way into the ocean, it takes these salts with it, so the seas get saltier, but only pure water is removed again.

In places where water is pumped from underground for irrigation it's possible for a layer of salt to gradually accumulate on the soil surface as a lot of that water will evaporate and leave its salt behind. It can also happen in places where forest is removed and the water tables change so that they emerge in unexpected places and evaporate from the surface.


Image credit: By kind permission of Informed Farmers - http://informedfarmers.com/soil-salinity-in-australia/

It's something I've become very interested since moving to a drier climate and I've just spent a week at Tamera attending the international water symposium there learning all about ways to re-hydrate landscapes.

[Drewage]

Rainwater permeates the earth, dissolving the minerals which then find their way to our oceans. Our fresh water does contain trace amounts of salt, harldy detectable when drunk but overtime the cycle of evaporation and weathering has built up the levels of saline concentration within our oceans.