FIRE, DROUGHT, AND VEGETATION CYCLES ON THE NORTHERN GREAT PLAINS

GRIMM, E.C. (1), BROWN, K.J. (2), CLARK, J.S. (3), DONOVAN, J.J. (4).

(1) Illinois State Museum, Research and Collections Center 1011 East Ash Street, Springfield, IL 62703, (2) Geological Survey of Denmark and Greenland (GEUS), Dept. of Quaternary Geology, Øster Volgade 10, DK-1350 Copenhagen K, Denmark, (3) Duke University, Biology Department Box 90338, Durham, NC 27708, (4) West Virginia University, Department of Geology and Geography 425 White Hall P.O. Box 6300 , Morgantown, WV 26506.

We have reconstructed a high-resolution (decadal scale) fire, drought, and vegetation time series from the mixed grass prairie in northwestern North Dakota. The data are from a 22-m long core from Kettle Lake. The core was sampled at 1-cm resolution for particulate charcoal, sediment mineralogy, loss-on-ignition, and pollen. Spectral and wavelet analyses reveal that that multi-decadal to centennial drought cycles have persisted on the northern plains for the last 10,000 years, though there were intervals where the cycles were muted, further compounding the dynamics of climate on the plains. A major environmental shift occurred about 5000 years ago, probably associated with a change in the seasonality of precipitation. A dominant 160-yr cycle of drought, fire, and vegetation has existed for the past 5000 years. Sediment mineralogy is a principal indicator of drought. Aragonite precipitation, which is dependent on ground water sources of calcium carbonate, distinguishes wet periods; whereas high quartz from eolian and slopewash sources, typifies dry periods. Fires were more common during wet phases because moist conditions enhance grassland productivity, resulting in greater fuel loads. In addition, wetter conditions favored grasses, which declined during droughts, when Ambrosia, Artemisia, and Chenopodiineae dominated the pollen record.

Fig. 1. Charcoal and Poaceae anomalies from Kettle Lake, North Dakota, for the past 5000 years.