HOLOCENE PALYNOLOGY AND S E DI M E NT O LO GY 21 



Pollen Assemblage Zones and Pollen Abundance 



Comparison of our Lake Michigan pollen diagrams with the work of others 

 on the region is difficult. The pollen assemblage zones apparent within the late 

 Holocene Lake Michigan sediments are not easily correlated with those at inland 

 sites, and only a few pollen diagrams are available from southwestern lower 

 Michigan. Several pollen diagrams from the Lake Michigan region, however, do 

 show some trends which may correlate with those seen in our lake diagrams. In 

 Lake Michigan pollen diagrams, the pine maximum (zone 3) below the Ambrosia 

 increase is a common feature. A similar pre- Ambrosia pine peak is seen in the 

 pollen diagram of a core from Kirchner Marsh, southeastern Minnesota, in sedi- 

 ments deposited after 1600 B.P. (Winter, 1962) and in a diagram of a core from 

 the upper sediments of Iola Bog, Wisconsin (Schweger, 1969). Pollen diagrams 

 of cores from central lower Michigan (Gilliam, Kapp, and Bogue, 1967) and eas- 

 tern lower Michigan (Jones and Kapp, 19 72) also exhibit a pine peak immediately 

 preceding the Ambrosia increase. Although it is difficult to determine a date for 

 the pine peak from published diagrams, the peak appears to be a consistent fea- 

 ture immediately preceding the ragweed rise in diagrams of pollen from throughout 

 the Lake Michigan area. 



The pollen concentration in the Lake Michigan cores varies from 4,000 to 

 90,000 grains per gram of sediment. The lowest concentrations are on the west 

 side, the highest on the east side. Before the Ambrosia increase, the highest num- 

 bers per gram are found in zone 3, the zone of the highest pine percentages. The 

 pollen in zone 3 is predominantly arboreal, and it is tempting to speculate about 

 relationships between the increase in numbers of tree pollen grains and the effects 

 of the late stages of the Little Ice Age, or Neoglacial (Flint, 1971, p. 524). Adam 

 (1967) ascribed increasing fir and heath pollen in the top of his pollen diagram 

 from the Sierra Nevada Mountains in California to the effects of this late Holo- 

 cene period of climatic cooling. Using the regression line plotted from core 1000- 

 3C (fig. 8, A), zone 3, the pine maximum, falls between 1075 and 130 radiocarbon 

 years B.P. These dates coincide with the late Neoglacial (Denton and Porter, 

 1970; Porter and Denton, 1967). Wendland and Bryson (1974) have identified 850 

 B.P. as the time of a "major botanical discontinuity" in North America. Additional 

 support for a climatic cooling during the deposition of zone 3 is shown by the in- 

 crease in Pioea in core 1000-3C. This spruce increase, reflected both in relative 

 abundance (fig. 4) and in pollen concentration (fig. 5), peaks after 1075 B. P., 

 probably at about 800 B.P. Applying the sedimentation rates (fig. 8) to date the 

 other pollen assemblage zones places the boundary of zones 4b and 4a at 71 years 

 B.P. in core 1000-3C. Extrapolating these sedimentation rates to core 969-2A 

 places the boundary of zones 2 and 1 at 2210 years B.P. 



The absolute numbers of pollen grains found in Lake Michigan sediments 

 are lower than those reported from Lake Ontario (McAndrews, 19 72) . In Lake On- 

 tario values as high as 240,000 grains per gram were found, and most samples 

 contained more than 60,000 grains per gram. The low values in Lake Michigan 

 suggest that very little air-borne pollen is settling on the surface of the lake. 

 The major sources of pollen, therefore, may be the rivers that drain into the lake. 

 As most of these rivers are located on the eastern side of the basin, the highest 

 pollen concentrations are found in cores from the east side. 



Cores from the east side of the southern basin of Lake Michigan show the 

 highest modern sedimentation rates in the lake (Lineback and Gross, 1972). The 

 base of the Ambrosia increase lies at a depth of 10 to 14 cm in cores from the 



