Compare and contrast the main salts in river water and seawater. What will be an ideal response?

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Much of the sea’s dissolved material originated from the ability of rain, groundwater, or
crashing surf to dissolve crustal rock, crustal rock is not the only source of the ocean’s
solutes. If crustal rock is the only source, then the salts in the ocean should be like those of
concentrated river water. But they are not. River water is usually a dilute solution of
bicarbonate and calcium ions, while the principal ions in seawater are chloride and sodium.
The magnesium content of seawater would also be higher if seawater were simply
concentrated river water. The proportions of salts in isolated salty inland lakes, such as
Utah’s Great Salt Lake or the Dead Sea, are much different from the proportions of salts in
the ocean. So weathering and erosion of crustal rocks cannot be the only source of sea salts.
The components of ocean water whose proportions are not accounted for by the weathering
of surface rocks are called excess volatiles. To find the source of these excess volatiles, we
must look to Earth’s deeper layers. The upper mantle appears to contain more of the

substances found in seawater (including the water itself) than are found in surface rocks, and
their proportions are about the same as found in the ocean. Some of the ocean’s solutes are
hybrids of the two processes of weathering and outgassing. Table salt, or sodium chloride, is
an example. The sodium ions come from the weathering of crustal rocks, while the chloride
ions come from the mantle by way of volcanic vents and outgassing from mid-ocean rifts. As
for the lower-than-expected quantity of magnesium and sulfate ions in the ocean, research at
a spreading center east of the Galápagos Islands suggests that the chemical composition of
seawater percolating through mid-ocean rifts is altered by contact with fresh crust. The water
that circulates through new ocean floor at these sites is apparently stripped of magnesium and
a few other elements. The magnesium seems to be incorporated into mineral deposits, but
calcium is added as hot water dissolves adjacent rocks.