Holocene paleoclimate and paleoecology of the western Great Lakes region: Relationships among peatland paleohydrology, Great Lakes water levels, and vegetation history

The Holocene of the western Great Lakes region was characterized by dramatic changes in regional hydrology, vegetation, and climate. Sedimentological studies have revealed water-level variability in the upper Great Lakes at millennial to sub-centennial timescales during the Holocene, although these records have not been adequately verified by other independent paleoclimate records. Vegetation changes during this time period have been well-documented from palynological studies in the region, but independent records of climate variability are necessary to differentiate between potential causes of vegetation change. I investigated the influence of Holocene climate variability on regional hydrology and vegetation by comparing records of Lake Michigan water-levels, inland lake-levels, terrestrial vegetation, and peatland paleohydrology.; Testate amoebae and other proxies for substrate moisture (e.g., humification, macrofossils) were used to reconstruct the paleohydrology of several Sphagnum-dominated peatlands. Modern ecological studies were undertaken to confirm that substrate moisture was the dominant control on distribution of peatland-inhabiting testate amoebae in the region. A calibration dataset was then developed to infer substrate moisture conditions from fossil testate amoebae. The hydrologic histories of several peatlands in the region were reconstructed using this calibration dataset, and compared to records of terrestrial vegetation and lake-levels.; Comparisons revealed coherent patterns of hydrological, vegetational, and climatic changes, suggesting that all responded to regional changes in effective moisture related to shifts in atmospheric circulation. Lake Michigan water-level variability corresponds well to centennial-scale hydrologic variability in the investigated peatlands, and major prolonged high-water events (highstands) correlate well with major vegetation changes. Peatland records also document increased moisture during the Algoma highstand (∼3200–2300 BP) and a later highstand (∼1900–1300 BP) of Lake Michigan. Peatland hydrologic changes coincide with major vegetation changes, including the late Holocene expansions of Tsuga, Betula, and Fagus in Upper Michigan and the decline of Fagus in southeastern Michigan. A denser network of proxy-climate records would potentially help reconstruct the atmospheric circulation patterns that drove Great Lakes water-level fluctuations and vegetation change during the late Holocene.