A DUNE-BUILDING SIGNATURE OF OUTBURSTS OF GLACIAL LAKE AGASSIZ ACROSS EASTERN UPPER MICHIGAN

Recent focus on the Holocene climate history of North America has fostered several studies of stabilized sand dunes far inland of Lake Superior in Upper Michigan. Deterioration of vegetation due to Altithermal drought and fire has been suggested as the primary agent of increased sand supply to these dunes, recently optically aged at ~7-6 ka. However, such destruction of forests in humid eastern North America is supported neither by regional pollen studies nor by integrated models of change in biome boundaries during the Holocene. To further investigate the history of inland dunes, we collected and processed 50 OSL samples from stabilized sand dunes in interior eastern Upper Michigan. Our samples yielded optical ages ranging from ~12.1 to 6.5 ka and suggest that emplacement of most interior dunes is not related to Altithermal drought. Rather, major dune genesis appears temporally correlated with dramatic changes in lake level caused by outbursts from glacial Lake Agassiz into Lake Minong, ~10.5 to 8.5 ka. These outbursts repeatedly raised Minong’s level by tens of meters, back flooded tributaries and eventually led to breaching of the Nadoway-Gros Cap Barrier and the destruction of Lake Minong. Physical disturbance wrought by these floods led to most dune building. Altithermal drought may be responsible for delayed recovery of vegetation in some areas until ~6.5 ka. The largest dunes appear to have been nourished by deflation of lake-edge sand flats that expanded rapidly coeval with regressive Lake Minong. Radiocarbon ages from organic debris in wetland and lake sediment cores and ground penetrating radar (GPR) data transecting buried valleys are consistent with flood-induced drainage alteration, ~10.5-8.5 ka. Cross-sectional gravity surveys of some stream valleys suggest buried tunnel channels which may have greatly enhanced groundwater flux and downstream emergence of surface water during outbursts. Such events likely disrupted riparian landscapes and led to dune building.