The Shaky Solution for a Too-Salty Sea

With the aftershocks from the latest earthquakes in San Francisco and China, millions of people received lessons about seismic events. Diagrams of faults and epicenters filled TV screens and newspaper pages; radio commentators lectured about plate tectonics and recurrence intervals. Amid the tragedies and terrors, no one had much good to say about earthquakes; yet, like the strains and pain of giving birth, the processes creating earthquakes are essential to the continuation of life.

For one thing, the sea would be poisonously briny without them.

Proving that requires considering first why the sea is salty. An old Scandinavian folktale says that a magic salt grinder was tossed into the ocean, where it functions eternally for want of the right charm to stop it. Oceanographers joke that the tale is quite true, except for details.

In the real world, the grinding is done by rivers and their tributaries. As rains wash the land, they carry some of it away. Raw material for the sea's salt, minerals slip into the waterways and head downhill to the ocean. Even the most transparent mountain stream carries a mineral burden, and some rivers (like the Colorado) are salty even before they meet ocean waters.

The oceans steadily evaporate, providing water vapor to the air. As fog and rain clouds, the water comes ashore again. It carries very little of the ocean's salt--only the tiny particles lifted in spray may provide nuclei for water droplets that later rain down.

The salts left behind are continually joined by new contributions brought by the rivers. Logically, if slowly, the sea should get more salty. Extrapolating from a rough average of the minerals now carried by rivers, scientists estimate that it took a hundred million years for the oceans to achieve their present three percent salinity.

But oceans have been around for a lot longer than 100 million years. Furthermore, analysis of 200-million-year-old oceanic sediments shows the ancient seas also had a salinity of just about three percent. If ocean basins were simply passive receptacles for drainage from the land, the seas should have grown increasingly salty. Because they haven't, we know something else is going on.

Anyone who's hefted a clamshell might make a quick guess about where some of the minerals go: living organisms sop them up. Sodium is necessary for all creatures. Calcium, a common river-borne element, is especially in demand for skeletons and shells. Some magnesium is needed for these structural components as well.

While biochemical processes take care of some minerals, geochemical ones account for others. Potassium binds to clays and rocks; so does sodium, although very slowly. (The average time that an atom can remain in solution in the ocean, the residence time, is about one million years for high-demand calcium, 68 million years for sodium. Chlorine, the other part of the commonest sea salt--sodium chloride, like table salt--has a practically infinite residence time.)

Eventually the residue of these activities lies as sediment on the seafloor, slowly building up. Yet over millennia, neither biology nor geochemistry could control the steady buildup of minerals. If these were the only ways to handle the land's donations, the sea still would grow more saline. The saving grace lies in the same geophysical processes that cause earthquakes.

As we've been dramatically reminded recently, the great slabs constituting the earth's crust skid about in a slow skater's waltz. As they grind past one another, or as one dips beneath another, sometimes the rocks stick--but only for a while. The driving forces continue, the strain becomes unbearable, the rocks break: earthquake. Then the inevitable movement can continue.

When crustal plates collide, the ocean floor generally is pushed under a continent-bearing block. This is what's happening from southcentral Alaska through the entire Aleutian chain, as the Pacific plate descends beneath the North American plate on which Alaska rides. In this subduction process, the old ocean floor and all the sediments atop it are recycled deep into the earth. The process causes volcanoes to erupt, laying new stone on the land; old shorelines can lift and tilt-- which is why the fossilized remains of sea creatures can lie on slopes now thousands of feet above sea level (as at Baked Mountain in Katmai National Monument, for example).

Thus it is possible, and perhaps best, to think of earthquakes as the ratcheting of a great geological conveyor belt. Through its action, the land is continually rebuilt, and the sea continually refreshed.