HIGH RESOLUTION HYDROLOGICAL VARIATIONS IN SOUTHEAST MINNESOTA DURING THE MID-HOLOCENE

Mid-Holocene climate variations played an important role in the development of the Neolithic Age culture over the world. Here we reconstruct the hydrological variability history in SE Minnesota during 3000-7269 yr BP, by using high resolution stalagmite records from Holy Grail Cave. The grey level variations show significant negative correlation with the Sr variations in the stalagmite, which indicates that they were both controlled by the hydrological conditions outside the cave. More rainfall could enhance the vegetation development over the cave, wash more organic and detrital materials into the cave, and enrich grey level in stalagmite. In contrast, more rainfall could reduce the resident time of seepage water in the cave system and decrease the Sr concentration in stalagmite. In general, rainfall decreased in SE Minnesota from 7269 yr BP to 6560 yr BP. It maintained dry climate during 6560 and 5890 yr BP, and the rainfall then gradually increased. There was a relative humid period during 5460-4530 yr BP followed by a sharp decrease of rainfall in SE Minnesota during 4530-4400 yr BP. Rainfall gradually increased after 4400 yr BP. This trend was similar to the d13C record of another stalagmite from Spring Valley Caverns in the same region. Our records also suggest two notable centennial drought events occurred around 4350-4450 yr BP and 5830-6000 yr BP. Three most significantly wet periods were observed in 4950-5050 yr BP, 6440-6500, and 7050-7269 yr BP. Spectrum analysis suggests there are significant 22, 10, and 4.8-2.6 yr cycles in the grey level record, further indicating solar activity and ENSO’s possible influences on rainfall variations in SE Minnesota during the mid- Holocene.