Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Ians Daddy on 20/10/2003 05:10:11
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My son posed a question today that I didn't know. I told him we'd ask you guys. "Why is the ocean salty, but rivers and lakes arent?"[?]
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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.
One more interesting fact is that like many things on earth, the level of salt in the ocean has reached an equilibrium at 35 parts per million. It is thought that there are many factors responsible for this including biological and tectonic (formation of new minerals etc).
Hope this helps.
Richard.
Resident Tour Operator - The Naked Scientists
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That's not ture in the Dead sea though is it ?
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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You also forgot the salt lake in Utah. Although the salinity of the lake has been decreasing over time due to dilution. [:D]
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There's heaps of salt lakes in Australia too, how do you explain those?
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The answer is quite simple.
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.
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It would be good to add that lakes and rivers, even though we call them "freshhwater", ARE salty. Un-polluted rainwater contains no salt, just water and dissolved carbon dioxide gas. So rainwater is a weak, carbonic acid solution, with an equilibrium pH of about 5.8. It falls to the ground, and begins dissolving minerals in the rocks and soil. Rocks and soil contain minerals such as calcium (bi)carbonate, and magnesium (bi)carbonate. Typically one finds salts of calcium and magnesium dissolved in river and lake water, that came from the groundwater. The metallic part of this is called water "hardness", and the (bi)carbonate part is called "alkalinity". In adition, there can be many other chemicals that dissolve in the water, depending on the local soil chemistry. Some are also man-made pollutants. Eventually this water flows to the sea, where evaporation removes the water and leaves the salts behind, repeating the water cycle.
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Wow that is really interesting. I never really thought about why oceans were salty.
Am I dead? Am I alive? I'm both!
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Hmm has anyone seen that movie "the day after tomorrow" The problem in the movie is global warming melted a massive sheet of the polar ice caps into the ocean, and since the ice is fresh water it disrupted the ratio of salt to fresh water. And then all life on Earth basicly ended. Is that feasible?
To see a world in a grain of sand.
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Erm, well, the film is kind of feasable. The stopping of the Atlantic Conveyor by the melting of ice caps is a serious and definite threat although there is no prediction on when it could actually occur... but to put into context what effect it would have on the UK... there would be sea ice in Dover!
As fot the other bits of the film, i went with some of my geoscience friends and found it a bit crap. The part about the downward falling air column made me laugh and as for the stupid love story side of things... enough to make you want to chunder! You Americans need to make more films where everyone dies... ditch these crappy happy endings! [:(!]
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I don't know about the movie, but I've read scientific papers about the stopping of the mid-Atlantic current. This happened about 13,000 years ago and lasted about 3000 years. Basically, europe's climate changed from what it is now to what it "should be" based on the latitude, similar to Siberia. The change occurred within 30 years. And then the change back was just as fast.
Think of the implications of one of the most densly populated regions on the earth suddenly changing that drastically.
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John - The Eternal Pessimist.