LAKE MICHIGANS LATE QUATERNARY LIMNOLOGICAL AND CLIMATE HISTORY FROM OSTRACODE, OXYGEN-ISOTOPE, AND MAGNETIC-SUSCEPTIBILITY

The limnology of Lake Michigan has changed dramatically since the late Pleistocene in response to the expansion and contraction of continent al glaciers, to differential isostatic rebound, and to climate change. The lake sediment's stratigraphic trends, magnetic susceptibility, de ltaO-18, and ostracode species abundance ratios provide criteria to id entify the lake's response to glacial ice and to differential isostati c rebound. The latter phenomena dominate the lake's late Pleistocene a nd early Holocene history. The lake's hydrological budget provides the primary linkage between the lake and climate, particularly effective moisture. Dissolved salts were stored in the lake's water column when the lake's output shifted toward evaporation, but were flushed when ou tput shifted toward outflow. The lake's salt storage history ay be int erpreted from some ostracode, deltaO-18, and magnetic susceptibility r ecords found in sediment cores. Climate change influenced the entire l ake's limnological history, but became the primary limnological driver from about the middle-Holocene to the present. The complex limnologic al history of Lake Michigan resulted in substantial changes in the ost racode species assemblages; from about 12,000 ka to about 5,500 ka, fi ve ostracode intervals can be identified. These ostracode intervals pr ovide a within-lake biostratigraphy and a stratigraphic reference for reconstruction of the paleoenvironmental dynamics of the lake.