Amazing Nature: Where did the ocean come from?

The view from Fort Abercrombie State Historical Park. (Switgard Duesterloh photo)

When you drive to work, climb a mountain, catch a fish or take a boat ride, the ocean is always there, a seemingly endless, vast, and powerful presence. Yet most textbooks about the ocean first ask the intriguing question: Where did it come from?

As I write this, I am consulting a book entitled “Introductory Oceanography” by Harold Thurman and Elizabeth Burton. The numbers used in the following text are scrounged from Chapter 2.

The cover, however, shows a picture much like the view from Fort Abercrombie State Historical Park in Kodiak with rocky reefs protruding from a sea of fog and light, reflecting on the ocean under a full moon in the background.

When we look at images from Mars, projections from the surface of other planets and images from their moons, it becomes clear that the ocean is a special feature of our homeworld. To ask where oceans come from, we first need to figure out where the water in the oceans came from.

Two of the most accepted theories could have produced several times the amount of water contained in the ocean today. The story starts 4.5 billion years ago. At that time, the earth’s crust had cooled enough to harden and turn to rock. However, there was still a lot of volcanic activity on this lifeless young planet. As the crust cooled, the sun became the main source of heat and energy at the surface, and by lucky circumstances the earth’s distance from the sun and its orbit are just right to keep parts of the surface at temperatures between 0 and 100 degree Celsius, the necessary conditions for liquid water.

Scientists with a knack for working with big numbers have calculated that the steam released from the earth’s crust through volcanic eruptions over the last 4 billion years (give or take a few), could have produced enough water to fill the ocean 100 times. So even if 99 percent of that steam were recycled, there would still be enough water for us.

Another theory looks to the sky for the source of water. Meteorites mostly made of ice hit the atmosphere at a rate of 20 per minute. Most of these never hit the surface, but the water within them stays trapped to the Earth by gravity.

Again, with a lot of calculations with big numbers, astronomers decided these would put enough water into the atmosphere to raise the water level in the ocean about .0001 inch per year. With a lot of patience, after about 4 billion years, the bathtub world ocean would also have filled up.

Water alone does not make the ocean. Where then did the salt come from? Again, we need to look to the creation of the Earth. Remember those volcanoes 4.5 billion years ago?

Besides steam, they transported a lot of other gases into the atmosphere, and some of those dissolved and eventually ended up in the ocean.

When water encounters rock, the processes of freezing, thawing, grinding and chemical weathering break down the rock. Salts within the rock leach out and dissolve, making their way in streams and rivers to the ocean. If you have seen any of the pictures from Seward and Talkeetna this week, you could tell by the muddy color of the flood that a load of dirt and rock were flushed toward the ocean.

When seawater evaporates, it leaves the salts behind, which is why the ocean near the equator is generally saltier. When water freezes, it also leaves the salt behind.

In the Greenland Sea, the cold and very salty seawater left behind by ice formation has a higher density than the seawater below it, which is warmer and less salty.

As a result, the saltier, colder water sinks to the bottom. In the Greenland Sea, a lot of water sinks to the bottom and flows southward with a deep current. After a long journey around the globe at the bottom of the sea, the same water surfaces in the North Pacific about 1,000 years later, bringing to us a lot of nutrients and salts it has picked up over time. This is the very source for all the good stuff that makes our phytoplankton grow, which feeds our zooplankton, which feeds our herring and smelt, which feed bigger fish, birds, marine mammals and the Kodiak fishing industry.

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