Pittsburg Basin Research Description

Rebecca Teed


My dissertation research was an analysis and interpretation of microfossils from sediment cores taken from lake sediment from central Illinois. One core, from Pittsburg Basin, spans about 150,000 years: back through the Wisconsinan glacial period (the last ice age), the interglacial before it, and into the Illinoian glacial. It is the longest such record east of the Rocky Mountains. I determined what the past vegetation around the lakes was like using fossil pollen, which I could determine to genus (for trees) or to family (for most herbaceous plants). The reconstructed vegetation record, along with a fire history obtained from charcoal particles in the same sediment layers, gave me information about succession, climate change, and human impact. I compared the fossil pollen assemblages to those from the surface sediments of lakes all over the U.S. and Canada to find modern analogs of the ancient plant communities. Fossil diatoms, the remains of siliceous algae, were also preserved in some levels of the main core, and provided a record of conditions in the lake itself, particularly of salinity and trophic status.
The base of the Pittsburg Basin core is Illinoian (penultimate-glacial) outwash, deposited at the end of the penultimate glacial. These sediments are rich in spruce pollen, mixed with pine. The levels immediately above the outwash contain a mixture of deciduous-forest and prairie pollen types. The deciduous-forest pollen consists of oak, mixed with hickory, ash, elm, sweetgum, hackberry, hazel, hornbeam, beech, sycamore, and others. The prairie types include grass, sedge, and ragweed. Forest types dominate the beginning and end of this zone and prairie types dominate the middle. I interpret these assemblages as interglacial, as they represent vegetation much like that of Illinois before homesteading: oak-hickory forests along streams and rivers, and prairie on the uplands. Between these sediments and those of the Holocene interglacial are those of the latest glacial period. The Wisconsin assemblages indicate dry conditions. The diatom assemblages indicate that the lake was eutrophic, with wetlands and slightly alkaline water. The local vegetation is prairie and wetland (with sedge and chenopod) in the early glacial and pine/spruce/oak parkland in the Late Wisconsinan. There are peaks of watermilfoil and cattail pollen in the late-glacial assemblages as well. The top two meters of sediment at Pittsburg Basin are oxidized (the lake was drained in 1926) and most of the pollen assemblages were either destroyed or biased in favor of durable types, but oak and hickory are present, along with prairie and wetland types. However, the upper levels of Catfish Pond, another lake 2.5 km away, contain well-preserved pollen. The assemblages deposited the last few millennia at Catfish Pond are similar to those of the previous interglacial from Pittsburg Basin, but with lower percentages of hardwoods other than oak.

A complete article on this topic has been published in Quaternary Research




© 2000 Rebecca Teed