2 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 496 



of the southern part of the Lake Michigan drainage basin about 1840. By deter- 

 mining the depth at which the ragweed increase starts, a horizon dated at 1840 A. D. 

 can be established for calculating sedimentation rates and for comparing modern 

 pollution levels and sedimentation rates with those of the presettlement period. 

 This paper reports on the analysis of the pollen in four short cores from surficial 

 Holocene bottom sediments of the southern basin of Lake Michigan. 



Previous Studies of Pollen in Great Lakes Sediments 



There are few published studies of pollen in Great Lakes sediments, and 

 none has examined the sediments from Lake Michigan. Benninghoff (1962) studied 

 sediments from Lake Superior, but found pollen poorly preserved and sparse. 

 Recent studies of pollen in western Lake Superior (Bruland et al., 1975; Maher, 

 1975), however, have identified the depth of the ragweed increase and have sug- 

 gested an increase in sedimentation with the settlement of the Duluth, Minnesota, 

 area. Studies of Lake Erie sediments (Lewis, Anderson, and Berti, 19 66) and of 

 surface sediments from Georgian Bay (Anderson and Terasmae, 1966) have found 

 pollen generally well preserved. 



The most extensive program of pollen analysis of Great Lakes sediments 

 has been conducted on sediments of Lake Ontario. Analysis of surface sediments 

 from 9 stations on a 16 -km grid has resulted in maps showing the pollen content 

 of sediment for the entire lake basin (McAndrews, 19 72; McAndrews and Power, 

 1973). The pollen in Lake Ontario was poorly preserved, the majority of grains 

 being badly corroded, crumpled, and broken. A long core through Holocene and 

 late Wisconsinan sediments within the lake contained pollen assemblage zones 

 which correlate with zones determined for inland lakes in southern Ontario 

 (McAndrews, .1972). From short "Benthos" cores from six stations in deep water, 

 McAndrews (1972) found the increase in Ambrosia starting between 15 and 20 cm 

 below the surface of the sediment. 



Studies of bog and pond sediments on Beaver Island in northern Lake Mich- 

 igan (Kapp, Bushouse, and Foster, 1969) correlated lake-level fluctuations with 

 the sedimentological history of the island. However, the study did not show an 

 Ambrosia zone near the top of the sediments. Zumberge and Potzger (1956) corre- 

 lated pollen in a buried peat horizon on the Lake Michigan shore near South Haven . 

 Michigan, with high-water stands of glacial Lake Chicago. The top of this peat 

 section was dated at 4000 radiocarbon years B.P., however; and it did not contain 

 more recent sediments . 



In many cases, the pollen found in the Great Lakes sediments is neither well 

 preserved nor abundant, and the thickness of Holocene sediments varies greatly. 

 The poor preservation of pollen, the variability of sediment, and the difficulty of 

 deep-water coring have limited the number of pollen studies in the Great Lakes. 



Geology of Southern Lake Michigan 



Lake Michigan began as a proglacial lake more than 13,000 radiocarbon 

 years ago. It occupies a basin carved in Paleozoic bedrock by the Lake Michigan 

 Lobe of the Wisconsinan ice sheet. The lake basin is surrounded by moraines of 

 late Wisconsinan glaciations. The southern one-third of the lake fills one large 

 circular basin in which the water is more than 170 m deep. A bedrock high sepa- 

 rates the southern basin from several smaller basins and intervening ridges in the 

 northern two-thirds of the lake. Four glacial tills underlie the lake (Lineback, 



