Why are the oceans saltwater? Where did the salt come from? The Great Lakes prove large masses of water can be fresh.
Because salts are soluble in water.
Rain falls, leaches soluble compounds out the ground it travels through and over, and end up in the oceans. When ocean water moves on to the next step in its "life"cycle–evaporation into vapor before falling on the land again as rain–it leaves the minerals behind.
Multiply this cycle by billions of years and you get an earth “rinsed” nearly clean, and oceans of concentrated salt.
The Great Lakes are up-land and drain down to the oceans. Most bodies of water (streams, rivers, lakes, groundwater) lead to the ocean eventually.
The Great Lakes prove large bodies of water CAN be fresh, but not MUST be fresh. Not to mention that the Great Lakes are miniscule compared to the oceans.
The salinity of the oceans is basicly a function of how prevalent chlorine is in the Earth’s crust. Alkaline elements such as sodium or potassium will bond with extremely common silica to form complex silicates. However, where chlorine ions are available (or more rarely, other halogens like fluorine), the alkaline elements can be dissolved in water, where the salts eventually end up concentrated in the ocean.
Lumpy wrote:
This isn’t quite the whole story, as flourine is more common in the earth’s crust (544 ppm) than chlorine (126 ppm) is. The reason is that the same solubility selection process occurs for the anions as well: flouride is generally less soluble with common cations than is chloride. There’s more magnesium (27640 ppm) than sodium (22700 ppm) in the earth’s crust, but magnesium salts often aren’t soluble where sodium salts are, so magnesium stays solid while more sodium is in solution in the oceans.
Where else are you going to put enough salt to build a wall around the earth’s equator thousands of feet high and thousands of feet thick?
Also, consider that the Great Lakes haven’t had enough time to build up any minerals in solution. They have only been around for the past 12,000 years or so, and water is still draining out of them. (Contrast this with the Great Salt Lake, of about the same vintage, but it doesn’t drain anywhere.) The Ocean has had some four billion years longer to accumulate stuff. And the Ocean doesn’t have anyplace to drain to.
A little thing we likes to call the “Hydrologic Cycle”, but with a geochemical twist:
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Start with some pretty dilute meteoric water (that’s rain for the uninitiated) with a slightly acidic pH (~5.6) thanks to dissolved atmospheric CO2.
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Rain falls, runs-off and/or percolates into the groundwater system. Now on terra firma, this slightly acidic water (a good solvent) starts interacting with rock. Limestone and dolostone may dissolve into Ca, Mg, and CO3 ions in solution. Feldspars alter into clays and give up Na, K, and Ca to solution. Likewise with other minerals; all but the silica polymorphs (eg., quartz) are suceptible to weathering. After a while, the water isn’t so dilute anymore.
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Surface and groundwater, now laden with (mostly) Ca, Mg, Na, K, CO3, SO4, PO4, F, Cl, NO3, and countless trace constituents, makes its way back to the ocean.
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So, all these ions are dumped into the ocean. When water from the ocean evaporates, the ions stay behind and become increasingly more concentrated. Return to step 1, repeat for 4.5 billion years, and there’s your answer.
As has been mentioned, the Great Lakes and other bodies of water with external drainage to the ocean–while by no means as dilute as meteoric water–are continuously replenished and washed out. For surface water with no external drainage (like Salt Lake), it’s the same story as the ocean except with considerably more evaporation.
I maybe wrong, but IIRC salt also will lower the freezing temperature of water, thus making it harder for a whole body of water to freeze. This probably doesn’t matter anymore, but maybe during the ice-age or something this would’ve been pretty important.
Actually, I believe that most of the salts in salt water are now believed to have been derived from underwater volcanism rather than being washed off the continents, although that’s a factor too.
Just because.
Well if you’re going to take that attitude, we might just as well say the ocean is salty because of that magic salt grinder churning away at the bottom of the ocean all these centuries. :rolleyes: