THE ROLE OF AQUIFERS IN PALEOCLIMATIC RECONSTRUCTIONS OF THE CROW

Facies analysis of sediment cores collected from transects at Lake Mina located at the upstream boundary of the Crow Wing Watershed in northcentral Minnesota and at Moody Lake located near the confluence of the Crow Wing and Mississippi Rivers reveals a dramatically different hydrologic response of these lakes to the mid Holocene dry period. Low lake levels were identified along the transect by the presence of sandy shoreline facies. Between approximately 4000 to 7700 14C years before present, upland Lake Mina experienced over a 10 meter drop in lake level due to warmer and dryer conditions while the lowland Moody lake only saw about a 2 meter decline. These data support the hypothesis that lakes within different portions of a watershed respond differently to the same climatic forcing. Upland lakes will respond with higher amplitude and shorter frequency fluctuations to periodic climatic forcing than lakes located near the outflow point of a watershed. This response is due to the buffering effects of groundwater hydrodynamics.

In this study, we assess the effects of aquifer hydrodynamics on paleoclimatic records from the Crow Wing Watershed using a coupled surface water/groundwater hydrologic model. The model was first calibrated to a 50-year historical record of average annual surface water discharge, monthly groundwater, and lake level fluctuations. The calibrated model was then used to reproduce paleo-groundwater and lake levels using climatic reconstructions based on pollen-transfer functions from Williams Lake located just outside the watershed.