Nav: Home

Ancient poop helps show climate change contributed to fall of Cahokia

February 25, 2019

MADISON, Wis. -- A new study shows climate change may have contributed to the decline of Cahokia, a famed prehistoric city near present-day St. Louis. And it involves ancient human poop.

Published today [Feb. 25, 2019] in the Proceedings of the National Academy of Sciences, the study provides a direct link between changes in Cahokia's population size as measured through a unique fecal record and environmental data showing evidence of drought and flood.

"The way of building population reconstructions usually involves archaeological data, which is separate from the data studied by climate scientists," explains lead author AJ White, who completed the work as a graduate student at California State University, Long Beach. "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."

Last year, White and a team of collaborators -- including his former advisor Lora Stevens, professor of paleoclimatology and paleolimnology at California State University, Long Beach, and University of Wisconsin-Madison Professor of Anthropology Sissel Schroeder -- showed they could detect signatures of human poop in lake core sediments collected from Horseshoe Lake, not far from Cahokia's famous mounds.

These signatures, called fecal stanols, are molecules produced in the human gut during digestion and eliminated in feces. As the people of Cahokia pooped on land, some of it would have run off into the lake. The more people who lived and defecated there, the more stanols evident in lake sediments.

Because the sediments of a lake accumulate in layers, they allow scientists to capture snapshots of time throughout the history of a region through sediment cores. Deeper layers form earlier than layers found higher up, and all of the material within a layer is roughly the same age.

White found that fecal stanol concentrations at Horseshoe Lake rise and fall similarly to estimates of Cahokia's population from better-established archaeological methods.

Schroeder, a scholar of the Cahokia area, says that excavations of the houses in and near Cahokia show human occupation of the site intensified around A.D. 600, and by 1100, the six-square-mile city reached its peak population. At the time, tens of thousands of people called it home.

Archaeological evidence also shows that by 1200, Cahokia's population was on the decline and the site was abandoned by its mound-building Mississippian inhabitants by 1400.

Scientists have uncovered a number of explanations for its eventual abandonment, including social and political unrest and environmental changes.

For instance, in 2015, co-author Samuel Munoz, a former UW-Madison graduate student and now a professor at Northeastern University, was actually the first to collect one of the Horseshoe Lake sediment cores White used in his study and he found evidence that the nearby Mississippi River flooded significantly around 1150.

White's latest study ties the archaeological and environmental evidence together.

"When we use this fecal stanol method, we can make these comparisons to environmental conditions that hither to now we haven't really been able to do," says White, now a PhD student at UC Berkeley.

Using Munoz's core and another White collected on Horseshoe Lake, the research team measured the relative amount of fecal stanols from humans present in sediment layers. They compared these to stanol levels known to come from bacteria in the soil in order to establish a baseline concentration for each layer.

They examined the lake cores for evidence of flooding and also looked for climate indicators that would inform them whether climate conditions were relatively wet or dry. These indicators, the ratio of a heavy form of oxygen to a light one, can show changes in evaporation and precipitation. Stevens explains that as water evaporates, the light form of oxygen goes with it, concentrating the heavy form.

The lake core showed that summer precipitation likely decreased around the onset of Cahokia's decline. This could have affected the ability of people to grow their staple crop maize.

A number of different changes begin to happen in the archaeological record around 1150, Schroeder explains, including the number and density of houses and the nature of craft production.

These are all indicators of "some kind of socio-political or economic stressors that stimulated a reorganization of some sort," she says. "When we see correlations with climate, some archaeologists don't think climate has anything to do with it, but it's difficult to sustain that argument when the evidence of significant changes in the climate shows people are facing new challenges."

This has resonance today, she adds.

"Cultures can be very resilient in face of climate change but resilience doesn't necessarily mean there is no change. There can be cultural reorganization or decisions to relocate or migrate," Schroeder says. "We may see similar pressures today but fewer options to move."

For White, the study highlights the nuances and complications common to so many cultures and shows how environmental change can contribute to social changes already at play.
-end-
The study was supported by the Geological Society of America and California State University, Long Beach.

Kelly April Tyrrell, kelly.tyrrell@wisc.edu, 608-262-9772

CONTACT: AJ White, 949-521-0060, ajwhitesemail@gmail.com; Lora Stevens, 562-985-4817, lora.stevens@csulb.edu; Sissel Schroeder, 608-262-0317, sissel.schroeder@wisc.edu

University of Wisconsin-Madison

Related Climate Change Articles:

Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).
Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.
Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.
Could climate change cause infertility?
A number of plant and animal species could find it increasingly difficult to reproduce if climate change worsens and global temperatures become more extreme -- a stark warning highlighted by new scientific research.
Predicting climate change
Thomas Crowther, ETH Zurich identifies long-disappeared forests available for restoration across the world.
Historical climate important for soil responses to future climate change
Researchers at Lund University in Sweden, in collaboration with colleagues from the University of Amsterdam, examined how 18 years of drought affect the billions of vital bacteria that are hidden in the soil beneath our feet.
Can forests save us from climate change?
Additional climate benefits through sustainable forest management will be modest and local rather than global.
From crystals to climate: 'Gold standard' timeline links flood basalts to climate change
Princeton geologists used tiny zircon crystals found in volcanic ash to rewrite the timeline for the eruptions of the Columbia River flood basalts, a series of massive lava flows that coincided with an ancient global warming period 16 million years ago.
Think pink for a better view of climate change
A new study says pink noise may be the key to separating out natural climate variability from climate change that is influenced by human activity.
Climate taxes on agriculture could lead to more food insecurity than climate change itself
New IIASA-led research has found that a single climate mitigation scheme applied to all sectors, such as a global carbon tax, could have a serious impact on agriculture and result in far more widespread hunger and food insecurity than the direct impacts of climate change.
More Climate Change News and Climate Change Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Rethinking Anger
Anger is universal and complex: it can be quiet, festering, justified, vengeful, and destructive. This hour, TED speakers explore the many sides of anger, why we need it, and who's allowed to feel it. Guests include psychologists Ryan Martin and Russell Kolts, writer Soraya Chemaly, former talk radio host Lisa Fritsch, and business professor Dan Moshavi.
Now Playing: Science for the People

#538 Nobels and Astrophysics
This week we start with this year's physics Nobel Prize awarded to Jim Peebles, Michel Mayor, and Didier Queloz and finish with a discussion of the Nobel Prizes as a way to award and highlight important science. Are they still relevant? When science breakthroughs are built on the backs of hundreds -- and sometimes thousands -- of people's hard work, how do you pick just three to highlight? Join host Rachelle Saunders and astrophysicist, author, and science communicator Ethan Siegel for their chat about astrophysics and Nobel Prizes.