Hydrosphere Contents

4. Oceans

The residence time of the dissolved salts in the oceans can be calculated

The Amazon: contributing substantial amounts of sodium to the oceans

The total amount of sodium present in the ocean is 144 x 10E14 metric tonnes. If the flux of sodium estimated as flowing down the rivers is 0.68 x 10E8 metric tonnes per year, then it can be calculated that it would take 210 million years to add all the sodium at present in the ocean. This is obviously a far shorter time than the oceans have existed. The reason is that there are removal processes occurring in the ocean as well as supply processes. Sodium is removed within the pore waters of sediments that subduct at the subduction zones. It also can be removed in those small areas of the ocean where evaporites are forming. In fact we think the sodium in the world's oceans is actually in steady state with the amount of sodium being supplied to the oceans by rivers and other processes, exactly equalling the amount which is removed by subduction and evaporite formation.

Calculate the residence time of sodium in the ocean relative to the flux from rivers. Show all workings. What processes hold oceanic sodium in steady state?