New ISGS Publication Documents Paleoecology at Illinois Site

Cover of Circular 571A recently completed study took place at Pratt's Wayne Woods Forest Preserve at a site near Brewster Creek in northwestern DuPage County. The study determined that loess deposition ceased at that location about 14,670 years ago and documents paleoecological and environmental changes during the last glacial to interglacial transition. An 8-m long core was sampled from near the deepest part of a kettle basin located in a wetland that archives fossils and sediment deposited during the last glacial to interglacial period dating from about 17,000 to 10,500 calibrated years ago. Twelve radiocarbon ages indicate that the sediment succession (from bottom to top) of silt, marl, and peat corresponds with the major climatic events interpreted from Greenland ice cores. The sediment accumulation rate was found to be nonlinear and was slowest at the base of the major lithologic units. Additional cores of the kettle basin show that the zones of slow sediment accumulation grade shoreward to unconformities that are in places marked by deposits of fine sand and, occasionally, fossil remains such as a recently discovered mastodon.

The pollen record at Brewster Creek differs from that at other sites in the region, indicating significant local variation in vegetation growing under the same climatic regime. During the waning parts of the last glaciation, dominant plant types included spruce, fir, and black ash. Other species, including pine, birch, oak, and elm, became important later during the transition. Spruce pollen persisted until about 10,800 years ago at Brewster Creek, which is about 1,000 years longer than its persistence at other sites in the area, such as Nelson Lake in Kane County. Evidence from fossil ostracodes and diatoms, microscopic lake dwelling organisms, indicates that Brewster Creek initially was a lake with water enriched in sulfate and bicarbonate ions. At the beginning of the transition to the last interglacial period, the lake was depleted in ulfate ion (likely because of bacterial activity) and stayed that way for the remainder of the record. The evidence for the change in water chemistry coincides with a change in lithology from smectite bearing silt to marl. The collective evidence thus indicates that windblown silt, the parent material for most of Illinois' rich soil, ceased to be deposited at the end of the last glaciation, the Older Dryas, about 14,600 years ago. Climatic reconstructions based on ostracodes indicate that temperatures were similar to that of west central Minnesota (mean annual temperature about 5°C) through the transition and increased to about 8°C at the beginning of the last interglaciation; mean annual precipitation increased from about 650 to 700 mm/yr to 850 to 950 mm/yr. Present conditions include a mean annual temperature of 9°C and mean annual precipitation of 950 mm/yr.

These research results are now published in the ISGS Circular 571, The Late Glacial and Early Holocene Geology, Paleoecology, and Paleohydrology of the Brewster Creek Site, a Proposed Wetland Restoration Site, Pratt's Wayne Woods Forest Preserve and James "Pate" Philip State Park, Bartlett, Illinois, by B.B. Curry, E.C. Grimm, J.E. Slate, B.C. Hansen and M.E. Konen, 2008.