THE POLLEN RECORD OF THE GLACIAL-INTERGLACIAL TRANSITION IN STEVENSON'S BOG, OHIO

The late Wisconsin Laurentide ice sheet began its retreat from southern Ohio approximately 18,000 years bp. Associated with this retreat are a series of conspicuous recessional moraines, outwash deposits and topographic depressions. Stevenson's Bog, a closed basin of approximately 60m^2 located in northwestern Ohio near Indian Lake, was selected to examine the paleoclimate record. The objective of this study is to document late glacial and early postglacial climate fluctations as reflected in variations in the proxy records. The bog, located on argillose glacial till on the northern slope of the Union recessional moraine, south of the St. Johns, is surrounded by classic outwash features including kettles and other small basins. Through patterns of sedimentation and captured biota, these basins preserve a record of the post-glacial climatic maximum and erosional events on a centennial scale. Two 5-meter long cores were taken three meters apart from the bog. The cores consist, at the bottom, of 1.5m of mottled silt with sand lenses grading up into 2m of alternating red and green beds overlain by 1.5m of gytja and then peat. A date of 14,360 yrs bp at 452cm and 13,490 yrs bp at 358cm constrain the massive silt unit. This provides a chronology for ice retreat along the transect and establishes an initial time scale for post-glacial environmental variations inferred from changes in the sediment. These cores were analyzed using several proxies (primarily fossil pollen, but also grain size, magnetic susceptibility, total organics and total carbonate) to investigate corresponding environmental conditions of the Pleistocene-Holocene transition. Preliminary analyses show initial spruce-dominated pollen assemblages at and below 420cm with increasing hardwoods and pine above 330cm, suggesting that additional analyses will permit the identification of climatic events such as the Younger Dryas.