OSTRACODE AND LITHOLOGIC EVIDENCE AT CRYSTAL LAKE, ILLINOIS, FOR COLDER, DRIER CONDITIONS DURING THE 8.2 KA EVENT

Well-dated, high-resolution lake records from the interior of North America so far have not discerned an environmental response to the 8.2 ka event. Most records indicate persistence of warm-moist conditions from the onset of the early Holocene to drier conditions. The character and timing of the latter change are different from place to place, but is notably younger than 8.2 ka such as 7.8 ka at Steele Lake, MN, 6.3 ka at Nelson Lake, IL. An ostracode record from Crystal Lake, Illinois, is unique among records from this region in that profound changes in lake level and temperature are suggested to have occurred at about 8.2 ka. A 9.8-m core sampled in 12.9 m of water has been processed at 1-cm intervals. Unlike other sediment cores of lakes in this region, ostracode valves are present throughout the record. Ten radiocarbon ages on charcoal flakes sampled every 50 cm indicate a near constant Holocene sediment accumulation rate of 4.5 x 10-3cm /yr. The lithology and ostracodes from a 6-cm-thick layer are notably different than the rest of the core. It contains abundant Limnocythere herricki, an ostracode whose modern distribution is limited to lakes on the Canadian prairies, and abundant nektic ostracodes such as Cypridopsis vidua. The abundance of these species is at the expense of Candona ohioensis, a benthic ostracode with an affinity for hard-water lakes surrounded by trees. Lithologically, the 6–cm layer is notably sandy and has abundant aquatic snail shells and calcified rootlets. The collective evidence strongly suggests that the 6-cm layer was deposited under cooler and drier conditions than adjacent layers. Linear extrapolation puts the age of the 6-cm thick layer between 8340 to 8090 cal yr BP; the control ages are located 14 cm below and 36 cm above the 6-cm layer. Two samples from the 6-cm layer have been submitted for radiocarbon assay to verify and refine the age model. An explanation for Crystal Lake's sensitivity to climatic change may involve the short residence time of the lake water and high rates of groundwater throughflow.