HOLOCENE PALYNOLOGY AND S E D I M E N T O L O G Y 7 



even though the samples were of approximately uniform volume. Because of this 

 variation, dry weight rather than sample volume was used for determining pollen 

 concentrations (Fletcher and Clapham, 1974). After the dry weight was obtained, 

 10 Stockmarr tablets, each containing 12, 500 ± 500 Lycopodium spores, were added 

 to each sample (Stockmarr, 1971). 



Pollen was extracted from the lake sediments, first by differential settling 

 to remove the heavy particles and then by acid digestion of the remaining inorganic 

 fraction. The carbonates were removed with hydrochloric acid, and the silicates 

 were digested by hydrofluoric acid. The remaining material was treated with a 

 1 -minute acetolysis, stained with basic fuchsin, and mounted in glycerin on 

 slides. Counting was done immediately to avoid any problems resulting from 

 pollen swelling due to the glycerin. Calculation of pollen concentrations was 

 made by the Benninghoff (1962) method. 



From all but three samples, a total of 200 fossil pollen grains each were 

 counted; 300 grains each were counted from the top three samples of core 1001-3A. 

 The pollen percentages were calculated on the total number of pollen grains 

 counted. The exotic grains ( Lyo op odium spores) were counted as an outside sum 

 and the fossil pollen concentration in grains per gram dry weight was calculated 

 by the formula 



(125,000) X (number of fossil grains) 

 Pollen concentration = . 



(number of exotic grains) X (g dry wt) 



RESULTS 



Diagrams of percentages of pollen were constructed for three cores (1001-3A ; 

 1000-3C, and 969-2A) (figs. 3, 4, 6). Diagrams of absolute pollen concentra- 

 tions were drawn for selected taxa from two cores, for which pollen concentration 

 data were available (figs. 5,7). Pollen concentration diagrams present the num- 

 ber of pollen grains per gram of dry weight sediment. The use of pollen concen- 

 tration data avoids the constraints imposed by the relative, or percentage, method, 

 Paleoecological interpretations are usually based on the percentage diagrams, 

 however, because concentration diagrams are not available for most geographic 

 areas. When concentration diagrams are available, they generally confirm the 

 trends in the percentage diagrams. 



Most of the pollen in southern Lake Michigan sediments is poorly pre- 

 served and badly eroded. Numerous pollen grains exhibit evidence of microbial 

 destruction. Many of them are severely thinned, and others show the character- 

 istic rosette pattern of fungal attack (Elsik, 19 71; King, Klippel, and Duffield, 

 1975). The pollen from the center of the basin is more highly degraded than that 

 from the eastern side. The degradation of pollen in the sediments probably 

 occurred before the sediments were deposited, and it indicates a long period of 

 transportation through surface streams into the lake. However, a number of per- 

 fectly preserved grains of delicate types such as Cyperaceae are present in the 

 sediments; and they probably indicate wind transport of some pollen directly into 

 the lake. Several types of Paleozoic spores also were encountered regularly 

 during analysis. These have likely undergone multiple cycles of erosion and 

 redeposition. 



