ENVIRONMENTAL MAGNETIC STUDIES IN THE GREAT LAKES WATERSHED: CLOSED LAKE STATUS FOR THE GREAT LAKES FROM 9400-7700 CAL BP

Previous reconstructions of late glacial and post-glacial lake phases in the Great Lakes have attributed major changes in lake levels to non-climatic processes (e.g. isostatic rebound and shifts in outlet elevation). New findings of early-middle Holocene lake closure events that could only have been caused by abrupt periods of severe dry climate contrast with relatively small changes in historical records of the last two centuries and the paleo-record of the late Holocene. We propose that lake levels were driven tens of meters below their outlets after 9000 cal BP first by incursions of dry cold Arctic air masses and then by dry warm Pacific air after 8200 cal BP and these low lake levels persisted until about 7800 cal BP. Environmental magnetic studies play a key role in the overall lake-level reconstruction. This role includes dating using SV records, site-site correlation and dating using SV records, and the use of mineral magnetic parameters to reconstruct lake level and paleoclimate variations. We have done magnetic studies of cores from Lakes Erie, Ontario, Huron,and Michigan and several small headwater lakes within the Great Lakes watershed. Multiproxy studies of these cores are currently underway.