SNOWY, WET AND CALM CONDITIONS DURING THE YOUNGER DRYAS IN NORTHEASTERN ILLINOIS

In the late 1990’s, Eric Grimm continued his interest in the paleoecology of the last deglaciation by pursuing multidisciplinary studies of sites in northeastern Illinois. He was keenly interested in the radiocarbon chronology and wanted to test the idea that in the western Great Lakes bulk conventional C-14 ages of lake sediment needed reevaluation through AMS dating of syndepositional terrestrial plant macrofossils. He soon discovered the age of the “double spruce peak” in most late midwestern late glacial records was more than a thousand years too old; the younger spruce peak correlated with the Younger Dryas. Funded by grants from local county agencies, the first two sites to demonstrate his hunch involved records from Nelson Lake and Brewster Creek (BC). Eric’s interpreted pollen records, my ostracode profiles, and our age models were presented in two contract reports, and one ISGS circular.

The data from these and other studies have been used in literature review papers to imply the Younger Dryas was dry and windy in northern Illinois, claims that we never made. Ostracode and pollen records from five NE IL basins (Nelson, Crystal and Mammoth lakes; Chatsworth Bog; BC), collectively indicate cool, moist (snowy) and calm conditions during the Younger Dryas. It was Eric’s desire to pull the five stories together using Neotoma to marry the insight from combined pollen and ostracode records, in addition to incorporating soil drainage characteristics of the watershed.

The record from BC is the more recent one to have been interpreted counter to our published reconstructions (Curry et al., 2006). Its continuous ostracode and pollen records span 16.25 to 10.50 cal. kyr B.P. (IntCal20). Aquatic and terrestrial ecology was strongly influenced the basin’s geologic setting (pitted outwash plain) and open hydrology. BC's subdued double spruce peak was attributed to well-drained soils and elucidated by separation of black and white spruce pollen. Analogs of the ostracode record indicate the Younger Dryas is part of an overall trend during the last deglaciation in NE IL towards moister and warmer conditions. There is no evidence at BC or in any of the other four basins that the YD was dry and windy. I attribute the uniquely white sediment color of YD age to abundant micrite and biogenic carbonate to high productivity typical of temperate hard-water lakes; ostracode ecology further indicates significant groundwater influence; abundant Abies pollen (from Eric) suggest abundant moisture from infiltration of snow melt through porous soils.