Why the Sea Is Salt

[A] Rivers are fresh and the sea is salt, although the sea is fed almost entirely by rivers. The explanation is that river water is not quite fresh: it carries a small quantity of dissolved mineral, picked up from the rock it has run over, and it delivers that quantity to the ocean and leaves it there. Water evaporates from the sea surface and returns as rain; the dissolved salt does not evaporate and stays behind. [B]

[C] A simple version of this argument was once used to calculate the age of the earth, by dividing the salt now in the ocean by the amount rivers deliver each year. The answer came out at about a hundred million years, and it was wrong by a factor of forty. [D] The error was not in the arithmetic but in an assumption so natural that it was never stated: that salt only ever arrives. In fact it also leaves, carried down in sediment, locked into new minerals on the sea floor and blown inland as spray. The ocean is not a bath filling up but a basin with an inflow and an outflow, and its saltiness has been roughly constant for a very long time.