A high-resolution record of late-Holocene moisture variability from a Michigan raised bog, USA

Abstract

We reconstructed the late-Holocene surface-moisture history and vegetation dynamics of a raised bog in Michigan using testate amoebae, peat humification, pollen, stomata and plant macrofossils. Our primary objective was to compare bog palaeohydrology with the water-level history of Lake Michigan, and to regional and local vegetation changes. Hydrologic histories inferred from testate amoebae and humification show similar trends, and correspond with records of past water-level variability in Lake Michigan. The bog clearly shows effective-moisture increases during the Algoma highstand (∼3200–2300 cal. BP) and a later unnamed highstand (∼1900–1300 cal. BP). Some higher-frequency fluctuations are also similar. The good correspondence indicates that bog hydrology and the water levels of Lake Michigan have been driven by changes in large-scale atmospheric circulation patterns for at least the past 3500 years. Climate during the Algoma highstand may have been spatiotemporally complex, with increasing moisture occurring several hundred years earlier in the northern portion of the western Great Lakes basin. Locally, the expansion and contraction of Picea populations on the bog surface was probably directly related to moisture conditions. The well-documented decline in Fagus populations in the central Great Lakes region after 1000 cal. BP is contemporaneous with a major shift towards drier conditions, suggesting that relative dryness caused the decline.