Mason Martinez Abstract

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Recording Patterns of Prehistoric Land Use Using the Loss-on-Ignition Method in Lake Sediment Cores from the Cahokia Region, Illinois, USA

Mason Martinez with Sam Munoz, PhD candidate and Jack Williams, PhD

UW Department of Geography

The area around East St. Louis, Illinois, was once a major Native American settlement called Cahokia.  The Cahokians were of an unknown tribe, and traded with many other settlements throughout the Mississippi River basin.  Because of this, it had a great economy, and an organized social structure.  However, starting in the 13th century, the population began to decline, and Cahokia was completely abandoned by 1350.  The exact reason why is not known.  Some evidence suggests resource overexploitation; other evidence suggests that severe droughts or floods led to crop failure which caused people to evacuate.  Yet, there is not enough evidence to evaluate these hypotheses.

To address this question, we collected lake sediment cores from lakes of increasing distance from Cahokia.  Closest to Cahokia is Horseshoe Lake in Madison County, Illinois (HORM).  Further south is Grassy Lake, Union County, Illinois (GRAS), and the furthest south is Horseshoe Lake in Alexander County, Missouri (HORX).  As HORX had no human activity near it at the time of Cahokia, it was considered the control lake where the signal of prehistoric human land use should be minimal.  Once all cores were collected, the method of loss-on-ignition was used to determine the percentage of organic substances, calcium carbonate, and minerals.  The cores were cut up into 1 cm sections, and sub-samples of 1 cm3 were taken from each section and put into weighed crucibles.  The samples were then heated in a muffle furnace in three stages: 100 Celsius for 12 hours, 550 Celsius for 4 hours, then 1000 Celsius for 2 hours to dry the sample, burn off organic carbon, and inorganic carbon, respectively.

Results reveal major shifts in sediment composition among all cores related to natural changes within the lake, flooding from the Mississippi River, and human activities along the lakeshore.  Our results show four distinct zones: The fourth, which is present day to 1800, has a medium amount of organic material and low amounts of CaCO3.  The third zone has relatively high organic carbon, and still low amounts of carbonate.  In the second zone, there is a sharp dip in both organic and inorganic carbon, and a very high mineral content.  In zone 1, there is a relatively high amount of carbonate, and a low amount of organic carbon.

The high mineral content in zone 2 could be caused by flooding eroding the banks and causing sediment buildup.  In zone 1, the lake was deeper than today, so there weren’t as many plants to contribute to the buildup of organic carbon, which could explain the high inorganic carbon content.  Around 1800, the settlement by Euro-American farmers accelerated the lake fill-in process, which accounts for the high organic to inorganic carbon ratio.