HOLOCENE PALYNOLOGY AND S E DI M E NT O LO G Y 



19 



i o 



20 



\ 1.02 mm/yr 

 • Ambrosia 



30- 



40- 



50 



o 60- 



I 10- 



120 



Core I000-3C 



0.90 mm/yr 



a to- 



Q 



\ 

 \ 



Wood date 



80- 



1 



_9I0 ± 140 

 UISGS-100) 



90- 





\Core 836-5 

 \ 



100- 





\ 

 \ A. 



4070 ± 130 

 "(ISGS-219) 



4400 ±200 

 (ISGS-234) 



\ 



20 



0.83 mm/yr 

 Ambrosia 



1 



I 



Core 969-2A 



5I40± 150 

 (ISGS-220) 



Radiocarbon years B.P. (in thousands! 



Fig. 8 - Sedimentation rates from eastern Lake Michigan. A. Plot of the depth 

 of the Ambrosia increase in core 1000-3C and a radiocarbon date from 

 core 836-5; B. Plot of the radiocarbon dates from core 1000-3C; C. Plot 

 of the Ambrosia increase in core 969-2A. Rates are determined from the 

 slopes of the lines and are given in mm/yr. 



Developing agriculture and the resulting forest clearance in the Lake 

 Michigan region was a time-transgressive event moving from south to north. The 

 first major land clearing and logging began in southern Michigan in the 1830's 

 and proceeded slowly northward until about 1900, by which time the vast pine 

 forests of northern Michigan had been destroyed. By 1840 southern Michigan was 

 undergoing extensive forest clearance, and this date is assigned to the beginning 

 of the Ambrosia increase. Varying this date by as much as a decade changes the 

 sedimentation rates by only about 10 percent. 



Sedimentation Rates 



Based on the depth of the beginning of the Ambrosia increase, the calcu- 

 lated sedimentation rates for the settlement period (1840 to 1973) are 0.83 mm/yr 

 in core 969-2A and 1.02 mm/yr in core 1000-3C, both from the eastern side of the 

 lake (fig. 8), and 0.49 mm/yr for core 1001-3A, taken from the center of the south- 

 ern basin of the lake (fig. 9) . Although there was insufficient pollen for analysis 

 in the sediments from the western side of Lake Michigan, geochemical data (Line- 

 back and Gross, 1972) suggest a slow rate of pollen accumulation. A sedimentation 

 rate of less than 0.1 mm/yr in the area of core 1002-3A is indicated by radiocarbon 

 dates. 



