CONTRASTS IN RAINFALL AND VEGETATION BETWEEN THE HOLOCENE AND LAST INTERGLACIAL IN THE NORTH AMERICAN MIDCONTINENT

The expansion of prairie from west to east during the middle-Holocene is a well-documented climatic feature throughout much of the Upper Midwest. However, the southern limb of this mid-Holocene prairie wedge is poorly understood, as it lies outside of the glacial margin, and few lakes and wetlands are available for vegetation reconstruction based on pollen or plant macrofossils. Crevice Cave, Missouri is situated along this southern margin, and offers an alternative means of reconstructing past vegetation based on carbon isotopes. The d¹³C record of the last 8,000 years shows variation of around 3 per mil, which is a modest range of variability and suggests, perhaps, that a full-blown prairie never established itself during this time, but instead savanna, like that of pre-settlement times, existed throughout this long interval. In contrast, the d¹³C record of the last interglacial period between 130-120 ka shows a sharp excursion of nearly 6 per mil between 128-125 ka, and is interpreted as a distinct interval of prairie. Another climate proxy within Crevice Cave is that of detrital laminae preserved in stalagmite calcite, which are interpreted as flood layers, and times of very high rainfall. This proxy suggests that, in general, interglacial periods are times of frequent heavy rainfall events compared to glacial times. Perhaps surprising is that the most intense period of heavy rainfall of the past 130,000 years is that of the last 3,000 years.