Chuck Bednar for redOrbit.com – @BednarChuck
At its peak, it was the largest native community in North America, and recent research led by University of Wisconsin-Madison geography experts Samuel Munoz and Dr. Jack Williams have discovered the possible reason for the sudden disappearance of Cahokia.
According to National Geographic, Cahokia makes its first appearance in archaeological data around AD 400, and by AD 1050, it had become a “major political and cultural center” that was home to tens of thousands of people. Some 300 years later, the hub that once occupied the land near modern-day St. Louis had suddenly disappeared for long-unknown reasons.
Now, in the journal Proceedings of the National Academy of Sciences (PNAS), Munoz, Dr. Williams, and their colleagues reported that both the emergence and decline of this ancient city-state coincided with shifts of flood frequency on the Mississippi River.
What led to Mississippi megafloods?
The researchers analyzed sediment cores to examine the timing of major floods that occurred along the Mississippi over the past 1,800 years. They found that Cahokia originally emerged in what they described as a period of reduced megaflood frequency associated with increasingly arid conditions throughout the middle part of North America.
Furthermore, its decline and eventual abandonment coincided with changing conditions that led to the return of large-scale flooding driven by waning aridity in the area after AD 1200. The team reached those conclusions after studying sediment samples from Horseshoe Lake, an oxbow lake close to Cahokia, using performing laser diffraction particle size analysis, Nat Geo said.
They also believe that these shifts in the frequency and magnitude of flooding events may have played a role in the rise and fall of other early agricultural societies, not just Cahokia. They called these fluctuations “an under-appreciated but critical factor” in both the formation and dissolution of social complexity in these types of early, farming-based city-states.
Right place at the right time
Oddly enough, as Dr. Wilson told redOrbit via email, the discovery “was a bit of being in the right place as the right time. Our original research question was quite different – we originally wanted to understand the extent and intensity of early Native American land use.”
“So we collected a transect of lake-sediment cores from oxbow lakes in the Mississippi River floodplain, along a gradient of archaeological site density, from densely populated Cahokia to southerly sites with few known archaeological sites. We have been using fossil pollen extracted from the lake sediments to identify what kinds of plants have lived in those areas over the last 2000 years and study changes in forest composition.”
As they started looking at the sediments, however, he and his colleagues found distinct, light-colored silty sediments interspersed with the dark organic-rich muds, which they believe was put there when the Mississippi River flooded beyond its banks and into the oxbow lakes. In all, they were able to identify at least eight major flooding events over the past 1,800 years.
On the heels of their findings, Dr. Williams said that they “now have one of the best long-term records of flood variability in the Mississippi River,” which provides insight into how common these events are. In addition, they found that Cahokia “grew and flourished” during a period in which megafloods were uncommon, leading them to suggest that “shifts in flood regime may have been a contributing factor to the rise and decline of Cahokia.”
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