Stalagmites composed of calcium carbonate from Crevice Cave, southeastern Missouri were studied to reconstruct changes in climate and vegetation during the last interglacial-glacial cycle. Variations in the isotopic composition of oxygen and carbon reflect long-term changes in the mean annual temperature of the area and changes in the plant communities above the cave, respectively. A precise timeline was established by utilizing the radioactive decay of uranium to thorium, a technique known as U-Th disequilibrium dating. The period covered by the Crevice Cave record extends from the beginning of the last interglaciation, circa 130 thousand years ago (ka), to the height of the last ice age circa 23 ka. Much of the record is therefore older than the 45 ka limit of radiocarbon dating. The cave records were compared to other global and regional paleoclimatic records to better understand the nature and timing of midcontinental climatic changes during the last interglacial-glacial cycle.

Several conclusions can be drawn from the Crevice Cave record. The pattern and timing of large, long-term changes in the vegetation (e.g. forest versus grassland) can be correlated to orbitally-induced changes in solar insolation and to the major stages of sea-level change. Also, the abrupt shifts in climate known from polar ice cores that took place throughout much of the last glacial period can be identified in the oxygen isotope record from Crevice Cave. These observations suggest that midcontinental climate is controlled to first order by large, global-scale factors. However, the accurate dating and the high-resolution nature of the Crevice Cave record reveal important details that run counter to conventional paleoclimatic wisdom. One example is the period of rapid ice build-up between 76 and 71 ka, the 5a/4 sea level transition, which has traditionally been considered to be a period of dramatic global cooling. The Crevice Cave record reveals a unique midcontinental climate during this interval that featured wet, temperate conditions, with winter temperatures possibly less extreme than today. A final contribution of the Crevice Cave record involves using the high-precision radiometric dating of the stalagmites to improve the chronology of the Greenland ice core records.

More information is available at: http://www.missouri.edu/~geolwww/faculty/dorale.html