To the editors:

Marcia Bjornerud has written an excellent and accurate portrait of the modern scientific understanding of how the sea gets its salts. One minor improvement could be made in the discussion of chlorine, the element that is most abundant and has the longest residence time in seawater. In the essay, the discussion on chlorine is immediately followed by another on the discovery of mid-ocean-ridge hydrothermal vents. Although the author does not explicitly link chlorine to hydrothermal vents, readers may make this connection.

The immense amount of chlorine in seawater and its very long residence time are not caused by the vents. Chloride, the negative ion of chlorine, is extremely chemically stable. This is due to the fact that it has eight electrons in its outermost (valence) electron shell. The chloride ion is effectively a noble gas and does not need to borrow electrons from other atoms. As a result, it has no chemistry to speak of.

The inert chloride ion has accumulated in the ocean over 120 million years or so, mainly from inputs such as rivers and volcanoes. Only a small amount has come from the vents. Chloride is primarily removed from the ocean in evaporite deposits, such as those that occur in the Persian Gulf or other desert locales where mixing with the open ocean is somewhat restricted. Chloride removal from the ocean is not so much a chemical process as a physical process. For these reasons, it has an exceedingly long residence time and high concentration.

A similar argument can be made for sodium. A small amount comes into the ocean from vents, but this is not the main reason for its immense concentration in seawater. The sodium ion also has eight electrons in its valence shell and so does not participate in chemistry. Like the chloride ion, sodium only leaves the ocean reluctantly, by the physical process of evaporation and not by a chemical process.

Many other elements come from the vents and do not have long residence times or high concentrations, such as iron.