SPATIOTEMPORAL PATTERNS OF CLIMATIC AND VEGETATIONAL CHANGE ASSOCIATED WITH THE ALGOMA HIGH-WATER EVENT (CA. 3200-2300 CAL-YR BP) IN THE WESTERN GREAT LAKES REGION

Water-levels of the Great Lakes have fluctuated during the Holocene. Prolonged periods of high water-levels, commonly referred to as highstands, have been variously attributed to differential isostatic rebound, outlet incision, and climatic change. During the Algoma highstand (ca. 3200 to 2300 cal-yr BP), Lakes Superior, Michigan, and Huron were all confluent. Paleoecological and paleohydrological records from the western Great Lakes region provide evidence for regional increases in effective moisture during this time. We reconstructed high-resolution records of moisture variability using testate amoebae from peatlands in northern and southern portions of the region. We used these records in conjunction with other records of paleoclimate (inland lake-levels, speleothems) and vegetation (pollen, macrofossils) from sites throughout the western Great Lakes region to assess spatial and temporal dynamics of climatic and vegetational change associated with the Algoma highstand. Sites from throughout the region show increased moisture around the time of the highstand, supporting the hypothesis that climatic change caused the high water-levels. However, climatic changes during this time period were spatially and temporally variable within the region, with the northern portions of the region experiencing increased moisture before and during the first half of the highstand (3900-2700 BP), and southern portions of the region experiencing increased moisture only after ca. 2700 BP. Thus the Algoma highstand of the western Great Lakes represented a regional integration of spatially and temporally variable climatic changes. Vegetational changes also occur throughout the region during the Algoma highstand, with the expansion and/or arrival of mesic taxa (e.g., Betula alleghaniensis, Fagus grandifolia, Tsuga canadensis) in northern portions and the expansion of mesic taxa and contraction of prairie in southern portions. A denser network of well-dated sites would allow reconstruction of the temporal and spatial patterns of increasing effective moisture 3200-2300 BP, and potentially reveal the atmospheric circulation patterns that drove these dynamics.