HOLOCENE DROUGHTS INFERRED FROM LOW LAKE LEVELS IN NORTHERN WISCONSIN

Paleoclimate records independent of pollen records are lacking in the western Great Lakes region. In this study we reconstructed lake level changes in small seepage lakes to document changes in regional water balance (precipitation - evaporation) in order to better understand how climate has changed in this region and how vegetation responded to these changes. Ground penetrating radar (GPR) was used to locate changes in sediment density. Magnetic susceptibility and loss-on-ignition (LOI) in sediment cores from near shore were used to identify dry periods when sand or peat was deposited in locations that currently collect organic lake sediments. Changes in sediment composition were radiocarbon dated.

The lowest lake-levels at both sites were in the early Holocene. Lake levels began to rise by 8000 BP. At Fallison Lake (FL) in north-central Wisconsin peat was replaced by silty sediment, followed by organic sediment. At this time, white pine populations were expanding. Organic sediment also began accumulating in near-shore cores at Cheney Lake (CL) in northwest WI at this time.

A second period of sustained low water levels began ~5800 BP at both sites and lasted until water began to rise ~3900 BP (CL) and continued rising to approximately modern levels by 3000 BP (FL & CL). The timing of this dry period is consistent with the hypothesis that dry conditions during the mid-Holocene influenced the sudden decline in hemlock populations (5400 BP) and that wetter conditions led to the subsequent recovery of hemlock (3500-3000 BP).

At CL, there is an additional brief period of low lake level ~1500 BP, a low stage that has not yet been detected at FL. This drought occurs during a time when oak decreased and jack-red pine increased at several nearby sites. Major vegetation changes do not occur at sites that were already pine dominated, suggesting that some types of vegetation may be more sensitive to climate changes than. During the most recent 1000 years neither site showed changes in sediment composition in near-shore locations suggesting that this period did not experience droughts of the magnitude experienced previously. This is consistent with pollen records that show decreases in the percentages of jack-red pine pollen and increases in white pine pollen in response to high moisture availability and/or less fire.