Holocene climate and vegetation change are well documented in many parts of the western Great Lakes and prairie-forest border region of North America. Uncertainties remain, however, and sometimes even the direction of climate change remains controversial. For instance, the Little Ice Age (LIA) ~700-100 BP in this region has traditionally been considered to be wetter and cooler than the previous several thousand years, resulting in the expansion of the Big Woods of Minnesota. An alternative hypothesis suggests that perhaps dry conditions may have led to a decrease in fuel and fire, which allowed expansion of taxa less tolerant of fire. Our previous data from the northwest Wisconsin sand plain (NWSP, about 200 km NE of the Big Woods) demonstrate that charcoal accumulation decreased in lake sediments after about 700 years ago, as pollen of jack/red pine (P. banksiana/resinosa) decreased and more mesic species such as white pine (Pinus strobus) increased. We interpreted these ecological changes to be the result of increased available moisture. In this study we analyze shallow-water sediment cores from Cheney Lake, on the NWSP, and use sediment composition, loss-on-ignition (LOI) and diatom-inferred lake level history to test alternative hypotheses about changes in relative moisture after 700 BP.
Results/Conclusions
Our results are consistent with high long-term average moisture availability during the last 700 years, supporting the hypothesis that increased moisture availability led to the increase in mesic taxa during the LIA. Sediment composition and LOI indicate a large increase in the lake-level between 4000-3000 years ago, consistent with studies throughout the region that indicate increased moisture at this time. LOI decreased sharply 1500 years ago, indicating an extended dry period during which sand was deposited deeper in the lake basin, and then rose again, reaching approximately modern levels by 700-800 years ago. Analyses of diatom assemblages also indicate a sharp decrease in moisture availability 1500 years ago, with water levels returning by at least 500 years ago. Additional radiocarbon dates would improve the certainty of the timing of moisture increases, but our data show no evidence of an extended moisture deficit in the last millennium. This result supports the hypothesis that LIA expansion of mesic taxa on the NWSP 700 years ago was favored by increased moisture.