Where did the elements in seawater come from?
Most elements in seawater came from chemical weathering of minerals in the Earth’s crust and other elements came from the atmosphere. The crust is made up mostly of silicates and aluminosilicates of metals in Groups 1 and 2 of the periodic table and of iron. The crust also contains carbonates of these metals. In weathering, acidic rainwater percolates through the soil and underlying rocks and acts on metal silicates and carbonates to dissolve them. Carbonates weather more rapidly. Rainwater is usually acidic because it reacts with carbon dioxide in the atmosphere and soil to form carbonic acid (H2CO3). The dissolved ions are ultimately washed into the sea (Fig. 30). At the air-sea interface, the atmospheric gases (nitrogen, oxygen, carbon dioxide, etc.) enter the seawater in which they dissolve. The atmosphere also contains small amounts of sulfuric and nitric acids produced when oxides of sulfur and nitrogen respectively react with rainwater.
The presence of fluoride, chloride, bromide, sulfate, and borate ions, which are present in insignificant levels in the Earth’s crust, suggests that they probably originate from volcanic gases and are then dissolved in rainwater.
In spite of all the dissolved material that is transported by rivers (2.5 x 1012 t each year), the sea does not get saltier. We believe that the oceans reached their present level of saltiness quite rapidly in the early history of the Earth and since then they have maintained the same salt content. The salinity of the sea does vary from place to place. While the open ocean contains 35 ppt dissolved solids, the Mediterranean Sea (39 ppt) and the Red Sea (41 ppt) are much saltier.