Climate variability in ecotonal west-central Minnesota (AD 1116--2002): Implications for the North American Great Plains

Drought is endemic to the North American Great Plains, causing severe economic consequences. However, instrumental climatological data only exist from ca. AD 1895. The dendrological drought record exists from ca. AD 1650, but has been spatially extrapolated to the Prairies. Limited paleolimnological, archeological and eolian activity records document more than two millennia, but at coarse resolution or with large dating uncertainty. This lack of monitoring data inhibits understanding of drought or climate in this region. To address this problem, I analyze the pollen, diatoms, and chrysophytes preserved in the sediments of Lake Mina, Minnesota, located just south of the coniferous/deciduous forest ecotone. We show, using a new statistical method based on differential seasonal pollen deposition, that the visible laminae in Lake Mina's well-preserved sediment are annually deposited varves, which allows high-resolution dating and sampling (i.e. every four years). We demonstrate that using a typical modern pollen-climate calibration set to infer paleoclimate causes significant signal distortion. We remove this distortion by using a new pre-settlement pollen-climate calibration set which we compile. May and February mean temperatures and "annual precipitation minus potential evapotranspiration", the climate variables that pollen shows a significant, independent response to, are reconstructed using the new calibration set.;The controversial Little Ice Age (LIA) (AD 1500-1870) was colder than the Medieval Climate Anomaly (MCA) (AD 1100-1500) in west-central Minnesota. Winter temperatures in the LIA declined more than summer ones. The LIA occurred in three phases: an initial phase from AD 1505-1575, near recovery, and then the coldest phase from AD 1625-1775. The diatom and chrysophyte records show that Lake Mina during the LIA was characterized by weak spring circulation and summer nutrient-poor waters, suggesting late ice-out conditions. The MCA was not homogenous: a severe century-scale drought occurred during approximately AD 1300-1400. This drought is the well-documented continent-wide "13 th century mega-drought", which greatly exceeded the severity of 20 th century droughts. The MCA was characterized by periods of strong, prolonged spring mixing, probably due to early ice-out, as shown by the diatom and chrysophyte records, except during the 1300s megadrought when warm conditions caused the lake to stratify intensely after ice-out.