Passage IV

NATURAL SCIENCE: This passage is adapted from the article “Flotsam Science” by Sid Perkins (©2007 by Science Service).

In January 1992, a freighter crossing the Pacific ran into rough weather. As the ship heaved through the storm-tossed seas, several cargo containers—including one filled with tens of thousands of plastic tub toys—came loose, fell overboard, and broke apart. Seven months after the spill, the plastic ducks, beavers, turtles, and frogs began washing up on beaches. Scientists who track ocean currents were ecstatic.

Even today, members of the tub-toy armada occasionally make landfall. The date and place of each of the nearly 1,000 toys recovered to date provide a data point, says Curtis Ebbesmeyer, a retired oceanographer in Seattle. Most of the drifters have remained stuck in the Pacific Subarctic Gyre, a set of deep-water and surface currents spanning an area the size of the continental United States that generally flows counterclockwise around the northern Pacific Ocean.

In most of the world, the dispersal of flotsam (floating debris) isn’t of major interest to researchers. But along the bustling trade routes that link eastern Asia to North America, the stuff that drops off ships is enabling scientists to fill in details of how the Pacific Subarctic Gyre works.

The ocean is teeming with a variety of scientific instruments. When measuring surface currents, however, these devices have their limitations. Probes specifically designed to ride surface currents have sensors which can quickly become obstructed by algae, barnacles, and other organisms that thrive in the sunlit portion of the ocean.

What’s more, probe batteries fail within months. Generally, probes haven’t traveled more than 1,000 km in that time, says Thomas C. Royer, an oceanographer at Old Dominion University in Norfolk, Va. That’s only a small fraction of the path around the gyre.

“We’ve never had a good handle on how long it takes [floating] objects to go around the gyre, or even if they do,” Royer adds.

To map the currents and clock their speeds, Ebbesmeyer, Royer, and their teammates circumvented the disadvantages of modern electronic probes by harnessing the power of floating junk. Because the Pacific is crisscrossed by major trade routes, “there’s a lot of stuff out there,” Ebbesmeyer notes. Many of those items can be traced back to specific spills, and if the lost objects are durable, they can drift in currents for years.

The team’s oldest data points—and the most ecofriendly—result from the eruption of Alaska’s Mount Katmai on June 6, 1912. Some of the pumice spewed by that volcano fell into the Gulf of Alaska. In mid-August 1914, large chunks of that frothy rock washed up on beaches of British Columbia’s Queen Charlotte Islands.

Each entry in the researchers’ flotsam database includes the latitude and longitude of the place where the item entered the ocean and of the site where it was discovered—in essence, a start point and an end point.

Because of the sheer volume of flotsam data, the scientists have been able to discern the typical configuration and average speed of the currents in the Pacific Subarctic Gyre, even though their overall pattern continually shows slight shifts and speed changes in response to the passage of large ocean eddies or variations in weather patterns caused by climate cycles such as El Ninos.

Results of computer simulations indicate that floating objects can be swept along several paths in the Pacific Subarctic Gyre. An item following the shortest loop would make a circuit in just over 2 years, the researchers estimate, and items taking the longest route would make a lap every 3.6 years. The average trip around the gyre took 3 years. This conclusion fits with observed recoveries of items that came at intervals of several years rather than being spread evenly through time.

According to the simulations, current speeds in the gyre range between 11 and 13 cm per second, or about one-fifth the speed of a typical human swimmer.

Once the researchers came up with these answers, they analyzed long-term records of water temperature and salinity at various sites in the North Pacific. They observed 3-year-long cycles in the data一"a pattern nobody noticed until the ducks came along,” says Royer.

Studying the dispersal patterns of flotsam is “interesting and creative,” says Howard J. Freeland, an oceanographer at the Institute of Ocean Sciences in Sidney, British Columbia. However, he cautions, the date of recovery of an item found on a beach may not reflect when it actually washed ashore. Ebbesmeyer and his colleagues “have good measurements, but I’m not sure of what,” he notes.