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As rivers flow into the sea, rivers contain a high concentration of calcium and carbonates, and a low concentration of sodium and chlorine, and for marine water it is vice versa. How come?

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  • $\begingroup$ Without knowing too much, I'd say that the sodium and chlorine (and magnesium and sulfate) are mostly coming from hydrothermal volcanic vents. Calcium and carbonate are being used up in living things like forams and stuff and then sink to the ocean floor, making limestone. $\endgroup$ – Gimelist Mar 7 '16 at 5:38
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The oceans are not saturated in sodium or chlorine. In fact, the chemical makeup of the oceans has remained fairly constant for hundreds of millions of years. Yet rivers and other sources continually bring sodium, chlorine, and other dissolved elements and compounds to the oceans. This means some processes must exist that remove constituent elements at more or less the same rate that the world's rivers and other sources supply these constituents.

It is true that the Earth's rivers bring substantially more calcium and carbonate ions to the oceans than they bring sodium and chlorine ions. Dissolved carbon dioxide from the atmosphere provides another source of carbonate ions. Yet the oceans are indeed substantially depleted of calcium and carbonate compared to the supply from rivers. So why is this?

The oceans are substantially saltier than are the rivers that supply the oceans. This is primarily a result of evaporation. Even with that, sodium and chlorine are but a fraction of their saturation levels in the world's oceans. This is not the case for calcium and carbonate. Carbonates in general have a low saturation level, and amongst common cations, calcium carbonate has a very low saturation level. Calcium carbonate is typically at or near the saturation level in the oceans, and oftentimes above it, particularly near the surface. This makes it very easy for calcium carbonate to come out of solution.

Because calcium and carbonate are at or above the saturation level, just about anything can cause calcium carbonate to form. The key "just about anything" process that causes calcium and carbonate to come out of solution is life.

This means that calcium and carbonate have a rather low residence time in the oceans. The processes that remove sodium and chlorine are much, much slower. The slowness by which sodium and chlorine are removed versus the rapidity by which calcium and carbonate are removed are why the oceans are "upside down" compared to rivers with regard to calcium, carbonate, sodium, and chlorine.

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  • $\begingroup$ What are the processes that remove sodium and chlorine? $\endgroup$ – Peter Mortensen Mar 8 '16 at 18:17
  • $\begingroup$ This would make a good follow up question, Peter. Sodium, chloride, and other ions in seawater are removed as aerosols and transported to the land, either as dry deposition or in rainfall. I think that there is some removal during lithification and subduction. I was not aware that the removal was significant in controlling the ocean concentration. $\endgroup$ – haresfur Mar 8 '16 at 23:59
  • $\begingroup$ Other processes that remove sodium and chlorine include chemical interactions with clays, and small seas being locked away from the main oceans by plate tectonics. Those small seas generally don't last long. They were the source of much of the salt mined in salt domes nowadays. $\endgroup$ – David Hammen Mar 9 '16 at 0:07
  • $\begingroup$ But, as @haresfur mentioned, this would be better answered as a follow-on question as opposed to in comments. Comments get deleted after a while. $\endgroup$ – David Hammen Mar 9 '16 at 0:08
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Rivers from land that flow into the ocean supply the oceans salts and chlorine from billions of years​ of weathering, minerals break down and the salts accumulate in the ocean. The oceans don't flow through land. The river water will never taste salty because the salts are released in small amounts over billions of years.

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    $\begingroup$ Who upvoted this? This does not come anywhere close to answering the question. $\endgroup$ – David Hammen Mar 8 '16 at 13:15

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