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 two millennia, but at coarse resolution or with large dating uncertainty. To address this lack of monitoring data, we analyze the pollen, diatoms, and chrysophytes preserved in the sediments of Mina Lake, Minnesota, a prairie-forest ecotone lake. We show, using a new statistical method based on differential seasonal pollen deposition, that the visible laminations in Lake Mina’s well-preserved sediment are annually deposited varves, which allows high-resolution dating. May and February mean monthly temperatures and “annual precipitation minus potential evapotranspiration” are reconstructed using a pre-settlement pollen-climate calibration set. The need for using a pre-settlement pollen-climate calibration set is demonstrated by the bias and signal distortion that arises from using a corresponding modern pollen-climate calibration set. The high-resolution (four years) of this study allows extension of the tree-ring record. By analyzing tandem pollen and siliceous micro-fossil samples, events in the aquatic and terrestrial ecosystems can be directly compared.

The 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 tandem diatom and chrysophyte records during the LIA show that Lake Mina was characterized by weak spring circulation and stable summer stratification, suggesting late ice-out conditions. The MCA was not homogenous: there was a severe century-scale drought from roughly AD 1300-1400. This century-scale drought is the widely reported continent-wide “13th century mega-drought”, which greatly exceeded the severity of 20th century droughts. The MCA was characterized by periods of strong and prolonged spring mixing, probably due to early ice-out, as is shown in the diatom and chrysophyte records, except during the 1300s megadrought when warm conditions caused the lake to stratify quickly after ice-out.
email:jmcheval@sympatico.ca