The good news is that 18 years after the Clean Air Act banned lead from gasoline, concentrations of lead in the Atlantic Ocean had dropped dramatically. The bad news is that since those measurements were made in the 1980s, the decline slowed in the 1990s, leaving present lead concentrations more than twice their pre-industrial levels.
In a recent study -- the first to document the decline in ocean lead over the past two decades -- Edward A. Boyle, professor of chemical oceanography, provides scientific proof that lead in the ocean comes from people's use of substances such as leaded gasoline. He announced his findings yesterday (February 10) at the 1998 ocean sciences meeting of the American Geophysical Union and the American Society of Limnology and Oceanography.
Since unleaded gasoline was first introduced in the early 1930s, the use of leaded gas in the United States has dropped, hitting a low point at the end of the 1980s. "Europe began slowly phasing out leaded gasoline in the 1980s and mandated elimination in the 1990s, but we suspect a lot of the lead that is in the Atlantic Ocean now comes from high-temperature industrial activities," Professor Boyle said.
While researchers have argued for years that ocean-borne lead is tied to human activities because levels are highest in waters close to emission sources, there have been no data to definitively link the time dependence of the two. Metals such as aluminum have the same distribution patterns in our oceans but occur naturally.
Professor Boyle studies trace elements that turn up in the ocean from human activities and natural causes. By understanding how trace metals in shells and sediments weave their way through moving water, researchers can use that knowledge as a tool to study how ocean circulations have changed over time, he said. Changes in ocean circulation have been hypothesized as a major fact in the evolution of ice ages.
By using the thermocline -- the subsurface layers of sea water with differing temperatures -- as a sort of tape recorder, researchers can track water after it cools and sinks from the surface into the depths.
This process can take a few years or many decades, so samples from different layers of water document the movement of lead into the deep ocean over time.
The data Professor Boyle and his colleagues have analyzed in this manner since 1979 show that lead levels peaked when the United States reported peak emissions, and dipped in the years following the phase-out of leaded gasoline.
Even before Professor Boyle collected samples of surface and subsurface seawater near Bermuda, a student provided him with a novel way to track Atlantic lead levels. Glen Shen (SB '79), who received the PhD from MIT in 1986, found that analyzing the composition of Bermudian coral, which grows a new layer each year like a tree adds a ring, provides an accurate record of its environment.
"The coral provides an environmental record that dates back to the 1880s," Professor Boyle said. "You can see the effects of the Industrial Revolution in its layers, as well as the subsequent imposition of leaded gas, which was invented in the 1920s by the same person who invented freon. At the time, both were seen as great discoveries of modern science."
Lead from gasoline gets into seawater when lead in automobile exhaust attaches to fine particles in the atmosphere. About 10 percent of the particles, which include lead from the smokestacks of industrial countries, catch a ride on the wind to remote regions of the ocean and polar ice cores.
While lead from industrialized countries moves all around the oceans, the Atlantic in particular receives lead from the United States on westerlies and from Europe on the trade winds.
After it falls into surface ocean water, lead is converted into soluble form. It then falls to deeper water on sinking biological particles, such as fecal pellets. Lead remains in surface waters for about two years, and up to a few hundred years in deep water.
While the elimination of lead from gasoline helped reduce lead levels in food, and lead paint and lead piping regulations also have reduced exposure, high-temperature industrial activities such as smelting, coal combustion and cement production may be the most recent culprits behind the residual lead in ocean surface water, Professor Boyle said.
"It's apparent that significant concentrations of anthropogenic lead still exist in the environment," he said.
A version of this article appeared in MIT Tech Talk on February 11, 1998.