Many studies have documented that Minnesota experienced a relatively wet period in the early Holocene, followed by a rapid decrease in moisture availability ~8000 BP (cal. yr before present = 1950 CE). Moisture increased gradually but irregularly until ~4000 BP. The intensity and duration of arid conditions, however, are not well known in the Western Great Lakes region (WGL; northern WI and the Upper Peninsula, MI), east of the prairie forest border. We analyzed changes in pollen assemblages and organic content of peat and lake sediments to reconstruct changes in vegetation and moisture availability from several sites in northern Wisconsin to test the hypothesis that during this same time period, aridity also increased in the region east of the prairie-forest border in the WGL region.
Results/Conclusions
Four peat cores from sites in NE Wisconsin have a silty, inorganic layer of sediment at about 9000 BP, consistent with dry conditions 1000 years before aridity increased in Minnesota. Above the silt layer, the organic content of the sediment increases and there is a sharp increase in pollen of white pine (Pinus strobus) from 10% to 40%, as jack and/or red pine (P. banksiana, P. resinosa) decreases from 55% to 30%. Similar increases in the proportion of white pine are found at sites across the WGL region, suggesting a shift to increased moisture availability 8000 BP. Changes in organic content in shallow water cores collected from a lake in NW Wisconsin show an increase in organic sediment deposition after ~7200 BP, consistent with a rising lake-level, peaking ~6500 BP, and renewed dry conditions after 5800 BP. Our results indicate that while prairie was reaching its maximum extent in Minnesota, white pine was expanding in the WGL, leading to a 2000-year mid-Holocene wet period (8000-6000 BP). If this interpretation is correct, then climate changes may have been in opposite directions from ~10,000 to 6000 BP, creating a sharpened climate gradient between Minnesota and the WGL.