This may seem odd but I see the oceans as a collective 'sump' that is gradually becoming more saline. The salt and other minerals in the oceans must have been dissolved into the oceans as a result of the perpetual rainfall cycle of evaporation and precipitation and emptying into the seas through the earth's rivers. If that could be said to be the case, how far back can we extrapolate until the seas were fresh? There must be some measure of the amount of solvents entering the oceans through the rivers, so given the present state of the seas we ought to be able to calculate/estimate how long it has taken to reach the present concentrations. also, how long before the sea dies of salt?