CHANGES IN LATE HOLOCENE SEDIMENTATION IN PRAIRIE LAKE, DUPO, ILLINOIS

^{At Horseshoe Lake, Illinois (38.693874 N, -90.069508 W), Munoz et al. (2014, 2015) found evidence for two large flooding events at ca. 1200 and 1400 AD. These events, and the location of this oxbow lake, were given as evidence of environmental stress on the decline of Mississippian culture as recorded in the Cahokia Mounds and the surrounding region. These interpretations were not necessarily well received by the archeological community (for example, see Baires et al, 2015). Our class decided to test these controversial results by coring another oxbow lake in the region. Prairie Lake was chosen because it is located in an area where the American Bottoms is narrow, and hence evidence of flooding would be from a more proximal source than the relatively distal location of Horseshoe Lake. We hypothesized that floods might be indicated by graded beds of sand with silt-rich waning periods (at Horseshoe Lake, most beds attributed to floods were not visible to the eye, and were interpreted from particle-size analyses). Earlier work at Prairie Lake had indicated the potential for a long record at 38.514310 N, -90.199806 W which we resampled with a 7-cm inner diameter Livingston corer. The total core length was 639 cm, and we bottomed out on sandy lacustrine sediment. The core was sampled every 0.5 m for grain size, clay mineralogy and microfossils. Core description and preliminary PSD results indicate a change in depositional environment around 500 cm. Below this horizon is organic-rich, rhythmically bedded fine to medium sand, and fine silty sand, while above this horizon, the sediment is mostly of silt and clay, with three thin sand beds like we had envisioned would mark individual flood events. We interpret this profound change in sedimentology to reflect a switch from deposition in a littoral zone that received high-frequency, low(er) magnitude floods, to a deeper water facies. If there is obvious correspondence with the Horseshoe Lake record, we hypothesize that the horizon at ca. 500 cm will date at about 1200 AD, effectively signaling the end of the Medieval Climate Anomaly, and onset of higher water tables, and more frequent megafloods. We note that microfossils were largely absent in the core except for one bed containing five species of ostracods common to moderately deep fresh water lakes and their littoral zones.}