GO IN DEPTH

Ancient Lead Pollution

Ancient Lead Pollution La Laguna de Rio Seco lagoon in Spain, where researchers recovered sediments containing signs of lead pollution from Neolithic times.
Photo Credit: University of Granada

Humans were polluting the air with lead as far back as 3,900 years ago.


Transcript

Prehistoric lead pollution. I’m Bob Hirshon and this is Science Update.

Researchers in Spain have found evidence of man-made lead pollution—as far back as 3,900 years ago. The evidence comes from sediments in an alpine lake. According to paleontologist Gonzalo Jimenez Moreno, the sediments’ chemistry reveals periodic spikes in atmospheric lead—starting in the Bronze Age, when humans there first started mining. To isolate metals, they’d burn away the surrounding rock, releasing other elements like lead in the process.

Moreno:
So in order to obtain the copper, which is what they really wanted to obtain, there’s always lead that goes into the atmosphere. This process is called smelting.

Other peaks occurred during the Roman Empire, and of course, the modern Industrial Revolution. One bit of good news: after hitting record highs, lead levels recently started declining again, presumably because of measures like banning leaded gasoline. I’m Bob Hirshon for AAAS, the Science Society.


Making Sense of the Research

Although humans today are impacting the environment at an unprecedented level, anthropogenic sources of pollution are hardly new. This study shows that people have been periodically polluting the air with lead, a toxic metal, for thousands of years. 

The researchers drew their conclusions by analyzing sediments at the bottom of a secluded lake in Spain's Sierra Nevada mountains, nearly ten thousand feet above sea level. The mud at the bottom of a lake accumulates gradually, layer by layer, over time. So if you can extract samples from deep beneath the mud's surface, you can essentially go back in time and see what the chemistry of the soil was like in different eras. And the chemistry of the air at the time would have a direct impact on the chemistry of the soil.

Moreno's team studied samples that dated back as far as 10,000 years. Starting 3,900 years ago, lead levels in the soil began to rise, peaking 1,000 years later. This corresponds with historical periods known as the Late Bronze Age and the Early Iron Age, in which people learned how to build things with metal. Part of that process was smelting: a form of mining in which parts of a rock are burned away until the desirable metals buried in it are left behind. This releases particles, including lead, into the atmosphere. 

The other peaks in the lead record also correspond to times when people in the area were doing a lot of mining and metal work: the Roman Empire, and the modern era, starting with the Industrial Revolution. In between these periods, when mining was less intense, lead levels dropped. 

The research also shows that lead levels in the atmosphere have finally started to decline, most likely because of massive efforts to reduce lead pollution. One of these is the elimination of leaded gasoline, which was phased out in the United States in the 1970s and 80s and in Western Europe shortly after. As a result of these efforts, emissions of lead from the transportation sector dramatically declined by 95 percent between 1980 and 1999, and levels of lead in the air decreased by 94 percent between 1980 and 1999. In parts of the developing world, however, leaded gasoline is still used, and presents a major public health risk. In addition to the leaded gasoline, other sources of lead emissions include the burning of coal, especially in developing countries such as China and India.

Now try and answer these questions:

  1. What is the major finding of this study?
  2. How did the researchers use lake sediments to measure lead pollution in earlier times?
  3. Over the last 4,000 years, levels of lead in the atmosphere appear to have risen and fallen in cycles. What makes the lead levels rise and fall?
  4. Did these findings surprise you? Why or why not? Does this change the way you look at modern-day pollution?
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Learn how humans may also have affected the climate thousands of years ago in Early Climate Change.

For one example of the effects of modern air pollution, see the Science Update Mountain Rain.


Going Further


For Educators

Learn how humans may also have affected the climate thousands of years ago in Early Climate Change.

For one example of the effects of modern air pollution, see the Science Update lesson Mountain Rain.


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