HOLOCENE PALYNOLOGY AND S E DI M E NT O L O GY 15 



Core 969-2A 



Core 969-2A was collected to the north of core 1000-3C;the 3 .2-m core was 

 entirely within the Waukegan Member of the Lake Michigan Formation. As noted 

 earlier, the upper 5 cm of the core was discarded. 



The percentage pollen diagram for core 969-2A (fig. 6) resembles the other 

 diagrams in that the pollen assemblage is dominated by oak and pine and it is 

 divisible into the same pollen assemblage zones, with the addition of zone 1 be- 

 low 230 cm. In zone 1, the pollen concentration is high and reaches a maximum 

 of 36,000 grains per gram; the percentage of pine declines and the percentage 

 of oak increases toward the boundary of zones 1 and 2 at 230 cm. 



Zones 2 and 3 in 969-2A, from the east side of the basin, contain more 

 deciduous tree pollen, especially Platanus , Carya, and Fraxinus , thancore 1001 - 

 3A, which was taken from the center of the lake. This may reflect differential 

 transport or different source areas of these types of pollen. The time required for 

 a pollen grain to reach the center of the basin is unknown, and the buoyancy of the 

 various pollen types could have an important effect on the resulting pollen assem- 

 blage deposited on the lake bottom. 



Zone 4, which encompasses the Ambrosia increase, starts between 9 and 

 11 cm in core 969-2A. The characteristic increase in pollen concentration of 

 zone 4b occurs above 7 cm. The boundary between zones 3 and 4 is clearly marked 

 by the increase in Ambrosia, Pine pollen decreases from the middle of zone 3 to 

 the top of the diagram, and oak pollen reaches a peak at the top of zone 3 (11 cm), 

 preceding the ragweed increase, and then decreases. The pollen concentration 

 diagram (fig. 7) for core 969-2A shows the decline in arboreal taxa preceding the 

 ragweed rise by 2 cm. The decline in numbers of pine and oak pollen grains in 

 zone 4 is less pronounced in this core than in core 1000-3C, but the ragweed in- 

 crease is equally large. Ragweed increases from 3 00 to 2 0,000 grains per gram 

 between 11 cm and 5 cm. The number of oak pollen grains increases in the top 

 sample, but that of pine grains does not. The rise in numbers of pine pollen 

 grains in response to reforestation is probably contained within the discarded 

 upper 5 cm of the core. The largest numbers of oak and pine pollen grains are 

 found in zones 3 and 1 . 



The x 2 tests show significant differences in pollen percentages between 

 the sample at 5 cm and the one at 7 cm (zones 4b and 4a), between the sample 

 at 9 cm and the one at 11 cm (zones 4a and 3), and between the one at 23 cm and 

 the one at 250 cm (zones 2 and 1) . The boundary between zones 2 and 3 was not 

 identified by the x 2 tests, presumably because it is not as clearly defined as in 

 the neighboring core, 1000-3C, and because the pollen concentrations were not con- 

 sidered by the tests. In general, the pollen zones in core 969-2A, especially 

 zones 2 and 3, are not as distinctive as the zones in the other cores. 



Sediment Dating 



The increase in Ambrosia pollen in southern Lake Michigan sediments 

 permits accurate dating of the upper few centimeters of sediment. A wood frag- 

 ment found between 72 and 94 cm in core 836-5, near cores 969-2A and 1000-3C, 

 was dated at 910 ±140 (ISGS-100) radiocarbon years B.P. (fig. 8) (Lineback and 

 Gross, 1972). Attempts to date the organic carbon in Lake Michigan sediments 

 have generally yielded dates much older than the wood date. For example, a sam- 

 ple just below the Ambrosia increase in core 1000-3C was dated at 4070 ±130 



