July 6, 2013
When a team of University of Pittsburgh researchers launched a boat off the Keweenaw Peninsula in northern Michigan, they were hoping to find evidence of an ancient society. They were looking for leftover lead pollution. They met their goal, but also found evidence of the oldest human-caused lead pollution in the world.
"There aren’t very many natural processes known that emit lead," says lead author David Pompeani, a Ph.D. candidate in geology and planetary science at the University of Pittsburgh. "So, lead is a very good tracer of these human activities.”
If lead spikes above a certain threshold—for Northern Michigan the baseline is 1 part per million—you can essentially assume that there was a human involved.
In this case, the lead was a result of ancient copper mining in the area. Scientists knew mining had occurred on the peninsula, but they didn't know when or to what extent. Thus, the presence of lead could help in understanding more about early North American cultures and how humans first started influencing the environment here.
Out on the water, the team shoved a long hollow tube into the lake bottom to collect a sample.
"This is similar to sticking a straw in a slushie and sticking your finger over the slushie straw and then you pull it out," says Pompeani.
Analyzing the sediment, the team found clear bands of lead that reached concentrations of up to five times the baseline. And the bands were much older than expected. The oldest was 8,000 years old—that’s about 5,000 years older than any other human-caused lead pollution known in the world. According to Pompeani, the impacts of this discovery span the fields of geology, anthropology, and climatology.
"I think one of the most important things for this discovery was that lead pollution was emitted far earlier than agriculture in North America," he says.
In other continents, agriculture and lead pollution more or less co-occurred, Pompeani says. Cultures grew more advanced, people had roles and specialties, and then lead pollution arose. This discovery, however, indicates that very early, relatively primitive societies in North America, were already working with metal. Pompeani says it also debunks a massive supposition about our environmental history.
"We have gigantic assumptions that humans weren’t modifying the environment in North America before Columbus," says Pompeani.
That is not the case, according to Pompeani's data. Instead, North Americans have been polluting for a long, long time. For Pompeani, this idea opens a door: If scientists can track this pollution, he says, they can compare it with climate records. And as more samples are taken and more historical context is generated, researchers can pinpoint with greater confidence which changes in climate are natural and which are man made.