Feb. 26 (UPI) -- By measuring fecal remains and climate data among ancient lake cores from Illinois' Horseshoe Lake, researchers were able to link climate change to the decline of Cahokia, a pre-Columbian Native American city located across the Mississippi River from present-day St. Louis.
"The way of building population reconstructions usually involves archaeological data, which is separate from the data studied by climate scientists," researcher A.J. White said in a news release. "One involves excavation and survey of archaeological remains and the other involves lake cores. We unite these two by looking at both kinds of data from the same lake cores."
White helped conduct the investigation of Cahokia while he was a grad student at California State University, Long Beach.
Researchers developed a technique for measuring the biochemical signature of human feces, called fecal stanols, among ancient lake core sediments. When the people of Cahokia relieved themselves on land, rains washed the human waste into the lake.
The more people living in the prehistoric city, the more feces was washed into lake. Scientists used stanol levels as a proxy for the city's population. By measuring stanol levels among different layers of sediment, researchers were able to plot population changes over time.
Previous analysis of archaeological evidence suggests Cahokia's population rose dramatically between 600 and 1100 AD. At its peak, the city housed some 20,000 people, making it easily the largest ancient city in what is now the United States.
Archaeological evidence also suggests the city's population was already starting to decline by 1200 AD, and by 1400, the city had been abandoned.
Archaeologists have previously uncovered evidence that political and social unrest played a role in Cahokia's decline, as well as evidence that climate change accelerated the city's demise.
The new lake core analysis provided scientists a more precise measure of changes in the city's population. The cores also allowed scientists to measure geochemical indicators of precipitation levels.
Researchers found evidence of declining summer precipitation levels starting around 1150 AD. The sediment cores also revealed evidence of a significant Mississippi River flood around the same time.
The new analysis -- published this week in the journal PNAS -- suggests the people of Cahokia were experiencing social and political changes at the same time that they faced a shift in the climate, a large flood coupled with prolonged drought conditions.
Authors of the new study contend the changes in climate may have triggered some of the social and political changes reflected in the archaeological record. Declining precipitation likely made it harder to grow corn, while dramatic flooding surely taxed the social and political structures that had remained stable for 600 years.
"Cultures can be very resilient in the face of climate change but resilience doesn't necessarily mean there is no change," said Sissel Schroeder, professor of anthropology at the University of Wisconsin-Madison. "There can be cultural reorganization or decisions to relocate or migrate. We may see similar pressures today but fewer options to move."