High-resolution speleothem record of late Quaternary climate change from the upper Midwest, USA

Speleothem samples were studied from southeastern Minnesota in order to understand high frequency climate variability in the mid-continental United States. Three Holocene stalagmites were studied from Spring Valley Caverns located in Fillmore County. The chronology for these samples was established with 230Th dates and annual band counting. Oxygen isotopic composition of these stalagmites was interpreted as warm/dry when heavy and cold/wet when light. The carbon isotopic variations represented changes in vegetation composition as well as aridity. These speleothem isotopic records were compared with regional paleoclimate records to establish conformity with regional climate. Next comparison was performed with other records from the North Atlantic region. A strong correlation was observed between the North Atlantic climate and the mid-continent suggesting a strong coupling between the climates of the two regions.; The stalagmite samples were also observed to contain fluorescent bands. These bands were imaged with a laser scanning confocal microscope. The band-counted ages were compared with 230Th ages and the bands were demonstrated to be annual with an uncertainty of 2%. The bands were used as additional chronometers. Detrital layer analysis from these samples revealed that these are composed of clay. Comparison with instrumental record and simple hydrological modeling showed that these layers are deposited on the growing stalagmite surface after an extreme rainfall event. Comparison with regional paleohydrological record showed increased frequency of cave floods and therefore extreme rain events during increased moisture availability in the region.; Stable isotope analyses from samples ranging in age from ∼62 ka to ∼44 ka revealed variation of mid-continental climate during Marine Isotope Stage 4. High resolution dating allowed comparison with other paleoclimate records. Millennial scale oscillations, similar to those observed in the Greenland ice core record and the Asian Monsoon record in China, were also observed in the Spring Valley Record. This similarity in climatic responses between spatially distant regions again point to a strong link between the mid-continental climate and the global climate system.