NORTHERN MIDWEST CLIMATE CHANGES LEADING UP TO THE WISCONSINAN GLACIAL MAXIMUM

A speleothem from Spring Valley Caverns (SVC) in Fillmore County, MN gives a record of Northern Midwest climate changes during the late Pleistocene. The ages of growth phases were obtained using the U/Th disequilibrium method. Consistent growth is observed from 105-88 ka, followed by short periods (on the order of several decades to several centuries) of growth at 60 ka, 53 ka, 41.5 ka, 40 ka, and 29 ka. Fluorescent banding was identified in portions of the speleothem using a Nikon A1R MP multiphoton confocal microscope. Where banding is well-developed, band counts are consistent with U-series dates and are likely annual. Analyses of stable oxygen and carbon isotopes were obtained at 5 mm intervals along the main growth phase and at 1 mm intervals across the small growth phases. The total range of δ¹⁸O values is 5‰, with a general trend from depleted values to enriched values with time. The δ¹⁸O record is interpreted as a function of temperature-dependent fractionation during calcite precipitation and fraction of convective rainfall at the cave site. This interpretation suggests a change from warmer climate with significant convective rainfall to colder climate with less convective rainfall from 105 ka to 29 ka. The δ¹³C record is consistent with this general idea with a trend from enriched δ¹³C values, consistent with C4-type prairie grasses at 105ka, to depleted δ¹³C values, consistent with a C3-type forest closer to glacial maximum. The trends in our δ¹³C values are similar to those from Crevice Cave, MO, suggesting some regional coherence in vegetation patterns. The smaller, more recent growth phases after 60 ka are likely linked to global events. The 60 ka growth phase correlates with the last centuries of Heinrich Stadial 6, immediately prior to Greenland Interstadial 17. The other short growth phases correlate with Greenland Interstadial Events.