Naked Science Forum
Life Sciences => The Environment => Topic started by: Meryn on 16/01/2009 09:01:33
-
Meryn asked the Naked Scientists:
Hello there naked scientists
Why is the sea salty?
regards
Tait Mc Jarrow ( I am 9 years old)
What do you think?
-
The sea is salty because the only place for the water to go when it has reached the sea is to evaporate, leaving the salt behind.
The same is true of the dead sea - with is an inland sea, but because it has no significant outflow, so there is nowhere for the slightly salty water to go except to evaporate.
The great lakes also carry a small amount of salts and minerals into the lakes, but not enough to make them appreciably salty; but they also carry most of the water out again before much of it has a chance to evaporate, so the outgoing water also carries the salt with it, whereas the evaporating water does not.
-
I think perhaps Meryn is asking where the sea salt comes from in the first place.
The oceans are enormous, covering the greater part of the Earths surface and are highly saline, yet smaller bodies of water have a very low salt content. Since rain water and 'fresh' water rivers contribute to the content of both the oceans and fresh water lakes, why should the oceans and some inland seas be so salty while the other bodies of water are not?
Come on all you marine scientists and geologists, give Meryn a good answer to a valid question, in terms a nine year old can understand....... and me, of course!!!
-
Ahh, as water flows from rivers to seas, it picks up small amounts of mineral salts from the rocks and soil of the river beds. The water in the oceans only leaves by evaporating, but the salt remains dissolved in the ocean - it does not evaporate. So the remaining water gets more and more salty as time passes.
And If all the oceans were fresh, well, life would not exist, simple as that, because water soluble salts would not exist on the planet, and about 75% of a human's body mass is a saline(salt content) solution
-
You need to picture our oceans as giant sinks. Water falls on land and often finds its way into either lakes or rivers. The water residency time in these is comparatively short, with rivers, fed by lakes, flowing out to the sea. Water also travels under the ground.
Salt can be found in its pure form or within other compounds but is readily dissolved when water flows through it. Water, laden with large amounts of various minerals, will eventually reach the ocean where the water cycle restarts. Water is evaporated from the ocean surface but the salt is too heavy to be evaporated and remains in the sea. Hence, since the oceans formed on the Earth, they have been recieving salt from land and have thus become increasingly more saline.
-
There are lakes that have no escape to rivers or the sea, and such lakes are generally more saline. They act in a similar way to the ocean with water evaporating off and leaving salts behind. In some cases enclosed lakes, like these, will evaporate completely leaving behind huge salt flats. However, due to the ease with which salts such as sodium chloride (NaCl) dissolve, the next rains will recreate the lake with its saline environment.
Interestingly, in our fossil record, you can find examples of such lakes which are often represented by rocks containing mud cracks, the cracks filled with sand.
Another interesting point is that some lakes are likely to vary in salinity according to the surrounding rocks/minerals.
Many large salt lakes around the world have been created by being closed off from the oceans... and thus have become hypersaline through evaporation. Around 6 million years ago during the Tertiary, the Mediterranean was closed off from the Atlantic and became very heavily saline before eventually drying out completely. Evidence of this is visible in the fossil record with large bands of gypsum and other evaporites found across southern Europe.
-
Listen to the answer to this question on our podcast. (http://www.thenakedscientists.com/HTML/podcasts/show/2009.02.08/)