Washington DC--It's often thought that humans began modifying our environment over large regions relatively recently, aided and abetted by modern technology. Now, a report by researchers at the University of Berne as well as universities in Liverpool, Kiev, and Copenhagen, helps show that humans have been dramatically altering their surroundings for thousands of years and chronicles the growing effect of humans on atmospheric pollution.
The researchers reconstructed a record of the past showing that human activity has caused lead levels to soar above what they once were naturally, and that this activity began as far back as 6,000 years ago. William Shotyk, of the University of Berne, and his colleagues analyzed changing levels and sources of lead in the atmosphere over the last 14,000 years by studying layers of peat in an ancient bog in Switzerland's Jura Mountains. The data suggest that everyone from the first plant cultivators in Europe, to miners of the Roman Empire, to Medieval silversmiths in Germany, to European petroleum companies in the 1970's, has left their mark on this peat bog by releasing lead or dust containing lead to the air. In the study, which will appear in the 11 September issue of Science, researchers traced natural variations in atmospheric lead and dust associated with the retreat of the large ice sheets from Scandinavia.
While peat bogs and ice cores have been used to analyze atmospheric lead levels for small increments of time and back to several thousand years ago, Shotyk and his colleagues present the first complete record to span approximately 14,000 years--nearly the entire time since the last ice age. They were therefore able to contrast the first 8,000 years of natural lead emissions with the contributions that humans made over the years that followed. To accomplish this task, the researchers took advantage of an unusual characteristic of most peat bogs, namely that their surface layers are isolated from ground and surface waters. Any inorganic substances or dust in the peat layers are therefore assumed to have settled out of the atmosphere. By analyzing the amount of heavy metals and dust that accumulated on the surface of the bog while it grew, scientists can create a detailed record of air quality throughout the bog's lifetime. What's more, Shotyk and his colleagues were able to identify the origin of the lead pollution--for example whether most of the lead was from dust in Scandinavia released as the glaciers receded or from lead ore brought into Europe from Australia and released through industrial use.
The researchers found that the release of lead to the atmosphere, probably from soil erosion produced when early peoples cleared land for agriculture, began to increase over natural levels about 6,000 years ago. (Tree and cereal pollen records also suggest that people were beginning to clear forested lands and grow crops at this time.) The record also shows large fluctuations that match up very well with historical events such as the rise and fall of the Roman Empire, and onset of the Industrial Revolution in Europe. In a Perspective that accompanies Shotyk and his colleagues' report, Jerome Nriagu notes that lead deposits have been valuable resources across a wide spectrum of cultures, including the ancient Greeks and Persians. "Because of the widespread commercial applications of lead," Nriagu writes, "the records of its mining and use have created a window for peeping into past cultures, customs, trade routes, and even early methods of mass manufacturing. The paper by Shotyk et al.'will expand our view into the history of lead and its impacts on human culture."
The peat bog data also indicate that lead levels appear to actually be decreasing today, as lead has been removed from gasoline and lead emissions from industrial sources have been reduced. The researchers note, however, that lead emissions are still several hundred times higher than the natural levels they found in samples over 6,000 years old.
Materials provided by American Association For The Advancement Of Science. Note: Content may be edited for style and length.
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